JEE Main & Advanced Physics Semiconducting Devices Working of Transistor

(1) There are four possible ways of biasing the two P-N junctions (emitter junction and collector junction) of transistor.

(i) Active mode : Also known as linear mode operation.

(ii) Saturation mode : Maximum collector current flows and transistor acts as a closed switch from collector to emitter terminals.

(iii) Cut-off mode : Denotes operation like an open switch where only leakage current flows.

(iv) Inverse mode : The emitter and collector are inter changed.  

Different modes of operation of a transistor

(2) A transistor is mostly used in the active region of operation i.e. emitter base junction is forward biased and collector base junction is reverse biased.

(3) From the operation of junction transistor it is found that when the current in emitter circuit changes. There is corresponding change in collector current.

(4) In each state of the transistor there is an input port and an output port. In general each electrical quantity (V or I) obtained at the output is controlled by the input.  

Circuit diagram of PNP/NPN transistor 

NPN - transistor	PNP - transistor
5% emitter electron combine with the holes in the base region resulting in small base current. Remaining 95% electrons enter the collector region.	5% emitter holes combine with the electrons in the base region resulting in small base current. Remaining 95% holes enter the collector region.
\[{{l}_{e}}>{{l}_{c}},\]  and \[{{l}_{e}}={{l}_{b}}+{{l}_{c}}\]	\[{{l}_{e}}>{{l}_{c}},\] and \[{{l}_{e}}={{l}_{b}}+{{l}_{c}}\]