Bipolar Junction Transistor
Introduction
Bipolar junction transistor definition
A bipolar junction transistor or BJT is a three terminal electronic device that amplifies the flow of current. It is a current controlled device. In bipolar junction transistor, electric current is conducted by both free electrons and holes.
Unlike a normal pn junction diode, the transistor has two p-n junctions.
Types of Bipolar Junction Transistors (BJTs)
The bipolar junction transistors are formed by sandwiching either n-type or p-type semiconductor layer between pairs of opposite polarity semiconductor layers.
Bipolar junction transistors are classified into two types based on their construction: They are
- NPN transistor
- PNP transistor
NPN transistor
When a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers, the transistor is said to be an npn transistor
PNP transistor
When a single n-type semiconductor layer is sandwiched between two p-type semiconductor layers, the transistor is said to be a pnp transistor.
Both PNP and NPN transistors consist of three terminals: they are emitter, base, and collector.
Terminals of BJT
Emitter:
As the name suggests, the emitter section supplies the charge carriers. The emitter section is heavily doped so that it can inject a large number of charge carriers into the base. The size of the emitter is always greater than the base.
Base:
The middle layer is called base. The base of the transistor is very thin as compared to emitter and collector. It is very lightly doped.
Collector:
The function of the collector is to collect charge carriers. It is moderately doped. That is the doping level of the collector section is in between emitter and base. The size of the collector is always greater than emitter and base. The collector area in the transistor is considerably larger than the emitter area. This is because the collector region has to handle more power than the emitter does and more surface area is required for heat dissipation.
In transistor, the amplification is achieved by passing input current from a region of low resistance to a region of high resistance.
Applications of bipolar junction transistor
The various applications of bipolar junction transistors include:
- Televisions
- Mobile phones
- Computers
- Radio transmitters
- Audio amplifiers
Bipolar Junction Transistor
BJT operation modes
The transistor can be operated in three modes: - Cut-off mode
- Saturation mode
- Active mode
In order to operate transistor in one of these regions, we have to supply dc voltage to the npn or pnp transistor. Based on the polarity of the applied dc voltage, the transistor operates in any one of these regions.
Applying dc voltage to the transistor is nothing but the biasing of transistor.
Cutoff mode
In the cutoff mode, both the junctions of the transistor (emitter to base and collector to base) are reverse biased. In other words, if we assume two p-n junctions as two p-n junction diodes, both the diodes are reverse biased in cutoff mode. We know that in reverse bias condition, no current flows through the device. Hence, no current flows through the transistor. Therefore, the transistor is in off state and acts like an open switch.
The cutoff mode of the transistor is used in switching operation for switch OFF application.
Saturation mode
In the saturation mode, both the junctions of the transistor (emitter to base and collector to base) are forward biased. In other words, if we assume two p-n junctions as two p-n junction diodes, both the diodes are forward biased in saturation mode. We know that in forward bias condition, current flows through the device. Hence, electric current flows through the transistor.
In saturation mode, free electrons (charge carriers) flows from emitter to base as well as from collector to base. As a result, a huge current will flow to the base of transistor.
Therefore, the transistor in saturation mode will be in on state and acts like a closed switch.
The saturation mode of the transistor is used in switching operation for switch ON application.
From the above discussion, we can say that by operating the transistor in saturation and cutoff region, we can use the transistor as an ON/OFF switch.
Active mode
In the active mode, one junction (emitter to base) is forward biased and another junction (collector to base) is reverse biased. In other words, if we assume two p-n junctions as two p-n junction diodes, one diode will be forward biased and another diode will be reverse biased.
The active mode of operation is used for the amplification of current.
From the above discussion, we can say that the transistor works as an ON/OFF switch in saturation and cutoff modes whereas it works as an amplifier of current in active mode.
