Problem 5.18

A particular enhancement MOSFET for which V_t = 0.5V and k'_n(W/L) = 0.1mA/V² is to be operated in the saturation region. If I_D is to be 12.5 uA, find the required V_GS and the minimum required V_DS. Repeat for I_D = 50 uA.

V_OV = V_DS at the edge of the saturation region.

For I_D = 12.5 uA

To find V_OV,

I_D = 1/2 k'_n (W/L) (V_OV)

.0125 = 1/2 (0.1) (V_OV)²

V_OV = 0.5 V

V_OV = V_DS = 0.5 V

V_OV = V_GS - V_t

0.5 V = V_GS - 0.5 V

V_GS = 1 V

For I_D = 50 uA

To find V_OV,

ID = 1/2 k'_n (W/L) (V_OV)²

.050 = 1/2 (0.1) (V_OV)²

V_OV = 1 V

V_OV = V_DS = 1 V

V_OV = V_GS - V_t

1 V = V_GS - 0.5 V

V_GS = 1.5 V