Ans -Power Transistor (BJT)
Power transistors are high-power versions of Bipolar Junction Transistors (BJTs), typically NPN type,
designed for handling large currents (up to 100 A) and power (up to 500 W). They operate as current-controlled switches or amplifiers.
Construction
A power BJT consists of three semiconductor layers: heavily doped N+ emitter, thin lightly doped P base, and large moderately doped N collector. The collector is larger for heat dissipation, and the device is often packaged with heat sinks (e.g., TO-220, TO-3).
Working Principle
In active mode, a small base current (IB) forward-biases the base-emitter junction, injecting electrons from emitter to base. Most electrons diffuse across the thin base to the reverse-biased collector-base junction, where they are swept into the collector as IC (IC * IB, = 20-200). This amplifies current.
In saturation, both junctions are forward-biased for low VCE; in cut-off, no current flows.
V-I Characteristics
Output characteristics (IC vs VCE at fixed IB) show cut-off (IC=0), active region (flat IC), quasi-saturation
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