# Confused on MOSFET terms (velocity limited, mobility limited, constant mobility)

So we're learning about transistors, but we don't much of the physics behind how they work (yet) so these terms are quite strange to me. The models we've used are "velocity limited", "mobility limited", and "constant mobility". In some models, the relation between drain current and gate-source voltage is linear and in others it's based on a square law. What do these terms mean, and where do the square or linear relationships come from?

Here's the equation for velocity-limited characteristics: $I_d=V_{sat} C_{ox} W_g (V_{gs}-V_t) (1+\lambda V_{ds})$

And this is for mobility limited (which they also refer to as constant mobility): $I_d=\frac{1}{2} \mu C_{ox} \frac{W_g}{L_g} (V_{gs}-V_t)^2 (1+\lambda V_{ds})$

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No answer, because it is complex and I cant write a better answer than these two: ece.ucsb.edu/Faculty/rodwell/Classes/ece2c/notes/… doe.carleton.ca/~tjs/21-mosfetop.pdf – posipiet May 20 '12 at 5:29