MOS transistors are just one type of field effect transistor. Another type is the junction field effect transistor or JFET. This device uses the depletion region surrounding the reverse junction as the gate dielectric. Figure 1.27A is a cross-sectional view of an N-channel JFET. This device has a piece of lightly doped N-type silicon called the body, in which there are two opposing P-type diffusion regions. The thin N-type silicon between the two junctions is the channel of the JFET. The two diffusion regions are the GATE and BACKGATE, and the opposite ends of the body are the source and drain.
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Figure 1.27 Cross-sectional view of an N-channel JFET transistor operating in the linear region (A) and in the saturation region (B). In the figure, S=Source, D=Drain, G=Gate, and BG=Backgate.
Assume that all four leads of an N-channel JFET are grounded. Depletion regions will form around the GATE-BODY junction and the BACKGATE-BODY junction. These depletion regions will extend into the lightly doped channel, but they will not touch each other. Therefore, there is a channel from drain to source. If the drain voltage exceeds the source voltage, a current will flow through the channel from drain to source. The magnitude of the current is related to the resistance of the channel, which in turn is related to its size and doping. As long as the drain-to-source voltage is still small, it will not significantly cause the depletion region to cross the channel. The channel resistance remains constant, and the drain-to-source voltage and drain current are linearly related. Under these conditions, the JFET is considered to be in the linear region. This region is relative to the linear (or triode) region of the MOS tube. Since the channel has been formed when V GS = 0, the JFET is more like a depletion-type MOSFET tube rather than an enhancement-type one.
The depletion region at the drain end of the JFET widens as the drain voltage increases. The channel becomes narrower and narrower as the opposing depletion region invades. Eventually the two depletion regions touch each other and pinch off the channel (Figure 1.27B). Even though the channel is pinched off, there is still a drain current flowing through the transistor. This current originates from the source end and consists of majority carriers (electrons). These carriers move along the channel until they encounter the pinched-off region. The large lateral electric field across this region helps these carriers enter the neutral drain.
Once the channel is pinched off, increasing the drain voltage will have little effect. The pinched-off region will widen slightly, but the channel size will remain the same. The resistance of the channel determines the drain current, so it also remains almost constant. Under these conditions, the JFET is said to be in saturation.
The gate and backgate terminals also affect the current flowing through the channel. As the gate-body and backgate-body voltages increase, the reverse bias voltage on the gate-body and backgate-body junctions slowly increases. The depletion region surrounding these junctions also widens, and the channel is compressed. Less and less current can flow through the compressed channel, and the drain-to-source voltage used to pinch off the channel is also reduced. As the gate and backgate voltages continue to increase, the channel is eventually pinched off at V DS = 0. Once this happens, no current can flow through the transistor no matter how high the drain-to-source voltage is, and the transistor is considered to be in the cutoff region.
Figure 1.28 shows the IV characteristic curve of an N-channel JFET with gate and backgate connected. Each curve has a different gate-to-source voltage V GS . When V GS = 0, the drain current has a maximum value, and it decreases as the gate voltage increases. When the gate voltage is equal to the turnoff voltage VT , conduction is completely terminated. The turnoff voltage is equivalent to the threshold voltage of the MOS tube. However, since the conduction principles of the two devices are very different, they cannot be compared exactly equally.
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Figure 1.28 Typical N-JFET transistor at V =IV curve at -8V
The drain current curve of the N-JFET is slightly tilted upward in the saturation region due to the channel length modulation effect. This effect is similar to the effect that occurs in MOS tubes. The pinched-off region of the JFET becomes longer as the drain-to-source voltage increases. The elongation of the pinched-off region will correspondingly shorten the channel. The channel length modulation effect is usually very small because the channel length is much longer than the length of the pinched-off region.
The source and drain terminals of a JFET can be interchanged without affecting the performance of the device. The structure of the JFET in Figure 1.27A is an example of a symmetrical device. Sometimes more complex JFET structures have a slight difference in the geometry of the source and drain, which makes them asymmetrical.
Almost all JFET structures shorten the gate and backgate terminals. Consider the device in Figure 1.27A. The channel has the source on the left, the drain on the right, the gate on the top, and the backgate on the bottom. The diagram does not show what is in front of and behind the channel. In most cases, the two sides of the channel are surrounded by reverse-biased junctions extending from the gate-body and backgate-body junctions. This arrangement inevitably shortens the gate and backgate.
Figure 1.29 shows the traditional circuit diagram symbols for N-channel and P-channel JFETs. The arrow on the gate indicates the direction of the PN junction between the gate and the body. The source and drain terminals are not clearly marked in the symbol, but most circuit designers put the drain of the N-JFET and the source of the P-JFET at the top.
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Figure 1.29 Symbols for N-channel JFET (A) and P-channel JFET (B).
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