Can overdrive voltage never be less than zero?

[mnvaughn]

I had a test recently and one of the problems was pretty basic. It gave you a table with the voltages at the gate, source, and drain of the device. V_t (Threshold voltage) was also provided, which was +1V. From there we were supposed to find V_gs, V_ov(Overdrive voltage), V_ds and the region of operation of the device. So for one part of the table I was provided with V_s (voltage at the source) is +1V, V_g (voltage at the gate) is +1V, and V_d (Voltage at the drain) is +2V. I know V_gs is V_g-V_s which in this case is 0V. And V_ds is V_d-V_s which is +1V. However for V_ov I got -1V which is solved by the equation (V_gs-V_t) so that would be 0-1=-1. This was wrong on my test. Does this mean the overdrive voltage can never be less than zero? Please help!

 

[yungman]

Vt is from gate voltage to source voltage. Vt=+1V means you need +1V at the gate RESPECTED to the source, NOT +1V respected to 0V.

In FET, you need to have Vgs exceed the Vt before the FET turns on. In your case, you correctly established that your Vgs=0V...Which is below the turn on threshold of Vt=1V. The transistor is not even turned on. That's all it means.

Another way to look at it is: Vt is the spec from the data sheet that you need Vgs to exceed Vt before the transistor turns on. This mean (Vgs-Vt) has to be slightly greater than 0V before the transistor turns on.

 

[Su3liminal]

You are right in that V_ov=V_gs-V_t
In your case, V_ov would be negative.

What does that mean? Well, there are two cases to consider in general when V_ov < 0 (although it is obvious that the transistor is NMOS since V_t > 0; unless, of course, it was given that |V_t|=1, then it is a PMOS and your answer would be wrong! Although the voltage levels at the gate and source doesn't make any resemblance with PMOS transistors bias conditions.):
1- If it was an NMOS (n-channel) FET, then it will be OFF.
It is clear because a positive potential at the gate (more than the threshold voltage when source is grounded) electrode is necessary to establish the electric field that will collect electrons from the source & drain, push free holes away from the substrate; that is, making a channel for current flow. So without the sufficient positive bias voltage you will have two back-to-back diodes in series between drain and source.
2- If it was a PMOS (p-channel) FET, then it will be ON .
It should be, since the condition V_ov < 0 is necessary for this type of transistor; the physical reasoning is the total opposite of what is said above about NMOS.

I hope that helps

/

One important note:
What I said above, is not "entirely" true in that for values of V_gs less than but close to V_t, a small drain current flows. This is called Subthreshold operation.
It has rare (but a growing number) of applications, and usually we ignore this region of operation, for simplicity.