Design the width of a MOSFET such that a specified current is induced for a given applied bias.
Consider an ideal n-channel MOSFET with parameters L=1.25 \mu \mathrm{m}, \mu_{n}=650 \mathrm{~cm}^{2} / \mathrm{V}-\mathrm{s}, C_{\mathrm{ox}}=6.9 \times 10^{-8} \mathrm{~F} / \mathrm{cm}^{2}, and V_{T}=0.65 \mathrm{~V} . Design the channel width W such that I_{D}(\mathrm{sat})= 4 \mathrm{~mA} for V_{G S}=5 \mathrm{~V}.
For the transition biased in the saturation region, we have, from Equation (10.66),
I_{D}(\mathrm{sat})=\frac{W \mu_{n} C_{\mathrm{ox}}}{2 L}\left(V_{G S}-V_{T}\right)^{2} (10.66)
or
4 \times 10^{-3}=\frac{W(650)\left(6.9 \times 10^{-8}\right)}{2\left(1.25 \times 10^{-4}\right)} \cdot(5-0.65)^{2}=3.39 W
Then
W=11.8 \mu \mathrm{m}
Comment
The current capability of a MOSFET is directly proportional to the channel width W. The current handling capability can be increased by increasing W.