1. Operation Theory BIPOLAR JUNCTION TRANSISTOR (BJT) Engr.Usman Ali khan

2. CONTENTS 1.BJT Formation 2.BJT Terminals 3.Biasing of BJT 4.BJT Currents 5.α of BJT

3. FORMATION OF BJT PNP Two Junctions NPN 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. BJT TERMINALS NPN EmitterCollector Base NNP E C B Base-Emitter JunctionBase-Collector Junction

5. BJT CIRCUIT SYMBOLS NNP E C B B C E C E B PPN E C B B C E C E B

6. FORWARD BIASED JUNCTION OPERATION ++++++ ------ E N P + - Opposing field due to battery 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 ++++++ ------ E N P + - Supporting field due to battery Electrons Push Minority Carrier electrons to the other side Positive terminal attracts Minority Carrier electrons Current flows due to minority carriers

8. BJT BIASING NPN EmitterCollector Base -+ - + V EE V CC -+ + - ON OFF

9. BJT CURRENTS NPN EmitterCollector Base -+ - + V EE V CC Negative Terminal pushes electrons to the other side When electrons come to Base, they become minority carriers Minority carriers are attracted to the positive terminal of the battery Some electrons escape to the ground

10. BJT CURRENTS NNP E C B Conventional Current Electron Current - + IBIB IEIE I C (INJ) I E = I B + I C(INJ)

11. COLLECTOR-BASE REVERSE CURRENT NNP E C B I CBO Conventional Current Electron Current 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 I CBO Total Collector current is = I C = I C(INJ) + I CBO

12. Α OF BJT α = I C(INJ) /I E I C = I C(INJ) + I CBO I CBO <<I C(INJ) I C = I C(INJ) α = I C /I E

13. HOME WORK Example: 4.1 End Problems: 4.1  4.5