1- TransistorTransistor is a 3-terminal device mainly used in signal amplification and switching circuits. Transistors are broadly classified to Bipolar Junction Transistors (BJT) and Field Effect transistors (FET)2- Bipolar Junction transistor (BJT)BJT consists of three semiconductor sections denoted as emitter (E), base (B), and collector (C). There are two types of BJT, npnand pnp2
npnCBEpnpBECCBE=CBE=The arrow in the symbol identifies the emitter and shows the direction of the current flowThe transistor may be considered as two back-to-back pnjunctionsIn the following the npntype will beconsidered for circuit analysisThe base-emitter (BE) junction acts as a diode when it is forward-biased.With the collector open, the charge carriers give rise to a netflow from the base to the emitter. 3
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When the base-collector (BC) junction is reverse-biased, the electrons emitted by the emitter when the BE junction forward-biased reach the narrow base region and after a few lost to recombination process in the base, most of the electrons are collected by the collector.The results is a net flow of currentfrom the collector to the emitterbeside the hole current from the base to the emitter.The electron current flowing into thecollector through the base is muchlarger than that flows into the base from the external circuit.Applying KCL: BCEIII+=4