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Bipolar Junction Transistor (BJT)

Bipolar Junction Transistor (BJT). Operation Theory. Engr.Usman Ali khan. Contents. BJT Formation BJT Terminals Biasing of BJT BJT Currents α of BJT. Formation of BJT. N. P. N. P. N. P. Two Junctions. Two Junctions.

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Bipolar Junction Transistor (BJT)

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  1. Bipolar Junction Transistor (BJT) Operation Theory Engr.Usman Ali khan

  2. Contents • BJT Formation • BJT Terminals • Biasing of BJT • BJT Currents • α of BJT

  3. Formation of BJT N P N P N P Two Junctions 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.

  4. N P N Emitter Collector C E Base N P N B BJT Terminals Base-Emitter Junction Base-Collector Junction

  5. C C E E N P P N N P B B BJT Circuit Symbols C E C B B E C E C B B E

  6. Opposing field due to battery E - - - + + + N P + - Forward Biased Junction Operation Electrons Push electrons to the other side Positive terminal attracts electrons Flow of electron current; but in semiconductor electronics the flow of positive or conventional current is considered

  7. Reverse Biased Junction Operation Positive terminal attracts Minority Carrier electrons Supporting field due to battery Electrons Push Minority Carrier electrons to the other side E - - - + + + Current flows due to minority carriers N P + -

  8. - + + BJT Biasing N P N Emitter Collector Base - + + - VEE VCC ON OFF -

  9. N P N Emitter Collector Base - + + - VEE VCC BJT Currents Minority carriers are attracted to the positive terminal of the battery When electrons come to Base, they become minority carriers Negative Terminal pushes electrons to the other side Some electrons escape to the ground

  10. C E N P N B BJT Currents IE IC (INJ) IB Conventional Current - + IE = IB + IC(INJ) Electron Current

  11. C E N P N B Collector-Base Reverse Current ICBO 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 Conventional Current Electron Current Total Collector current is = IC = IC(INJ) + ICBO

  12. α of BJT α = IC(INJ)/IE IC = IC(INJ) + ICBO ICBO<<IC(INJ) IC = IC(INJ) α = IC/IE

  13. Home Work • Example: 4.1 • End Problems: 4.1  4.5

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