P-N Junction Diode

Unit V: P-N Junction Diode

Duration: 4 Hours | Credit: ELX 133.3

Introduction to P-N Junction

A P-N junction is formed by joining p-type and n-type semiconductor materials. It is the fundamental building block of all semiconductor devices including diodes, transistors, and integrated circuits.

Junction Formation and Depletion Region

What Happens at the Junction

When p-type and n-type materials are joined:

1. Electrons from n-side diffuse toward p-side (driven by concentration gradient)
2. Holes from p-side diffuse toward n-side
3. Electrons and holes combine (recombine) near the junction
4. This creates a region depleted of mobile carriers called the Depletion Region
5. Charge separation creates an electric field (built-in field)
6. Eventually, diffusion and drift currents balance, reaching equilibrium

Depletion Width

W = √[(2ε₀ε_r/q) × (V_bi - V) × (N_a + N_d) / (N_a × N_d)]

Where V_bi is the built-in potential and V is the applied voltage.

Barrier Potential

Built-in Potential (V_bi)

The built-in potential is the potential difference across the junction at equilibrium (zero applied voltage).

V_bi = (k_BT/q) × ln(N_a × N_d / n_i²)

At room temperature (300K):

• Silicon: V_bi ≈ 0.6-0.7 V
• Germanium: V_bi ≈ 0.2-0.3 V

Forward Bias

Forward Bias Condition

Forward bias is applied when:

• Positive voltage on P-side (attracts electrons from n-side)
• Negative voltage on N-side (repels electrons toward p-side)

Effects of Forward Bias

• Reduces the depletion width
• Lowers the potential barrier
• Increases diffusion current
• Decreases drift current
• Net result: Large forward current (approximately exponential with voltage)

Forward Current Equation

I_F = I_S × [exp(qV/(nk_BT)) - 1]

Where:

• I_S: Reverse saturation current (typically 10^-12 to 10^-15 A)
• n: Ideality factor (typically 1-2)
• V: Applied voltage

Reverse Bias

Reverse Bias Condition

Reverse bias is applied when:

• Negative voltage on P-side
• Positive voltage on N-side

Effects of Reverse Bias

• Increases the depletion width
• Increases the potential barrier
• Decreases diffusion current (nearly zero)
• Small drift current due to thermal generation (reverse saturation current)
• Total current remains very small (ideally zero, but practically 10^-9-10^-12 A)

Breakdown Voltage

If reverse voltage exceeds a critical value (breakdown voltage V_BR), the junction breaks down:

• Zener Breakdown: Direct rupture of covalent bonds (primary in lightly doped junctions)
• Avalanche Breakdown: Carrier multiplication through impact ionization (primary in heavily doped junctions)

Diode Characteristic Curve

I-V Characteristics

The diode characteristic curve shows the relationship between voltage and current:

• Forward Region (V > 0): Current rises exponentially with voltage
• Reverse Region (V < 0): Current remains small until breakdown occurs
• Knee Voltage: Voltage at which forward current starts increasing significantly
• For silicon: Knee voltage ≈ 0.6-0.7 V
• For germanium: Knee voltage ≈ 0.2-0.3 V

Rectifier Diodes

Rectification Process

Rectifier diodes convert AC voltage to DC voltage by allowing current in only one direction:

Half-Wave Rectification

• Uses one diode
• Only positive (or negative) half-cycles appear at output
• Average output voltage: V_DC = V_m / π (for ideal diode)
• Ripple frequency: f (same as input)

Full-Wave Rectification

• Uses two diodes (center-tapped transformer) or four diodes (bridge configuration)
• Both positive and negative half-cycles appear as positive output
• Average output voltage: V_DC = 2V_m / π (for ideal diode)
• Ripple frequency: 2f (twice the input frequency)
• Lower ripple voltage than half-wave rectification

Zener Diodes

Zener Operation

A zener diode is designed to operate in the breakdown region:

• Heavily doped junction (both sides)
• Lower breakdown voltage (3-200 V typical)
• Sharp knee at breakdown voltage
• Used as voltage regulator or reference

Zener Voltage Regulation

Zener diode maintains a nearly constant voltage across its terminals despite variations in load or input voltage:

• Series resistor (R_S) limits current
• Zener voltage (V_Z) remains approximately constant
• Output voltage: V_out ≈ V_Z
• Useful for low-current applications (< 100 mA)

Zener Specifications

• V_Z: Zener voltage at specified test current (I_Z, typically 5 mA)
• I_Z,min: Minimum zener current (below which regulation breaks down)
• I_Z,max: Maximum zener current (limited by power dissipation)
• P_max: Maximum power dissipation (P_max = V_Z × I_Z,max)
• Temperature Coefficient: Rate of change of V_Z with temperature

Diode Parameters and Temperature Effects

Temperature Dependence

• Forward Voltage: Decreases by approximately 2 mV/°C as temperature increases
• Reverse Saturation Current: Doubles for every 25-50°C temperature rise
• Zener Voltage: Typically increases with temperature (positive TC) or decreases (negative TC) depending on doping

Diode Applications

Common Diode Applications

• Rectification: AC to DC conversion
• Voltage Regulation: Using zener diodes
• Switching: As ON/OFF switches in circuits
• Clamping: Limiting voltage peaks
• Detection: In AM radio receivers
• Multiplexing: Signal routing

Key Takeaways

• P-N junction is formed when p-type and n-type materials are joined
• Depletion region has a width that depends on doping levels and applied voltage
• Built-in potential barrier prevents current flow at zero bias
• Forward bias reduces barrier, allowing large current (exponential relationship)
• Reverse bias increases barrier, allowing only small leakage current
• Knee voltage indicates start of significant conduction (≈0.6V for Si, ≈0.2V for Ge)
• Zener diodes regulate voltage in reverse-bias breakdown region
• Rectifiers convert AC to DC using one or more diodes