Principles and Classification of Field-Effect Transistors
The field-effect transistor is commonly referred to as an FET. Model designations typically consist of a prefix (such as 3DJ, 3DO, or CS) followed by a serial number and specification code. While they resemble ordinary bipolar junction transistors (BJTs) in appearance and share advantages such as compact size and low power consumption, their control characteristics differ fundamentally. An ordinary BJT is a current-controlled device that regulates collector current by controlling base current, and it possesses relatively low input impedance. In contrast, an FET is a voltage-controlled device that regulates output current using the electric field effect generated by the input voltage; FETs are widely used due to advantages such as high input impedance, excellent thermal stability, and suitability for integration.
Based on their conduction mechanisms, FETs are classified into two types: junction FETs (JFETs) and insulated-gate FETs. The following discussion focuses primarily on JFETs, with a brief introduction to insulated-gate FETs.
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1. Junction Field-Effect Transistor (JFET)
If the size of a semiconductor's conductive region can be controlled—thereby altering its resistance—the current flowing through an N-type semiconductor can be regulated. A PN junction contains mostly immobile impurity ions, has very few charge carriers, and exhibits high resistivity; when a reverse bias voltage is applied, the depletion region of the PN junction widens. By creating two PN junctions on opposite sides of an N-type semiconductor and varying the magnitude of the reverse bias voltage, one can adjust the width of the PN junctions. This controls the size of the region available for electron flow, thereby regulating the current intensity within the N-type semiconductor. The JFET is constructed based on this fundamental conduction principle.
2. Insulated-Gate Field-Effect Transistor
An insulated-gate FET is a type of FET in which the gate is completely electrically isolated from the drain and source, resulting in even higher input impedance. The most widely used type of insulated-gate FET is the Metal-Oxide-Semiconductor FET, or MOSFET. MOSFETs come in two varieties—N-channel and P-channel (designated as NMOS and PMOS, respectively)—and each variety can be further classified into enhancement-mode and depletion-mode types.
Based on their conduction mechanisms, FETs are classified into two types: junction FETs (JFETs) and insulated-gate FETs. The following discussion focuses primarily on JFETs, with a brief introduction to insulated-gate FETs.
1. Junction Field-Effect Transistor (JFET)
If the size of a semiconductor's conductive region can be controlled—thereby altering its resistance—the current flowing through an N-type semiconductor can be regulated. A PN junction contains mostly immobile impurity ions, has very few charge carriers, and exhibits high resistivity; when a reverse bias voltage is applied, the depletion region of the PN junction widens. By creating two PN junctions on opposite sides of an N-type semiconductor and varying the magnitude of the reverse bias voltage, one can adjust the width of the PN junctions. This controls the size of the region available for electron flow, thereby regulating the current intensity within the N-type semiconductor. The JFET is constructed based on this fundamental conduction principle.
2. Insulated-Gate Field-Effect Transistor
An insulated-gate FET is a type of FET in which the gate is completely electrically isolated from the drain and source, resulting in even higher input impedance. The most widely used type of insulated-gate FET is the Metal-Oxide-Semiconductor FET, or MOSFET. MOSFETs come in two varieties—N-channel and P-channel (designated as NMOS and PMOS, respectively)—and each variety can be further classified into enhancement-mode and depletion-mode types.
Sep 08,2026