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current at zero bias voltage, this
f.e.t.
resembles a valve. But drain-
current/drain-voltage (I
D
/V
D
) curves, shown in Fig. 10.15, are more
like transistor curves; for quite a different reason however.
Increasing the drain voltage accelerates the electrons and so tends to
increase the current, as with ordinary resistance, but at the same time
it makes all parts of the channel progressively more positive towards
the drain end, thereby increasing the reverse bias across the junction
and narrowing the channel. Beyond a certain K
D
, called the pinch-off
voltage, the two effects almost balance, so the curve flattens out as
shown. The a.c. resistanc ...