CMOS Analog IC Review Part I

2.3 Second-Order Effects

Body Effect

A source-to-bulk (substrate) voltage affects the threshold voltage, the relationship is given by:

V_TH = V_TH0 + γ[√(2Φ_F + V_SB) – √|2Φ_F|]

Channel-Length Modulation

In pinched off channel, the channel length gradually decreases as V_DS increases in saturation region. Assume a linear relationship, I_D is given by:

I_D = (1/2)μ_nC_ox(W/L)(V_GS – V_TH)²(1 + λV_DS)

There is no channel-length modulation in triode region.

Subthreshold Conduction

For VGS < VTH, a weak inversion layer still exists and some current flows from drain to source. ID exhibits an exponential dependence on VGS:

I_D = I₀exp[V_GS/(ξV_T)],