William Shockley, in 1951 invented the first junction transistor with two back to back p-n junctions. Over the years, many types of transistors were invented and differentiate junction transistors from the new ones, we call them Bipolar Junction Transistors.
Understanding Junction Transistors
A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that consists of two p-n junctions and are able to amplify or magnify a signal. A BJT is a current controlled device. A bipolar junction transistor has three terminals: emitter, base and collector. A junction transistor uses both electrons and holes as charge carriers.
Working of Junction Transistor
In comparison to the emitter and collector, the base of a transistor is made thin and is lightly doped which means that the density of the majority carriers in the base is less than the density of majority carriers in the emitter and collector.
The flow of current is supplied through the emitter and the collector collects them. For a proper connection between the emitter and the collector, the base provides the interaction for the connectivity. In a junction transistor, the arrow points towards the conventional current. In n-p-n transistor’s emitter, the arrow points away from the base, and in a p-n-p transistor’s emitter, the arrow points towards the base. The base-emitter junction is usually forward biased and the base-collector junction is reversed biased when the junction transistor is used in a circuit.
Two Types of Bipolar Junction Transistors
There are two types of junction transistors and they are:
- NPN transistor
- PNP transistor
In an n-p-n junction transistor, p-type has the majority charge carriers and the other two ends have n-type majority carriers. On the other hand, in a p-n-p transistor, n-type has the minority charge carriers whereas the other two hands have p-type majority carriers.
In an NPN transistor, a p-type semiconductor base is sandwiched between an n-doped emitter and n-doped collector. NPN transistors are the highest used bipolar transistors due to the ease of electron mobility over electron-hole mobility.
The figure shows the construction and symbol of NPN transistors. The majority charge carriers in an n-p-n transistor are electrons and holes are the minority charge carriers. A small amount of current at the base terminal causes a large amount of current to flow from emitter to collector. Due to forward biasing of the transistor, the majority charge carriers in the emitter are repelled towards the base. The electron-hole recombination at the base is very small in the base region and most of the electrons cross into the collector region.
In a PNP transistor, an n-type semiconductor base is sandwiched between a p-doped emitter and p-doped collector. In this type of transistors, holes are the majority carriers and electrons are the minority carriers. In a PNP transistor, the emitter is forward biased and the collector is reverse biased.
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