BJT has revolutionized the field of electronics. BJT is a main building block of all electronic systems. It is a three terminal device whose output current , voltage , power are controlled by its input current. BJT has a very important property of raising the strength of a weak signal. This property is known as amplification. Therefore transistor ( BJT ) is the most widely used in the semiconductor devices.
Bipolar Junction Transistor Construction:
A bipolar junction transistor has three regions known as emitter , base and collector.
1. Emitter : It is a region situated in one side of transistor, which supplies charge carries ( i.e. electrons and holes ) to the other two regions. The emitter is a heavily doped region.
2. Base : It is the middle region that forms two P-N junctions in the transistor. The base of transistor is thin , as compared to the emitter and is a lightly doped region.
3. Collector : It is region situated in the order side of transistor ( i.e. , the side opposite to the emitter ), which collects charge carriers ( i.e. electrons or holes ). The collector of a transistor is always larger than the emitter and base of a transistor. The doping level of the collector is intermediate between the heavy doping of emitter and light doping of the base.
Unbiased Bipolar Junction Transistor : If the three terminals of transistor left open , is known as unbiased bipolar junction transistor.
In the unbiased condition , the diffusion of free electrons across the junction produces two depletion layers as shown in above figure. The barrier potential voltage is 0.7V is for silicon and 0.3V for germanium transistor. It is observed that an emitter - base depletion layer penetrates slightly into the emitter, as it is heavily doped, whereas it penetrates deeply into base as it is lightly base region Similarly, the collector-base depletion layer penetrates more into base region and less into the collector region. Note that emitter-base depletion layer width is smaller than that of collector-base depletion layer. Unbiased transistor is never used in practical practice.
Biased Bipolar Junction Transistor : The application of a suitable dc voltages across the transistor terminals, is called biasing. Each junction of a transistor may b forward biased or reverse-biased independently. Therefore , there are three different ways of biasing a transistor.
Forward Active : In this mode , the emitter-base junction of a transistor is forward biased and collector-base junction is reverse biased. In a forward active biasing the negative terminal of a battery is connected to N-side and positive terminal to P-side. The reverse biasing requires all the connections to be opposite to those for forward biasing.Saturation : In this mode, both the emitter-base and collector-base junctions of a transistors are forward-biased. In this mode, transistor has a very large value of current . The transistor is operated in this mode, when it is used as a closed switch.
Cut-off : In this mode, both the emitter-base and collector-base junctions of a transistor are reverse biased . In this mode, transistor has practically zero current. The transistor is operated in this mode , when it is used as an open switch.
Remember that for amplifier , a transistor is used in forward active mode and for switching applications, transistor is used alternatively in saturation and cut-off region.
Modes of operations are the same for both NPN and PNP transistors. However, in PNP transistor biasing , the polarity of battery voltage are opposite to that of NPN transistor.
