A complete guide to diodes — the PN junction, diode symbol, forward and reverse bias, and every major type of diode explained: rectifier, Zener, Schottky, LED, laser, photo, varactor, PIN, TVS, switching, alternator, and data diodes.
Last updated: September 22, 2026
A diode (pronounced DAHY-ohd) is a two-terminal semiconductor electronic component that allows electric current to flow easily in one direction only, while blocking or heavily restricting current in the opposite direction. It is built from a PN junction — the junction between P-type (positive, hole-rich) and N-type (negative, electron-rich) semiconductor material.
The word "diode" comes from "di-" (two) + "-ode" (electrode), referring to its two terminals: the anode (positive terminal) and the cathode (negative terminal). Diodes are among the most fundamental and widely used components in modern electronics — found in nearly every circuit board, power supply, and electronic device.
Figure 1: A diode acts like a one-way valve for electric current — allowing flow from anode to cathode (forward bias) while blocking flow in the reverse direction.
To understand how diodes work, you need to understand the PN junction at its core:
A diode is formed by joining P-type semiconductor material (doped to have an excess of "holes," or positive charge carriers) with N-type semiconductor material (doped to have an excess of free electrons). At the junction between them, a depletion region forms, where electrons and holes combine and cancel out, creating a barrier to current flow.
When the diode is forward biased — positive terminal of a voltage source connected to the P-side (anode) and negative to the N-side (cathode) — the depletion region narrows. Once the applied voltage exceeds the diode's threshold (turn-on) voltage (about 0.7V for silicon, 0.3V for germanium), current flows easily through the diode.
When the diode is reverse biased — polarity reversed — the depletion region widens, and the diode blocks current flow (except for a tiny leakage current in the microampere range). If the reverse voltage becomes too large, it exceeds the diode's breakdown voltage, and current suddenly flows in reverse (this is the principle exploited intentionally by Zener diodes).
Figure 2: In forward bias, the depletion region narrows and current flows freely. In reverse bias, the depletion region widens and current is blocked.
The standard diode symbol is a triangle pointing toward a bar — the triangle represents the anode, and the bar represents the cathode. The direction of the triangle shows the direction of conventional current flow when forward biased.
A diode bridge (also called a bridge rectifier, or pont de diodes in French) is an arrangement of four diodes in a bridge configuration that converts AC (alternating current) into DC (direct current) by allowing current to flow through the load in the same direction during both halves of the AC cycle. It is the most common rectification circuit used in power supplies worldwide.
Understanding what diodes do and what diodes are used for is key to understanding modern electronics. Diodes perform several critical functions:
There are many types of diodes, each engineered for a specific electrical characteristic or application. Below are the 12 most important types.
Rectifier diodes are general-purpose diodes designed to convert AC to DC by allowing current to flow in only one direction. They are used in half-wave, full-wave, and bridge rectifier circuits in virtually every power supply.
Power suppliesBattery chargersAC-DC adaptersA Zener diode is specially designed to operate safely in reverse breakdown at a precise, stable voltage (the Zener voltage). Unlike regular diodes, exceeding the reverse breakdown voltage does not damage a Zener diode — making it ideal for voltage regulation.
Voltage regulationReference voltageOvervoltage protectionA Schottky diode uses a metal-semiconductor junction instead of a standard PN junction, giving it a much lower forward voltage drop (0.15–0.45V) and extremely fast switching speed. This makes it highly efficient for high-frequency and low-power-loss applications.
High-frequency switchingSolar panel bypassRF circuitsLight emitting diodes (LEDs) emit visible (or infrared/ultraviolet) light when forward biased, as electrons recombine with holes and release energy as photons. LEDs are far more energy-efficient and longer-lasting than incandescent bulbs. A related advanced type is the OLED (organic light-emitting diode), which uses organic compounds for even thinner, flexible displays used in smartphones and TVs.
LightingDisplays & indicatorsOLED screensLaser diodes emit coherent, focused light through stimulated emission, unlike the incoherent light of an LED. Common examples include red and green laser diodes used in everyday devices.
Barcode scannersFiber-optic communicationLaser pointersA photodiode converts light into an electrical current. It is typically operated in reverse bias, where incoming photons generate electron-hole pairs, producing a current proportional to light intensity.
Light sensorsSolar cellsOptical communication receiversA varactor diode (also called a varicap) exploits the voltage-dependent capacitance of a reverse-biased PN junction. As reverse voltage changes, its junction capacitance changes, making it useful as a voltage-controlled capacitor.
Voltage-controlled oscillatorsRF tuning circuitsA PIN diode has an undoped intrinsic (I) semiconductor layer sandwiched between P-type and N-type layers. This structure makes it excellent as a fast electronic switch and variable resistor at radio frequencies.
RF switchesAttenuatorsPhotodetectorsA TVS diode protects sensitive electronic circuits from sudden, brief high-voltage transients or spikes (e.g., from electrostatic discharge or lightning-induced surges) by rapidly clamping the voltage to a safe level.
ESD protectionSurge protectionAutomotive electronicsSwitching diodes are small-signal diodes designed to switch on and off extremely quickly, used in digital logic and high-speed circuits. The 1N4148 is a classic example of a switching diode.
Digital logicSignal clippingHigh-speed switchingAlternator diodes are heavy-duty rectifier diodes built into a vehicle's alternator to convert the AC output generated by the rotating alternator into DC current, which is used to charge the car battery and power the vehicle's electrical system.
Automotive charging systemsAC-to-DC rectificationUnlike the other entries, a data diode is not a single electronic component but a network security hardware device that enforces strictly one-way data flow between two networks (e.g., from a secure system out to a monitoring system, but never back in) — named after the electronic diode because it mimics its one-directional behavior at the network level.
CybersecurityCritical infrastructureOne-way network gateways| Property | Silicon Diodes | Germanium Diodes |
|---|---|---|
| Forward voltage drop | ~0.7 V | ~0.3 V |
| Temperature tolerance | High (up to ~150°C) | Low (up to ~75°C) |
| Reverse leakage current | Very low | Higher |
| Common use | Most modern electronics, power rectification | Low-voltage signal detection, RF demodulation |
Semiconductor diodes today are overwhelmingly silicon-based because of silicon's superior temperature stability, availability, and manufacturing cost — germanium diodes remain in use mainly in niche low-voltage signal applications like vintage radio detectors.
| Part Number | Type | Rating | Common Use |
|---|---|---|---|
| 1N4007 | General-purpose rectifier diode | 1A, 1000V reverse | Power supply rectification |
| 1N4148 | Small-signal switching diode | 200mA, 100V reverse | Digital circuits, RF switching |
| 1N5819 (example) | Schottky diode | 1A, 40V reverse | Low-drop rectification |
| 1N4733A (example) | Zener diode (5.1V) | 1W | Voltage regulation |
You can build and test simple diode circuits (like a basic rectifier or LED circuit with a current-limiting resistor) virtually using free circuit simulation tools such as Tinkercad Circuits before building them on a physical breadboard.
Diodes are one of several fundamental electronic components. Here's how they compare:
| Component | Terminals | Primary Function |
|---|---|---|
| Diode | 2 | Allows current in one direction only |
| Resistor | 2 | Opposes/limits current flow (Ohm's Law: V = IR) |
| Capacitor | 2 | Stores electrical energy in an electric field |
| Transistor (BJT) | 3 | Amplifies or switches current using a small control current |
| MOSFET | 3 (or 4) | Amplifies or switches current using a control voltage (not current) |
While a diode only conducts in one direction and cannot amplify a signal, a transistor or MOSFET can act as both a switch and an amplifier, controlled by a third terminal. Diodes are essentially "one-way gates," while transistors are "controllable gates."
Practice these diode concepts with hands-on lab experiments that are part of standard worldwide electronics engineering curricula. Click any experiment to try the demo.
Plot the current-voltage characteristic curve of a PN junction diode under forward bias and determine the threshold/cut-in voltage.
Try DemoStudy the small reverse leakage current and reverse breakdown behavior of a PN junction diode.
Try DemoObserve the forward conduction behavior of a Zener diode, similar to a standard PN junction diode.
Try DemoDetermine the Zener breakdown voltage and verify its use as a voltage regulator by studying the sharp reverse breakdown curve.
Try DemoPlot the forward characteristic curve of an LED and determine its turn-on (threshold) voltage compared to a standard silicon diode.
Try DemoStudy how a photodiode's reverse current varies with incident light intensity — the standard operating mode for light detection.
Try Demo| What is a diode? | 2-terminal device allowing current in one direction only (PN junction) |
| Forward voltage drop | ~0.7V (silicon), ~0.3V (germanium), 0.15-0.45V (Schottky) |
| Main types | Rectifier, Zener, Schottky, LED, laser, photo, varactor, PIN, TVS, switching, alternator, data |
| Common part numbers | 1N4007 (rectifier), 1N4148 (switching) |
A diode is a two-terminal semiconductor electronic component that allows electric current to flow easily in one direction (forward bias) while blocking current in the opposite direction (reverse bias). It is built from a PN junction and is one of the most fundamental building blocks in electronics.
A diode works based on the PN junction. In forward bias, the depletion region narrows and current flows once the forward voltage exceeds a threshold (~0.7V for silicon). In reverse bias, the depletion region widens, blocking current except for a tiny leakage current, until breakdown voltage is reached.
Diodes convert AC to DC (rectification), protect circuits from reverse voltage and spikes, regulate voltage (Zener diodes), emit light (LEDs), detect light (photodiodes), generate coherent light (laser diodes), and perform switching in digital and RF circuits.
The main types include rectifier diodes, Zener diodes, Schottky diodes, LEDs, laser diodes, photodiodes, varactor diodes, PIN diodes, TVS diodes, switching diodes, and alternator diodes — each optimized for a specific electrical characteristic or application.
Silicon diodes have a ~0.7V forward drop and tolerate higher temperatures and reverse voltages, dominating modern electronics. Germanium diodes have a lower ~0.3V forward drop, useful for low-voltage signal detection, but are more temperature-sensitive with higher leakage current.
A Zener diode is designed to operate safely in reverse breakdown at a precise, stable voltage, making it primarily used for voltage regulation and maintaining a constant reference voltage in a circuit.
A Schottky diode uses a metal-semiconductor junction instead of a PN junction, giving it a much lower forward voltage drop (0.15-0.45V) and faster switching speed than a standard silicon diode, ideal for high-frequency and high-efficiency applications.
The 1N4007 is a very common general-purpose rectifier diode (1A, 1000V reverse), widely used in power supplies. The 1N4148 is a common small-signal switching diode used in high-speed digital and RF circuits.
