Diode Types
“All diodes conduct one way” is a good first lesson, but specialized diodes are designed to take advantage of different materials, junction structures, and operating regions.
Rectifier diode
A rectifier diode is optimized to carry useful power current and block reverse voltage. Power-supply rectifiers may be rated from fractions of an ampere to hundreds or thousands of amperes, with reverse-voltage ratings from tens of volts to many kilovolts.
Small-signal and switching diode
Small diodes such as the familiar 1N4148 family are optimized for low-current, fast switching and signal work. They are useful in logic steering, detectors, clamps, shaping networks, and protection circuits.
In the Variable RC Timer, a 1N4148 is used as a clamp diode. It does not light up like an LED. Its job there is to limit the unwanted opposite-polarity spike produced when a capacitor turns an output edge into a short trigger pulse.
Useful 1N4148 specifications
| Specification | Vishay 1N4148 value | Practical meaning |
|---|---|---|
| Device type | Silicon epitaxial planar, small-signal fast switching diode | Designed for fast signal switching, steering, clipping, and clamping rather than power rectification. |
| Package | DO-35 (DO-204AH), axial | Easy to use on a solderless breadboard. |
| Repetitive peak reverse voltage, VRRM | 100 V | Do not repeatedly reverse-bias it beyond this rating. |
| Forward voltage example | ≤ 1.0 V at 10 mA | A silicon diode normally drops well under the supply voltage while conducting; the exact drop depends on current and temperature. |
| Reverse leakage example | ≤ 25 nA at 20 V | Only a very small reverse current flows under this test condition. |
| Reverse recovery time, trr | 8 ns | Fast enough for many switching and pulse-shaping jobs. |
Schottky diode
A Schottky diode uses a metal-semiconductor junction rather than an ordinary PN junction. It can have a lower forward drop and very fast switching because it does not store minority-carrier charge in the same way as a PN junction.
Tradeoffs include greater reverse leakage and, depending on technology, lower reverse-voltage capability.
Zener diode
A Zener diode is designed to operate in reverse breakdown at a specified voltage. With proper current limiting it can provide a simple reference or voltage clamp.
LED
A light-emitting diode converts forward current into light. Semiconductor material and junction structure determine the emitted wavelength. LEDs are available from infrared through visible colors and into ultraviolet.
An LED needs current control. Directly connecting a bare LED to a stiff voltage source can destroy it even when the supply voltage looks only slightly greater than the LED's forward voltage.
Laser diode
A laser diode is related to an LED but uses an optical cavity and stimulated emission to produce a much narrower and more directional light output. Laser diodes generally require carefully controlled drive current and thermal management.
Photodiode
A photodiode produces current when light creates electron-hole pairs in or near its depletion region. Photodiodes are used in optical receivers, encoders, light meters, safety systems, medical instruments, and scientific detectors.
PIN diode
A PIN diode places a lightly doped or intrinsic region between P and N regions. At RF it can behave as a current-controlled resistance, making it useful in radio-frequency switches, attenuators, limiters, and protection networks. PIN structures are also used in photodetectors.
Varactor diode
A reverse-biased PN junction behaves like a voltage-dependent capacitor because the depletion region changes width with bias. A varactor is optimized for this effect and is used in electronically tuned oscillators, filters, radios, and phase-locked loops.
TVS diode
A transient-voltage-suppressor diode is designed to absorb short surges and clamp voltage before protected circuitry is damaged. TVS devices are selected by standoff voltage, breakdown voltage, clamping voltage, surge waveform, power capability, capacitance, and polarity.
Avalanche diode
Some diodes are specifically designed for controlled avalanche breakdown. Avalanche behavior is used in protection, voltage references, pulse generation, and specialized noise sources.
Photovoltaic cell
A solar cell is a large-area light-sensitive semiconductor junction designed to deliver electrical power. It is closely related in physics to the photodiode, but optimized for energy conversion rather than simply sensing light.