Question

NMOS and PMOS

Explain the difference between a NMOS and PMOS and plot their characteristics curves LTspice (you may use any standard MOSFET circuit). Distinguish between NMOS/PMOS and NMOS4/PMOS4.

Answer 1

NMOS is built with n-type source and drain and a p-type substrate, while PMOS is built with p-type source and drain and a n-type substrate. In a NMOS, carriers are electrons, while in a PMOS, carriers are holes. When a high voltage is applied to the gate, NMOS will conduct, while PMOS will not. Furthermore, when a low voltage is applied in the gate, NMOS will not conduct and PMOS will conduct. NMOS are considered to be faster than PMOS, since the carriers in NMOS, which are electrons, travel twice as fast as holes, which are the carriers in PMOS. But PMOS devices are more immune to noise than NMOS devices. Furthermore, NMOS ICs would be smaller than PMOS ICs (that give the same functionality), since the NMOS can provide one-half of the impedance provided by a PMOS (which has the same geometry and operating conditions).

Both the devices contain 4 terminals gate,drain,source and body,as moentioned above NMOS conducts when gate is high and PMOS conducts when gate is low.Below are the characteristics of PMOS and NMOS at different gate to source and drain to source voltages.Here plots are between drain current and drain to source voltage at different gate voltges

NMOS characteristics

PMOS charecteristis

Here we can observe that for PMOS porarities of gate and drain are reverse to that of NMOS and current is the drain terminal current which is negative for PMOS.This signifies that current is flowing out of drain terminal.

More over the difference between NMOS/PMOS and NMOS4/PMOS4 is that the later devices will have four terminals where as former will have only three terminals .The fourth terminal (i.e body terminal) is connected to the source to avoid body effect.In some cases we need to connect body some different potential other that that of source ,in those cases NMOS4/PMOS4 will be used