25. N 通道增強型MOSFET 之臨界電壓\( V_T=2V \),\( K=0.5mA/V^2 \),當MOSFET 導通且\( V_{GS}=4V \)時,\( I_D \)為多少?(A)1mA(B)2mA(C)3mA(D)4mA