Test: Parameters Of MOS Transistors

The work function difference between gate and Si (Φms) is negative for silicon substrate and polysilicon gate.

Substrate bias voltage Vsb is positive for pMOS and negative for nMOS.

According to body effect, substrate is biased with respect to the source. Body effect can be seen as a change in the threshold voltage.

 Increasing the substrate bias voltage Vsb, increases the threshold voltage because it depletes the channel of charge carriers.

Transconductance expresses the relationship between output current Ids and input voltage Vgs.

 Transconductance gm of a MOS device can be increased by increasing its width and it does not depend on length.

Increasing the transconductance gm results in increase in input capacitance and area occupied as it is directly proportional.

 Ids is inversely proportional to the length L of the channel and using this relationship strong dependence of output conductance on channel length can be demonstrated.

Switching speed of a MOS device depends on gate voltage above threshold and on carrier mobility and inversely as the square of channel length.

A fast circuit requires gm as high as possible as the switching speed depends on gate voltage above threshold and on carrier mobility and inversely to square of channel length.