This flow of charge carriers constitutes the emitter current, I E. When the emitter-base junction is forward biased, the majority of carriers from the n-type region, i.e., electrons start flowing towards the p-type base region as shown in the figure.This reduces the width of the depletion region in the emitter-base junction and increases the width of the depletion region in the collector-base junction. The transistor is in ON mode and acts like a closed switch.įor an NPN transistor, the emitter-base junction is forward-biased by the DC source V EB, and the collector-base junction is reverse-biased by the DC source V CB. Saturation Mode: When both the junctions of the transistor are in forward bias, current flows through the device, and the transistor is said to be in saturation mode. This mode has very limited application but is conceptually important. Reverse Active Mode: In this mode, the emitter-base junction is reverse-biased and the collector-base junction is forward-biased.This mode is used for amplification of current. Active Mode: In this mode, the emitter-base junction is forward-biased and the collector-base junction is reverse-biased.The transistor is in OFF mode and acts like an open switch. Cut-Off Mode: When both the junctions of the transistor are in reverse bias, no current flows through the device, and the transistor is said to be in cut-off mode.A BJT has different modes of operation depending on the bias condition (forward or reverse) of its emitter-base junction (EBJ) and the collector-base junction (CBJ). When an external voltage is applied to a transistor, it is said to be biased. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly as for the standard diode symbol. The arrow in the circuit symbol of the PNP and NPN transistor is always present on the emitter terminal and indicates the direction of “conventional current flow” between the base terminal and the emitter terminal. The area of the collector is larger than the emitter and the base. The doping level of a collector lies between the doping level of the emitter and the base.
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