Field Effect Transistor

Junction Field Effect Transistors | The Structure of a JFET | FET Operation | JFET Amplifiers


Junction Field Effect Transistors

Now that you know how a BJT device is designed and how it operates, let us proceed to a Junction Field Effect (JFET) transistor. A JFET transistor is similar to the following BJT device characteristics:

It has three terminals.
It can amplify small voltages.
It has an electrical field that controls the shape and conductivity of a channel in the semiconductor material.

There are two types of JFETs:

n-channel - Obtained by embedding a p-type material around an n-type channel.
p-channel - Obtained by embedding an n-type material around a p-channel.

Review Figures 3.1 and 3.2 for the respective JFET n-channel and p-channel structures and symbols:

Figure 3.1: n-Channel JFET

Figure 3.2: p-Channel JFET


The Structure of a JFET

While a BJT is a bipolar semiconductor device, a JFET is a unipolar semiconductor device containing a single p-n junction. Under normal operating conditions, a JFET is always reverse-biased. To understand how a JFET is constructed, review Figure 3.2. In Figure 3.2, a channel of an FET (a single semiconductor crystal) is doped to produce either an n-type or a p-type channel semiconductor.

Figure 3.2 also illustrates the three terminals of a JFET:

Gate (G) - Acts as a "gatekeeper" by aiding or obstructing the flow of electrons by creating or eliminating a channel between the source and drain.
Drain (D) - Acts as a "drain" for charge carriers that need to be drained; that is, removed from the device.
Source (S) - Acts as the source for charge carriers by connecting the source charge carriers to the channel current.


FET Operation

Given the three terminals of a JFET, it is important to recognize how these terminals interact with each other based on the voltage applied. The flow of electrons from the source towards the drain depends on the applied voltage between the source and gate terminals. Irrespective of whether you are working with an n-channel or a p-channel JFET, the charge carriers always travel from the source to the drain.

The gate source p-n junction is reverse-biased, as indicated in the Figure 3.3. In other words, there is reverse leakage current flowing between the source and the gate terminals. As you increase the reverse voltage V_GS, the depletion region extends into the channel. This indicates an increase in the resistance and a decrease in current flow through the channel. This current is the drain current and is denoted as ID. Therefore, as you increase the voltage you apply, the device turns off.

Figure 3.3: n-Channel JFET Circuit

In summary:

When there is no control voltage V_GS applied at the gate, the charges flow freely through the channel and make the drain current very high.
Result: The JFET acts like a switch that is turned on.
When you apply a reverse-biased control voltage at the gate, the depletion region keeps increasing in width. This obstructs the free flow of charges through the channel.
Result: This condition turns the JFET device off. Since the junction is reverse-biased, I_G = 0A and I_S = I_D.
Therefore, an FET device acts like a turn-off device while a BJT device acts like a turn-on device.


JFET Amplifiers

JFET amplifiers are classified as Common Gate, Common Drain, and Common Source amplifiers. To evaluate these circuits, a DC analysis must be performed in order to establish the DC operating point. Again, the AC signals will be riding on that DC voltage. The output signal will be amplified from the input signal.