What Is a Transistor?
A transistor is a semiconductor device used in electronic circuits to amplify electrical signals and to control the flow of electricity.
Transistors are used in various electronic devices. Applications that utilize the amplifying action include audio amplifiers and sensor detection circuits. Applications that utilize the switching action include logic circuits that make up integrated circuits and rectifier circuits for power supplies.
The structure of a transistor is a combination of n-type and p-type semiconductors. They are generally fabricated on silicon (Si) substrates, although compound semiconductors such as SiGe, GaAs, SiC, and GaN may also be used depending on the application.
How Transistors Work
A transistor is a three-terminal device consisting of a thin p-type semiconductor base sandwiched between an n-type semiconductor collector and emitter. This is known as an NPN-type transistor. There is also a PNP-type transistor, in which an n-type semiconductor is sandwiched between a p-type semiconductor. These NPN and PNP types can be combined to make electronic circuits with a various functionalities.
In the case of a PNP-type transistor, the voltage polarity and the direction of the current are reversed. When using an NPN-type transistor, a positive voltage is applied to the base and collector with respect to the emitter. As a result, a current tens to hundreds of times larger than the current flowing in the base will flow in the collector. In other words, the output of the collector current can be controlled by the input of the base current, providing amplification and switching effects.
The ratio of the base current to the collector current is called the current amplification factor, hFE, and is one of the important performance indicators of a transistor.
Types of Transistors
There are several types of transistors based on their structure. Transistors can be broadly classified into two categories: bipolar transistors and unipolar transistors.
The term bipolar means bi-polar because it involves electrons and holes to conduct an electric current. On the other hand, unipolar transistors are named as such because either electrons or holes are involved to conduct current.
1. Bipolar Transistor (BJT)
Bipolar transistors are made from a combination of P-type and N-type semiconductors, with either NPN or PNP junctions. The term transistor simply refers to a bipolar transistor, while an NPN or PNP transistor is called a BJT.
2. Phototransistor
A type of transistor in which the collector current can be controlled by incident light. It can take out a larger photocurrent compared to photodiode.
They should be selected appropriately according to the application.
3. Field Effect Transistor (FET)
While transistors control collector current by varying the base current, field-effect transistors control current by voltage.
Since transistors are controlled by voltage, they require less drive power and can be easily driven at high speeds compared to BJTs. On the other hand, they are inferior in terms of high withstand voltage and high current.
FETs also include junction FETs (JFETs) and metal oxide semiconductor FETs (MOSFETs), each of which has an N-channel type and a P-channel type. Transistors consume a lot of power and generate a lot of heat, so they cannot be used in dense circuits.
Unlike transistors, field-effect transistors consume less power and can be made smaller, as such are widely used in integrated circuits such as ICs and LSIs.
4. Insulated Gate Bipolar Transistor (IGBT)
IGBTs are one of the power transistors used in high-power applications, and their structure facilitates high current and high withstand voltage.
Transistor Operation
Transistors come in NPN and PNP types and are semiconductor devices with three electrodes: base, collector, and emitter.
When the positive electrode of a dry cell battery is connected to the base and the negative electrode to the emitter, electrons flow from the emitter into the base. Some of these electrons combine with holes in the base, while the remaining electrons flow into the junction surface of the base and collector. The current flowing from the base to the emitter is called the base current, while the current flowing from the collector to the emitter is called the collector current. The collector current is much larger than the base current, and a small change in the base current causes a large change in the collector current. This is called the amplifying effect of the transistor.