Semiconductor Physics and Logic Gates1. Semiconductors: Semiconductors do conduct electricity when an electric field is applied. But their

conductivity is in between that of conductors and insulators. They behave like insulators at 0 K. Astemperature increases, their conductivity increases.

2. Semi-conductors:a) If the forbidden energy gap between the conduction band and valence band is small (about 1 eV)

then such substances are called semiconductors. Eg : Silicon & Germanium.b) For silicon forbidden energy gap is 1.1 eV and for germanium 0.72 eV.c) At absolute zero, semiconductors behave as perfect insulators.d) Semiconductors are of two types.

1) Intrinsic 2) Extrinsic3. Hole: A hole is an unfilled covalent bond (or) A vacant energy state in the valance band of a

semiconductor is called hole.4. Fermi energy: The highest energy level which an electron can occupy at 0 k is called Fermi level. For

intrinsic semiconductors this level lies in the middle of the forbidden gap. It can also take as averageenergy of charge carriers.

5. p-n junction: When a semi conducting material such as silicon or germanium is doped with impurity insuch a way that one side has a large number of acceptor impurities and the other side has a large numberof donor impurities. The resulting semiconductor is called p-n junction.

6. The no charge carrier region formed at p-n junction due to the combination of electrons and holes iscalled depletion layer.

7. The potential difference across the barrier which is set up to prevent diffusion of charge carriers throughthe junction is called potential barrier or contact potential.

8. The potential barrier for silicon is 0.7 volts and for germanium is 0.3 volts.9. Forward Bias: In a p-n junction diode, if p-region is connected to +ve terminal

(relatively higher potential) of the battery and n-region is connected to ve terminal(relatively lower potential) of the battery then it is said to be forward biased.

10. Reverse Bias: In a p-n junction diode, if p-region is connected to ve terminal(relatively low potential) of the battery and n-region is connected to the +ve terminal(relatively high potential) of the battery then it is said to be reverse biased.

11. Rectifier:a) The conversion of A.C. voltage to D.C. voltage is called rectification.b) A p-n junction diode is used as a rectifier.

12. Efficiency of half wave rectifier:a) The ratio of D.C. power output to the applied input A.C. power is known as rectifier efficiency.b) Rectifier efficiency dc L

ac L f

P 0.406RP R r

= =+

where RL - load resistance, rf - diode resistance.

13. In half wave rectification, a maximum of 40.6% of A.C. power is converted into D.C. power. In a halfwave rectifier, if input frequency is n Hz A.C., then the output pulse frequency is n Hz D.C.

a) Maximum current im = mf L

Vr R+

where Vm = maximum voltage, rf = internal resistance of the diode,

RL = load resistance.

b) Average current Idc = mIpi

c) rms current irms = mi2

.

d) a.c power input = 2rms f Li x (r R )+e) d.c power output Pdc = (Idc)2xRL

p n

+p n

+

14. Efficiency of full-wave rectifier:a) The ratio of D.C. power output to the applied input A.C. power is known as rectifier efficiency.b) Full wave rectifier efficiency dc L

ac f L

P 0.812RP r R

= =+

where rf - diode resistance, RL - load resistance.

15. In full wave rectifier a maximum of 81.2% of A.C. power is converted into D.C. power.16. Zener diode:

a) It is a heavily doped p-n junction diode which is operated in the breakdown region in reverse biasmode.

b) Zener diode has a sharp breakdown voltage in the reverse bias because of heavy doping. This voltageis called Zener Voltage (VZ).

17. Transistors: A transistor is formed by sandwiching a thin layer of a p-type semiconductor between twolayers of n-type semiconductors or by sandwiching a thin layer of an n-type semiconductor between twolayers of p-type semiconductors.

18. Transistors are of two types i) n-p-n, ii) p-n-p 19. Transistor will mainly consists of three sections

i) emitter, ii) base, iii) collector20. Transistor can be connected in three different configurations.

i) Common base configurationii) Common emitter configurationiii) Common collector configuration

21. In any transistor circuit IE = IB + IC.

22. In common base configuration transistor, the current gain is Ca.cE

II

=

.

23. In common base configuration transistor value is less than 1 ( < 1).24. The practical value of lies between 0.95 and 0.995.

25. In common-emitter configuration transistor, the current gain is Ca.cB

II

=

.

26. The value of is greater than one ( > 1).27. The practical value of lies in between 20 to 500.

p n

V -break down voltageb

n pp nn p

B B

C C

E E

28. Relation between, : ;1 1

= = +

Comparative study of CB, CE and CC AmplifiersParameter CB Amplifier CE Amplifier CC Amplifier

1. Input Resistance Minimum (50 200K) Less (1-2K) Maximum (150 800K)2. Output Voltage Maximum (1-2M) More (5-50K) Minimum (1K)3. Current gain Minimum More (5-50K) Minimum (1K)4.Voltage gain Medium High Minimum5. Power gain Medium (20-30) Maximum (30-40) Minimum (10)6. Phase reversal No Yes No

LOGIC GATES:29. The basic building blocks of digital circuits are called as logic gates, since they perform logic

operations.OR GATE:30. An OR gate has two or more inputs with the output.31. The Boolean expression is Y = A + B (Y equals A or B).32. The output (Y) of OR gate will be 1 when the inputs A or B or both 1.

a) circuit symbol b) truth table

AND GATE:33. An AND gate has two or more inputs with one output.34. The Boolean expression is Y = A, B (Y equals A and B).

a) circuit symbol b) truth table

NOT GATE:35. It has a single input and a single output.36. The Boolean expression is Y A= . (Y equals not A).37. The output of NOT gate is the inverse of the input or it performs negation operation.

a) Circuit symbol b) truth table

NOR GATE:1. It has two or more inputs and one output. A negation (NOT operation) applied after OR gate, gives a

NOT-OR gate or simply NOR gate.2. NOR gate output is inverse of OR GATE output.

The output of NOR gate is 1 only when all the inputs are simultaneously 0.3. The Boolean expression is Y A B= +

YB

AA B Y0 0 00 1 11 0 11 1 1

A B Y0 0 00 1 01 0 01 1 1

YB

A

A Y A Y0 11 0

a) Circuit symbol b) Truth table

NAND GATE:1. It has two or more inputs and one output. A negation (NOT operation) applied after AND gate, gives a

NOT-AND gate or simply NAND gate.2. NAND gate output is inverse of AND gate output.3. The Boolean expression is Y A . B= .4. The output of NAND gate is 1 only when atleast one input is 0.5. The NOR and NAND gates are considered as universal gates, because we can obtain all the gates like

OR, AND and NOT by using either NOR or NAND gates repeatedly.a) Circuit symbol b) truth table

XOR GATE :1. XOR gate is obtained by using OR, AND and NOT gate.2. It is also called exclusive OR gate.3. The output of two input XOR gate is 1 only when the two inputs are different.4. The Boolean equation is Y A.B B.A= +

a) two input XOR gate

b) circuit symbol c) truth table

XNOR GATE:1. XNOR gate is obtained by using OR, AND and NOT gates.

a) two input XNOR gate

b) circuit symbol c) truth table

A B Y0 0 10 1 01 0 01 1 0

YB

A

A B Y0 0 10 1 11 0 11 1 0

YB

A

A B Y0 0 00 1 11 0 11 1 0

Y

B

A

A

B Y

A

Y

B

YB

A

YB

A

A B Y0 0 10 1 01 0 01 1 1

YB

A

De-Morgans Theorems:i) A B A.B+ =ii) A.B A B= +iii) A B A B A.B+ = + =iv) A.B A B A B= + = +