Discussion
For an N-channel depletion type MOSFET, the value of drain current at zero \(V_{GS}\) (gate to source voltage), \(I_{DSS} = 10\) mA. What is the value of drain current at \(V_{GS} = +1\) V? The value of pinch off voltage \(V_P = -4\) V.
Answer: B
The drain current of a depletion type MOSFET follows the Shockley equation
\[ I_D = I_{DSS}\left(1 - \frac{V_{GS}}{V_P}\right)^2 \]
\[ \begin{aligned} I_D = 10\left(1 - \frac{1}{-4}\right)^2 \\ &= 10\left(1 + 0.25\right)^2 = 10 \times 1.5625 \\ &= 15.63\ \text{mA} \end{aligned} \]
The depletion MOSFET can work with a positive gate voltage (enhancement mode), so the current is greater than \(I_{DSS}\). The value 1.563 mA is a factor of ten too small, 156.3 mA is a factor of ten too large, and 0.1563 mA is a hundred times too small. The answer is option B.
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