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JFET Construction and Operation

A schematic representation of an n channel JFET is shown in Figure 118. An n-type channel is formed between two p-type layers which are connected to the gate. Majority carrier electrons flow from the source and exit the drain, forming the drain current. The pn junction is reverse biased during normal operation, and this widens the depletion layers which extend into the n channel only (since the doping of the p regions is much larger than that of the n channel). As the depletion layers widen, the channel narrows, restricting current flow.

  

Figure 118: n-channel JFET structure.

When $\vert v_{GS} \vert >> \vert v_{DS} \vert$, there is little voltage drop along the length of the channel, and the depletion regions are parallel, Figure 119. As v_GS is increased negatively, they eventually touch reducing i_D to zero. The value of v_GS at which this occurs is called the pinch-off voltage, V_p (or v_GS(off)).  

  

Figure: n-channel JFET structure for $\vert v_{GS} \vert >> \vert v_{DS} \vert$ showing parallel depletion regions.

When $\vert v_{DS} \vert >> \vert v_{GS} \vert$, there is a voltage drop along the length of the channel, and the depletion regions are no longer parallel, but are closer together towards the drain, Figure 120. As v_DS is increased, they will touch (pinch-off) towards the drain, and the drain current i_D can increase no longer. At the threshold of pinch-off, v_GS-v_DS=V_p. As v_DS is further increased, i_D remains constant, and the JFET is in its current saturation region, the normal mode of operation. (This constant current region is a characteristic feature of any transistor, FET or BJT.) The channel shape remains unchanged, with a small region of touch near the drain, and further increases in v_DS occurs across this small region.  

  

Figure: n-channel JFET structure for $\vert v_{DS} \vert >> \vert v_{GS} \vert$ showing non-parallel depletion regions.

JFETS are high input impedance devices, and so $i_G \approx 0$ (due to the reverse bias pn junctions).

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