Diode
A diode is a two-terminal semiconductor device that allows current to flow primarily in one direction (forward bias) and blocks it in the reverse direction (reverse bias), acting as an electrical one-way valve. It is formed by a p-n junction — a contact between p-type (positive hole-rich) and n-type (negative electron-rich) semiconductor material. Diodes are used in rectifiers to convert AC to DC, in signal clipping and clamping, as protection devices, and in light-emitting diodes (LEDs) and photodiodes for optoelectronics.
Key Formula
I = I₀ × (e^(V / nVT) − 1) — Shockley diode equation
LaTeX: I = I_0 \left( e^{V / nV_T} - 1 \right)
| Symbol | Meaning | Unit |
|---|---|---|
| I | Diode current | Ampere (A) |
| I_0 | Reverse saturation current (typically 10⁻¹² A) | Ampere (A) |
| V | Voltage across the diode | Volt (V) |
| n | Ideality factor (1 for ideal, 1–2 for real diodes) | dimensionless |
| V_T | Thermal voltage = kT/q ≈ 26 mV at 300 K | Volt (V) |
Worked Example
Problem
A silicon diode (forward voltage drop V_D = 0.7 V) is connected in series with a 1 kΩ resistor to a 5 V supply. Find the current through the circuit.
Solution
Step 1: Apply KVL: V_supply = V_D + V_R. Step 2: Voltage across resistor: V_R = V_supply − V_D = 5 − 0.7 = 4.3 V. Step 3: Current: I = V_R / R = 4.3 / 1000 = 4.3 mA. Step 4: Verify power: P_R = I² × R = (0.0043)² × 1000 = 18.5 mW.
Answer
Current through circuit = 4.3 mA
Types of Diodes and Their Key Characteristics
| Diode Type | Forward Voltage (V) | Key Property | Primary Application |
|---|---|---|---|
| Silicon rectifier diode | 0.6–0.7 V | Standard p-n junction | AC to DC rectification |
| Germanium diode | 0.2–0.3 V | Lower forward voltage | Low-signal detection |
| Zener diode | 2.4–200 V (reverse) | Constant reverse breakdown voltage | Voltage regulation |
| LED (Light Emitting Diode) | 1.8–3.5 V (colour-dependent) | Emits light when forward biased | Displays and lighting |
| Schottky diode | 0.15–0.45 V | Very fast switching, low V_f | High-frequency circuits, power |
| Photodiode | Reverse biased | Generates current in light | Optical sensors, solar cells |
Interactive Tools
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Related Terms
Transistor
A transistor is a semiconductor device with three terminals that can amplify electrical signals or act as an electronic switch by controlling current flow between two terminals using a small input signal at the third. The two main types are the Bipolar Junction Transistor (BJT), which is current-controlled, and the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), which is voltage-controlled. Transistors are the fundamental building blocks of modern electronics, with billions packed into a single integrated circuit chip.
Operational Amplifier
An operational amplifier (op-amp) is a high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance, designed to perform mathematical operations such as addition, subtraction, integration, and differentiation on electrical signals. The ideal op-amp has infinite open-loop gain, infinite input impedance, zero output impedance, and infinite bandwidth. Op-amps are fundamental building blocks in analogue electronics, used in signal conditioning, filters, oscillators, comparators, and instrumentation.
Kirchhoff's Voltage Law
Kirchhoff's Voltage Law (KVL) states that the algebraic sum of all voltages around any closed loop in a circuit equals zero. This principle is a direct consequence of the conservation of energy — as a charge traverses a complete loop, the energy gained from sources equals the energy lost across resistances. KVL is fundamental for analysing series circuits, mesh analysis, and determining unknown voltages in complex networks.
The word "diode" comes from Greek "di" (two) + "hodos" (path or way), meaning a device with two terminals or two paths. It was coined by William Henry Eccles in 1919. The semiconductor p-n junction diode was developed in the 1940s, with the point-contact diode preceding it as the first practical rectifier.