EC > GATE 2014 SET-1 > MOSFET
A depletion type N-channel MOSFET is biased in its linear region for use as a voltage controlled resistor. Assume threshold voltage VTH = -0.5 V, VGS = 2.0 V, VDS = 5 V, W/L = 100, Cox = 10-8 F/cm2 and μn = 800 cm2/V-s. The value of the resistance of the voltage controlled resistor (in Ω) is __________.
Correct : 500
The answer is 500 Ω.
When a MOSFET is biased in the linear region with a small VDS, it acts as a voltage controlled resistor. The resistance is given by:
R = 1 / [μn × Cox × (W/L) × (VGS - VTH)]
Plugging in the values — μn = 800 cm2/V-s, Cox = 10-8 F/cm2, W/L = 100, VGS = 2.0 V, VTH = -0.5 V:
VGS - VTH = 2.0 - (-0.5) = 2.5 V
R = 1 / (800 × 10-8 × 100 × 2.5)
R = 1 / (800 × 10-8 × 250)
R = 1 / (2 × 10-3)
R = 500 Ω
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