Field Effect Transistor is a three terminal device electronic component that uses electric field to control the conductivity of a channel. Output current is proportional to the input voltage applied to its gate terminal. This creates an electric field that controls the current flowing through it. Field effect transistor or FET transistor is a unipolar device, that is current through the device is only due to majority carriers. It can be made smaller than an equivalent BJT transistor and therefore, device density in the integrated circuit can be increased. Another important advantage is that, its input impedance is higher than BJT.
In the figure, you can see three terminals, Source, Drain and Gate of the JFET or FET Transistor. Current flow from source to drain is controlled by the voltage applied at the gate terminal.
Junction Field Effect Transistor: FET Transistor
As shown in the figure, source and drain is connected by a narrow, high-resistivity semiconductor material. Channel width can be varied by changing the voltage applied at the gate. Depending on the type of majority carrier in the channel they can be of two types, P-channel and N-channel. Electrons have higher mobility than holes and this makes N-channel JFET electronic component more efficient over P-channel JFET electronic component. Under reverse-biased conditions, gate current is negligible. This improves the efficiency of JFET or FET transistor over BJT, since in BJT base current will always be a non-zero value.
Working of JFET
By changing the width of the channel, current flow through this electronic component can be controlled. Channel has the maximum width when no voltage is applied across the gate. In N-channel JFET, if the negative voltage at the gate is increased, it reverse biases the junction. This in turn, reduces the width of the channel. When reverse bias is increased the width of the depletion region will also increase. Depletion region extends to the channel which reduces effective channel width. At one point, channel ‘pinch off ' occurs, which stops the conduction through the channel.
Working of JFET or FET Transistor
The JEFT characteristics show the different regions of operation of JFET electronic component. There are basically 4 regions of operation for JFET.
- Ohmic Region: In this region the JFET will act as a voltage controlled resistor. The JFET will be in this mode when Vgs=0.
- Cut off Region: JFET will act in the cut off state when Vgs is sufficient to cause the JFET to act as an open circuit.
- Active Region: This region is also called as the saturation region. In this region, the JFET will act as a conductor.
- Breakdown region: When Vds becomes very high, JFET will move to a breakdown region.
JFET- Mode of operation
- CS (Common Source Configuration)
In this mode of operation, JFET will have the input given to the gate terminal and the output taken from the drain terminal. This mode has high input impedance and better amplification features. The main application of this mode of JFET is in the amplifier section.
- CG (Common Gate Configuration)
Here, the input is given to the source and the output is taken from the drain terminal In this mode of operation, the major disadvantage is it has low input impedance. But here the output impedance is high. Impedance matching circuits makes use of this mode.
- CD (Common Drain Configuration)
In this mode of operation, the input is given to the gate and the output is taken from the source. This mode is also known as Source Follower. Since it has a voltage gain near to unity, it can be used in buffer amplifiers.