Overview: The article reviews bipolar junction transistors and their significance in electronic circuits, highlighting their role as switches and amplifiers. It covers the construction and working principles and their types, such as NPN and PNP transistors.

The invention of the transistor is a significant milestone that has paved the way for the development of numerous semiconductor devices capable of functioning both in discrete form and integrated circuit form. 

These transistors are utilized in various applications, such as modern computers, electronic gadgets, controllers, scientific equipment, etc. The transistors come in many forms and sizes, with the two primary varieties being bipolar junction transistors (BJT) and field effect transistors.

What is a BJT?

A BJT is a three-terminal semiconductor device that can serve two primary functions: as a switch for controlling electronic circuits or as an amplifier. Two types of silicon semiconductors are used in the production of BJT.

Construction of P-Type and N-Type of Semiconductor Material

Four silicon atoms surround each silicon atom. The silicon atom typically possesses four electrons in its valence shell. However, silicon seeks to achieve eight electrons in its valence shell to attain a stable structure as per the octet rule. 

Therefore, these silicon atoms form covalent bonds by sharing electrons with nearby atoms, resulting in a lack of free electrons for conducting electricity under normal conditions, as shown in Fig. 1. 

Therefore, two types of silicon semiconductors, namely p-type and n-type, are created through a process called doping. Doping is the intentional incorporation of impurities into the semiconductor material to modify its conductivity. 

Fig. 1: Covalent bonding of silicon atoms forms a stable structure with no free electrons. Source: Rakesh Kumar, Ph.D.

As shown in Fig. 2, N-type semiconductor materials are doped with pentavalent impurities, such as phosphorous, which have five electrons in their valence shell. Four of these electrons form covalent bonds with nearby atoms, while the remaining electron is free to travel and conduct electricity. Therefore, n-type semiconductor material contains an abundance of free electrons.

Fig. 2: Illustration of doping procedure Source: Rakesh Kumar, Ph.D.

In the case of p-type semiconductor material, trivalent impurities such as aluminum (which has three electrons in the valence shell) are used for doping. This leads to a deficiency of one electron available for covalent sharing with an adjacent atom, forming a hole. 

Therefore, p-type semiconductor material contains an abundance of holes. Both n and p-type semiconductor materials are used in the construction of BJT.

Construction of BJTs

A BJT comprises a thin slice of doped semiconductor material sandwiched between two other layers of doped semiconductor material, which can be p-type or n-type, as shown in Fig. 3. The three pins are doped differently and vary in size. The three doped regions of a transistor are:

  • Emitter
  • Base
  • Collector

As its name implies, the emitter plays a crucial role in the supply of electrons and is severely doped. The base is lightly doped and has a reduced thickness. The collector is moderately doped, and the width is comparatively greater. It functions to gather electrons from the emitter.

Fig. 3: Illustration of two types of BJT Source: Rakesh Kumar, Ph.D.

The term bipolar signifies that both electrons and holes have a role in the conduction of electric current. 

Types of BJTs

BJTs can be classified into two varieties based on the arrangement of the doped material, as shown in Fig. 3.

  • NPN transistor
  • PNP transistor

The NPN transistor has its emitter and collector doped with n-type semiconductor material, while the base is doped with p-type semiconductor material. The PNP transistor has its emitter and collector doped with p-type semiconductor material, while the base is doped with n-type semiconductor material. It consists of two PN junctions. 

The symbol used to denote the two types of BJT is shown in Fig. 4:

Fig. 4: Symbol of NPN and PNP BJT Source: Rakesh Kumar, Ph.D.

Typically, low-power transistors are housed in a resin case, whereas high-power transistors are partially contained in a metal case to effectively dissipate the heat they generate. The high-power transistors are also connected to heat sinks, which help remove the heat that may harm components over time.

Regions of Operation in BJTs

A BJT can function in three distinct areas.

  • Active region
  • Cut off region
  • Saturation region

Active Region

In the active region, it is essential to apply forward bias to the base-emitter junction and reverse bias to the base-collector junction. The operation of an NPN transistor in the active region is schematically shown in Fig. 5.

Fig. 5: Illustration of working of BJT Source: Rakesh Kumar, Ph.D.

The operation of the transistor is illustrated by the formulas below. The voltage at the base is denoted as VB, whereas the voltage at the collector is denoted as VC, and the voltage at the emitter is denoted as VE. For an NPN BJT to operate in the active area, VC > VB > VE. The transistor works as an amplifier in this active region.

Working of BJTs

The depletion zone in the base-emitter PN junction diminishes because of the forward bias of the base-emitter junction. This pulls electrons toward the base region. Only a few electrons recombine with holes in the base region and are subsequently drawn toward the positive terminal of the base, and the remaining electrons are drawn toward the collector. This is due to the following reasons: 

  • The base region is sparsely doped, resulting in a low concentration of holes available for recombination
  • Additionally, the width of the base region is much reduced, allowing electrons from the emitter to pass through this region and reach the collector quickly

Once the electron reaches the collector, due to the reverse bias of the base-collector junction, the electrons are drawn toward the positive terminal of the collector. IB, IC, and IE indicate the direction of current flow in the base, collector, and emitter. Emitter current can be derived using the following equation:

IE= IB+IC

Only a small proportion of electrons contribute to the base current, and the current through the collector can be represented as, 

IC =  aIE

where   is one crucial factor that determines the efficiency of the transistor and is a constant value specific to the transistor, which depends on various factors such as size doping, etc.

Secondly, the ratio of the collector current to the base current is referred to as the current gain or amplification factor (b). This is another important parameter that determines the transistor's efficiency.

Therefore, by regulating the minimal base current on the input side, the current on the output side may be managed, which is why BJT is referred to as a current-controlled current source.

Cut-off Region

In the cut-off region, both the base-emitter and the base-collector junction are reverse-biased, which may be expressed as VC > VB < VE. In this region, the transistor functions as an open switch, effectively interrupting the flow of current.

Saturation Region

Both the base-emitter and base-collector junctions of the transistor are forward-biased in the saturation region. This can be written as VE < VB > VC. In this state, the transistor functions as a conductor. 

Active regions are utilized for amplification, whereas the cut-off and saturation regions are used for switching purposes.

Practical BJT Product to Consider 

The BC857C,215, as shown in Fig. 6, is a PNP transistor specifically engineered for various purposes, such as switching and amplification, in various applications. The BC857 series of transistors, made by Nexperia, is well known for its capacity to function at low current and low voltage levels. 

Fig. 6: BC857C,215 BJT Source: oemsecrets

To conclude, understanding the construction and working principles of BJTs, including the roles of the emitter, base, and collector, is essential for their effective utilization.

Summarizing the Key Points

  • BJTs are crucial semiconductor devices used as switches and amplifiers in electronic circuits, offering versatile functionality
  • BJTs operate in distinct regions, such as the active, cut-off, and saturation regions, each requiring specific biasing conditions for optimal performance
  • The two types of BJTs, NPN and PNP transistors, based on doping materials, provide unique characteristics and applications in various electronic systems

Reference

S. a. L. Maftunzada, “The Structure and Working Principle of a Bipolar Junction Transistor (BJT),” Physical Science International Journal 26, no. 11–12 (December 31, 2022): 35–39, https://doi.org/10.9734/psij/2022/v26i11-12772.

Luiz a. P. Santos, “An Overview on Bipolar Junction Transistor as a Sensor for X-ray Beams Used in Medical Diagnosis,” Sensors 22, no. 5 (March 1, 2022): 1923, https://doi.org/10.3390/s22051923.