A diode is a two-terminal semiconductor device that allows current to flow readily in one direction (forward bias) while blocking it in the other (reverse bias). The most common type is the p-n junction diode, though several variants exist with significantly different electrical characteristics suited to specific applications.
In practice
Diodes appear throughout embedded hardware in rectification, protection, and signal-conditioning roles. The most common uses in embedded designs are reverse-polarity protection on power inputs, flyback suppression across inductive loads such as relay coils and motor windings, and steering current in power supply circuits. For flyback suppression, a general-purpose silicon diode (e.g., 1N4007) is adequate at low switching frequencies, but Schottky diodes are often preferred when fast turn-off or low forward-voltage drop matters, provided their reverse leakage and reverse-voltage limits are acceptable for the application.
Forward voltage drop is a critical parameter to account for in circuit design. Standard silicon p-n junction diodes typically drop around 0.6 V to 0.7 V at moderate forward currents, while Schottky diodes commonly drop 0.2 V to 0.45 V depending on the part and current level. Germanium diodes drop approximately 0.2 V to 0.3 V at typical operating currents, though this range varies with device type and current level, and they are rarely used in new designs. These figures are not constants: forward voltage varies with current and temperature, decreasing roughly 2 mV per degree Celsius for silicon as a rule of thumb (the actual coefficient varies with current, device construction, and operating point), which can lead to thermal runaway in parallel diode configurations where current-balancing is not addressed. The post "Voltage Drops Are Falling on My Head" covers operating-point analysis, temperature coefficients, and thermal runaway in detail.
Zener diodes, a closely related variant, are operated in reverse breakdown and are widely used for voltage clamping and simple shunt regulation. TVS (transient voltage suppression) diodes are structurally similar but optimized for clamping fast, high-energy transients on I/O lines and power rails, a common requirement in designs with exposed connectors. Schottky diodes, LEDs, and PIN diodes each represent further specialized variants discussed in the post "10 More (Obscure) Circuit Components You Should Know."
In gate drive and power switching circuits, diodes appear as body diodes within MOSFETs, as bootstrap diodes in half- and full-bridge drivers, and as freewheeling diodes across the switch. The reverse recovery behavior of diodes in these positions directly affects switching losses and EMI; the post "Modeling Gate Drive Diodes" addresses how to model these devices accurately in simulation. When evaluating thermal performance under high-current or high-frequency conditions, the post "Hot Fun in the Silicon: Thermal Testing with Power Semiconductors" provides practical guidance on measurement and characterization.
Frequently asked
What is the difference between a Schottky diode and a standard silicon p-n junction diode?
A
Schottky diode uses a metal-semiconductor junction instead of a p-n junction. This gives it a lower forward voltage drop (typically 0.2 V to 0.45 V vs. 0.6 V to 0.7 V for silicon) and much faster switching, because the metal-semiconductor junction does not store minority carriers in the same way a p-n junction does, making conventional reverse recovery time largely inapplicable. In practice, Schottky diodes switch far faster than standard rectifier diodes, which can exhibit hundreds of nanoseconds or more of reverse recovery. Schottky diodes are preferred in high-frequency switching supplies, synchronous rectification circuits, and anywhere conduction loss is critical. The trade-off is higher reverse leakage current and lower reverse breakdown voltage compared to many standard diodes.
What is reverse recovery time and why does it matter in switching circuits?
When a forward-biased diode is suddenly reverse-biased, stored minority carriers must be swept out before the diode blocks current. The time this takes is the reverse recovery time (t_rr). In low-frequency rectifiers this is usually negligible, but in switching power supplies or gate drive circuits operating at tens to hundreds of kilohertz, a slow diode can conduct a significant reverse current spike during every switching transition. This wastes power, generates heat, and increases
EMI. Selecting a diode with a rated t_rr appropriate to the switching frequency -- or using a
Schottky diode where minority-carrier recovery is not a concern -- is important in these applications.
How do I choose a flyback suppression diode for a relay or solenoid?
The diode must withstand the supply voltage in reverse with adequate margin; as a starting point, a diode rated at least 2x the supply voltage is a common heuristic, though the appropriate margin depends on supply tolerance, transients, and load topology. The diode must also handle the peak current when the inductive load switches off. For a relay coil driven from 12 V and drawing 50 mA, a general-purpose 1N4148 or 1N4007 is typically sufficient. Place the diode as close as possible to the inductive load with short, low-inductance routing. If the relay must release quickly, a standard flyback diode slows de-energization because it clamps the kickback voltage near 0.7 V; adding a
Zener in series (Zener cathode toward supply) clamps at a higher voltage and speeds release at the cost of slightly more stress on the switch.
What is a TVS diode and when should I use one instead of a Zener?
A TVS (transient voltage suppression) diode is optimized for clamping fast, high-energy transients such as
ESD events or cable-induced surges. Compared to a
Zener, a TVS has lower dynamic impedance during conduction and is rated for peak pulse power (e.g., 400 W, 1500 W, 5000 W) rather than continuous power dissipation. Use a TVS on signal and power lines exposed to external environments -- connectors, motor drive lines, or any trace that leaves the PCB. A Zener is appropriate for DC voltage clamping and simple regulation, but many Zeners can be damaged by the energy of a substantial transient; whether a given Zener survives depends on the pulse width, energy level, and package rating.
Why does forward voltage drop change with temperature, and why does that matter for circuit design?
The forward voltage of a silicon diode decreases approximately 2 mV per degree Celsius increase in junction temperature as a rule of thumb, though the actual coefficient varies with current and device construction. This has two practical consequences. First, a diode voltage used as a reference or in a level-shifting network will drift with temperature, which must be accounted for in margin calculations. Second, if two diodes share a current without ballasting resistors, the one that runs slightly hotter will have a lower forward voltage, draw more current, heat further, and draw still more current -- a positive feedback loop that can lead to thermal runaway. The post 'Voltage Drops Are Falling on My Head' covers this behavior with detailed analysis.
Differentiators vs similar concepts
Diodes are often discussed alongside
Zener diodes, TVS diodes, and
Schottky diodes as if they are interchangeable. They share the same two-terminal p-n (or metal-semiconductor) structure but are optimized for different operating regimes. A standard rectifier diode (e.g., 1N4007) is operated in forward bias and is not designed for controlled reverse breakdown. A Zener diode is designed to break down at a precise reverse voltage and is used in that mode intentionally. A TVS diode is optimized for absorbing high-energy transient pulses and is designed and rated specifically for transient pulse handling in a way that standard Zeners are not. A Schottky diode uses a metal-semiconductor junction for low forward voltage and near-zero reverse recovery time. An LED is a p-n junction diode that emits photons when forward biased; its forward voltage is determined by the semiconductor bandgap (typically 1.8 V to 3.5 V depending on color), not the 0.6 V to 0.7 V of silicon rectifiers.
