In this content, you can learn about introduction of unbiased transistor and it's different types of modes.
UNBIASED TRANSISTOR
When no external voltage source is connected between the different terminals of a transistor, the transistor is said to be an unbiased state or in an open-circuit state.
As we know that a p-n junction (depletion layer) formed between two different semiconductor layer without any biasing voltage, so in transistor two pn-junction are formed I.e. emitter- base junction and collector- base junction.
The depletion layer consist only immobile atom or covalent bond. No conduction take place between in unbiased transistor.
Biasing of a Transistor
When a voltages are applied across the different terminals of a transistor, then this process called biasing. Two voltage sources (battery) are required to bias a transistor, one of the voltage source connected between Emitter and base terminal for Emitter-bias junction of biasing. Other voltage source is connected between collector and base terminal for collector-base junction of biasing.
Both of the junction either be forward biased or reverse-biased. Therefore, there are four possible ways of biasing for these junction. These possible ways are also called modes of operation of a transistor. These different modes are following:-
(a) Active Mode.
(b) Saturation Mode.
(C) Cut-off Mode.
(d) Inverted Mode.
(a) Active Mode:-
In this mode, the emitter-base junction is forward-biased and Collector-base junction is reverse-biased. In the active mode, the transistor is used for amplification. In this mode, the Collector current depends upon the base current (input current). Fig 1.19 shows that a NPN transistor in active mode of biasing.
(b) Saturation Mode:-
In this mode, both the junction are forward - biased. In the saturation mode, the transistor acts like a closed switch (ON-Switch).
The Collector current is independent of the base current. Fig 1.20 shows that NPN Transistor is biased in saturation mode.
(C) Cut-off Mode:-
In this mode, both the junction are reverse-biased. In this mode, no current flow through the transistor. In the cut-off mode transistor behave as a open switch. Fig 1.21 show that the NPN transistor biased in cut-off mode.
(d) Inverted Mode:-
In this mode, emitter-base junction is reverse-biased and Collector-base junction is forward-biased. In the inverted mode, the action of transistor is poor. This region is of little importance. Fig 1.22 shows that NPN transistor biased in inverted mode.
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