Bipolar junction transistor - The BJT theory

Both types of BJT function by letting a small current input to the base control an amplified output from the collector. The result is that the transistor makes a good sitch that is controlled by its base input. In the NPN in what is called active mode, the base-emitter voltage and collector-base voltage       are positive, forward biasing the emitter-base junction and reverse-biasing the collector-base junction. In the active mode of operation, electrons are injected from the forward biased n-type emitter region into the p-type base where they diffuse as minority carriers to the reverse-biased n-type collector and are swept away by the electric field in the reverse-biased collector-base junction.