Difference between a field-effect transistor and a bipolar transistor

The field-effect transistor (FET) is a type of transistor in which conduction is controlled by an electric field. There are two main types: the junction FET (JFET) and the metal–oxide–semiconductor FET (MOS‑FET). Since conduction involves only majority carriers, it is also referred to as a unipolar transistor. It is a voltage‑controlled semiconductor device that offers numerous advantages, including very high input impedance (10⁷–10¹⁵ Ω), low noise, low power consumption, wide dynamic range, ease of integration, absence of secondary breakdown, and a broad safe operating area. As a result, it has become a strong competitor to bipolar junction transistors and power transistors.

What is a field-effect transistor?

The field-effect transistor (FET) is a type of transistor in which conduction is controlled by an electric field. It comes in two main types: the junction FET (JFET) and the metal–oxide–semiconductor FET (MOS‑FET). Since conduction involves only majority carriers, it is also referred to as a unipolar transistor. As a voltage‑controlled semiconductor device, it boasts several advantages, including very high input impedance (10⁷–10¹⁵ Ω), low noise, low power consumption, wide dynamic range, ease of integration, absence of secondary breakdown, and a broad safe operating area. Consequently, it has become a formidable competitor to bipolar junction transistors and power transistors.

A field-effect transistor (FET) is a semiconductor device that uses the electric field effect in the input circuit to control the current in the output circuit, hence its name. Because it conducts current solely through majority carriers in the semiconductor, it is also referred to as a unipolar transistor.

Compared with bipolar junction transistors, field-effect transistors have the following characteristics: ：

(1) A field-effect transistor is a voltage-controlled device; it regulates the drain current ID by means of the gate-source voltage VGS.

(2) The control input terminal of a field-effect transistor draws very little current, resulting in a very high input resistance (10⁷–10¹² Ω).

(3) It operates by majority-carrier conduction, and therefore exhibits good temperature stability.

(4) The voltage gain of the amplifier circuit it constitutes must be lower than that of an amplifier circuit composed of a transistor.

(5) Field-effect transistors exhibit strong radiation resistance;

(6) It exhibits low noise because it is free from shot noise caused by the random motion of electrons.

What is a transistor?

A transistor, formally known as a semiconductor transistor, is also referred to as a bipolar junction transistor or a crystal transistor. It is a current‑controlled semiconductor device that amplifies weak signals into electrical signals with larger amplitudes and can also serve as a contactless switch. The crystal transistor is one of the fundamental semiconductor components; it exhibits current amplification and is a core element in electronic circuits. A transistor is fabricated by forming two closely spaced PN junctions on a single semiconductor substrate. These two PN junctions divide the semiconductor into three regions: the central region is the base, while the two outer regions are the emitter and collector. Transistors come in two configurations: PNP and NPN.

Working principle

Bipolar transistors (hereafter referred to as “transistors”) are classified by material into two types: germanium transistors and silicon transistors. Each of these comes in two structural configurations—NPN and PNP. However, the most widely used are silicon NPN and germanium PNP transistors. (Here, “N” stands for negative, referring to the negative terminal; an N-type semiconductor is created by doping high-purity silicon with phosphorus, which replaces some silicon atoms and generates free electrons that conduct electricity under an applied voltage. Meanwhile, “P” denotes positive, meaning that boron is added to replace silicon atoms, creating a large number of holes that facilitate electrical conduction.) Apart from differences in power‑supply polarity, the operating principles of both types are essentially the same.

Difference between a field-effect transistor and a bipolar transistor

Like bipolar transistors, field-effect transistors can both control and amplify signals; however, due to their fundamentally different structures and operating principles, the two types exhibit significant differences. In certain specialized applications, field-effect transistors outperform bipolar transistors and are irreplaceable by them. The key distinctions between bipolar transistors and field-effect transistors are summarized in the table below.

A field-effect transistor is a voltage-controlled device, whereas a bipolar transistor is a current-controlled device. When only a small current needs to be drawn from the signal source, a field-effect transistor should be selected. Conversely, when the signal-source voltage is low and a relatively large current can be sourced, a bipolar transistor is more appropriate. In a field-effect transistor, conduction is carried by majority carriers, meaning that only one type of charge carrier—either electrons or holes—moves through the device; in contrast, a bipolar transistor utilizes both majority and minority carriers. Because the concentration of majority carriers is less susceptible to external influences, field-effect transistors are particularly well suited for applications involving significant environmental variations. Field-effect transistors have high input impedance, making them ideal for circuits requiring high input resistance. Additionally, they exhibit a low noise figure, which makes them suitable for the front stage of low-noise amplifiers.

1. A bipolar transistor is a bipolar device, meaning that during operation, both holes and free electrons—two types of charge carriers—participate in current conduction within the device. In contrast, a field-effect transistor is a unipolar device; during operation, either only holes or only free electrons contribute to conduction, involving just one type of charge carrier.

2. A bipolar transistor is a current-controlled device: output current flows only when there is input current. A field-effect transistor, on the other hand, is a voltage-controlled device; it can produce output current even in the absence of input current.

3. Bipolar transistors have low input impedance, while field-effect transistors have high input impedance.

4. For some field-effect transistors, the source and drain terminals are interchangeable, whereas for bipolar transistors, the collector and emitter terminals are not interchangeable.

5. The frequency characteristics of a field-effect transistor are inferior to those of a bipolar transistor.

6. Field-effect transistors have a low noise figure, making them suitable for the front stage of low-noise amplifiers.

7. If you want the signal source current to be low, a field-effect transistor is preferable; otherwise, a bipolar transistor is more suitable. 。