A silicon n-channel MOSFET has a doping concentration of $N_{a}=2 \times 10^{16} \mathrm{~cm}^{-3}$ and a threshold voltage of $V_{T}=0.40 \mathrm{~V}$. Determine the change in channel length, $\Delta L$, for $(a) V_{G S}=1.0 \mathrm{~V}, V_{D S}=2.0 \mathrm{~V} ;(b) V_{G S}=1.0, V_{D S}=4.0 \mathrm{~V} ;(c) V_{G S}=2.0 \mathrm{~V}$
$V_{D S}=2.0 \mathrm{~V} ;$ and $(d) V_{G S}=2.0, V_{D S}=4.0$