Electronic Devices

Electronic devices are based on semiconductors. This chapter covers the physics of semiconductor devices and digital electronics.

Overview

graph TD
    A[Electronic Devices] --> B[Semiconductors]
    A --> C[p-n Junction]
    A --> D[Applications]
    B --> B1[Intrinsic]
    B --> B2[Extrinsic]
    C --> C1[Diode]
    C --> C2[I-V Characteristics]
    D --> D1[Rectifier]
    D --> D2[Zener]
    D --> D3[Logic Gates]

Semiconductors

Types

Intrinsic (Pure):

• Si, Ge
• Equal electrons and holes: $n_i = p_i$
• Conductivity increases with temperature

Extrinsic (Doped):

Type	Dopant	Majority Carrier	Example
n-type	Pentavalent (P, As)	Electrons	Si + P
p-type	Trivalent (B, Al)	Holes	Si + B

Carrier Concentration

$$np = n_i^2$$

where $n_i$ = intrinsic carrier concentration

Conductivity

$$\sigma = e(n\mu_e + p\mu_h)$$

where μ = mobility

p-n Junction

Formation

When p and n materials join:

1. Diffusion of majority carriers
2. Depletion region forms
3. Built-in potential barrier develops

Depletion Region

• Width: $W \propto \sqrt{V_0}$
• Electric field points from n to p
• No mobile charges present

Biasing

Forward Bias:

• p connected to positive terminal
• Barrier reduced
• Current flows easily

Reverse Bias:

• p connected to negative terminal
• Barrier increased
• Very small (leakage) current

p-n Junction Diode

I-V Characteristics

Forward bias:

$$I = I_0(e^{eV/kT} - 1)$$

Reverse bias:

$$I \approx -I_0$$

(saturation current)

Key Parameters

• Knee voltage: ~0.7 V (Si), ~0.3 V (Ge)
• Breakdown voltage: Reverse voltage where current suddenly increases
• Forward resistance: Low (~few Ω)
• Reverse resistance: High (~MΩ)

Rectifier

Half-Wave Rectifier

• Uses one diode
• Output frequency = input frequency
• Ripple factor = 1.21

Full-Wave Rectifier

• Uses two diodes + center-tapped transformer OR four diodes (bridge)
• Output frequency = 2 × input frequency
• Ripple factor = 0.48

Special Diodes

Zener Diode

• Operates in reverse breakdown
• Voltage regulator
• Breakdown due to Zener effect (low voltage) or avalanche (high voltage)

As voltage regulator:

$$V_{out} = V_Z = \text{constant}$$

LED (Light Emitting Diode)

• Emits light when forward biased
• Wavelength depends on band gap

Photodiode

• Operates in reverse bias
• Current proportional to incident light intensity
• Used in optical communication

Solar Cell

• Converts light to electricity
• Open circuit voltage: 0.5-1 V
• Short circuit current proportional to intensity

Logic Gates

Basic Gates

Gate	Symbol	Boolean Expression	Output
AND	·	Y = A·B	1 only if all inputs 1
OR	+	Y = A+B	1 if any input 1
NOT	'	Y = A'	Complement

Universal Gates

Gate	Expression	Made from
NAND	Y = (A·B)'	AND + NOT
NOR	Y = (A+B)'	OR + NOT

NAND and NOR are called universal gates because any logic function can be implemented using only NAND (or only NOR) gates.

Boolean Laws

• A + 0 = A
• A + 1 = 1
• A · 0 = 0
• A · 1 = A
• A + A = A
• A · A = A
• A + A’ = 1
• A · A’ = 0

De Morgan’s Theorems

$$(A + B)' = A' \cdot B'$$ $$(A \cdot B)' = A' + B'$$

Practice Problems

1. For a silicon diode in forward bias, the current is 5 mA. Find the voltage drop across it.

2. A Zener diode has breakdown voltage 6 V. Design a voltage regulator circuit for 12 V input.

3. Implement AND gate using only NAND gates.

4. Find the output of the following: Y = (A’B + AB’)'

Further Reading

• Current Electricity - Circuit fundamentals
• Dual Nature - Quantum basis of semiconductors