shown in Figure 7-3(a). This is as we would expect, since increasing the voltage across the fixed-resistance channel simply causes an Ohm’s law increase in the current through it. As we continue to increase VI)S, we find that noticeable depletion regions begin to form in the channel, as illustrated in Figure 7-3(b). Note that the
depletion regions are broader near the drain end of the channel (in the vicinity of point A) than they are near the source end (point B). This is explained by the fact that current flowing through the channel creates a voltage drop along the length of the channel. Near the top of the channel, the channel voltage is very nearly equal to Nos, so there is a large reverse-biasing voltage between the N channel and
the P gate. As we proceed down the channel, less voltage is available because of the drop that accumulates through the restive N material. Consequently, the reverse-biasing potential between channel and gate becomes smaller and the depletion regions become narrower as we approach the source. When Nos is increased expand Ana toe Cezanne becomes very narrow in the vicinity of point A, causing the total resistance of the channel to increase. As a consequence, the rise in current is no longer directly proportional to Nos. Instead, the current begins to level off, as shown by the curved portion of the plot in Figure 7-3(a). Figure 7-4(a) shows what happens when VS is increased to a value large
enough to cause the depletion regions to meet at a point in the channel near the drain end. This condition is called pinch-off At the point where pinch-off occurs, the gate-to-channel junction is reverse biased by the value of Nos, so (the negative of) this value is called the pinch-off voltage, VI’. The pinch-off voltage is an important JET parameter, whose value depends on the doping and geometry of the device. VI’ is always a negative quantity for an N-channel JET and a positive quantity for a P-channel JET. Figure 7-4(b) shows that the current reaches a maximum value at pinch-off and that it remains at that value as Nos is increased beyond I VI’I. This current is called the saturation current and is designated loss-the D rain-to-Source current with the gate Shorted. Despite the implication of the name pinch-off. note again that current continues to flow though the device when exceeds.I. The value of current remains constant at loss because of a kind of self-regulating or equilibrium process that controls the current when exceeds I Suppose that an increase in did cause Io to increase; then there would be in the channel an increased voltage drop
43
