GATE 2017 EC (EC2) – Question 19
An n-channel enhancement-mode MOSFET is biased at $V_{GS}>V_{TH}$ and $V_{DS}>(V_{GS}-V_{TH})$, where the symbols denote gate-to-source voltage, drain-to-source voltage and threshold voltage. Considering channel-length modulation to be significant, the MOSFET behaves as a
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Correct answer: (C) current source with finite output impedance
Explanation
The stated inequalities place the MOSFET in saturation. Its drain current is controlled primarily by gate overdrive, which gives current-source behavior.
Channel-length modulation makes the current also increase with $V_{DS}$, often modeled by $I_D=I_{D0}(1+\lambda V_{DS})$. The nonzero slope gives finite output resistance, approximately $1/(\lambda I_D)$.
Therefore the correct model is a **current source with finite output impedance, C**.