What is a transistor and what is it for?

Do you know what a transistor is? This device is used in most electronic circuits, being the main component of modern electronic equipment. Its characteristics are of great importance in the study of electronics and will be seen below.

What is a transistor?

The transistor is a three-layer semiconductor device, widely used in the construction of electronic chips for the most varied applications. Mainly composed of silicon or germanium, the transistor is used in signal amplification and production processes and in switching operations.

A transistor can be created by joining three semiconductor materials obtained by adding impurities, alternating between those of type P (which have positive polarity) and type N (which have negative polarity). In this way, the two main types of transistors used in electronics originate, which are the PNP transistor and the NPN transistor.

When was it discovered?

The first transistor was developed on December 16, 1947, by William Shockley, John Bardeen and Walter Brattain in the laboratories of Bell Telephone. Shortly after being announced and perfected, transistors replaced valves in electronic applications, as they are more compact, robust and efficient.

What are the main types of transistors?

Transistors can be divided according to their type and application. Among the various types of transistors on the market, bipolar junction transistors and field-effect transistors are the most used.

The Bipolar Junction Transistor – TBJ

This category of transistor receives this name because it has two combined PN junctions and involves both positive and negative charges in the conduction process. The two most common types of TBJ are NPN and PNP. In PNP transistors, the current is composed mostly of positive charges (gaps), while in NPN the current is composed mostly of electrons. In both cases, there is also a flow of minority charges.

Symbols used for NPN and PNP transistors.

The three terminals of the bipolar junction transistor are called the base, collector, and emitter. The base terminal is responsible for controlling the driving process, while the emitter and collector are the input and output terminals of the main driving current. The order of the terminals on each transistor can switch between different models, types and manufacturers, making it necessary to consult its datasheet to know the correct order.

Because it is a device that has three alternating semiconductor layers, it is possible to treat it as two junctions of type P and type N semiconductors. There are then the base-emitter junctions and the base-collector junction. When correctly polarized, these PN junctions can be considered as two diodes, with the base-emitter junction equivalent to a directly polarized diode and the base-collector junction to an inversely polarized diode.

Because it presents this behavior between its junctions, the bipolar junction transistor has three different operating regions, depending on the type of polarization applied between its terminals. The active region of operation occurs when the base-emitter junction is directly polarized and the base-collector is inversely polarized. The saturation region occurs when both junctions are directly polarized. The cutting region, on the other hand, occurs when both junctions are inversely polarized, with no current flowing between the collector and the emitter.

The Field Effect Transistor – FET

Another type of transistor widely used is the FET, which unlike TBJ, is controlled according to the voltage at its control terminal (gate). This device has three terminals: the gate, which is analogous to the base; the source, which has the same role as the issuer; and the drain that has the same function as the collector.

The FET is a unipolar device, that is, its conduction involves only one type of load at a time. There are no PN junctions defining the type of load conducted, just a semiconductor channel connecting source and drain. This characteristic gives rise to the two main types of FET, those of the N channel (conduct electrons) and those of the P channel (conduct gaps).

There are two main types of field effect transistors: the JFET (Junction Field Effect Transistor), which translating is Field Effect Junction Transistor; and the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) which translating is Oxide Metal Semiconductor Field Effect Transistor. Both transistors are more compact than TBJ and have a high input impedance as the predominant characteristic in applications, which is the reason why they do not act depending on the current.

What are the main uses of transistors?

The two main uses of TBJ are in amplification and switching operations in circuits. The first occurs because when a current is applied to the base of the transistor, a collector current proportional to the base current multiplies by its gain. In the datasheets, this gain is called hFE, which is the current gain in the transistor collector, when it operates in the active region. The video below shows an example of how to measure this gain with a digital multimeter.

The second use of the transistor, and the main one in control operations, is switching circuits. In this function, the transistor operates in the cut and saturation regions. When the current applied to the base is equal to zero, the transistor operates as an open switch in the cut-off region. When the base is supplied with a current in order to obtain the maximum collector current (specified in the datasheets), the device goes from the cut-off state to saturation, thus acting as a closed switch to the minimum increase of current above the limit.

Example of applying the transistor as a switch. The LED lights up when the transistor is saturated.

What is the most used transistor?

Transistors have the most diverse applications and function of their characteristics. Examples of transistors most used in electronic circuits are the NPN and PNP transistors BC548 and BC558, along with similar ones from the same family. The difference between them is only in the nominal characteristics exposed in the datasheets. BCs transistors are general-purpose devices used in circuits such as oscillators, audio amplifiers, LED exciter , among others, and are present in most electronic circuits.