CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
Transistor is an n-p-n or p-n-p junction device. The central thin and lightly doped block is called ‘Base’ while the other electrodes are ‘Emitter’ and ‘Collectors’. The emitter-base junction is forward biased while collector-base junction is reverse biased. Since other types of transistors are also known, the junction transistor is called the Bipolar Junction Transistor (BJT).
Types of transistors
- n-p-n transistor
The two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type semiconductor (base).
The schematic representations of n-p-n transistor
The symbolic representations of n-p-n transistor
- p-n-p transistor
The two segments of p-type semiconductor (emitter and collector) are separated by a segment of n-type semiconductor (base).
The schematic representations of p-n-p transistor
The symbolic representations of p-n-p transistor
- All the three segments of a transistor have different thickness and doping levels.
- In the symbols used for representing p-n-p and n-p-n transistors the arrowhead shows the direction of conventional current in the transistor.
- In a p-n-p transistor the current enters from emitter into base whereas in a n-p-n transistor it enters from the base into the emitter.
- Emitter is the segment on one side of the transistor. It is of moderate size and heavily doped. It supplies a large number of majority carriers for the current flow through the transistor.
- Base is the central segment in the transitor. It is very thin and lightly doped.
- Collector is the segment on other side of the transistor. It collects a major portion of the majority carriers supplied by the emitter. The collector side is moderately doped and larger in size as compared to the emitter.
- Two depletion regions are formed at the emitter-base junction and the base-collector junction.
- The charge carriers move across different regions of the transistor when proper voltages are applied across its terminals.
- The biasing of the transistor is done differently for different uses.
Saturated, active and cutoff states of transistor
- The transistor can be used in two ways, as an amplifier and as a switch. When the transistor is used in the cutoff or saturation state it acts as a switch. On the other hand for using the transistor as an amplifier, it has to operate in the active region.
- Transistor in saturation region: The transistor acts like a short circuit. Current freely flows from collector to emitter.
VC < VB and VE < VB for n-p-n
- Transistor in cut-off region: The transistor acts like an open circuit. No current flows from collector to emitter.
VC > VB and VE > VB for n-p-n
- Transistor in active region: The current from collector to emitter is proportional to the current flowing into the base.
VC > VB > VE for n-p-n