Bipolar junction transistor (BJT):

  Bipolar junction transistors are those which contain both holes and free electrons for the transmission of electrons, as the diode is made up of two semiconductors join together to form a PN-junction, BJT is two diodes join together forming a three-layer, two junction semiconductor material. Physically it has three basic terminals through which it is attached in the circuit, these terminals are known as base, emitter, and collector.

Bipolar junction is a combination of Type semiconductor material sandwich between two N-type semiconductor material NPN bipolar transistors or N-type sandwich between two P-type transistors forming PNP transistor. N-type and P-type semiconductors are made by doping. Doping is a process through which semiconductors were treated through some other chemicals to enhance their properties like conductivity by injecting more free electrons and holes.

These transistors are used in the circuits in such a way that by providing a small current through them we can manage or control huge current passing through a circuit that’s why they can be used in switching or amplification of signals especially radio signals.

Based on the functionality BJT transistors are divided into two types PNP and NPN Transistors. Symbolic views of both the transistors are shown in the figure below. In both, the cases emitter-base junction is forward biased where the collector-base junction is reversed biased.

NPN Transistor working:

    AS the name represent the P-Type region is between two N-type regions to form an NPN transistor. The P-type region has an excess amount of holes as the majority charge carrier while the N-type region has an excess amount of electrons as the majority charge carrier. In an NPN transistor, the emitter is heavily doped whereas the base is moderately doped and the collector is lightly doped.

Working:

    NPN transistors are attached din the circuit in such a way that the emitter is connected with a negative terminal and is forward biased whereas the collector is attached with the positive terminal so the collector-base junction is reverse biased. The electrons from the emitter start traveling towards the base region where only 5 % of them were attached to the holes present there and cause emitter current to flow which is shown in the figure below.

The remaining 95 % of electrons which are left behind as the base layer is very small were traveled towards the collector where they cause collector current o flow. The motion of electrons in the circuit can be observed by the figure showing this illustration. Meanwhile, the Emitter current is equal to the base current and collector current in the circuit.

Working of PNP transistor:

    PNP transistor as the name represents P-type Semiconductor is between two N-type Semiconductors, P-type doping contains an excess amount of holes present in it. Emitter which is larger and heavily doped contains a large number of holes in it, the base is smaller in size having an excess amount of electrons as majority charge career and Emitter is again P-type and less doped structure.

The emitter-base junction is forward biased and the emitter is connected to the positive terminal where the collector is attached to the negative terminal and biasing of the collector-base junction is reverse biased. When the current has been applied the holes from the emitter start attracting towards the negative terminal which is the base side and produces emitter current, only 5 % of the holes produced emitter current.

The remaining holes are transferred to the collector region from where they are attracted toward the negative side of the source as shown in the figure in this way collector current is produced. So from here we observe that by using a small amount of electric charge a large amount of current is controlled. The biasing and the production of current can be observed by the figure.

Characteristics of Bipolar junction Transistors:

    Based on Functionality and performance in electric circuits BJT has three modes of operation. In these modes, one of the terminals is selected as the base or main terminal whether the other two can be used as input and output terminals.  The arrangements of all these modes were displayed in the figure below.

• Common Base
• Common Emitter
• Common Collector

Common Base characteristics:

Input characteristics:

    In a common base PNP transistor, the emitter current and Ie and voltage Veb are taken as the input signals, and our graphs are shown below. In PNP type the emitter-base junction is forward biased that is shown as Vcb is increasing continuously.

   

Output terminal characteristics:

    Output characteristics are based upon the output current and the collector-base voltage as the collector base terminal is reversed biased in the PNP transistors so the graphs that at a specific value of emitter current the collector current is increased initially at the certain point then shows a constant behavior as we increase the voltage of collector base terminal. This illustration is shown in the figure below.

Common Emitter Characteristics;

    For common-emitter, we observe that as the base-emitter voltage increases which are forward-biased P-N junction then we observe that the base current also increases as we change the voltage along with the terminals of collector and Emitter.

The behavior of Collector current can be observed when we put Collector-Emitter terminal reverse biased. We have a clear parameter of increasing collector current as the base current is decreasing, from here we have a clear image of BJT characteristics in these modes.

Input characteristics:

Applications of BJT:

• The invention of transistors changes the electronic world from mobile phones to space satellites they are everywhere some of their uses were discussed below.
• The main application of BJT is it is used as Switch, BJT is very useful where sudden action related to supply and cutoff was necessary.
• Thousands of BJT transistors were used in the Microprocessors, Cpu’s, Nano chips, operating or controlling machines as a switch.
• The second most important use of BJT transistors is to work as amplifiers they were used to amplify radio signals, manual transmissions, music systems, military purposes, and much more.
• Electronic switching
• Used as temperature sensors
• Used as clippers
• Used for modulation and demodulation
• Used in microprocessors and microcontrollers.