What is Pinch off in a transistor?

[Hyo X]

Suppose I have a channel between a source and a drain in a FET.
What is 'pinch-off' and how does it affect my I-V curves?
I don't really understand 'space charge' yet.
Thanks

 

[carlgrace]

When you have a bias on the gate and you slowly increase the drain-source voltage, the depletion region (the channel) at the drain gets smaller and smaller. Eventually, you will reach a point where the depletion region goes to zero right at the drain. This is pinch off, or otherwise called "the edge of saturation".

1. When your channel is pinched off, then, to first order, the current through the FET is constant as you vary the drain-source voltage and only responds to changes in the gate-source voltage. This is called the "Saturation region".

In practice, of course, the FET current responds a little bit to changes in the drain-source voltage, and this is captured in the concept called "output resistance".

2. Space charge is a pretty simple concept, it just has a fancy name.

Imagine you have some doped silicon. If you bias your silicon to push out the free carriers, you still have some charge there because the dopant atoms themselves are fixed in the lattice. This is called space charge.

 

[dnyberg2]

In an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch. This is called pinch-off, and the voltage at which it occurs is called the pinch-off voltage. Conversely, a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.