Why Are Electrons Not Attracted to Holes in a MOSFET Without Gate Voltage?

[Denver Dang]

Homework Statement

I currently reading about transistors in a solid state course. And the transistor is a MOFSET, which I have to describe. I can't really find a nice figure of the one I have in my book, so I will just try to explain my confusion.

It says that you make a MOFSET by having a p-doped substrate in which you have two n-doped regions (source and drain), which are connected by an oxide layer, which again is connected to a metal electrode (gate).

Now, when you "open" the gate, you repel the holes underneath the oxide layer, making "room" for electrons and attracting them, right ? And first when you apply a voltage, there is enough energy to make the electrons move from source to drain, or maybe I have misunderstood something ?

So, my main question is: Since the p-substrate has a lot of holes, why are the electrons from the two n-type regions not attracted to them, even when there is no gate or voltage applied ? Why do they first start to be attracted when the gate is opened, and making the oxide layer positive ?

Don't know if I have explained it in a good way, but I hope you understand.

Homework Equations

EDIT:
I actually have the figure from my book right here:

http://i.imgur.com/Qda9msK.png

The Attempt at a Solution

 

[phyzguy]

So, my main question is: Since the p-substrate has a lot of holes, why are the electrons from the two n-type regions not attracted to them, even when there is no gate or voltage applied ?

The positive charge of the holes in the p-substrate is balanced by the negative charge of the acceptor atoms in the p-substrate. So the p-substrate is charge-neutral and there is no net charge to attract electrons from the n-type source and drain. It is only when a positive voltage (larger than the threshold voltage) is applied to the gate that an n-type channel forms between the source and drain. Does this answer your question?

 

[Denver Dang]

Indeed it did :)

Although, the threshold voltage, how is that defined ?

 

[phyzguy]

Indeed it did :)

Although, the threshold voltage, how is that defined ?

Try this:

http://en.wikipedia.org/wiki/Threshold_Voltage