What Is a Diode and How Does It Work?

Detailed Explanation

Diodes are among the oldest and most fundamental semiconductor devices. Understanding the variety of diode types — and matching each type to the right application — prevents common design errors, from wasted power in switching supplies to ESD damage on board connectors.

The P-N Junction Diode

A silicon diode is formed at the junction between p-type semiconductor (doped with boron, creating holes as majority carriers) and n-type semiconductor (doped with phosphorus, creating free electrons). At the junction, electrons and holes combine, creating a depletion region with a built-in potential.

Forward bias: Apply positive voltage to the anode (p-side) relative to the cathode (n-side). When the applied voltage exceeds the built-in potential (~0.6 V for silicon), the depletion region narrows and current flows — holes from the p-side and electrons from the n-side recombine in the junction, carrying current. Current rises exponentially with applied voltage (Shockley equation).

Reverse bias: Apply positive voltage to the cathode. The depletion region widens, and only a tiny reverse saturation current (typically nA) flows due to thermally generated minority carriers.

Breakdown: At sufficiently high reverse voltage (typically 50–1000 V for rectifier diodes), the electric field in the depletion region is strong enough to create avalanche multiplication — the diode conducts heavily in reverse. This is destructive in a standard rectifier diode but is used deliberately in Zener diodes.

Silicon Rectifier Diodes

The general-purpose rectifier converts AC to pulsating DC. Standard silicon rectifier diodes (1N4001 through 1N4007 series) have forward voltage of 0.6–1.0 V (higher at high currents), reverse voltage ratings from 50 V to 1000 V, and reverse recovery time of 0.5–2 µs.

The 1N4001 (50 V, 1 A) and 1N4007 (1000 V, 1 A) are the workhorse through-hole rectifiers. SMD equivalents include the M1 through M7 and MBRS/MBRD series. For higher currents, Schottky rectifiers are generally preferred.

Schottky Diodes

Schottky diodes use a metal-semiconductor junction (aluminium or titanium silicide on n-type silicon) instead of a P-N junction. This gives two important properties:

Lower forward voltage: Typical Vf of 0.2–0.4 V, compared to 0.6–0.7 V for silicon. At 2 A load current, a switching supply with a silicon rectifier wastes 1.2–1.4 W in forward drop; a Schottky wastes only 0.4–0.8 W. For battery-powered designs, this directly extends runtime.

No reverse recovery time: The metal-semiconductor junction has no minority carrier storage, so the diode switches from conducting to blocking in picoseconds. In a buck converter, the freewheeling diode (or body diode of the synchronous FET) must recover before the high-side switch turns on. Slow recovery causes shoot-through current and losses. Schottky freewheeling diodes eliminate this.

Limitations: Higher reverse leakage current than silicon (important in precision rectifiers and small-signal circuits). Lower breakdown voltage than equivalent silicon types (most Schottky diodes are 20–60 V; high-voltage Schottky up to 200 V are available but expensive). Reverse leakage increases significantly with temperature — verify leakage at the maximum operating temperature.

Common Schottky parts: 1N5819 (40 V, 1 A through-hole), BAT54 (30 V, 200 mA SOT-23 dual), SS14 (40 V, 1 A SMA SMD), MBRS340T3G (40 V, 3 A SMB).

Zener Diodes

Zener diodes are designed for operation in reverse breakdown. Two mechanisms are used depending on the breakdown voltage:

• Zener effect: Quantum tunnelling through a thin depletion region, dominant below ~5 V. Has a negative temperature coefficient (Vz decreases with temperature).
• Avalanche effect: Impact ionisation in wider depletion regions, dominant above ~5 V. Has a positive temperature coefficient (Vz increases with temperature).

At approximately 5.1 V, the temperature coefficients of both mechanisms cancel, producing a very temperature-stable Zener breakdown. The 5.1 V Zener is one of the most temperature-stable voltage reference points achievable with a simple diode (though precision reference ICs outperform it).

Zener shunt regulator circuit:

1. Series resistor R from supply voltage Vs to Zener anode
2. Zener cathode connected to the regulated output node
3. Load connected from output to GND

Rseries = (Vs − Vz) / (Iz_min + IL_max)

The Zener must carry at least Iz_min (typically 1–5 mA) to stay in regulation. The maximum Zener current is limited by power: P = Vz × Iz_max ≤ rated power dissipation.

Zeners are commonly used to clamp MCU input pins, protect ADC inputs from overvoltage, create simple 3.3 V or 5 V rail references from a higher supply, and set threshold voltages in comparator circuits. See what is a comparator? for examples of Zener-set thresholds.

TVS (Transient Voltage Suppression) Diodes

TVS diodes are designed for very high peak current and power handling during brief transients (ESD strikes, inductive switching spikes, load dump in automotive environments). Key parameters:

Clamping voltage (Vc): The voltage across the TVS when the specified peak current (Ipp) is flowing. This is what the protected circuit sees. Choose Vc below the IC's absolute maximum input voltage.

Breakdown voltage (VBR): The voltage at which the TVS begins to conduct reverse current significantly (at 1 mA). Should be above the circuit's maximum operating voltage.

Peak pulse power (Pppm): The maximum instantaneous power the TVS can absorb in a single transient (e.g. 600 W, 1500 W, 3000 W for 1 ms). Repetitive pulses reduce the effective rating.

Unidirectional vs bidirectional: Unidirectional TVS conducts only in one direction (suitable for protecting positive DC signals). Bidirectional TVS conducts symmetrically (needed for AC signals, data lines, and any line that may go negative).

TVS diodes should be placed at every external connector: USB, RS-485, CAN bus, digital I/O headers, and any RF coaxial connector where ESD is possible. For the MOSFET switching forum discussion, the freewheeling diode (flyback diode) across an inductive load serves a similar TVS function for inductively generated voltage spikes specifically.

LED (Light-Emitting Diode)

An LED is a P-N junction diode that emits photons when forward-biased. The emission wavelength (colour) depends on the semiconductor bandgap (GaAs, GaP, InGaN, etc.). Forward voltage varies by colour: red LEDs (GaAs/GaAlAs) Vf ≈ 1.8–2.2 V; green LEDs (InGaN) Vf ≈ 2.9–3.5 V; blue LEDs (InGaN) Vf ≈ 3.2–3.5 V; IR LEDs Vf ≈ 1.1–1.5 V.

LEDs are current-controlled devices — forward voltage changes only slightly with current, but brightness and lifetime are strongly current-dependent. Always use a series resistor to limit current: R = (Vsupply − Vf) / If_desired. Typical LED current: 5–20 mA for indicator LEDs, 350 mA to several amps for power LEDs (requiring dedicated constant-current drivers).

For a practical circuit involving transistors switching diode loads and the inductive kickback issue, see what is a transistor?.

For component selection guidance as part of a complete hardware design, Zeus Design's electronics engineering team can advise on protection topology and component specification.

Design Considerations

• Always include a flyback (freewheeling) diode with inductive loads: Any inductive load (relay, solenoid, motor) driven through a transistor or MOSFET will generate a large reverse voltage spike when the switch turns off. A Schottky diode (anode to switch drain/collector, cathode to supply) clamps this spike. Without it, the switching transistor will fail over time or immediately.
• Verify Schottky reverse leakage at maximum temperature: Schottky reverse leakage increases rapidly with temperature. A Schottky rated 10 µA at 25°C may leak 1 mA at 85°C. This matters in precision rectifiers or battery backup circuits where leakage current must stay below a budget.
• Choose TVS clamping voltage conservatively: The TVS clamping voltage Vc is specified at the rated peak current, but real ESD events can exceed the datasheet test current for longer durations. Select Vc at least 20% below the IC's absolute maximum input rating to account for this.

Common Mistakes

• Using a Zener without a series current-limiting resistor: A Zener connected directly from supply to GND without a series resistor will draw unlimited current and either burn out or blow the supply protection. The series resistor is not optional.
• Selecting TVS maximum working voltage equal to supply voltage: If the TVS begins to conduct at exactly the supply voltage (due to tolerance), it will dissipate continuous power as the supply holds it at its breakdown knee — potentially overheating. Choose maximum working voltage (standoff voltage) at least 10% above the highest expected supply voltage, including any transient overshoot.
• Forward-biasing a Schottky in a reverse battery protection circuit incorrectly: A common reverse polarity protection circuit uses a series Schottky diode in the supply path. This works but wastes 0.2–0.4 V in the normal forward-conducting state. An alternative using a MOSFET and body diode (ideal diode controller) avoids this loss. Choose based on the system's power budget.