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BIPOLAR JUNCTION TRANSISTOR (BJT) Operation Theory Engr.Usman Ali khan CONTENTS 1. BJT Formation 2. BJT Terminals 3. Biasing of BJT 4. BJT Currents 5. α of BJT FORMATION OF BJT P N Two Junctions P N P N Two Junctions N-Type material has excessive free electrons and P-Type material has excessive holes NPN and PNP BJTs have same characteristics. The only difference is in their polarities of applied voltages. The polarities are reversed for the two types of transistor. BJT TERMINALS N P N Emitter Collector Base-Emitter Junction Base-Collector Junction Base C E N P B N BJT CIRCUIT SYMBOLS C C E N P E C N B B B E C C E P N B E C P B B E FORWARD BIASED JUNCTION OPERATION Opposing field due to battery E Positive terminal attracts electrons - + - + - + Electrons Push electrons to the other side N P + - Flow of electron current; but in semiconductor electronics the flow of positive or conventional current is considered REVERSE BIASED JUNCTION OPERATION Supporting field due to battery Electrons Push Minority Carrier electrons to the other side E - + - + - + N P - + Positive terminal attracts Minority Carrier electrons Current flows due to minority carriers BJT BIASING N P N Emitter Collector - + Base + VEE ON VCC OFF - + - + BJT CURRENTS When electrons come to Base, they become minority carriers Negative Terminal pushes electrons to the other side N P Minority carriers are attracted to the positive terminal of the battery N Emitter Collector - + Base + VEE VCC Some electrons escape to the ground BJT CURRENTS C E IE IC (INJ) N P N IB - Conventional Current + B IE = IB + IC(INJ) Electron Current COLLECTOR-BASE REVERSE CURRENT C E ICBO N When Collector-Base Junction is reverse biased and Emitter-Base Junction is Open, a reverse current flows in this reverse biased junction which is called ICBO P N Conventional Current B Electron Current Total Collector current is = IC = IC(INJ) + ICBO Α OF BJT α = IC(INJ)/IE IC = IC(INJ) + ICBO IC = IC(INJ) α = IC/IE ICBO<<IC(INJ) HOME WORK • Example: 4.1 • End Problems: 4.1 4.5