Question

is MOSFET inverstion charge is fromulated as Qinv=Cox(Vg-V(y)-Vt)

where V(y) is the voltage on a point in the channel,so near the Drain Qinv decreases .the charge density "n" in the formula bellow decreases as well (pinch off)

J=q*n*v=q*n*miu*E (because J is constant ,n decreases) so E field goes up near drain (speed saturation)

So why in long channnel the predominant saturation mechanism is pinch off

and in short channel its velosity saturation?

Thanks

Answer 1

The velocity of charge carriers, such as electrons or holes is proportional to the electric field that drives them, but that is only valid for small fields As E = V/LAs length of the channel decreases, Electric field increases and field gets stronger As the field gets stronger, their velocity tends to saturate This means that above the critical field they tend to stablize their speed and eventually cannot move faster velocity satiration occurs in short channel MOST &Ttransistors, because they have high electric fields So answer to your questions is that in short Channel devices, the electric field is very high and predominant saturation mechanism is velocity saturation (E = V/L)as downarrow E uparow and stronger electric field is responsible for velocity saturation \r\n When does the velocity of charge carrier's saturate ? the electrical velocity is defined by the materials the charge carriers are flowing through carrier velocity is given by Vd = mu E/ 1+ e/E_c straightline 1