Explain BJT Transistor construction and working principle

Bipolar Junction Transistor (BJT) is three layers and a three-terminal device having two PN junctions. The bipolar junction BJT transistor is made up of two N-type and P-type materials. In the case of NPN or two P-type and single N-type materials making three two junctions. Transistors are mainly used as switching devices or as an amplifier.

Construction of BJT Transistor

A Bipolar Junction Transistor is a three-layer device where every layer is connected by an electrical. The three terminals are called Emitter E, Base B, and Collector C. An NPN transistor can be formed by doping a sandwich of P-type between two N-types. Similarly, a PNP transistor is formed by doping a sandwich of N-type between two P-type materials. There are two PN junctions at a transistor. The first junction is a base-emitter BE junction and the second one is called a base-collector BC junction.

In either NPN or PNP, the emitter E is heavily doped, base B is lightly doped and collector C is moderately doped. The outer layers are wide as compared to the central layer. The ratio of the total size of a transistor to the base is 150:1. The doping of the central layer is also less as compared to the outer layers by a 10:1 ratio. Which increases the resistance of the layer by limiting the number of carriers. The transistor is called Bipolar because the transistor current is made up of holes and electron current.

Symbol of BJT Transistor

The symbol of the transistor contains a three-terminal representing the Emitter, Base, and Collector. The Emitter terminal is differentiated by an arrowhead from the Collector terminal. The direction of the arrowhead shows the type of transistor (NPN or PNP). The direction of the arrowhead shows the conventional current flow. In the case PNP transistor the arrow points inside and for NPN the arrow points outside the transistor.

Transistor construction, symbol and equivalent circuit

Equivalent circuit

The transistor equivalent circuit contains two back-to-back diodes. In the case of the NPN transistor, the anode terminals of both diodes are shorted and named as a base. Where in the case of the PNP transistor, the cathode terminal is shorted and named as a base terminal of the diode.

Working principles of BJT transistor

For a properly working transistor, a transistor should be connected to DC voltages at all three terminals such that both of the PN junctions are correctly biased. The base-emitter junction should be forward-biased and the base-collector junction should be reversed-biased. The working of NPN and PNP transistors is the same except for the role of electrons and holes, biasing voltage polarities, and direction of current flow.

As the BE junction is forward biased and the BC junction is reversed biased the BE depletion region becomes narrow and the BC depletion region widens. The heavily doped emitter can easily inject electrons (in NPN) to baes and is called emitter current IE. As the base is lightly doped, only a few electrons recombine with holes at the lightly doped base and become base current IB. Where the rest of the electrons cross the BC junction due to the electric field developed by the positive and negative ions. It becomes the collector’s current IC.

Transistor working circuit diagram


Can we use BJT as a diode?

Though a bipolar transistor comprises two diodes, it’s not intended for direct use as a diode.

Why is BJT called a current-controlled device?

As the base current changes, it influences the collector current proportionately, demonstrating the direct relationship between input and output currents in a BJT.

Is a BJT symmetric or asymmetric?

Bipolar junction transistors (BJT) are asymmetric, interchanging the collector and emitter terminals can cause the transistor to operate in reverse mode rather than the forward active mode.

Which is faster BJT or MOSFET?

MOSFETs are typically faster than BJTs due to their majority carrier current conduction. The absence of minority carriers results in quicker switching times compared to BJTs.

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