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:

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