Question

7. What happens with drain current, ip in MOSFET saturation? Why a MOSFET saturates at a high Vos ? The ideal current-voltage (I-V) relationship in the triode or the non-saturation region of a MOSFET is given as.ip K,12 GS VTN)UDS vBs. Prove the I-V relationship in the saturation region to be: lp Kn (vGS VN)2 from this equation 110]

Answer 1

As the drain voltage increases, the voltage drop across the oxide near the drain terminal decreases which means that the induced charge density near the drain also decreases. As the result I_D vr V_DS curve will decreases V_DS > V_DS sat at the saturation when a larger V_DS > V_DS sat the is under saturation region and the channel get highly tapered and will be broken. The drain current is a draft current. When the channel is broken, the channel length reduces. The inversion charge density Q�_n can be written as -E_ox E_ox = -E_ox/tox [(v_qs - v_t) - (phi_ms + 2 phi_fp)] V_T rightarrow Threshold voltage phi_ms rightarrow metal semiconductor work function Now integrate over the length of the channel integral^L_0 t_x dx = -w c_ox integral [(v_qs - v_t) - v_x] dV_x I_D = w mu n c_ox/2L [2 (v_95 - v_t) v_DS - V^2_DS] I_D (sat) = w mu n c_ox/2L (v_95 - v_t)^2