Q: JFET structure terminology: The U-shaped, opposite-polarity region formed near the channel to control conduction in a JFET is called the:

Introduction / Context: In a JFET, conduction occurs through a lightly doped semiconductor channel. Regions of opposite polarity form PN junctions that, when reverse biased, deplete the channel and regulate current. Recognizing the name and function of this region is vital for understanding control of channel width and device biasing. Given Data / Assumptions: n-channel JFET with p-type gate regions (or the complement for p-channel). The U-shaped control region surrounds or abuts the channel. Gate and channel form a reverse-biased PN junction during normal operation. Concept / Approach: The JFET gate is the opposite-polarity material diffused adjacent to the channel. Applying a reverse bias to the gate expands the depletion region into the channel, reducing its effective cross section and hence the drain current. This mechanism enables voltage control of current without gate current flow (ideally), yielding high input resistance devices compared to BJTs. Step-by-Step Solution: Identify opposite-polarity region next to the channel → forms a PN junction.Name of that control region → the gate.Reverse bias at the gate widens depletion → modulates channel conduction.Therefore the correct term is “gate”. Verification / Alternative check: Cross-sectional diagrams of JFETs in textbooks show p+ “fingers” or a U-shaped p region tied together as the gate around an n-type channel (or vice versa). Biasing the gate more negative (for n-channel) reduces ID. Why Other Options Are Wrong: Block/heat sink/substrate are mechanical or bulk structures, not the control region of the channel. Drain is the terminal where carriers leave the channel; it is not the U-shaped control region. Common Pitfalls: Confusing the physical shape (U) with a unique component; the function defines it as the gate. Assuming the gate conducts significant DC current; ideally it is reverse-biased and draws negligible current. Final Answer: gate

Q: Common-gate JFET characteristics: What is the typical input impedance seen at the input of a common-gate configured JFET stage?

Introduction / Context: Different FET configurations (common-source, common-gate, common-drain) present different input impedances, gains, and bandwidths. The common-gate (CG) configuration is often used for wideband or low-noise RF front-ends precisely because of its relatively low input impedance at the source node, which can match transmission lines. Given Data / Assumptions: Idealized small-signal JFET model. Gate is AC-grounded; signal applied to the source; output taken at the drain. Neglect parasitic capacitances for the basic impedance discussion. Concept / Approach: The small-signal input resistance of a common-gate JFET is approximately 1/gm (the inverse of transconductance), often only a few hundred ohms to a few kilo-ohms, which is much lower than the megaohm-level input resistance of common-source or source-follower stages. Hence, the qualitative description is “low” input impedance, not “very high.” Step-by-Step Solution: Small-signal model shows source input looking into a controlled current source with transconductance gm.Input resistance Rin ≈ 1/gm.Typical gm values in mS range → Rin in hundreds of ohms to a few kΩ → “low”.Thus, the common-gate JFET offers low input impedance suitable for matching. Verification / Alternative check: RF amplifier design texts use CG JFETs and MOSFETs to match 50 Ω or similar lines by choosing devices with suitable gm, confirming the “low input impedance” characterization. Why Other Options Are Wrong: Very high/high: describe common-source or common-drain behavior, not common-gate. Very low: overly strong; while low, it is typically not near-zero ohms. Approximately infinite: applies to gate input of CS, not CG source input. Common Pitfalls: Confusing gate input resistance (very high) with source input resistance in CG (low). Ignoring frequency-dependent capacitive reactance that can further shape input impedance. Final Answer: low

Q: Enhancement-mode MOSFET (E-MOSFET) at zero gate bias: When the gate-to-source voltage VGS = 0 V for an E-MOSFET, what is the drain current?

Introduction / Context: Enhancement-mode MOSFETs are normally-OFF devices. Unlike depletion-mode devices (JFETs or D-MOSFETs), an E-MOSFET requires a gate voltage above a threshold to induce a conducting channel. Knowing this default state is crucial for switch design, power stages, and fail-safe behavior. Given Data / Assumptions: Ideal n-channel E-MOSFET (the same logic applies for p-channel with polarities reversed). VGS = 0 V relative to the source. Leakage currents neglected for the ideal statement. Concept / Approach: With VGS = 0 V, no inversion channel exists between source and drain, so the device is OFF and conducts only negligible leakage. Once VGS exceeds the threshold voltage (Vth), an inversion layer forms, allowing significant current to flow for adequate VDS. Step-by-Step Solution: Set VGS = 0 V on an E-MOSFET → no induced channel.Drain current through the channel is absent.Thus, ideal ID = 0 A (practically, only small leakage remains).Therefore, the correct qualitative answer is “zero”. Verification / Alternative check: Datasheets show ID(off) (leakage) in the microamp or nanoamp range at VGS = 0; switching circuits rely on this normally-OFF property for safety and control. Why Other Options Are Wrong: At saturation/widening the channel: requires VGS above threshold. IDSS: JFET parameter, not applicable to E-MOSFETs. Leakage only (option e) is technically true in practice, but the standard textbook answer for ideal behavior is “zero.” Common Pitfalls: Confusing depletion-mode and enhancement-mode FET behavior. Assuming MOSFETs always pass current with any VDS; gate control is essential. Final Answer: zero

Q: Biasing methods for JFETs: Which JFET biasing scheme specifically requires a separate negative gate supply to establish the operating point (for an n-channel device)?

Introduction / Context: Biasing a JFET can be done in several ways: gate-bias (fixed bias), self-bias using a source resistor, voltage-divider bias, or feedback arrangements. Knowing which approach needs a negative gate supply (for n-channel devices) is key when selecting a topology compatible with available rails. Given Data / Assumptions: n-channel JFET requiring VGS < 0 to set the quiescent current. Single-supply versus dual-supply design considerations. Idealized components for conceptual clarity. Concept / Approach: Gate-bias (fixed-bias) sets the gate at a fixed negative potential relative to the source, typically using a dedicated negative supply VGG. Self-bias uses a source resistor to raise VS and naturally make VGS negative without a negative supply. Voltage-divider bias can reference the gate to a mid-rail point to achieve the desired VGS using one supply. Step-by-Step Solution: Identify bias schemes: fixed gate-bias, source self-bias, voltage-divider bias, feedback.Fixed gate-bias sets VG below VS using a negative rail → requires negative supply for n-channel.Self-bias lifts VS positive with a resistor → VGS becomes negative using a single positive supply.Therefore, the scheme needing a negative rail is “gate” bias. Verification / Alternative check: Typical textbook schematics show VGG negative for n-channel JFET fixed-bias. SPICE simulations confirm identical operating points can be reached via self-bias without a negative supply by choosing an appropriate source resistor. Why Other Options Are Wrong: Source and voltage-divider bias: designed to avoid a negative rail, using resistor networks. Feedback bias: often leverages drain feedback and source resistor, again avoiding a negative supply. Current-source self-bias: similarly avoids a separate negative rail. Common Pitfalls: Assuming all JFET biases need dual supplies; many single-supply options exist. Confusing MOSFET threshold behavior with JFET depletion-mode bias needs. Final Answer: gate

Question 1

JFET drain-current parameter IDSS: Which definition correctly describes IDSS for a JFET (shorted-gate drain current)?

Accepted Answer

Introduction / Context: IDSS is a key datasheet parameter for depletion-mode JFETs. It sets the upper bound of drain current when the gate-source PN junction is unbiased (VGS = 0 V). Understanding IDSS is essential for biasing, small-signal design, and interpreting spread in device characteristics. Given Data / Assumptions: Depletion-mode n-channel JFET. VGS = 0 V (gate shorted to source) while sweeping VDS into the saturation region. Temperature/time effects ignored. Concept / Approach: By definition, IDSS is the drain current at VGS = 0 V with the device biased so that it operates in the constant-current (saturation) region. Any negative VGS reduces channel width and decreases drain current below IDSS; more negative VGS drives the device toward cutoff at VGS(off). Step-by-Step Solution: Set VGS = 0 V (short gate to source).Increase VDS until the device is in saturation (beyond pinch-off).Measure ID; this value is IDSS.Note that for VGS < 0, ID < IDSS; for VGS → VGS(off), ID → 0. Verification / Alternative check: Device transfer characteristics (ID vs. VGS) show maximum current at VGS = 0 V; datasheets label this as IDSS. Curve tracers confirm this condition practically. Why Other Options Are Wrong: Minimum current (option a) is cutoff, not IDSS. VGS = –4 V (option b) is arbitrary; any negative VGS reduces ID below IDSS. “Source shorted” (option d) is ambiguous; the standard definition is the gate shorted to source (VGS = 0). “At threshold” (option e) is MOSFET terminology; JFET uses VGS(off) rather than a positive threshold. Common Pitfalls: Mixing MOSFET threshold concepts with JFET depletion behavior. Forgetting that IDSS varies widely between parts and is temperature dependent. Final Answer: the maximum possible current with VGS held at 0 V

Question 2

JFET structure terminology: The U-shaped, opposite-polarity region formed near the channel to control conduction in a JFET is called the:

Accepted Answer

Introduction / Context: In a JFET, conduction occurs through a lightly doped semiconductor channel. Regions of opposite polarity form PN junctions that, when reverse biased, deplete the channel and regulate current. Recognizing the name and function of this region is vital for understanding control of channel width and device biasing. Given Data / Assumptions: n-channel JFET with p-type gate regions (or the complement for p-channel). The U-shaped control region surrounds or abuts the channel. Gate and channel form a reverse-biased PN junction during normal operation. Concept / Approach: The JFET gate is the opposite-polarity material diffused adjacent to the channel. Applying a reverse bias to the gate expands the depletion region into the channel, reducing its effective cross section and hence the drain current. This mechanism enables voltage control of current without gate current flow (ideally), yielding high input resistance devices compared to BJTs. Step-by-Step Solution: Identify opposite-polarity region next to the channel → forms a PN junction.Name of that control region → the gate.Reverse bias at the gate widens depletion → modulates channel conduction.Therefore the correct term is “gate”. Verification / Alternative check: Cross-sectional diagrams of JFETs in textbooks show p+ “fingers” or a U-shaped p region tied together as the gate around an n-type channel (or vice versa). Biasing the gate more negative (for n-channel) reduces ID. Why Other Options Are Wrong: Block/heat sink/substrate are mechanical or bulk structures, not the control region of the channel. Drain is the terminal where carriers leave the channel; it is not the U-shaped control region. Common Pitfalls: Confusing the physical shape (U) with a unique component; the function defines it as the gate. Assuming the gate conducts significant DC current; ideally it is reverse-biased and draws negligible current. Final Answer: gate

Question 3

Common-gate JFET characteristics: What is the typical input impedance seen at the input of a common-gate configured JFET stage?

Accepted Answer

Introduction / Context: Different FET configurations (common-source, common-gate, common-drain) present different input impedances, gains, and bandwidths. The common-gate (CG) configuration is often used for wideband or low-noise RF front-ends precisely because of its relatively low input impedance at the source node, which can match transmission lines. Given Data / Assumptions: Idealized small-signal JFET model. Gate is AC-grounded; signal applied to the source; output taken at the drain. Neglect parasitic capacitances for the basic impedance discussion. Concept / Approach: The small-signal input resistance of a common-gate JFET is approximately 1/gm (the inverse of transconductance), often only a few hundred ohms to a few kilo-ohms, which is much lower than the megaohm-level input resistance of common-source or source-follower stages. Hence, the qualitative description is “low” input impedance, not “very high.” Step-by-Step Solution: Small-signal model shows source input looking into a controlled current source with transconductance gm.Input resistance Rin ≈ 1/gm.Typical gm values in mS range → Rin in hundreds of ohms to a few kΩ → “low”.Thus, the common-gate JFET offers low input impedance suitable for matching. Verification / Alternative check: RF amplifier design texts use CG JFETs and MOSFETs to match 50 Ω or similar lines by choosing devices with suitable gm, confirming the “low input impedance” characterization. Why Other Options Are Wrong: Very high/high: describe common-source or common-drain behavior, not common-gate. Very low: overly strong; while low, it is typically not near-zero ohms. Approximately infinite: applies to gate input of CS, not CG source input. Common Pitfalls: Confusing gate input resistance (very high) with source input resistance in CG (low). Ignoring frequency-dependent capacitive reactance that can further shape input impedance. Final Answer: low

Question 4

Enhancement-mode MOSFET (E-MOSFET) at zero gate bias: When the gate-to-source voltage VGS = 0 V for an E-MOSFET, what is the drain current?

Accepted Answer

Introduction / Context: Enhancement-mode MOSFETs are normally-OFF devices. Unlike depletion-mode devices (JFETs or D-MOSFETs), an E-MOSFET requires a gate voltage above a threshold to induce a conducting channel. Knowing this default state is crucial for switch design, power stages, and fail-safe behavior. Given Data / Assumptions: Ideal n-channel E-MOSFET (the same logic applies for p-channel with polarities reversed). VGS = 0 V relative to the source. Leakage currents neglected for the ideal statement. Concept / Approach: With VGS = 0 V, no inversion channel exists between source and drain, so the device is OFF and conducts only negligible leakage. Once VGS exceeds the threshold voltage (Vth), an inversion layer forms, allowing significant current to flow for adequate VDS. Step-by-Step Solution: Set VGS = 0 V on an E-MOSFET → no induced channel.Drain current through the channel is absent.Thus, ideal ID = 0 A (practically, only small leakage remains).Therefore, the correct qualitative answer is “zero”. Verification / Alternative check: Datasheets show ID(off) (leakage) in the microamp or nanoamp range at VGS = 0; switching circuits rely on this normally-OFF property for safety and control. Why Other Options Are Wrong: At saturation/widening the channel: requires VGS above threshold. IDSS: JFET parameter, not applicable to E-MOSFETs. Leakage only (option e) is technically true in practice, but the standard textbook answer for ideal behavior is “zero.” Common Pitfalls: Confusing depletion-mode and enhancement-mode FET behavior. Assuming MOSFETs always pass current with any VDS; gate control is essential. Final Answer: zero

Question 5

Biasing methods for JFETs: Which JFET biasing scheme specifically requires a separate negative gate supply to establish the operating point (for an n-channel device)?

Accepted Answer

Introduction / Context: Biasing a JFET can be done in several ways: gate-bias (fixed bias), self-bias using a source resistor, voltage-divider bias, or feedback arrangements. Knowing which approach needs a negative gate supply (for n-channel devices) is key when selecting a topology compatible with available rails. Given Data / Assumptions: n-channel JFET requiring VGS < 0 to set the quiescent current. Single-supply versus dual-supply design considerations. Idealized components for conceptual clarity. Concept / Approach: Gate-bias (fixed-bias) sets the gate at a fixed negative potential relative to the source, typically using a dedicated negative supply VGG. Self-bias uses a source resistor to raise VS and naturally make VGS negative without a negative supply. Voltage-divider bias can reference the gate to a mid-rail point to achieve the desired VGS using one supply. Step-by-Step Solution: Identify bias schemes: fixed gate-bias, source self-bias, voltage-divider bias, feedback.Fixed gate-bias sets VG below VS using a negative rail → requires negative supply for n-channel.Self-bias lifts VS positive with a resistor → VGS becomes negative using a single positive supply.Therefore, the scheme needing a negative rail is “gate” bias. Verification / Alternative check: Typical textbook schematics show VGG negative for n-channel JFET fixed-bias. SPICE simulations confirm identical operating points can be reached via self-bias without a negative supply by choosing an appropriate source resistor. Why Other Options Are Wrong: Source and voltage-divider bias: designed to avoid a negative rail, using resistor networks. Feedback bias: often leverages drain feedback and source resistor, again avoiding a negative supply. Current-source self-bias: similarly avoids a separate negative rail. Common Pitfalls: Assuming all JFET biases need dual supplies; many single-supply options exist. Confusing MOSFET threshold behavior with JFET depletion-mode bias needs. Final Answer: gate

Question 6

“OFF” versus “ON” devices: Among the listed components, which one is considered a normally-OFF device (no conduction at zero gate bias under ideal conditions)?

Accepted Answer

Introduction / Context: Switching applications and bias planning require knowing whether a device conducts by default. Depletion-mode devices (JFETs, D-MOSFETs) are normally ON and must be biased to reduce conduction, whereas enhancement-mode MOSFETs are normally OFF and need a gate drive to conduct. BJTs require base bias to conduct but are not categorized as depletion/enhancement devices. Given Data / Assumptions: Ideal device behavior at VGS = 0 for FETs. No leakage or parasitics considered for the conceptual answer. Room-temperature, low-frequency operation. Concept / Approach: An E-MOSFET lacks a physical channel at zero gate bias; it needs a gate voltage above threshold to induce an inversion layer and allow current. JFETs and D-MOSFETs have pre-existing channels and conduct at VGS = 0, requiring reverse gate bias to reduce current. BJTs are current-controlled devices; without base current they do not conduct significantly, but the “normally-OFF” classification is conventionally applied in the FET context to enhancement-mode MOSFETs. Step-by-Step Solution: Evaluate each device at zero gate/base bias.JFET/D-MOSFET → ON at VGS = 0 (depletion devices).E-MOSFET → OFF at VGS = 0; needs VGS > Vth to turn on.Therefore, the normally-OFF device is the E-MOSFET. Verification / Alternative check: Datasheets list ID(off) for E-MOSFETs at VGS = 0 and specify threshold voltages where conduction starts. Application notes for power switching emphasize the fail-safe OFF state with no gate drive. Why Other Options Are Wrong: JFET and D-MOSFET: normally ON (conduction at VGS = 0). BJT: needs base bias; classification context here is FET “OFF” vs. “ON.” Vacuum diode: conducts with correct polarity and sufficient voltage; not a gate-controlled OFF device. Common Pitfalls: Assuming “no gate drive” equals “no conduction” for all FETs; depletion devices contradict this. Confusing leakage currents with purposeful conduction. Final Answer: E-MOSFET (enhancement mode)

Question 7

Vertical-channel E-MOSFET usage: A vertical-channel (power) E-MOSFET structure is primarily selected for which application requirement?

Accepted Answer

Introduction / Context: Power MOSFETs commonly use a vertical current flow architecture (VMOS, trench MOSFETs). The vertical structure shortens the current path and increases the effective channel area, enabling low on-resistance and large current handling. Recognizing why we choose a vertical channel informs device selection for converters, motor drivers, and power supplies. Given Data / Assumptions: Power-class E-MOSFETs designed for switching. Thermal constraints managed via packages and heatsinks. Basic trade-offs among voltage rating, current capability, and switching speed. Concept / Approach: A vertical conduction path lets many unit cells be paralleled on the die, reducing RDS(on) and spreading current. While such devices can also be designed for high voltage, the core reason for the vertical approach is to deliver high current with low conduction loss. High-frequency capability is more limited by charge storage and capacitances than by channel orientation alone. Step-by-Step Solution: Identify vertical MOSFET benefit: large cross-sectional path for carriers.Result: low RDS(on) → supports high currents at acceptable losses.Therefore, the primary application reason is “high currents.”High-voltage variants exist, but current capability is the central architectural advantage. Verification / Alternative check: Datasheets show trench/vertical devices with milliohm-level RDS(on) and high continuous/peak drain currents. Lateral MOSFETs of similar area have much higher RDS(on), limiting current. Why Other Options Are Wrong: High frequencies: determined by gate charge/capacitance and driver strength, not simply vertical orientation. High resistances: opposite of the design goal (we want low on-resistance). High voltages: achievable, but the key advantage asked is high current capability. Common Pitfalls: Equating high-voltage rating with high-current capability; both depend on die design and cell density. Ignoring thermal limits; high current requires adequate heat removal. Final Answer: for high currents

Question 8

Biasing a depletion-mode MOSFET (D-MOSFET): What is the very simple bias method, often used because the device conducts at VGS = 0 V?

Accepted Answer

Introduction / Context: D-MOSFETs, like JFETs, are depletion-mode devices with a conducting channel at zero gate bias. This lets designers use extremely simple biasing in some applications, particularly as variable-resistance elements or for constant-current configurations with minimal parts. Given Data / Assumptions: D-MOSFET with channel present at VGS = 0 V. Small-signal or low-power usage where drift and tolerance are acceptable. Temperature effects ignored for the conceptual answer. Concept / Approach: Because the device naturally conducts at VGS = 0 V, one can simply tie the gate to the source (or otherwise ensure VGS ≈ 0) to obtain a baseline operating current. More elaborate schemes (self-bias, voltage-divider) add resistors to stabilize the operating point, but the simplest is often called zero-biasing. Step-by-Step Solution: Consider D-MOSFET at VGS = 0 → conduction occurs.Tie gate to source → VGS = 0 steadily.This simple approach is termed “zero biasing.”Other methods add complexity to improve stability or set specific currents. Verification / Alternative check: Manufacturer application notes show D-MOSFETs used as two-terminal current regulators or variable resistors with VGS around 0 V, demonstrating zero-bias practicality. Why Other Options Are Wrong: Self biasing and voltage-divider biasing: valid, but not the “very simple” method exploiting VGS = 0 conduction. Gate biasing/fixed negative-gate bias: require an additional supply or network, defeating the simplicity goal. Common Pitfalls: Expecting identical current across parts; zero-bias current varies with device IDSS and temperature. Using zero-bias in precision circuits without feedback or trimming. Final Answer: zero biasing

Question 9

Counting PN junctions in a JFET: How many effective diodes (PN junctions) are present in the structure of a typical JFET?

Accepted Answer

Introduction / Context: A JFET controls current via PN junctions between the gate regions and the channel. In many constructions, p+ gate regions are formed on both sides of an n-type channel (or vice versa), tied together electrically. Understanding how many PN junctions are effectively present helps when visualizing depletion regions and interpreting equivalent diagrams used in education. Given Data / Assumptions: Typical symmetrical JFET structure with gate diffusions flanking the channel. Both gate regions are electrically common (same terminal). Idealized representation for teaching purposes. Concept / Approach: Each gate diffusion forms a PN junction with the channel, so there are two PN junctions arranged around the channel and connected in parallel at the gate terminal. These junctions are reverse-biased in normal operation, expanding depletion into the channel to modulate conduction. Step-by-Step Solution: Identify p+ gate on one side of the channel → one PN junction.Identify p+ gate on the opposite side → another PN junction.Both gates are tied together → two effective PN junctions associated with the single gate terminal.Therefore, the count is “2.” Verification / Alternative check: Cross-sections and device models in textbooks frequently depict two junctions controlling a central channel; SPICE models abstract this, but the teaching diagrams remain consistent. Why Other Options Are Wrong: 1: ignores the bilateral gate structure common in many JFETs. 3 or 4: overstates the number for a standard single-gate JFET. 0: contradicts the “Junction” in Junction FET. Common Pitfalls: Assuming the two junctions behave as two separate gates; they are one terminal electrically. Confusing JFET with MOSFET, which uses an insulated gate (oxide) rather than a PN junction. Final Answer: 2

Question 10

p-channel JFET conduction direction: In a p-channel junction field-effect transistor (JFET), considering conventional current and electron flow, in which direction do electrons actually move through the device under normal operation?

Accepted Answer

Introduction / Context: A p-channel JFET is a unipolar transistor whose majority carriers are holes. While circuit analysis usually uses conventional current (the flow of positive charge), understanding the actual electron flow clarifies device physics, terminal naming, and sign conventions in equations for gate–source voltage and drain current. Given Data / Assumptions: Device is a p-channel JFET operated in its normal region (gate reverse-biased with respect to the channel). Majority carriers in the channel are holes. Conventional current direction is opposite to electron flow direction. Concept / Approach: In a p-channel JFET, conventional current flows from source to drain through a p-type channel when the gate is reverse-biased. Because electrons carry negative charge, their motion is opposite to conventional current. Therefore, if conventional current goes source → drain, electron flow must go drain → source. This is consistent with the electrostatic potentials used to bias a p-channel device (source more positive relative to drain in many textbooks’ sign conventions for p-channel JFETs). Step-by-Step Solution: Identify majority carrier type: p-channel → holes are majority carriers.Conventional current in p-channel: source → drain (carried by holes).Electron flow is opposite conventional current: drain → source. Verification / Alternative check: Think of a complementary n-channel JFET: conventional current goes drain → source, and electrons go source → drain. The p-channel case must invert both. Datasheet arrows and bias diagrams also reinforce these directions when comparing n- and p-channel devices. Why Other Options Are Wrong: From source to drain: This is conventional current (holes) in p-channel, not electron flow. From source to gate / from drain to gate: With reverse-biased gate, only small leakage exists; the main conduction is channel, not to the gate. Common Pitfalls: Confusing conventional current with electron flow and mixing n-channel vs. p-channel direction conventions. Always invert directions when switching between charge types. Final Answer: from drain to source

Question 11

Transconductance application: An input change of 2 V produces a transconductance gm = 1.5 mS. What is the resulting change in drain current (ΔID)?

Accepted Answer

Introduction / Context: Transconductance (gm) links an input voltage change to an output current change in field-effect transistors. It is a core small-signal parameter for amplifiers, directly influencing gain and bandwidth. Here we perform a straightforward calculation using the definition of gm to find the current change. Given Data / Assumptions: Input voltage change ΔV_in = 2 V (e.g., ΔV_GS for a FET). Transconductance gm = 1.5 mS = 1.5 × 10^-3 S. Linear small-signal approximation applies around the operating point. Concept / Approach: By definition: gm = ΔI_out / ΔV_in. For a FET, gm = ΔI_D / ΔV_GS. Rearranging gives ΔI_D = gm * ΔV_GS. We simply multiply the transconductance by the input voltage change, keeping units consistent (S * V = A). Step-by-Step Solution: gm = 1.5 mS = 1.5 × 10^-3 S.ΔV_GS = 2 V.ΔI_D = gm * ΔV_GS = (1.5 × 10^-3) * 2 = 3.0 × 10^-3 A = 3 mA. Verification / Alternative check: Unit check: siemens * volts = amperes, confirming dimensional correctness. Magnitude (milliamp-level) is reasonable for small-signal changes in many JFET/MOSFET amplifier stages. Why Other Options Are Wrong: 666 mA: Orders of magnitude too large for the given gm and ΔV. 0.75 mA: Would correspond to gm = 0.375 mS or ΔV = 0.5 V, not our values. 0.5 mA: Would require gm = 0.25 mS at 2 V; not given. Common Pitfalls: Forgetting to convert mS to S; mixing up ΔI_D = gm / ΔV_GS instead of multiplication; or assuming gm is constant over very large signal swings (it is a small-signal parameter around bias). Final Answer: 3 mA

Question 12

JFET terminology — are pinch-off voltage and breakdown voltage the same?  Consider a JFET: the gate-source pinch-off voltage (|V_P|) defines channel control, while breakdown (e.g., gate-to-channel avalanche) defines destructive or limiting condition…

Accepted Answer

Introduction / Context: Properly biasing a JFET requires distinguishing between the pinch-off voltage, which is a normal operating parameter, and breakdown voltages, which are stress or limit ratings. Confusing them can lead to device damage or misinterpretation of I–V curves. Given Data / Assumptions: Pinch-off voltage V_P (often specified as V_GS(off)) is the gate-source voltage at which the channel current is reduced to a small value (near cutoff). Breakdown voltages (e.g., BV_GD, BV_GS, BV_DS) indicate avalanche or tunneling onset; exceeding them risks permanent damage. Concept / Approach: V_P defines the control range of the JFET channel and is typically a few volts (negative for n-channel devices). Breakdown voltages are usually larger in magnitude and mark the onset of high, potentially destructive currents. They serve entirely different purposes: one is an operating parameter, the other a limit rating. Step-by-Step Solution: Identify V_P (or V_GS(off)) from the transfer curve: current approaches zero as V_GS → V_P.Identify breakdown on I–V curves: sharp current increase at a high reverse bias.Compare magnitudes and roles: they do not coincide; breakdown exceeds normal control range.Conclusion: calling them the same voltage is incorrect. Verification / Alternative check: Datasheets list V_GS(off) separately from breakdown ratings (BV_GS, BV_GD, BV_DS), confirming distinct definitions and values. Why Other Options Are Wrong: Correct / conditional qualifiers: contradict standard datasheet terminology and device physics. Common Pitfalls: Misreading the term “pinch-off” used for channel current saturation in FETs versus catastrophic junction breakdown events. Final Answer: Incorrect

Question 13

Control nature of a JFET — current-controlled or voltage-controlled?  Evaluate the statement: “A JFET can be either a current-controlled device or a voltage-controlled device.” Choose whether this characterization matches standard JFET operation.

Accepted Answer

Introduction / Context: Device control type—current-controlled vs. voltage-controlled—guides how we build amplifiers and bias networks. BJTs are typically modeled as current-controlled (collector current set by base current), while JFETs and MOSFETs are modeled as voltage-controlled (drain current set by gate-source voltage). Given Data / Assumptions: JFET has a reverse-biased gate-channel p-n junction under normal operation. Gate current is extremely small; input impedance is very high. Small-signal drain current depends on gate-source voltage (Id ≈ f(V_GS)). Concept / Approach: In a JFET, the electric field from the reverse-biased gate modulates the channel width. Thus, the drain current is primarily a function of V_GS, not gate current. Modeling uses transconductance gm = dId/dVGS, underscoring voltage control. Only if the gate is (undesirably) forward-biased would appreciable current flow into the gate, but this is outside proper linear operation and risks damage or distortion. Step-by-Step Solution: Bias gate reverse with respect to source (for n-channel: V_GS ≤ 0) to maintain high input impedance.Recognize Id vs. V_GS relationship via transfer curve (e.g., Shockley equation for JFETs).Note that gate current ≈ 0 in normal operation, confirming voltage control rather than current control.Hence, calling a JFET “either current- or voltage-controlled” is inaccurate. Verification / Alternative check: Datasheets specify transconductance (gm) and gate leakage currents in nanoamps or microamps, emphasizing voltage control and negligible input current. Why Other Options Are Wrong: Correct / configuration-specific / forward-bias qualifiers: still misrepresent JFETs. Proper operation keeps the gate reverse-biased, making it voltage-controlled. Common Pitfalls: Projecting BJT intuition onto JFETs; confusing rare, abnormal forward-gate conditions with intended device use. Final Answer: Incorrect

Question 14

JFET carrier flow directions: In a junction field-effect transistor, do charge carriers move from source to drain in a p-channel device and from drain to source in an n-channel device?

Accepted Answer

Introduction / Context: Understanding current and carrier flow directions in junction field-effect transistors (JFETs) is foundational in electronics. The statement mixes up conventional current with actual charge-carrier motion for p-channel and n-channel JFETs. Clarifying this distinction helps prevent sign and polarity mistakes when biasing or analyzing JFET circuits. Given Data / Assumptions: N-channel JFET uses electrons as majority carriers; p-channel JFET uses holes. Standard terminal naming: gate (G), drain (D), source (S). Biasing is such that the gate-channel junctions remain reverse-biased in normal operation. Conventional current direction is defined as positive charge flow. Concept / Approach: In an n-channel JFET, conventional drain current (ID) flows from drain to source, but electrons (negative carriers) physically move from source to drain—opposite to conventional current. In a p-channel JFET, holes are the carriers and move from source to drain; conventional current also flows from source to drain. The statement claims carriers move source→drain in p-channel (true) and drain→source in n-channel (false; electrons move source→drain). Step-by-Step Solution: Identify the device: n-channel or p-channel.Recall carrier type: electrons (n-channel), holes (p-channel).Relate carrier motion to conventional current: carrier motion is opposite to conventional current for electrons and the same for holes.Evaluate claim: p-channel portion is consistent; n-channel portion is not—hence the overall statement is incorrect. Verification / Alternative check: Use the sign of VDS and the established symbol conventions: for an n-channel device, typical operation has the drain at a higher potential than the source. Electrons flow toward the drain terminal internally from the source side, confirming source→drain carrier motion for electrons. Why Other Options Are Wrong: Depends only on VDS: channel type and carrier charge determine direction, not VDS alone.Correct: contradicts n-channel carrier motion.Undefined: carriers absolutely move in the channel under bias.Always drain→source: true only for conventional current in n-channel, not for carrier motion. Common Pitfalls: Confusing conventional current with electron flow; assuming both channel types share the same carrier direction; forgetting that diagrams typically show current arrows, not carrier drift direction. Final Answer: Incorrect

Question 15

Voltage-divider biased JFET: Is the drain current ID at its maximum value when VGS = 0 V?

Accepted Answer

Introduction / Context: Bias curves for JFETs show how gate-to-source voltage (VGS) controls the drain current (ID). A common memory point is that the maximum ID under normal operation occurs at VGS = 0 V and is denoted by IDSS. Recognizing this helps designers quickly estimate operating points in voltage-divider bias networks. Given Data / Assumptions: JFET is operating in its normal region with the gate-channel junction reverse-biased. VGS controls channel pinch through depletion width. IDSS is defined as ID at VGS = 0 V. Shockley equation models behavior: ID = IDSS * (1 − VGS/VP)^2 for VGS between 0 and VP (negative for n-channel). Concept / Approach: As VGS becomes more negative (for n-channel) or more positive (for p-channel), the channel is pinched more strongly, reducing ID. Therefore, when VGS is at 0 V, the gate exerts the least depletion effect, giving the maximum drain current IDSS for a given device and temperature. Step-by-Step Solution: Recall the defining parameter: IDSS is ID at VGS = 0 V.Apply Shockley relation: ID decreases as |VGS| increases toward |VGS(OFF)|.Conclude: maximum ID in passive gate bias occurs at VGS = 0 V.Use divider bias to set VGS near desired operating point below IDSS for linear operation. Verification / Alternative check: Examine ID–VGS transfer curve in any JFET datasheet: the peak of the curve at the left (for n-channel) corresponds to VGS = 0 V with value IDSS, confirming the claim. Why Other Options Are Wrong: Incorrect: conflicts with the definition of IDSS.Only p-channel: it is true for both polarities with respective sign conventions.Only if VDS = 0 V: at VDS = 0 V there is no channel voltage; IDSS is defined under proper VDS (saturation region), not at zero.True only when IDSS = 0: if IDSS were zero, the device would not conduct at VGS = 0 V. Common Pitfalls: Mixing up conventional current with carrier motion; forgetting that IDSS assumes operation in the saturation (active) region; assuming IDSS at any VDS without checking datasheet test conditions. Final Answer: Correct

Question 16

Enhancement-type MOSFET (E-MOSFET): Can it be turned on when the channel is already depleted, or does it require an induced channel via a threshold gate voltage?

Accepted Answer

Introduction / Context: Metal-oxide-semiconductor field-effect transistors (MOSFETs) come in depletion and enhancement varieties. Enhancement-type MOSFETs are normally off at VGS = 0 V and require a gate voltage exceeding a threshold to create (enhance) a conductive inversion channel. Misstating this behavior leads to biasing errors and device misuse. Given Data / Assumptions: Enhancement MOSFET: no strong channel at VGS = 0 V. Turn-on occurs when VGS reaches or exceeds VTH (n-type positive, p-type negative). Depletion MOSFETs differ: they have a channel at VGS = 0 V and can be pinched off by opposite VGS polarity. Gate is insulated; gate current in normal operation is negligible. Concept / Approach: For an E-MOSFET, conduction begins only after the gate-to-source voltage induces an inversion layer. Saying it turns on “when the channel is depleted” confuses the depletion device mechanism with enhancement behavior. An enhancement device cannot rely on a pre-existing channel; it creates one by field effect once |VGS| reaches VTH and grows with further |VGS| increases. Step-by-Step Solution: Classify device: enhancement vs depletion.Recall condition: E-MOSFET needs |VGS| ≥ |VTH| to form a channel.Interpret the claim: “channel depleted” describes turning a depletion device off, not turning an enhancement device on.Conclude: statement is incorrect for enhancement MOSFETs. Verification / Alternative check: Look at ID–VGS transfer curves: for enhancement devices, ID ~ k*(VGS − VTH)^2 (simple square-law model) for VGS above VTH, and ~0 below threshold—reinforcing that an induced channel is required. Why Other Options Are Wrong: Correct: contradicts E-MOSFET physics.Depends only on ID rating: ratings do not change the turn-on mechanism.Only true for depletion MOSFETs: this clarifies the confusion but is not the given statement.Works with gate shorted to source: with VGS = 0 V an E-MOSFET remains off (neglecting leakage). Common Pitfalls: Confusing depletion and enhancement types; assuming any MOSFET conducts at VGS = 0 V; ignoring threshold spread and temperature effects. Final Answer: Incorrect

Question 17

MOSFET structure and biasing: Does a MOSFET feature an electrically insulated (isolated) gate that draws negligible steady-state current?

Accepted Answer

Introduction / Context: The defining characteristic of MOSFETs is their insulated gate structure, typically using a thin oxide between the gate electrode and the semiconductor. This isolation enables extremely high input impedance and allows voltage control with virtually no steady-state gate current, a key advantage over junction-gate devices and BJTs. Given Data / Assumptions: Gate dielectric is usually silicon dioxide or high-k material. Gate–channel structure forms a capacitor; gate current is leakage-limited. Normal operation avoids oxide breakdown and ESD damage. Comparison with JFET/BJT is for context only. Concept / Approach: Because the gate is insulated, the control mechanism is purely electric field. The gate voltage modulates channel charge without requiring continuous charge injection. As a result, DC gate current is near zero (aside from leakage on the order of nanoamps or less), allowing high impedance interfaces and minimal loading on preceding stages. Step-by-Step Solution: Identify device: MOSFET with oxide-isolated gate.Relate to operation: gate voltage creates or modulates a channel.Assess gate current: negligible in steady state, limited by leakage.Conclude: statement is correct. Verification / Alternative check: Datasheets list gate leakage (IGSS) values orders of magnitude smaller than source/drain currents, confirming isolation. Contrast with a JFET or BJT, where junctions draw bias currents or have diode conduction under various biases. Why Other Options Are Wrong: Incorrect: contradicts MOS structure.Only true for JFETs: JFET gates are PN junctions, not insulated oxides.Only at cryogenic temperatures: insulation holds at normal temperatures.Forward-biased diode: that describes BJT base–emitter or a JFET gate if misused, not MOSFET gates. Common Pitfalls: Assuming the insulated gate is immune to ESD; forgetting that transient charging/discharging of the gate capacitance does require current; exceeding maximum VGS can puncture the oxide. Final Answer: Correct

Question 18

JFET characteristic information: Do the gate-biased JFET characteristic curves explicitly involve the drain–source current ID (often written as I_D), including the special case IDSS at VGS = 0 V?

Accepted Answer

Introduction / Context: Characteristic curves are the primary visual tool for understanding JFET behavior. They plot relationships among drain–source current (ID), drain–source voltage (VDS), and gate–source voltage (VGS). Knowing that ID, including the noted parameter IDSS, is central to these curves helps with design and bias calculations. Given Data / Assumptions: ID is the controlled current through the channel. Curves typically show ID vs VDS for several fixed VGS values. IDSS is the maximum ID at VGS = 0 V in the saturation region. Shockley equation relates ID to VGS in the transfer characteristic. Concept / Approach: The output characteristics display families of ID–VDS curves parameterized by VGS. The transfer characteristic shows ID versus VGS for a fixed VDS in the saturation region, highlighting IDSS at VGS = 0 V and the cutoff at VGS(OFF). Thus, drain current ID is explicitly part of all these presentations. Step-by-Step Solution: Identify the axes on the output characteristic: vertical is ID; horizontal is VDS.Observe parameter lines: each curve corresponds to a specific VGS.Note special markers: IDSS at VGS = 0 V; cutoff near ID ≈ 0 at VGS(OFF).Conclude: the curves explicitly include ID, validating the statement. Verification / Alternative check: Cross-check with any JFET datasheet: the characteristic graphs always include ID either as the graphed variable (output characteristic) or as the dependent variable of VGS (transfer characteristic). Why Other Options Are Wrong: Incorrect: ignores what is plotted.Only VDS: current is essential on the other axis.Gate current plotted: gate current is negligible and not the focus.Only VGS(OFF): cutoff is a point on the ID curve, not the entirety of the data. Common Pitfalls: Confusing IDSS with IDS (notation variations); overlooking the required region (saturation vs ohmic) for applying specific equations; reading curves at the wrong VGS line. Final Answer: Correct

Question 19

JFET terminals: Does a junction field-effect transistor have exactly three terminals named gate, drain, and source?

Accepted Answer

Introduction / Context: Device terminal naming distinguishes FETs from BJTs and helps avoid schematic errors. A JFET’s three accessible terminals—gate (G), drain (D), and source (S)—mirror the structure and control method of the device. Getting the names right is key when reading datasheets or wiring test circuits. Given Data / Assumptions: JFET structure employs a conductive channel between source and drain. The gate forms reverse-biased PN junctions to control the channel. No separate body terminal is normally used in discrete JFETs. Comparison with MOSFET/BJT is for context only. Concept / Approach: Unlike BJTs (emitter, base, collector), the JFET uses source, gate, and drain. The gate controls channel width by reverse-biasing, modulating depletion regions. This three-terminal arrangement is standard for discrete parts and symbols used in schematics. Step-by-Step Solution: Identify the controlling node: gate.Identify the current path: source ↔ drain channel.Confirm there are only three external connections on common packages.Conclude: statement is correct. Verification / Alternative check: Inspect any JFET datasheet pinout (e.g., TO-92 or SOT-23 packages): exactly three pins labeled G, S, and D. Why Other Options Are Wrong: Incorrect: contradicts standard device pinouts.Four terminals including body: this applies to some MOS devices, not typical JFETs.Collector/emitter: BJT terminology; not applicable to JFETs.Only two terminals: gate is very real and externally accessible. Common Pitfalls: Mixing BJT and FET nomenclature; swapping source/drain in circuits where orientation matters; ignoring package-specific pin numbering. Final Answer: Correct

Question 20

Device comparison: Is a JFET “very similar” to a BJT in operating principle and control, or are they fundamentally different (voltage-controlled vs current-controlled)?

Accepted Answer

Introduction / Context: Though both are called “transistors,” BJTs and JFETs operate on different principles. BJTs are current-controlled devices relying on carrier injection across a forward-biased junction, while JFETs are voltage-controlled devices using electric field to modulate a channel via reverse-biased junctions. Treating them as “very similar” leads to wrong biasing and modeling choices. Given Data / Assumptions: BJT terminals: base, collector, emitter; JFET terminals: gate, drain, source. BJT needs base current; JFET gate current is negligible (reverse-biased). Small-signal models and transfer functions differ substantially. Thermal and noise behaviors are different in practice. Concept / Approach: The JFET controls conduction by changing depletion width with gate voltage; the BJT controls collector current by base-emitter junction charge injection. One is predominantly voltage-controlled with high input impedance; the other is current-controlled with finite input resistance. Therefore, they are not “very similar” in operation, despite both being three-terminal semiconductor amplifying devices. Step-by-Step Solution: Identify control variable: VGS for JFET vs IB (or VBE) for BJT.Note input impedance: very high for JFET, moderate for BJT.Recognize transfer forms: JFET often follows Shockley law; BJT follows exponential relation between IC and VBE.Conclude: statement claiming high similarity is incorrect. Verification / Alternative check: Compare small-signal parameters: transconductance expression for JFET differs from BJT (gm behavior vs IC and device constants), reinforcing different physics. Why Other Options Are Wrong: Very similar/current-controlled: JFET is not current-controlled.Correct: contradicts device fundamentals.Identical transfer characteristics: false; laws differ.Equivalent with matched gain: matching gain does not equal identical operation. Common Pitfalls: Porting BJT bias networks to JFETs unchanged; expecting the same temperature drift or noise characteristics; mixing up terminal names. Final Answer: Incorrect

Field Effect Transistors (FET) Questions