A transistor is a semi-conductor device corresponding to a thermionic vacuum triode. It can amplify electric signals and can also be used as an oscillator. In its original form it was invented by John Bardeen and Walter House Brattain in 1948. An improved form was devised by William Bradford. Transistors operate at much lower power and are better than electronic tubes in many respects. These are widely used in portable radios and television receivers. These form the key elements in computers, space vehicles, satellites and all modern communication and power systems.
A transistor is formed by growing either a very thin layer of n-type crystal between two very thick layers of p-type crystals or a thin layer of p-type crystal between two very thick layers of n type crystals. The, first type of transistor is known as a p-n-p transistor and the second type is called an n-p-n transistor.
In both these the middle thin layer is called the base of the transistor and can be compared roughly to the control grid of a thermionic vacuum tube. One of the outer thick layers is called the emitter, often compared to the cathode of a vacuum tube. The emitter is highly doped and is the source of majority carriers i.e., holes in case of p-n-p transistor and electrons in case of n-p-n transistor. The second thick layer is known as the collector, often compared to the plate
of a vacuum tube. The collector is comparatively less doped and its function is to collect the charge carriers. Moreover, the length of the collector is greater than that of the emitter and hence the emitter and the collector cannot be interchanged. An arrow head is drawn on the emitter to distinguish it from the collector, pointing towards the direction of flow of positive charge i.e., holes. Thus, the direction of the arrow head is from p to n in the emitter base combination i.e., from emitter to base in a p-n-p transistor and from base to emitter in an n-p-n transistor.
The junction between the emitter and the base is called the emitter base junction or simply the emitter junction (JEB) and the junction between the collector and base is called the collector base junction or simply the collector junction (JCB).
The entire semi-conductor material is hermetically sealed against moisture inside a metal or plastic case with metal leads coming out for connection to emitter, base and collector respectively. In junction transistors both majority and minority carriers are involved. These are therefore, called bipolar junction transistors (BJT).
Origin of the term transistor:
A transistor consists of two p-n junctions. One junction (emitter junction) is forward biased and the other junction (collector junction) is reverse biased. The forward biased junction has a low resistance path, whereas the reverse biased junction has a high resistance path. The weak signal is introduced in the low resistance circuit and the output is taken from the high resistance circuit. Therefore, a transistor transfers a signal from a low resistance to a high resistance. The prefix ‘ trans’ means the signal transfer property of the device while ‘istor’ classifies it as a solid-state element in the same general family with resistors. So, a transfer resistor is a transistor.
Working of a p-n-p transistor:
To study the working of a p-n-p type transistor, the emitter base circuit is forward biased with a small voltage VEB while collector base circuit is reverse biased with a voltage VCB.
The emitter base circuit, being forward biased helps, the movement of the holes in p-type and electrons in n-type towards the junction between the emitter and base. On the other hand, the collector base junction being reverse biased does not allow the holes in p-type and electrons in n-type to move towards the collector base junction. The holes in the emitter and the electrons in the base are repelled by the positive and the negative terminals respectively of the battery VEB. On reaching the emitter base junction a small fraction about 5% of the total number of holes
combine with the electrons in the base to get neutralised. As the base region is very thin and · the collector is given a high negative potential the holes will rush towards the collector and about 95% of them are thus collected by it. For each hole reaching the collector, an electron is released from the negative of the collector base battery VCB and neutralises the hole.
Also, for each hole which is lost in the collector region a covalent bond near the emitter breaks down and an electron is liberated which enters the positive terminal of the emitter-base battery. Thus, we see that the current is carried by the holes inside the crystal and by electrons in the external circuit.
In the emitter base external circuit, the flow of electrons is from p to n and, therefore, the conventional current IE flows from n to p or base to emitter. Similarly, in the collector base circuit the flow of electrons is from n top in the external circuit and hence a conventional current IC. Flows in the collector base circuit from p to n or collector to base. The difference between IE and lC is the base current IB which flows in the direction of lC, Thus
IB = IE – IC
IE = IB + IC
Working of n-p-n transistor:
In this case also the emitter base junction is forward biased while the collector base circuit is reverse biased. The holes in the base being repelled by positive terminal of the forward bias and the electrons in the emitter being repelled by the negative terminal move towards the emitter base junction. On reaching the junction a small fraction of about 5% of total number of electrons combine with the holes at the base to get neutralise.
As the base layer is very thin and the collector is given a very high positive potential the electrons will rush towards the collector and about 95% of them are collected by it. These electrons collected by the collector flow towards the positive of battery VCB. The deficiency of the electrons in the emitter is made up by the negative of the emitter base battery VEB· Thus the current is carried by the electrons both inside the crystals as well as in the external circuit. In the emitter base external
circuit the flow of electrons is from p to n. Hence the conventional current h is from n to p or emitter to base. In the collector base circuit, the flow of electrons in the external circuit is from n to p resulting in the conventional current lC from p to n or base to collector. In the base circuit the current,
IB = IE – IC
IE = IB + IC
