ABSTRACT
Sidney Soclof
Joseph Watson
John R. Brews
Harvey J. Stiegler
James E. Morris
A junction field-effect transistor (JFET) is a type of transistor in which the current flow through the device
between the drain and source electrodes is controlled by the voltage applied to the gate electrode. A simple
physical model of the JFET is shown in Figure 3.1. In this JFET an n-type conducting channel exists between
drain and source. The gate is a p
region that surrounds the n-type channel. The gate-to-channel pn-junction
is normally kept reverse-biased. As the reverse bias voltage between gate and channel increases, the depletion
region width increases, as shown in Figure 3.2. The depletion region extends mostly into the n-type channel
because of the heavy doping on the p
side. The depletion region is depleted of mobile charge carriers and
thus cannot contribute to the conduction of current between drain and source. Thus, as the gate voltage
increases, the cross-sectional areas of the n-type channel available for current flow decreases. This reduces the
current flow between drain and source. As the gate voltage increases, the channel gets further constricted and
the current flow gets smaller. Finally, when the depletion regions meet in the middle of the channel, as shown
in Figure 3.3, the channel is pinched off in its entirety between source and drain. At this point the current flow
between drain and source is reduced to essentially zero. This voltage is called the pinch-off voltage, V
.
The pinch-off voltage is also represented by V
