The voltage along the channel is defined as Vc(x) (Fig. 3.5), with the range 0 at x = 0 (source) to Vc(L) = VD (drain). The device is in the linear-region mode, that is, VD < Veffn. The charge in the channel at x is
Equation 3.20
where the charge at the source is Qchan = Cox(VGS – Vtno), as in the case of (3.2).
A generalization of (3.2) for the incremental conductance, dGchan(x), at x over a length dx is
Equation 3.21
The voltage drop across the length dx, for a drain current ID, is
Equation 3.22
where Rchan(x) = 1/Gchan(x). Using (3.21), the incremental voltage across dx is
Equation 3.23
Rearranging the equation and integrating gives
Equation 3.24
which leads to the result (3.5), repeated here
18.222.35.21