JFET Transistors
The Junction field effect transistor
JFET’s are constructed in two types. They can either be N-channel, or P-channel. In an N-channel JFET, There is a solid layer of N-type semiconductor, with two layers of P-type material attached to the sides. These two P-type materials are calledd the gate, and the two ends of the N-type material are called the source and the drain.
In diagrams, the drain is at the upper end, and the source is at the bottom end. Currrent in the drain circuit flows from the source to the drain.
The JFET is always operated with the gate-source junction reverse-biased. This reverse biasing of the gate-source junction with a negative gate voltage produces a depletion region in the p-n junction, which extends into the N-channel and increases the resistance between the source and the drain terminals.
In an example with two power supplies, one is attached from the drain to the sourceand is called Vdd, and is known as the drain circuit. The negative terminal is connected to ground, as well as to the source of the JFET. The positive end is connected to a series limiting resistor (Rs) and also to the source terminal of the JFET.
The gate supply (Vgg) is connected with the positive end to ground, and the negative end to the gate. This creates a negative gate voltage, which is needed for the reverse biasing of the gate source pn junction.
A greater value of Vgg narrows the channel, which increases the resistance of the JFET, and decreases drain current (Id).
Less Vgg widens the channel, which decreases resistance and increases drain current(Id).
Pinch-off voltage
The Pinch-off voltage (Vp) is the value of voltage from drain to source at which drain current (Id) becomes constant. In this area, known as the constant-current area, drain current will remain constant until it reaches breakdown. Once breakdown occurs, the JFET is being operated out of range and current will increase quite rapidly until it is destroyed.
Cutoff voltage
The value of voltage from the gate to the source that produces a drain current of approximately zero is called the cutoff voltage, or Vgs(off). For N-channel JFET’s, this will be a negative voltage, and this causes the delpetion region to become so large that current flow is stopped.
There is a relation between the pinch-off voltage and the cutoff voltage. Vgs(off) and Vp are always equal, but opposite in sign. That is, if Vgs(off) is -3 volts, then pinch-off voltage is 3 volts.
to be continued…
