Test: The Junction Transistor - Electrical Engineering (EE) MCQ

A junction transistor is an analogous to a vacuum triode. The main difference between them is that a transistor is a current device while a vacuum triode is a voltage device. The advantages of a transistor over a vacuum triode are long life, high efficiency, light weight, smaller in size, less power consumption.

The main function of this section is to supply majority charge carriers to the base. Hence it is more heavily doped in comparison to other regions. This forms the left hand section of the transistor.

As regards to the symbols, the arrow head is always at the emitter. The direction indicates the conventional direction of current flow. In case of PNP transistor, it is from base to emitter.

 In most of the transistors, the collector is made larger than emitter region. This is due to the fact that collector has to dissipate much greater power. The collector and emitter cannot be interchanged.

The electrons in the emitter region are repelled by the negative terminal of the battery towards the emitter junction. The potential barrier at the junction is reduced due to forward bias and base region is very thin and lightly doped, electrons cross the P type base region.

 The 2 – 5% of holes is lost in recombination with electrons in the base region. The majority charge carriers are holes for a PNP transistor. Thus the collector current is slightly less than the emitter current.

In saturated mode, both emitter and collector are forward biased. The negative of the battery is connected to emitter and similarly the positive terminals of batteries are connected to the base. The transistor now acts like a closed switch.

In cut off region, both the junctions are reverse biased. The transistor has practically zero current because the emitter does not emit charge carriers to the base. So, the transistor acts as open switch.

 When the emitter-base junction is forward biased and the collector-base junction is reverse biased, the transistor is used for amplification. A battery is connected to collector base circuit. The positive terminal is connected to the collector while the negative is connected to the base.

A forward biased emitter base junction has a low resistance path. A reversed biased junction has a high resistance path. The weak signal is introduced in a low resistance circuit and the output is taken from the high resistance circuit.