Introduction to Transistors. Semiconductors Diodes Transistors

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1 Introduction to Transistors Semiconductors Diodes Transistors 1

2 Semiconductors Typical semiconductors, like silicon and germanium, have four valence electrons which form atomic bonds with neighboring atoms At room temperature silicon is an insulator Depending on external conditions, like temperature, semiconductors can become conductive by breaking some of the atomic bonds and allowing the liberated electrons to move Remaining holes, or missing electrons, can be filled by neighboring electrons, thus enabling a current to flow Energy e Conduction Band Valence Band = = = = 2

3 Doping of Semiconductors By integrating atoms with five electrons into the semiconductor crystal lattice, electrons not needed for the atomic bond are added This is called n-doping and the elements are called donors The valence band of the donor atom, like phosphorus, is energetically much closer to the conduction band of the semiconductor Energy e Conduction Band Valence Band of Donor Valence Band = = = = P 3

4 Doping of Semiconductors Instead of atoms with five electrons, elements with three valence electrons can be integrated into the crystal lattice This is called p-doping and the elements are called acceptors The valence band of the acceptor atom, like boron, is energetically much closer to the valence band of the semiconductor Instead of surplus electrons, surplus holes, or minority carriers, lead to conduction Energy e Conduction Band Valence Band of Acceptor Valence Band = = B = 4

5 pn-transition At the transition between a p-semiconductor and a n-semiconductor the electrons and holes diffuse towards the concentration slope from one to the other semiconductor Once a charge carrier concentration is reached the migration stops and a potential difference, or charge difference, remains An electronic diode is a pn-transition p n Potential Difference 5

6 Forward Biased Diode Applying a voltage to the diode with the positive pole connected to the p-semiconductor leads to an additional force to move the charge carriers across the pn-transition The diode is permitting current flow and is called forward biased p n 6

7 Reverse Biased Diode Applying a voltage to the diode with the negative pole connected to the p-semiconductor leads to a depletion zone of charge carriers at the transition No, or only a small, current is possible through the depletion zone The diode is blocking current and is called reverse biased p n 7

8 Diode Equation I I = I 0 eu/kt (e 1) I 0 ~ 0.7 V U 8

9 npn-transistor A transistor can be described as two diodes with a common anode Connecting a voltage between E and C results in no current flow as either one of the diodes is reverse biased In the below connection the actual small current flow depends on the electron concentration in the BC diode B n p n E C Emitter Base Collector 9

10 npn-transistor To increase the electron, majority carrier, concentration in the BC diode, the EB diode can be forward biased A current can flow from the collector to the emitter, depending on the base current B U CE E C U BE C n p n B E Emitter Base Collector 10

11 pnp-transistor The pnp-transistor is similar with the cathodes of the two diodes being joint B Current E C I E I E = I B E α = > 0.97 I E I B B β = 100 I B C 11

12 npn-transistor Circuits Common-emitter circuit Common-base circuit I B U CE I B U BE U BC U BE I E I E = β I B = -α I E 12

13 Limits of Transistors Maximum current through basis I Bmax Maximum voltage across basis and emitter U BE0 Four boundary conditions on the output Maximum collector current max Maximum power loss P vmax = U CE Breakdown voltage U CE0 P vmax = 300 mw I cmax 100 ma I B = 20 ma I B = 15 ma I B = 10 ma I B = 5 ma Operate transistor as amplifier in region where I c does not depend on U CE ( but on I B ) 1 V U CE0 45 V U CE 13

14 Common-Emitter Circuit Common-emitter circuit with added resistive load R L Basis resistor R B determines the working point through fixed I BA R B R L U CC U CE = U CC R L U = CC 1 U CE R L R L I Bmax I BA U CE I cmax Working Point A I BA U BE I E A Load line I B = 0 U CEA U CC 14 U CE