A JFET (Junction Field-Effect Transistor) is a type of transistor that works by controlling the width of a conducting channel between two semiconductor regions (N-type and P-type) using an electric field. The device consists of a single piece of N-type or P-type semiconductor material with two P-N junctions on either side forming a channel.
The operation of a JFET is based on the reverse-biasing of the P-N junctions, which creates a depletion region in the channel. The width of the depletion region determines the resistance of the channel, and thus, the current flow through the device. When a voltage is applied across the JFET, the electric field across the depletion region changes, which alters the width of the channel and hence the resistance.
There are two types of JFETs – N-channel and P-channel. In an N-channel JFET, the channel is made of N-type material, and a negative voltage is applied to the gate relative to the source to increase the width of the depletion region and reduce the current flow. In a P-channel JFET, the channel is made of P-type material, and a positive voltage is applied to the gate relative to the source to decrease the width of the depletion region and increase the current flow.
JFETs are commonly used in amplifier circuits, switching circuits, and voltage regulators. They have some advantages over other types of transistors, such as high input impedance, low noise, and simple biasing circuits. However, they also have some disadvantages, such as lower gain, lower bandwidth, and higher output impedance.
p-n Junction
When a semiconductor is doped with impurities, it creates excess electrons (n-type doping) or holes (p-type doping) in the material, which can carry electrical charge. These excess electrons or holes can move around the material, allowing for the flow of electric current.
When two differently doped regions of a semiconductor material are brought together, a p-n junction is formed. At the p-n junction, the excess electrons from the n-type region and the holes from the p-type region diffuse across the junction and combine, creating a region that is depleted of charge carriers called the depletion region.
The p–n junction possesses a useful property for modern semiconductor electronics. A p-doped semiconductor is relatively conductive. The same is true of an n-doped semiconductor, but the junction between them can become depleted of charge carriers, and hence non-conductive, depending on the relative voltages of the two semiconductor regions. By manipulating this non-conductive layer, p–n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction.
Bias is the application of a voltage relative to a p–n junction region:
- Forward bias. When a voltage is applied across the p-n junction in the forward bias direction (i.e., the positive terminal is connected to the p-type region and the negative terminal to the n-type region), the depletion region becomes narrower and allows the flow of current through the material.
- Reverse bias. In the reverse bias direction (i.e., the positive terminal is connected to the n-type region and the negative terminal to the p-type region), the depletion region becomes wider, preventing the flow of current through the material. However, if the reverse voltage is increased to a certain threshold value, the material can undergo a process called avalanche breakdown, in which the depletion region suddenly collapses and allows a large amount of current to flow through the material.
The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. When the p–n junction is forward-biased, electric charge flows freely due to reduced resistance of the p–n junction. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal.
