This tutorial gives the reader a basic understanding of the junction field effect transistor.
This article contains the following topics:
The N-channel JFET
Before discussing the junction field effect transistor, here is a quick review of semiconductor theory.
An neutral atom contains an equal number of positive charges (protons) and negative charges (electrons). A neutral atom has no net charge.
A neutral atom that loses an electron attains a positive charge and is called a positive charged ion.
Likewise, a neutral atom that gains an electron attains a negative charge and is called a negative charged ion.
A diode is made of two different materials: N-type material (the anode) and P-type material (the cathode). The anode contains positive ions meaning atoms with a positive net charge. The cathode contains negative ions meaning atoms with a negative net charge.
In a forward biased diode, the negative charges travel from the cathode to the anode because they are attracted to the positive voltage external to the anode. These charges are associated with electrons that broke free from their atoms. The atoms in the cathode contain extra electrons. Hence a small electrical potential can remove these electrons from the atom. The electron is the medium of electricity. It contains a negative charge. When an electron breaks away from an atom, the atom loses the negative charge. However it could be interpreted as the atom gaining a positive charge.
In a reverse biased diode, the charges move away from the junction of the anode and the cathode leaving an uncharged area known as a depletion region. In this depletion region each atom have no net charge. The depletion region has an extremely high impedance.
When a diode is reverse biased, a finite amount of leakage current flows through it. This current flows from the type N material to the type P material and is called leakage current. Some of this leakage current is thermal current and some of this current may be due to impurities.
The bipolar transistor
The same phenomenom happens between collector and base of an NPN transistor.
The NPN transistor’s layout is shown in Figure 1a. The symbol for an NPN transistor is shown in figure 1b. The symbol for this leakage current is Icbo. The transistor is controlled by the base-emitter current. This is easily realized when looking at the ratio of collector current to base current.
current gain = (collector current)/(base current)
A emitter common transistor that amplifies a signal is always on. This means that the base emitter junction is always forward biased. This yields a low input impedance because that impedance is the impedance of a foreward biased diode.
The N-channel junction field effect transistor
The junction field effect transistor (JFET) is a voltage controlled device. It’s layout is shown in Figure 1c. The symbol for a JFET is shown in figure 1d. There are two types of junction FETs: the N-channel JFET and the P-channel JFET. Their operation is similar except that the biasing is opposite. This article will focus on the N-channel JFET.
In figure one; G = gate, S = source and D = drain.
The gate is the controlling input of the N channel JFET. The gate controls the current through the channel via increasing or decreasing the size of the channel depletion region along the junction of the gate and the channel. The channel has two external connections: the drain and the source. The source literally is the source of electrons. The drain is typically connected to a positive voltage.
Take a look at figure two. It shows the bias for a JFET.
The output current flows between drain and source. This path is called the channel. This current is controlled by the reverse biased gate-channel junction. Since the gate-channel junction is always reverse biased, the input impedance of a JFET is very high.
The more negative the gate voltage, the larger the depletion region in the channel. As the gate voltage decreases, the depletion region spreads across the channel pinching off the current through the channel.
The junction field effect transistor has three modes of operation: saturation, active and pinch off.
Saturation occurs when the gate is shorted to the source. No depletion region exists in the channel and therefore, the maximum amount of current flows from drain to source.
In the active mode, the gate-channel junction is reverse biased and the channel current is controlled by the width of the depletion region. The larger the depletion region, the narrower the conducting region of the channel and consequenly; less current flows through the channel. In this mode of operation, the JFET has a very low output impedance. The output impedance is the impedance from drain to source.
Pinch off happens when the depletion region extends across the channel cutting off all channel current flow.
One popular use of JFETs is as a switch. The low impedance of the channel when the channel is conducting and the extremely high impedance of the channel during pinch-off make the JFET a great switch.
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