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If you look at the load line of collector current vs collector
voltage of a BJT, it has a slope. If you extrapolate that
slope back to the X axis, the intercept is the Early voltage.
The effect comes from the modulation of C-B depletion
width by the collector0base junction reverse voltage (Vcb).
As the depletion region grows, the base width narrows.
As the base width narrows, the current gain rises and so
does the collector current.
Because collector current follows collector voltage, there
appears to be a conductance in parallel with the "ideal"
current controlled current source. This detracts from the
stage gain (gm*rout). The gain related figure of merit
in BJT processes is BF*VA (beta*Early voltage product).
Early voltage is improved by increasing the ratio of base
doping to collector doping. But that has its own costs,
for example doing it by upping the base doping loses
some emitter injection efficiency and hurts base lifetime,
both costing gain. Doing it by lowering collector doping
raises collector resistivity and Rc, making a poorer power
These trades and all of their permutations account for
the wide variety of part types, in what is conceptually
a rather simple sort of device.
in mosfet, this effect is referred to as channel length modulation. after Vds exceeds Vgs - Vt , the layer of inversion charge near to the drain disappears and we say that the channel has pinched off. Further increase in Vds will cause the layer of the inversion charge in the channel to shrink away from the drain further, this reduces the effective channel length. As a result, the current, Ids increases a little bit since its proportional to W/Leffective. When L effective decreases, I goes up.