In an n-type junction field-effect transistor (JFET), under steady-state operation with the gate-to-channel junction reverse biased, will the device conduct when the drain is held at a positive potential and the source at a negative potential (so that VDS is positive)? Explain the correct operating condition for channel conduction in an n-type JFET.
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ACorrect — with the drain at a higher potential than the source (VDS > 0) and the gate reverse biased, the n-type JFET channel conducts.
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BIncorrect — an n-type JFET requires the drain to be negative with respect to the source to conduct.
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CIncorrect — an n-type JFET conducts only when VGS is positive and forward biases the gate-channel junction.
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DIncorrect — current will not flow unless the gate is shorted to the source in all cases.
Answer
Correct Answer: Correct — with the drain at a higher potential than the source (VDS > 0) and the gate reverse biased, the n-type JFET channel conducts.
Explanation
Introduction / Context:JFETs are voltage-controlled devices where the gate-to-channel junction is normally reverse biased. Understanding which terminal polarities allow conduction is essential for biasing and small-signal design. This item focuses on the polarity requirements for current to flow in an n-type JFET channel.
Given Data / Assumptions:
- Device is an n-type JFET.
- Gate-to-channel junction is kept reverse biased (VGS ≤ 0 referenced to the source).
- Polarities in question: drain positive, source negative, implying VDS = VD − VS > 0.
- Normal room-temperature operation; no breakdown or forward gate conduction.
Concept / Approach:In an n-type JFET, the channel is n-type semiconductor. Electrons are the majority carriers. With the gate reverse biased, depletion regions pinch the channel to a degree set by VGS. Drain current ID flows when VDS > 0, pulling electrons from source to drain (conventional current from drain to source). The current magnitude follows ID = f(VGS, VDS), saturating when VDS exceeds a threshold often called VDS(sat) in the JFET context, with control primarily through VGS.
Step-by-Step Solution:
Identify correct polarity: For n-type JFET, require VDS > 0 (drain at higher potential).Maintain reverse bias of the gate-channel junction: VGS ≤ 0 to avoid forward conduction.Conclude: With drain positive and source negative (making VDS positive), the channel conducts as long as VGS is not too negative to pinch off completely.Verification / Alternative check:Datasheets show characteristic curves ID versus VDS for different VGS ≤ 0; conduction increases with more positive VDS until saturation. This confirms that a positive VDS enables current for an n-channel device.
Why Other Options Are Wrong:
- Drain negative (option b) describes p-channel behavior, not n-channel.
- Positive VGS (option c) forward-biases the gate junction, which is incorrect for JFET operation and risks damage.
- Gate shorted to source (option d) is a possible bias (VGS = 0) but not a necessity for every case; it is not a precondition for conduction.
Common Pitfalls:Confusing JFET polarity with MOSFET conventions, or assuming VGS must be positive for all field-effect devices. JFET gates are reverse biased; MOSFET gates are insulated and can take positive VGS for n-channel operation.
Final Answer:Correct — with VDS > 0 and the gate reverse biased, an n-type JFET conducts.