Complete Electronic Devices Summary

Semiconductors, Diodes, Transistors, and Logic Gates

Semiconductors
PN Junction Diodes
Transistors
Logic Gates

1. Semiconductors

Solid semiconductors are substances which have their electrical conductivities lying between that of good conductors and insulators.

Intrinsic Semiconductors

Semiconductors without any impurity are called intrinsic semiconductors.

Examples:

  • Germanium (Ge)
  • Silicon (Si)

Properties:

  • At temperatures near absolute zero, pure Ge and Si behave like perfect insulators
  • Their conductivities increase with increase in temperature
  • For Germanium, the binding energy of an electron in the covalent bond is 0.7 eV
  • Both electrons and holes act as charge carriers
Temperature Dependence

Conductors:

  • Resistance increases with temperature
  • Free electrons act as charge carriers

Semiconductors:

  • Resistance decreases with temperature
  • Both electrons and holes act as charge carriers

1.1 Extrinsic Semiconductors

The conductivity of a pure Ge or Si crystal can be considerably increased by adding small quantities of impurities. This process is called doping.

Doping concentration: Approximately one impurity atom in 100 million atoms of Ge or Si.

n-type Semiconductor

Doped with pentavalent elements (5 valence electrons):

  • Arsenic (As)
  • Antimony (Sb)
  • Phosphorus (P)

Charge Carriers:

  • Majority: Electrons
  • Minority: Holes
p-type Semiconductor

Doped with trivalent elements (3 valence electrons):

  • Indium
  • Aluminium
  • Boron

Charge Carriers:

  • Majority: Holes
  • Minority: Electrons

Hole: The absence of an electron in a covalent bond, regarded as a positive charge carrier.

2. PN Junction Diodes

2.1 Forward Biasing

Forward Bias Configuration

P-side: Positive

N-side: Negative

Characteristics:

  • When applied p.d. > barrier p.d., diode starts conducting
  • Direction of hole current is same as conventional current
  • Depletion layer width decreases

2.2 Reverse Biasing

Reverse Bias Configuration

P-side: Negative

N-side: Positive

Characteristics:

  • Width of depletion region increases
  • Acts like an insulator
  • Current flow due to majority carriers is zero

2.3 PN Junction as Rectifier

Half-wave Rectifier

Operation:

  • Positive half cycle: Diode is forward biased, conducts current through RL
  • Negative half cycle: Diode is reverse biased, no current flows through RL

Output voltage V0 is unidirectional - hence called half-wave rectifier.

Full-wave Rectifier

Positive Half Cycle:

  • A is positive w.r.t. C, B has equal negative voltage
  • D1 is forward biased and conducts
  • D2 is reverse biased and does not conduct

Negative Half Cycle:

  • B is positive w.r.t. C, A has equal negative voltage
  • D2 is forward biased and conducts
  • D1 is reverse biased and does not conduct

Current flows through RL in both halves of input cycle - hence called full-wave rectifier.

3. Transistors

3.1 Types of Transistors

p-n-p Transistor

p | n | p

E | B | C

n-p-n Transistor

n | p | n

E | B | C

3.2 Transistor Action

Biasing:

  • Emitter-base junction: Forward biased
  • Collector-base junction: Reverse biased

Current Flow:

  • Some electrons combine with holes in base → base current Ib
  • Electron current from emitter to base → emitter current IE
  • Electrons attracted towards collector → collector current IC
According to Kirchoff's law: \( I_E = I_C + I_B \)

3.3 Amplifying Action

Common Emitter Configuration

DC current gain: \( \beta_{dc} = \frac{I_C}{I_B} \)
AC current gain: \( \beta_{ac} = \frac{\Delta I_C}{\Delta I_B} \)
Voltage gain: \( = \frac{R_L \Delta I_C}{R_i \Delta I_B} = \beta \frac{R_L}{R_i} \)
Power gain: \( = \beta \frac{R_L}{R_i} \times \beta = \beta^2 \frac{R_L}{R_i} \)

Common Base Configuration

DC current gain: \( \alpha = \frac{I_C}{I_E} \)
AC current gain: \( \alpha_{ac} = \frac{\Delta I_C}{\Delta I_E} \)
AC voltage gain: \( = \frac{\Delta V_C}{\Delta V_i} = \frac{\Delta I_C R_L}{\Delta I_E R_i} = \alpha_{ac} \times \frac{R_o}{R_i} \)
AC power gain: \( = \frac{\Delta V_C \times \Delta I_C}{\Delta V_i \Delta I_E} = \alpha_{ac}^2 \times \frac{R_o}{R_i} \)

4. Logic Gates

A digital circuit with one or more input signals but only one output signal is known as logic gate.

OR Gate

Boolean Expression: \( A + B = Y \)

Functional statement: Output (Y) will be 1 when input A or B or both are 1.

A B Y
0 0 0
0 1 1
1 0 1
1 1 1
AND Gate

Boolean Expression: \( Y = A \cdot B \)

Functional statement: Output Y is 1 if all inputs simultaneously have state 1.

A B Y
0 0 0
0 1 0
1 0 0
1 1 1
NOT Gate

Boolean Expression: \( Y = \overline{A} \)

Functional statement: Performs negation operation on input.

A Y
0 1
1 0
NOR Gate

Boolean Expression: \( Y = \overline{A + B} \)

NOT Gate at output of OR Gate gives NOR gate.

A B Y
0 0 1
0 1 0
1 0 0
1 1 0

Special Property:

NOR Gate is a universal gate because we can obtain all possible gates by using NOR gate as basic building block.