An introduction to transistors

Transistors are the parts that can be found throughout electronic circuits. They are employed as switching devices and amplifiers and employed as amplifiers in high- and low-level circuits, frequency stages, oscillators, modulators, detectors and any circuit that requires a function—commonly used as switches in digital circuits.

Our technological world revolves around transistors. In almost every modern circuit, they are essential as a control source. Sometimes visible, although usually hidden deep within an integrated circuit’s die.

Three layers of a semiconductor make up a transistor. Each one is capable of transferring current. Semiconductors, such as germanium or silicon, is a substance that conducts electricity in a “semi-enthusiastic” manner. 

Doping is a chemical technique that gives specific features to semiconductor materials. Doping produces some material that either adds some extra electron (referred to as N-type due to the additional negative charge carrier) or develops some “holes” in the crystal structure. The three-layer structure of the transistor has an N-type semiconductor layer between the P-type layer or P-type layer placed between N-type layers (a PNP configuration) (an NPN configuration).

Transistor symbol

There are many different varieties of transistors, each with its own set of properties,advantages and disadvantages. Some transistors are primarily used in switching applications. Others are used for switching and amplification at the same time. Other transistors, such as phototransistors, are unique in that they react to the quantity of light shining on them to produce current flow.

Types of transistors

1. PNP Transistor

This transistor has two p-type semiconductor materials and is another form of BJT – Bipolar Junction Transistors. There is an n-type semiconductor used that separates the materials. Many of the charge carriers are holes and a few are electrons. The arrow symbol in the transistor shows the normal current flow. From the emitter terminal, the current flows to the collector terminal. Now, when the base terminal is pulled to LOW compared to the emitter terminal, the transistor will switch on. 

Three layers of semiconductor material make a bipolar PNP junction transistor, with two P-type regions and one N-type area. It consists of three terminals:

• Emitter.

• Collector.

• Base.

PNP transistor construction

The emitter and collector of P-type semiconductors are doped more strongly than the base of N-type semiconductors. As a result, the depletion zone penetrates the N-type layer at both junctions. Most charge carriers in PNP transistors are holes, while the minority charge carriers are electrons. The emitter creates holes, which the collector collects. The base current that enters the collector of a PNP transistor is amplified. The base is usually in charge of controlling current flow. In the base, current flows in the opposite direction. The emitter of a PNP transistor releases “holes,” which the collector gathers. There are a lot of free electrons in the base area. On the other hand, the intermediate layer is relatively narrow and minimally doped. As a result, there are much fewer free electrons in the base area.

2. NPN Transistor

NPN transistors are a type of three-layer bipolar transistor used for signal amplification. It is a device powered by electricity. The abbreviation NPN stands for the negative-positive-negative transistor. In this structure, a p-type semiconductor is fused between two n-type semiconductor materials. The emitter, base and collector are the three sections. The passage of electrons in an NPN transistor leads it to conduct.

NPN transistor construction

There are two techniques to construct an NPN transistor. NPN transistors are made by fusing a p-type semiconductor material (such as Silicon or Germanium) between two n-type semiconductor materials. A lot of distinct components make up an NPN transistor.

The emitter, collector and base are the three parts. The emitter-base junction is the part of the circuit that links the emitter to the base. On the other hand, the collector-base junction is the meeting point of the collector and base areas. Due to two junctions in between three areas, it works as two PN junction diodes. Each of the three regions has distinct levels of doping. There is extensive doping in the emitter region and the base region. The doping level in the collector region is moderate, somewhere between the emitter and the base region.

On the other hand, the PNP transistor has a P-region sandwiched between two N-Type regions. It is worth mentioning that you cannot swap the emitter and collector zones. It is because the collection region is slightly thicker than the emitter region. 

The structure and function of transistors

The three layers of semiconductor material are stacked together to make transistors. Some of these layers have some extra electrons added, while the others have electrons removed (a process known as “doping”). An n-type semiconductor material has more electrons than a p-type material (p for positive because electrons have a positive charge) . A transistor is made by stacking an N on top of a p on top of an n or by stacking p above n over p.

Conclusion

In this section, we learned about what a transistor is, the symbol of a transistor, types of transistors, its structure, functioning and working. We learned that a transistor is a semiconductor device. Also, 3 layers are used to make a transistor. It has two main types, the PNP and the NPN junction transistor, apart from which there are also a few other types of the transistor, but these two are the most common ones. The functioning and working are also well explained.