--- title: PN Junction url: https://doc.liz6.com/en/hardware/03-semiconductor/02-pn-junction locale: en area: hardware tags: - hardware - semiconductor date: 2026-06-30 modified: 2026-07-16 description: 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 se… --- # 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*