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PN Junction
Basic Concepts
PN Junction — A special region formed at the interface between P-type and N-type semiconductors, serving as the foundation for all semiconductor devices.
Formation Principle
1. Carrier Diffusion
- Holes in the P region diffuse into the N region
- Electrons in the N region diffuse into the P region
- They recombine and disappear upon meeting
2. Formation of the Space Charge Region
- Accumulation of positive and negative charges → Built-in electric field
- The electric field prevents carriers from continuing to diffuse
- Dynamic equilibrium is reached
3. Built-in Potential
Vbi ≈ 0.7V (Silicon) / 0.3V (Germanium)
Depends on doping concentration and temperature
Bias States
Forward Bias
P Region ─┤+├─── N Region
├──+
│ ← Applied Voltage
├-+
Current Direction: P → N
| Characteristic | Description |
|---|---|
| Applied Voltage | > Vbi (approx. 0.7V) |
| Current | Exponential growth: I = Is(e^(V/VT) - 1) |
| Resistance | Low |
Reverse Bias
P Region ─┤-├─── N Region
│+|
│ ← Applied Voltage
│+
├-
Current Direction: Almost zero (except for leakage current)
| Characteristic | Description |
|---|---|
| Applied Voltage | > 0V |
| Current | Very small (Reverse leakage current) |
| Resistance | High |
Breakdown Mechanisms
Breakdown occurs when the reverse voltage is too high:
1. Zener Breakdown
- Vz < 5V
- Strong electric field directly breaks covalent bonds
- Reversible, Zener diodes utilize this principle
2. Avalanche Breakdown
- Vz > 5V
- Carrier acceleration causes impact ionization
- Chain reaction
- Reversible
Parasitic Parameters
┌─────────────────────┐
│ ┌───┐ │
────┤ │ Vd│ ├────
│ └───┘ │
│ ┌─────────┐ │
│ │ Cj │ ← Junction Capacitance
│ └─────────┘ │
└─────────────────────┘
- Cj: Junction Capacitance (Significant under reverse bias)
- Reverse Recovery Time (Switching Characteristics)
Energy Band Diagram
Equilibrium State
Energy
↑
│ P Region N Region
│ ════ ════ Valence Band
│ ↑ ↑
│ Holes Electrons
│ │ │
│ ────────────────── Fermi Level
│ │ ↓ │
│ │ Built-in Electric Field│
│ │ │ │
└─────┴──────┴─────┴──
Forward Bias
- Potential barrier decreases
- Carriers can diffuse across
Reverse Bias
- Potential barrier increases
- Depletion layer widens
Temperature Characteristics
| Parameter | Temperature Effect |
|---|---|
| Vd (Forward Voltage Drop) | For every 1°C increase, Vd decreases by 2mV |
| Is (Reverse Saturation Current) | For every 10°C increase, Is doubles |
| Breakdown Voltage | Generally, as temperature increases, Vz increases (Avalanche) |
Key Formulas
Shockley Equation
I = Is × (e^(V/(n×VT)) - 1)
Is: Reverse Saturation Current
VT: Thermal Voltage = kT/q ≈ 26mV (Room Temperature)
n: Ideality Factor (1~2)
Depletion Layer Width
W = √(2εSi × Vbi / q × (1/NA + 1/ND))
NA, ND: Doping Concentrations
Keywords: PN Junction, Forward Bias, Reverse Bias, Depletion Layer, Built-in Electric Field, Breakdown, Zener, Avalanche