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In JFET, the Pinch‐off Voltage can be defined as: 

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UGC NET Paper 2: Electronic Science 3rd Dec 2021 Shift 2
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  1. \(\rm \frac{qN_Da^2}{\in_s}\)
  2. \(\rm \frac{qN_Da}{2\in_s}\)
  3. \(\rm \frac{qN_Da^2}{2\in_s}\)
  4. \(\rm \frac{qN_D}{2\in_s.a}\)

Answer (Detailed Solution Below)

Option 1 : \(\rm \frac{qN_Da^2}{\in_s}\)
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Pinch off voltage in a JFET is defined as the gate to source voltage at which drain current reaches a maximum current IDSS which is saturation current,

qImage14826

Consider the n channel JFET as shown in the diagram above, the width of the depletion region extending into the channel is W. The width between two metallurgical junction is 2a. Now, the channel is pinched off when, 

 \(V_p = \rm \frac{qN_Da^2}{\in_s} - V_o\)

Where, Vo is the built in potential.

If the built in potential is zero, then the most closest option is option 1. 

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