Question

A basic cmos inverter will have a P-transistor upside and N-transistor down side. what happens if we reverse the p and n transistors?

Answer 1

A P-transistor needs to have the gate at a lower potential than the source by a threshold for a good channel to exist. An N transistor needs to have the gate at a higher potential than the source by a threshold for a good channel to exist.



When in=0, Vgs of the N transistor is negative so it is off. The P transistor's channel will exist as long as the output is Vt volts or higher. In other words the output is pulled down to Vt volts and no lower. It's a 'soft 0'.

When in=Vdd Vgs of the P transistor is always positive so it is off. The N transistor's channel will exist as long as the output is Vdd-Vt volts or lower. In other words the output is pulled high up to Vdd-Vt volts and no higher. It's a 'soft 1'.

The result: you've got a bad inverter :)