Definition: A transistor consists of two PN junctions formed by sandwiching either p-type or n-type semiconductors between a pair of opposite types.
Types of Transistors
There are two types of transistors:
- n-p-n transistor
- p-n-p transistor
An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type in fig (i).A p-n-p transistor is formed by two p-sections separated by a thin section of n-type as in fig (ii).
There are two pn junctions. Therefore, a transistor may be regarded as a combination of two diodes connected back to back. One junction is forward biased and the other is reverse biased. The forward junction has a low resistance path whereas a reverse biased junction has a high resistance path. The weak signal is introduced in the low resistance circuit and output is taken from the high resistance circuit. Therefore ,a transistor transfers a low resistance to high resistance. The prefix ‘trans’ means the signal property of the device while ‘istor’ classifies it as a solid element in the same general family with resistors.
- There are three terminals ,one taken from each type of semiconductor.
- The middle section is very thin layer. This is the most important factor in the function of a transistor.
An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type.
A p-n-p transistor is formed by two p-sections separated by a thin section of n-type.
The arrow in the symbol indicates the direction of flow of conventional current in the emitter with forward biasing applied to the emitter-base junction .There is the difference in the direction of current in NPN and PNP transistors.
A transistor(pnp or npn) has three sections of doped semiconductors. The section on one side is the emitter and the section on the opposite side is collector. The middle section is called the base and forms two junctions between the emitter and collector.
Emitter: The emitter is heavily doped so that it can inject a large number of charge carriers(electrons and holes) into the base. The emitter is always forward biased w.r.t base so that it can supply a large number of majority carriers. The junction between emitter and base may be called emitter-base diode or emitter diode. The emitter diode is always forward biased. The resistance of emitter diode is very small as compared to collector diode. Therefore ,forward bias applied to the emitter diode is generally very small.
Collector: The collector base junction is always the reverse biased. The collector junction is wider than both emitter and base. The collector junction is moderately doped. The junction between the base and collector may be called collector-base diode or collector diode. During transistor operation, much heat is produced at the collector junction. The collector is made larger to dissipate the heat.
Base: The base is the middle section of the transistor. The base is much thinner than the emitter. The base is lightly doped and very thin. The emitter-base junction is always forward biased so that it can supply a large number of majority carriers. The collector-base junction is always reverse biased.
Working of Transistors
The emitter-base junction of a transistor is forward biased whereas the collector-base junction is reverse-biased. If the emitter-base junction is omitted, then practically no current would flow in the collector circuit because of the reverse bias. However, if the emitter-base junction is present, then forward bias on it causes the emitter current to flow. The emitter current almost entirely flows in the collector circuit. The current in the collector circuit depends upon the emitter current. If the emitter current is Zero, then the collector current is nearly zero. In actual practice, very little current(a few μA) would flow in the collector circuit. This is called collector cut-off current and is due to minority carriers.
A transistor is a current-controlled device i.e. input current controls the output current. In a transistor, emitter current(IE) is equal to the sum of collector current(IC) and base current(IB) i.e.
The base current IB is much less than collector current IC so that IC≅IE. This relation is true for both npn and pnp transistors.
Frequently Asked Questions (FAQ)
Why is the base of transistor made thin and lightly doped?
The base is made thin and lightly doped so that the majority carriers supplied by the emitter do not combine in the base region and most of them pass on the collector.
What of the key factor responsible for amplifying capability of a transistor?
In a transistor, the weak input signal current is introduced in the low-resistance input circuit, and output is taken forms the high-resistance output circuit. The property of the transistor to transfer the input signal current from a low-resistance circuit to a high-resistance circuit is the key factor responsible for amplifying the capability of a transistor
Why is collector of a transistor made wider than emitter and base?
During transistor operation, much heat is produced at the collector junction. The collector is made larger to dissipate the heat.
Why does a transistor posses low input resistance and high output resistance?
Because the input circuit is forward biased and the output circuit is reverse biased.
What is origin name of transistor?
A transistor transfers a low resistance to high resistance. The prefix ‘trans’ means the signal property of the device while ‘istor’ classifies it as a solid element in the same general family with resistors.
Why are most of transistors npn and not npn type?
In an NPN transistor, the current conduction is mainly by free electrons whereas, in a PNP transistor, it is mainly by holes.