The junction-gate field-effect transistor ( JFET) is one of the simplest types of field-effect transistor. The symbols differ for N-type and P-type channel materials and for enhancement-mode and depletion-mode MOSFETs.Electric current from source to drain in a p-channel JFET is restricted when a voltage is applied to the gate. Look closely at the four symbols used for the MOSFET, Figure 3. In an enhancement-mode MOSFET, a current through the source-drain circuit is increased by the gate voltage. In a depletion-mode MOSFET, a current through the source-drain circuit is reduced by the gate voltage. There are two main types of MOSFET, enhancement mode and depletion mode. A low impedance, or low resistance, a path through the channel is also very desirable for devices such as amplifiers. A high impedance input device is very desirable in circuits such as amplifiers.Īlso, the channel consisting of the same material from the source to the drain provides a very low impedance path. The high resistance between the gate and channel area makes for a very high impedance input device. The thin layer of high resistance insulation prevents electron flow between the gate and the channel material. The MOSFET is similar in construction to the JFET. Care must be taken when handling MOSFET transistors and devices.įigure 3. The layer of insulation is so thin that it can easily be damaged by static electricity. The difference is that the MOSFET has a very thin film of insulation (silicon dioxide) between the gate and the channel area. It is used extensively in digital circuits and memory circuits in computers. MOSFET ConstructionĪnother type of FET is the MOSFET.
The amount of voltage applied to the gate will determine the current value from the source to the drain. Notice how the potential applied at the gate pinches off the flow of electrical energy.įigure 2. If a large potential is applied to the gate, a small current will exist in the channel between the source and the drain. If a small potential is applied to the gate, a large current through the P channel will exist. The current through the device is controlled by the gate potential. The physical construction of typical JFETs and their schematic symbols. The channel through the center of the device can be P- or N-type material.įigure 1. The channel is a path from the source to the drain. The gate is diffused into the channel material.
These three parts are similar to the three main parts of a typical bipolar transistor. The three main parts of the JFET are the source, drain, and gate. The construction and symbols for the JFET are shown in Figure 1.
There are two types of FETs, the junction field-effect transistor (JFET) and the metal oxide semiconductor field-effect transistor (MOSFET). A FET controls current in the source-drain circuit by the amount of potential applied to the gate. The current through the collector-emitter circuit of a BJT is controlled by the amount of current in the base-emitter circuit.
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