Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction * Junction: device includes two p - n junctions (as opposed to a “point-contact” transistor, the first transistor) M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction * Junction: device includes two p - n junctions (as opposed to a “point-contact” transistor, the first transistor) * Transistor: “transfer resistor” When Bell Labs had an informal contest to name their new invention, one engineer pointed out that it acts like a resistor, but a resistor where the voltage is transferred across the device to control the resulting current. (http://amasci.com/amateur/trshort.html) M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction * Junction: device includes two p - n junctions (as opposed to a “point-contact” transistor, the first transistor) * Transistor: “transfer resistor” When Bell Labs had an informal contest to name their new invention, one engineer pointed out that it acts like a resistor, but a resistor where the voltage is transferred across the device to control the resulting current. (http://amasci.com/amateur/trshort.html) * invented in 1947 by Shockley, Bardeen, and Brattain at Bell Laboratories. M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction * Junction: device includes two p - n junctions (as opposed to a “point-contact” transistor, the first transistor) * Transistor: “transfer resistor” When Bell Labs had an informal contest to name their new invention, one engineer pointed out that it acts like a resistor, but a resistor where the voltage is transferred across the device to control the resulting current. (http://amasci.com/amateur/trshort.html) * invented in 1947 by Shockley, Bardeen, and Brattain at Bell Laboratories. * BJT is still used extensively, and anyone interested in electronics must have at least a working knowledge of this device. M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction * Junction: device includes two p - n junctions (as opposed to a “point-contact” transistor, the first transistor) * Transistor: “transfer resistor” When Bell Labs had an informal contest to name their new invention, one engineer pointed out that it acts like a resistor, but a resistor where the voltage is transferred across the device to control the resulting current. (http://amasci.com/amateur/trshort.html) * invented in 1947 by Shockley, Bardeen, and Brattain at Bell Laboratories. * BJT is still used extensively, and anyone interested in electronics must have at least a working knowledge of this device. * “A BJT is two diodes connected back-to-back.” M. B. Patil, IIT Bombay
Bipolar Junction Transistors Emitter p n p Collector Emitter n p n Collector Base Base pnp transistor npn transistor * Bipolar: both electrons and holes contribute to conduction * Junction: device includes two p - n junctions (as opposed to a “point-contact” transistor, the first transistor) * Transistor: “transfer resistor” When Bell Labs had an informal contest to name their new invention, one engineer pointed out that it acts like a resistor, but a resistor where the voltage is transferred across the device to control the resulting current. (http://amasci.com/amateur/trshort.html) * invented in 1947 by Shockley, Bardeen, and Brattain at Bell Laboratories. * BJT is still used extensively, and anyone interested in electronics must have at least a working knowledge of this device. * “A BJT is two diodes connected back-to-back.” WRONG! Let us see why. M. B. Patil, IIT Bombay
Bipolar Junction Transistors Consider a pnp BJT in the following circuit: R 1 R 2 E C p p n I 1 I 2 1 k 1 k B I 3 10 V 5 V M. B. Patil, IIT Bombay
Bipolar Junction Transistors Consider a pnp BJT in the following circuit: R 1 R 2 E C p p n I 1 I 2 1 k 1 k B I 3 10 V 5 V If the transistor is replaced with two diodes connected back-to-back, we get R 1 R 2 E C I 1 I 2 1 k 1 k D1 D2 B I 3 5 V 10 V M. B. Patil, IIT Bombay
Bipolar Junction Transistors Consider a pnp BJT in the following circuit: R 1 R 2 E C p p n I 1 I 2 1 k 1 k B I 3 10 V 5 V If the transistor is replaced with two diodes connected back-to-back, we get R 1 R 2 E C I 1 I 2 1 k 1 k D1 D2 B I 3 5 V 10 V Assuming V on = 0 . 7 V for D1, we get I 1 = 5 V − 0 . 7 V = 4 . 3 m A , R 1 I 2 = 0 (since D2 is reverse biased), and I 3 ≈ I 1 = 4 . 3 m A . M. B. Patil, IIT Bombay
Bipolar Junction Transistors Using a more realistic equivalent circuit for the BJT, we obtain, R 1 R 2 R 1 R 2 α I 1 E C E C p p n I 1 I 1 I 2 I 2 1 k 1 k 1 k 1 k B B I 3 I 3 5 V 10 V 5 V 10 V M. B. Patil, IIT Bombay
Bipolar Junction Transistors Using a more realistic equivalent circuit for the BJT, we obtain, R 1 R 2 R 1 R 2 α I 1 E C E C p p n I 1 I 1 I 2 I 2 1 k 1 k 1 k 1 k B B I 3 I 3 5 V 10 V 5 V 10 V We now get, I 1 = 5 V − 0 . 7 V = 4 . 3 m A (as before), R 1 M. B. Patil, IIT Bombay
Bipolar Junction Transistors Using a more realistic equivalent circuit for the BJT, we obtain, R 1 R 2 R 1 R 2 α I 1 E C E C p p n I 1 I 1 I 2 I 2 1 k 1 k 1 k 1 k B B I 3 I 3 5 V 10 V 5 V 10 V We now get, I 1 = 5 V − 0 . 7 V = 4 . 3 m A (as before), R 1 I 2 = α I 1 ≈ 4 . 3 m A (since α ≈ 1 for a typical BJT), and M. B. Patil, IIT Bombay
Bipolar Junction Transistors Using a more realistic equivalent circuit for the BJT, we obtain, R 1 R 2 R 1 R 2 α I 1 E C E C p p n I 1 I 1 I 2 I 2 1 k 1 k 1 k 1 k B B I 3 I 3 5 V 10 V 5 V 10 V We now get, I 1 = 5 V − 0 . 7 V = 4 . 3 m A (as before), R 1 I 2 = α I 1 ≈ 4 . 3 m A (since α ≈ 1 for a typical BJT), and I 3 = I 1 − I 2 = (1 − α ) I 1 ≈ 0 A . M. B. Patil, IIT Bombay
Bipolar Junction Transistors Using a more realistic equivalent circuit for the BJT, we obtain, R 1 R 2 R 1 R 2 α I 1 E C E C p p n I 1 I 1 I 2 I 2 1 k 1 k 1 k 1 k B B I 3 I 3 5 V 10 V 5 V 10 V We now get, I 1 = 5 V − 0 . 7 V = 4 . 3 m A (as before), R 1 I 2 = α I 1 ≈ 4 . 3 m A (since α ≈ 1 for a typical BJT), and I 3 = I 1 − I 2 = (1 − α ) I 1 ≈ 0 A . The values of I 2 and I 3 are dramatically different than the ones obtained earlier, viz., I 2 ≈ 0, I 3 ≈ 4 . 3 mA. M. B. Patil, IIT Bombay
Bipolar Junction Transistors Using a more realistic equivalent circuit for the BJT, we obtain, R 1 R 2 R 1 R 2 α I 1 E C E C p p n I 1 I 1 I 2 I 2 1 k 1 k 1 k 1 k B B I 3 I 3 5 V 10 V 5 V 10 V We now get, I 1 = 5 V − 0 . 7 V = 4 . 3 m A (as before), R 1 I 2 = α I 1 ≈ 4 . 3 m A (since α ≈ 1 for a typical BJT), and I 3 = I 1 − I 2 = (1 − α ) I 1 ≈ 0 A . The values of I 2 and I 3 are dramatically different than the ones obtained earlier, viz., I 2 ≈ 0, I 3 ≈ 4 . 3 mA. Conclusion: A BJT is NOT the same as two diodes connected back-to-back (although it does have two p - n junctions). M. B. Patil, IIT Bombay
Bipolar Junction Transistors What is wrong with the two-diode model of a BJT? M. B. Patil, IIT Bombay
Bipolar Junction Transistors What is wrong with the two-diode model of a BJT? * When we replace a BJT with two diodes, we assume that there is no interaction between the two diodes, which may be expected if they are “far apart.” Emitter p p n Collector Base Emitter Collector D1 D2 Base M. B. Patil, IIT Bombay
Bipolar Junction Transistors What is wrong with the two-diode model of a BJT? * When we replace a BJT with two diodes, we assume that there is no interaction between the two diodes, which may be expected if they are “far apart.” Emitter p p n Collector Base Emitter Collector D1 D2 Base * However, in a BJT, exactly the opposite is true. For a higher performance, the base region is made as short as possible, and the two diodes cannot be treated as independent devices. Emitter p n p Collector Base M. B. Patil, IIT Bombay
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