PMOS Transistor

In a PMOS transisor, the drain to source current I_{SD} is a function of all the terminal voltages V_{SG}, V_{SD} and V_{SB}

The drain current in a PMOS transistor is given by

(1)   \begin{equation*} I_{SD} = {1 \over 2}\mu_p C_{ox} {W \over L}\left( V_{SG} - V_{T,p}\right)^2 (1+\lambda V_{SD}) \end{equation*}

Small signal current is given by

(2)   \begin{equation*} i_{d} = \underbrace{{\partial I_{SD} \over \partial V_{SG}}}_{g_m} ~ v_{sg} + \underbrace{{\partial I_{SD} \over \partial V_{SB}}}_{g_{mb}} ~ v_{sb} + \underbrace{{\partial I_{SD} \over \partial V_{SD}}}_{g_{ds}} ~ v_{sd} \end{equation*}

g_m \rightarrow transconductance
g_{mb} \rightarrow backgate transconductance
g_{ds} \rightarrow output conductance

Figure 1. Small signal equivalent model of PMOS device in saturation

Figure 1. Small signal equivalent model of PMOS device in saturation

The small signal model in Figure 1(b) is much helpful while circuit analysis.
From Figure 1(b), we can say small signal model of PMOS appears almost same as NMOS, except the direction of drain current.

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