Describe the working principle of a bipolar junction transistor (BJT) and its configurations.
1 Answer
A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that operates as an amplifier or a switch in electronic circuits. It consists of three layers of semiconductor material: the emitter, the base, and the collector. There are two main types of BJT: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). The working principle of a BJT is based on the interaction of minority charge carriers (holes for NPN, electrons for PNP) and majority charge carriers (electrons for NPN, holes for PNP) in the semiconductor regions.
Working Principle:
NPN BJT:
The emitter region is heavily doped with electrons (N-type) to provide an abundant supply of majority charge carriers (electrons).
The base region is lightly doped compared to the emitter and is very thin. It acts as the control element in the transistor.
The collector region is moderately doped with electrons (N-type) and is larger in size than the base.
When a voltage is applied across the base-emitter junction (forward bias), it allows electrons to flow from the emitter to the base region. These electrons represent the minority carriers in the base region.
Due to the thinness of the base region, only a small number of electrons are required to cross it and reach the collector region, which is reverse biased with respect to the base-emitter junction.
The majority charge carriers (electrons) in the collector region are attracted by the positive voltage applied to the collector terminal, causing them to flow toward the collector terminal.
The current flow from the emitter to the base region is amplified in the collector region, making the NPN BJT act as an amplifier.
PNP BJT:
The emitter region is heavily doped with holes (P-type) to provide an abundant supply of majority charge carriers (holes).
The base region is lightly doped compared to the emitter and is very thin. It acts as the control element in the transistor.
The collector region is moderately doped with holes (P-type) and is larger in size than the base.
When a voltage is applied across the base-emitter junction (forward bias), it allows holes to flow from the emitter to the base region. These holes represent the minority carriers in the base region.
Due to the thinness of the base region, only a small number of holes are required to cross it and reach the collector region, which is reverse biased with respect to the base-emitter junction.
The majority charge carriers (holes) in the collector region are attracted by the negative voltage applied to the collector terminal, causing them to flow toward the collector terminal.
The current flow from the emitter to the base region is amplified in the collector region, making the PNP BJT act as an amplifier.
Configurations:
BJTs can be configured in three different ways based on the connection of their terminals: Common Emitter (CE), Common Base (CB), and Common Collector (CC).
Common Emitter (CE):
In the CE configuration, the emitter terminal is common between the input (base-emitter junction) and the output (collector-emitter junction).
It provides high voltage gain and current gain, making it a suitable choice for amplifier applications.
Common Base (CB):
In the CB configuration, the base terminal is common between the input (collector-base junction) and the output (emitter-base junction).
It provides high current gain and low voltage gain, making it suitable for applications where current amplification is essential.
Common Collector (CC):
In the CC configuration, the collector terminal is common between the input (base-collector junction) and the output (emitter-collector junction).
It provides voltage gain less than unity (less than 1), but it has a high current gain. It is also known as the emitter follower configuration and is often used as a buffer between circuits.
The choice of configuration depends on the specific requirements of the electronic circuit in which the BJT is used. Each configuration offers different advantages and characteristics.
Working Principle:
NPN BJT:
The emitter region is heavily doped with electrons (N-type) to provide an abundant supply of majority charge carriers (electrons).
The base region is lightly doped compared to the emitter and is very thin. It acts as the control element in the transistor.
The collector region is moderately doped with electrons (N-type) and is larger in size than the base.
When a voltage is applied across the base-emitter junction (forward bias), it allows electrons to flow from the emitter to the base region. These electrons represent the minority carriers in the base region.
Due to the thinness of the base region, only a small number of electrons are required to cross it and reach the collector region, which is reverse biased with respect to the base-emitter junction.
The majority charge carriers (electrons) in the collector region are attracted by the positive voltage applied to the collector terminal, causing them to flow toward the collector terminal.
The current flow from the emitter to the base region is amplified in the collector region, making the NPN BJT act as an amplifier.
PNP BJT:
The emitter region is heavily doped with holes (P-type) to provide an abundant supply of majority charge carriers (holes).
The base region is lightly doped compared to the emitter and is very thin. It acts as the control element in the transistor.
The collector region is moderately doped with holes (P-type) and is larger in size than the base.
When a voltage is applied across the base-emitter junction (forward bias), it allows holes to flow from the emitter to the base region. These holes represent the minority carriers in the base region.
Due to the thinness of the base region, only a small number of holes are required to cross it and reach the collector region, which is reverse biased with respect to the base-emitter junction.
The majority charge carriers (holes) in the collector region are attracted by the negative voltage applied to the collector terminal, causing them to flow toward the collector terminal.
The current flow from the emitter to the base region is amplified in the collector region, making the PNP BJT act as an amplifier.
Configurations:
BJTs can be configured in three different ways based on the connection of their terminals: Common Emitter (CE), Common Base (CB), and Common Collector (CC).
Common Emitter (CE):
In the CE configuration, the emitter terminal is common between the input (base-emitter junction) and the output (collector-emitter junction).
It provides high voltage gain and current gain, making it a suitable choice for amplifier applications.
Common Base (CB):
In the CB configuration, the base terminal is common between the input (collector-base junction) and the output (emitter-base junction).
It provides high current gain and low voltage gain, making it suitable for applications where current amplification is essential.
Common Collector (CC):
In the CC configuration, the collector terminal is common between the input (base-collector junction) and the output (emitter-collector junction).
It provides voltage gain less than unity (less than 1), but it has a high current gain. It is also known as the emitter follower configuration and is often used as a buffer between circuits.
The choice of configuration depends on the specific requirements of the electronic circuit in which the BJT is used. Each configuration offers different advantages and characteristics.