Diodes

Learning Objectives

By the end of this page, you should be able to:

• Explain how a p-n junction creates one-way current flow and describe the depletion region
• Distinguish forward bias from reverse bias and predict diode behaviour in each
• Identify the major diode types (rectifier, Zener, Schottky, LED, photodiode) and match each to its typical application
• Calculate output voltage and current in a simple diode circuit using the forward voltage drop
• Explain how a Zener diode is used deliberately in reverse breakdown for voltage regulation
• Describe how a half-wave rectifier converts AC to pulsating DC

Quick Answer

A diode is a semiconductor component that allows current to flow easily in one direction (forward bias) while blocking it in the other (reverse bias). It's built from a p-n junction — a boundary between p-type and n-type semiconductor material — that creates a one-way "gate" for current. In forward bias, a diode drops a fairly constant voltage (about 0.7 V for silicon) once conducting; in reverse bias, it blocks current until you exceed its breakdown voltage. This one-way behaviour is why diodes are used to convert AC to DC (rectification), protect circuits from reversed power or voltage spikes, emit light (LEDs), and — in the special case of Zener diodes — regulate voltage by operating deliberately in controlled reverse breakdown.

What Is a Diode?

A diode has two terminals: the anode and the cathode. Current flows easily from anode to cathode (in the direction of the arrow on its symbol) but is blocked in the reverse direction. This behaviour comes from how the diode is built.

How a Diode Works: The p-n Junction

A diode is formed where p-type semiconductor material (doped to have an excess of "holes," or missing electrons) meets n-type material (doped to have an excess of free electrons). At the junction, electrons from the n-side diffuse across and combine with holes on the p-side near the boundary, creating a region depleted of free charge carriers — the depletion region. This acts as a natural barrier to current flow.

• Forward bias: Connecting the positive terminal of a supply to the anode (p-side) and negative to the cathode (n-side) pushes charge carriers toward the junction, narrowing the depletion region. Once the applied voltage exceeds the diode's forward voltage drop (about 0.7 V for silicon, 0.3 V for germanium, roughly 2–3 V for LEDs depending on colour), current flows freely.
• Reverse bias: Connecting the supply the opposite way widens the depletion region, blocking current almost entirely — only a tiny reverse saturation current (microamps or less) leaks through. If reverse voltage exceeds the diode's breakdown voltage, it will conduct heavily in reverse, which destroys ordinary diodes but is exactly how a Zener diode is designed to operate.

Types of Diodes

Rectifier Diodes

Standard silicon diodes designed to convert AC to DC. Used in power supply bridge rectifiers.

Zener Diodes

Designed to operate safely in reverse breakdown at a specific, well-controlled voltage. Once past that breakdown point, the voltage across the Zener stays essentially constant even as current through it varies — making it a simple voltage reference or regulator.

Schottky Diodes

Use a metal-semiconductor junction instead of a p-n junction, giving a much lower forward voltage drop (around 0.2–0.3 V) and very fast switching. Common in high-frequency rectification and reverse-polarity protection where minimizing voltage loss matters.

Light-Emitting Diodes (LEDs)

When forward biased, electrons and holes recombine at the junction and release energy as photons — visible or infrared light, depending on the semiconductor material. Used for indicators, displays, and general lighting.

Photodiodes

Operate in reverse bias; incoming light generates electron-hole pairs that produce a measurable current proportional to light intensity. Used in light sensors, optical receivers, and solar cells (in a related, unbiased mode).

Varactor (Varicap) Diodes

Behave as a voltage-controlled capacitor when reverse biased — the depletion region width (and hence capacitance) changes with applied reverse voltage. Used in voltage-controlled oscillators and tuning circuits.

Tunnel Diodes

Heavily doped junctions that exhibit a region of negative resistance, where increasing voltage actually decreases current. Used in very high-frequency oscillators.

Diode Characteristics

• Forward voltage drop (Vf): The voltage needed across the diode before it conducts significantly (~0.7 V silicon, ~0.3 V germanium, higher for LEDs)
• Reverse saturation current (Irs): The tiny leakage current in reverse bias, usually in the nanoamp to microamp range
• Breakdown voltage (Vb): The reverse voltage at which the diode begins conducting heavily; for a normal diode this is destructive, for a Zener it is the designed operating point
• Junction capacitance: Varies with applied voltage, becomes important at high switching frequencies

Real-World Example

Every USB phone charger includes a bridge rectifier — four diodes arranged so that regardless of which half of the AC cycle mains voltage is in, current is always routed through the load in the same direction. The output is a pulsating DC waveform, which a filter capacitor then smooths. Without diodes, a switching supply couldn't convert the incoming AC into usable DC at all.

Applications of Diodes

• Rectification: Converting AC to DC in power supplies (half-wave and full-wave/bridge rectifiers)
• Voltage regulation: Zener diodes provide a stable reference voltage
• Reverse-polarity protection: A series diode blocks current if a battery is connected backwards
• Signal demodulation: Extracting the audio signal envelope from an AM radio carrier
• Clamping and clipping: Limiting a signal to a safe voltage range
• Light emission and detection: LEDs and photodiodes in indicators, displays, and optical sensing

Half-Wave Rectifier Circuit

During the positive half of the AC cycle, the diode is forward biased and conducts, so the load sees that half of the waveform (minus the diode's forward drop). During the negative half, the diode is reverse biased and blocks current, so the load sees zero. The result is a series of one-directional pulses — pulsating DC — which a filter capacitor smooths into something closer to steady DC. A full-wave bridge rectifier (four diodes) improves on this by using both halves of the AC cycle.

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Key Terms

Term	Definition	Related Concept
p-n junction	Boundary between p-type and n-type semiconductor forming the diode's core structure	Depletion region
Depletion region	Charge-carrier-free zone at the p-n junction acting as a barrier to current	Forward/reverse bias
Forward bias	Voltage polarity that narrows the depletion region and allows current flow	Forward voltage drop
Reverse bias	Voltage polarity that widens the depletion region and blocks current	Breakdown voltage
Forward voltage drop (Vf)	Voltage needed across a conducting diode (~0.7 V silicon)	Circuit voltage calculations
Zener breakdown	Controlled reverse breakdown used deliberately for voltage regulation	Voltage reference circuits
Rectification	Converting AC to DC using one-way current flow	Half-wave/full-wave rectifiers
Schottky diode	Metal-semiconductor diode with low forward drop and fast switching	High-frequency rectification
Photodiode	Diode that generates current proportional to incident light in reverse bias	Optical sensing
Reverse saturation current	Small leakage current that flows in reverse bias	Diode non-ideality

Common Mistakes

Misconception: A diode blocks all current in reverse bias with zero exceptions. Why it's wrong: A tiny reverse saturation current always leaks through, and if reverse voltage exceeds the breakdown voltage, the diode conducts heavily (destructively for a regular diode, by design for a Zener). Correct understanding: Reverse bias blocks current only up to the breakdown voltage; beyond that, current flows in reverse, and this is the deliberate operating mode of a Zener diode.

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Misconception: The forward voltage drop of a diode changes a lot as current increases, similar to a resistor's linear V-I relationship. Why it's wrong: A diode's V-I relationship is exponential, not linear. Over a wide range of forward current, the voltage drop stays relatively close to its typical value (about 0.7 V for silicon) — it takes a large current change to shift the voltage by even a few tenths of a volt. Correct understanding: Treat the forward voltage drop as approximately constant (0.7 V for silicon, ~2 V for a typical LED) for quick circuit calculations, understanding it's a simplification of an exponential curve.

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Misconception: Any diode can be used as a rectifier as long as its current rating is sufficient. Why it's wrong: Different diode types are optimized for different jobs. Using a slow standard rectifier diode in a high-frequency switching supply causes excessive switching losses and heat; a Schottky diode's fast recovery and low forward drop matter there instead. Correct understanding: Match diode type to application: standard rectifiers for line-frequency AC/DC conversion, Schottky diodes for high-frequency or low-voltage-drop needs, and Zener diodes specifically for voltage regulation.

Comparison and Connections

Feature	Rectifier Diode	Zener Diode	Schottky Diode	LED
Normal operating region	Forward bias	Reverse breakdown	Forward bias	Forward bias
Typical forward drop	~0.7 V	Not applicable (Vz set by design)	~0.2–0.3 V	~1.8–3.3 V (colour-dependent)
Switching speed	Moderate	Slow	Very fast	N/A
Primary use	AC-DC rectification	Voltage regulation/reference	High-frequency rectification	Light emission

Practice Questions

Recall

1. What are the two terminals of a diode called, and which one does current conventionally flow out of? Answer guidance: Anode and cathode. Conventional current flows into the anode and out of the cathode when forward biased.

2. What is the typical forward voltage drop of a silicon diode? Answer guidance: Approximately 0.7 V.

Understanding

3. Explain why a diode conducts in forward bias but blocks current in reverse bias. Answer guidance: Forward bias narrows the depletion region at the p-n junction, allowing charge carriers to cross and current to flow once the forward voltage threshold is exceeded. Reverse bias widens the depletion region, creating a much larger barrier that blocks all but a tiny leakage current.

4. How is a Zener diode's use different from that of a regular rectifier diode, even though both are p-n junction devices? Answer guidance: A rectifier diode is operated in forward bias and designed to avoid reverse breakdown. A Zener diode is deliberately operated in reverse breakdown at a controlled, well-defined voltage, exploiting the fact that voltage across it stays nearly constant over a range of currents — useful for voltage regulation.

Application

5. An LED with a forward voltage of 2 V needs to be driven from a 9 V supply with a forward current of 15 mA. What series resistor value is needed? Answer guidance: Voltage across resistor = 9 − 2 = 7 V. R = V/I = 7 / 0.015 ≈ 467 Ω. Nearest standard value: 470 Ω.

6. A 5.1 V Zener diode is used to regulate a supply that varies between 8 V and 12 V, with a series resistor and a load. Explain qualitatively how the circuit maintains a steady 5.1 V output. Answer guidance: As long as the Zener is kept in reverse breakdown by sufficient current through the series resistor, the voltage across it stays fixed near 5.1 V regardless of moderate changes in supply voltage or load current — the series resistor absorbs the varying excess voltage.

Analysis

7. Compare what happens in a half-wave rectifier versus a full-wave bridge rectifier in terms of output ripple and efficiency, and explain why. Answer guidance: A half-wave rectifier only uses one half of the AC cycle, leaving the load unpowered during the other half, resulting in higher ripple and lower average output. A full-wave bridge rectifier uses both halves of the cycle (inverting the negative half), doubling the output frequency of ripple and reducing peak-to-peak ripple for a given filter capacitor, improving efficiency.

8. A student wires an LED directly across a 9 V battery with no resistor, and it burns out immediately. Explain the failure using diode I-V behaviour. Answer guidance: An LED's forward voltage stays close to its rated value (~2 V) over a wide current range because of its steep exponential I-V curve, meaning any voltage above that value drives an extremely large, uncontrolled current. Without a series resistor to limit current, the LED draws far more current than it can handle and the junction overheats and fails.

FAQ

Why does a diode have a "knee" voltage instead of just turning on gradually? The forward current through a diode follows an exponential relationship with voltage (the diode equation). Below the knee, current is negligible; once voltage rises close to the material's characteristic drop (0.7 V for silicon), current increases so steeply that the voltage appears to "clamp" — practically constant over a wide current range. It's not a hard switch, just a very steep curve.

Why do LEDs of different colours have different forward voltage drops? The forward voltage of an LED corresponds to the energy of the photon emitted, which depends on the semiconductor material's bandgap. Blue and white LEDs use materials with a larger bandgap (higher photon energy, shorter wavelength) and need a higher forward voltage (~3 V) than red LEDs (~1.8–2.2 V), which use a smaller bandgap material.

Can a diode be damaged by too much reverse voltage even if it never actually breaks down? Yes, applying reverse voltage close to but below the rated breakdown voltage repeatedly can stress the junction over time, and any voltage spike (such as from a switching inductive load) can momentarily exceed the rated breakdown and cause permanent damage even if it's brief.

What's the difference between a diode's breakdown voltage and a Zener diode's Zener voltage? They describe the same physical phenomenon — reverse breakdown — but the design intent differs. A regular diode's breakdown voltage is a limit to avoid; exceeding it is usually destructive because current isn't limited. A Zener diode is manufactured and doped specifically so that its breakdown occurs at a precise, repeatable voltage and can safely sustain that breakdown current (within its power rating) indefinitely.

Why do some diodes have a stripe on one end? The stripe marks the cathode terminal. Getting this backwards in a circuit means the diode won't conduct where expected (if forward bias was intended) or will conduct where it shouldn't (if reverse bias was intended) — always check the stripe orientation against the circuit diagram before soldering.

Quick Revision

• A diode allows current in one direction (forward bias) and blocks it in the other (reverse bias)
• The p-n junction's depletion region is the barrier; forward bias narrows it, reverse bias widens it
• Silicon diodes have a forward voltage drop of about 0.7 V; germanium about 0.3 V
• Reverse breakdown is destructive for regular diodes but the designed operating mode for Zener diodes
• Zener diodes hold a nearly constant voltage in reverse breakdown, useful for voltage regulation
• Schottky diodes have a lower forward drop (~0.2–0.3 V) and switch faster than standard diodes
• LEDs emit light through recombination at the junction in forward bias; forward voltage depends on colour/material
• Photodiodes generate current proportional to light in reverse bias
• A half-wave rectifier passes only one half of the AC cycle; a full-wave bridge rectifier uses both
• Always use a series resistor to limit current through an LED — never connect it directly across a supply

Related Topics

Prerequisites: Semiconductor basics (p-type/n-type doping); Ohm's law; basic AC/DC concepts

Related Topics: Transistors (built from p-n junctions); rectifier and power supply design; LEDs and optoelectronics; Zener voltage regulators

Next Topics: Transistors; rectifier and filter circuit design; voltage regulator circuits; optoelectronic sensors