Diode - Coursedia

Okay, let’s break down what a diode is and how it works, especially from an electrical engineering point of view. Think of this as your guide to understanding these fundamental electronic components.

What is a Diode?#

Simply put, a diode is an electronic part with two terminals. Its main job is to act like a one-way valve for electric current. It lets current flow through easily in one direction but largely blocks it from flowing the other way. This is called “asymmetric conductance.”

A diode is a two-terminal electronic component that conducts electric current primarily in one direction (asymmetric conductance). It has low (ideally zero) resistance in one direction and high (ideally infinite) resistance in the other.

The most common type you’ll encounter today is the semiconductor diode. This little device is usually made from a piece of silicon or sometimes other materials like gallium arsenide or germanium, specially prepared to have this one-way behavior. It was actually one of the very first semiconductor electronic parts ever discovered.

There’s also an older type, the thermionic diode, which is a vacuum tube, but semiconductor diodes are the workhorses in modern electronics.

How Diodes Work: The Main Ideas#

Diodes have a few key characteristics that make them super useful:

Unidirectional Current Flow#

This is the star feature. Imagine a pipe with a check valve – water can only go one way. A diode does the same for electrical current.

Forward Direction: When voltage is applied in the “right” direction (positive on one side, negative on the other, based on how the diode is made), the diode offers very little resistance, and current flows easily.
Reverse Direction: When voltage is applied in the “wrong” direction, the diode offers very high resistance, blocking most of the current flow.

This ability to control the direction of current flow is fundamental. One of the most common uses is converting Alternating Current (AC), which changes direction constantly, into Direct Current (DC), which flows in only one direction. This process is called rectification. Diodes acting as rectifiers are essential in power supplies for almost all electronic devices. They were also historically used in early radio receivers to pull the audio signal out of the radio waves.

Threshold Voltage (Turn-On Voltage)#

While a diode ideally blocks everything in reverse and lets everything through in forward, in reality, it’s not quite perfect. For a semiconductor diode to start conducting significantly in the forward direction, the voltage across it needs to reach a certain level.

The forward threshold voltage (also called turn-on voltage or cut-in voltage) is the minimum voltage required across a diode in the forward direction for it to begin conducting a significant amount of current.

This threshold voltage depends on the material the diode is made from:

Silicon diodes typically have a threshold voltage around 0.6 to 0.7 Volts (V).
Germanium diodes are lower, around 0.2 to 0.3 V.
Light-Emitting Diodes (LEDs) have higher threshold voltages, often 1.8V to 3.5V or more, depending on the color (material).

Once the voltage reaches this threshold, the current starts to flow freely. The voltage across the diode in the forward-conducting state (the forward voltage drop) doesn’t increase much even if the current increases significantly. This makes the threshold voltage a good estimate for the typical voltage drop across a diode when it’s conducting. For higher currents, like in power diodes, this forward voltage drop might increase slightly, maybe up to 1V or 1.5V.

Think of it like pushing open a sticky door – you need a certain amount of push (voltage) before it opens (conducts), and once it’s open, the effort (voltage) doesn’t change much even if you push harder (increase current).

Reverse Breakdown#

What happens if you apply a very large voltage in the reverse direction? While the diode is designed to block current in reverse, there’s a limit. If the reverse voltage gets too high, it reaches the breakdown voltage. At this point, the resistance suddenly drops, and a large current can flow in the reverse direction. This can often damage or destroy a normal diode permanently.

The breakdown voltage is the reverse voltage at which a diode’s resistance suddenly drops, allowing a large current to flow in the reverse direction. Exceeding this in a standard diode can cause permanent damage.

Some diodes, like Zener diodes and avalanche diodes, are specifically designed to operate safely in this breakdown region and are used for voltage regulation or protection against voltage spikes.

Other Cool Things Diodes Can Do#

Besides basic one-way current flow and voltage characteristics, engineers can tweak how semiconductor diodes are made to give them special abilities. These include:

Changing capacitance based on voltage (Varactor diodes) for tuning radios or TVs.
Generating light when current flows (Light-Emitting Diodes - LEDs, Laser Diodes).
Generating radio-frequency signals (Tunnel, Gunn, IMPATT diodes).
Detecting light (Photodiodes).
Sensing temperature.

These specialized diodes open up a huge range of applications beyond simple rectification.

A Quick Look Back: History#

Diodes, both the old vacuum tube type and the solid-state semiconductor type, were actually developed around the same time in the early 1900s, primarily as detectors for early radio signals.

Thermionic Diodes: The effect was noticed as early as the 1870s by folks like Frederick Guthrie and Thomas Edison (the “Edison effect”). But it was John Ambrose Fleming, an advisor to Marconi, who patented the first practical vacuum tube diode, called the “Fleming valve,” in 1904. These were common in radios and TVs until the 1950s and 60s when semiconductor technology improved.
Semiconductor Diodes: The discovery of one-way conduction in crystals dates back to Ferdinand Braun in 1874. Early crystal detectors, often using a mineral like galena and a fine wire (“cat’s whisker”), were used in radio receivers by inventors like Jagadish Chandra Bose and Greenleaf Whittier Pickard in the early 1900s. Semiconductor theory wasn’t fully understood back then. During World War II, research into point-contact diodes ramped up for radar applications. Modern junction diodes, which form the basis of most current types, were developed in the early 1950s, eventually replacing vacuum tubes for most uses.

Where the Name “Diode” Came From#

The word “diode” wasn’t used right away. Early one-way conductors were just called rectifiers. The term “diode” was coined much later, in 1919, by William Henry Eccles. It comes from Greek roots: di meaning “two” and ode meaning “path,” referring to the two terminals. Although all diodes rectify, the term “rectifier” is often specifically used for diodes designed for higher power applications, typically in power supplies.

The Old School: Vacuum Tube Diodes#

Imagine a glass bulb with the air pumped out. Inside are two key parts:

Cathode: A piece of metal that gets heated up.
Plate (or Anode): Another metal plate.

The cathode is heated, either directly by passing a current through it or indirectly by a separate heater element nearby. When metal gets hot enough (around 800-1000°C), it starts spitting out electrons into the vacuum – this is called thermionic emission. The cathode is often coated with special materials (like barium and strontium oxides) to make it easier for electrons to escape.

The plate doesn’t get hot, so it doesn’t emit electrons. However, it can attract them.

If you apply a positive voltage to the plate compared to the cathode, the plate attracts the electrons floating around near the cathode, and a current flows from cathode to plate.

If you apply a negative voltage to the plate, it repels the electrons, and no current flows.

So, current only flows one way. Simple, but bulky and power-hungry compared to modern semiconductor diodes. They still have niche uses today where their ability to handle large, fast voltage spikes is an advantage, or in some audio equipment for their specific sound characteristics.

The Modern Mainstay: Semiconductor Diodes#

These are the little black cylinders or tiny surface-mount components you see everywhere in modern electronics. They work based on the magic of semiconductor materials and how they behave when different types are put together.

Point-Contact Diodes#

These are somewhat older than junction diodes but still used today, particularly for high-frequency applications like microwave circuits (3 to 30 GHz). They evolved from the early crystal detectors.

A point-contact diode uses a small, sharp metal wire pressed against a piece of semiconductor crystal. The rectification happens at the tiny point of contact.

Some types have a permanent electrical connection (“welded”), while others rely on just the physical contact. They generally have lower capacitance (good for high speeds) but also higher resistance and ‘leakier’ reverse current compared to modern junction diodes.

Junction Diodes (The Most Common Type)#

This is the core technology for most diodes today, including the standard “p-n junction diode.”

A p-n junction diode is made by joining two types of semiconductor material: p-type and n-type.

Here’s the simplified idea:

Semiconductor Material: Start with a material like silicon. It’s not a great conductor or a great insulator – it’s somewhere in between.
Doping: To make it useful, we add tiny amounts of other elements (impurities) in a process called doping.
- Adding certain impurities (like phosphorus to silicon) creates an n-type semiconductor. “n” stands for negative; this material has extra free electrons (negative charge carriers) that can move around.
- Adding different impurities (like boron to silicon) creates a p-type semiconductor. “p” stands for positive; this material has ‘holes’ (places where an electron is missing, which act like positive charge carriers) that can move around.
Forming the Junction: When you put a piece of p-type material next to a piece of n-type material, you get a p-n junction.
The Depletion Region: Right at the boundary, some of the free electrons from the n-side move over to fill holes on the p-side. When an electron meets a hole, they essentially cancel each other out in terms of being mobile charge carriers. This leaves behind fixed, immobile charged atoms (positive ions on the n-side, negative ions on the p-side) in a zone around the junction. This zone becomes empty of mobile charge carriers and acts like an insulator. It’s called the depletion region. This depletion region creates a small internal electric field and voltage barrier.

Now, let’s see how voltage affects this junction:

Reverse Bias: If you connect the positive terminal of a battery to the n-type side and the negative terminal to the p-type side, you’re pulling the free electrons (from n) and holes (from p) away from the junction. This makes the depletion region wider, increasing the insulating barrier. Very little current can flow, just a tiny reverse leakage current (unless you hit the breakdown voltage).
Forward Bias: If you connect the positive terminal of a battery to the p-type side and the negative terminal to the n-type side, you’re pushing the free electrons (from n) and holes (from p) towards the junction. If the applied voltage is greater than the built-in voltage barrier of the depletion region (the threshold voltage we talked about), you overcome the barrier. The depletion region shrinks, and charge carriers can flow across the junction. Electrons flow from n to p, and holes flow from p to n. This results in a significant current flow through the diode.

Schottky Diodes#

These are a specific type of junction diode, but instead of a p-n junction (semiconductor-semiconductor), they use a metal-semiconductor junction.

A Schottky diode is a semiconductor diode formed by a metal–semiconductor junction.

This structure has a few advantages:

Lower forward voltage drop (often 0.15V to 0.45V), meaning less power is wasted when current flows forward.
Faster switching speed because they don’t have a “reverse recovery” problem (explained below) that p-n diodes do.
Lower junction capacitance.

These features make Schottky diodes great for high-speed switching power supplies and RF (Radio Frequency) circuits. The trade-off is they generally have higher reverse leakage current than p-n diodes and a lower reverse breakdown voltage.

Current-Voltage (I-V) Characteristic#

This is a graph that shows how much current flows through the diode for a given voltage across it. It’s the diode’s “signature.”

For a semiconductor diode, the I-V curve looks roughly like this:

Reverse Bias (Voltage negative or slightly positive but below threshold): Almost no current flows. There’s a tiny leakage current.
Forward Bias (Voltage positive and above threshold): Current starts to increase rapidly, following an exponential curve.
Breakdown (Voltage very negative): If the reverse voltage gets too high, current suddenly spikes in the reverse direction.

The mathematical model that describes this exponential relationship in the forward direction (before the leveling off at high current) and in reverse bias (before breakdown) is the Shockley diode equation. We won’t go into the math here, but it’s the fundamental formula used to model diode behavior.

As current increases in the forward direction, eventually the curve levels off and becomes more like a straight line. This is because the resistance of the bulk semiconductor material itself (not the junction) starts to limit the current, adding an ohmic resistance effect. This is important for power diodes handling large currents.

Reverse-Recovery Effect#

This is a non-ideal behavior, but important, especially in fast switching circuits.

When a p-n diode is conducting in the forward direction, there’s a buildup of mobile charge carriers (electrons and holes) in the junction area. If you suddenly switch the voltage to reverse bias, the diode doesn’t instantly stop conducting. Those stored charge carriers need time to be removed from the junction area.

The reverse-recovery effect is a temporary current that flows in the reverse direction when a p-n diode is switched from forward conduction to reverse bias. The time it takes for this current to stop is called the reverse recovery time (t_rr).

During the reverse recovery time, the diode acts almost like a short circuit, allowing current to flow the wrong way for a brief period. This can cause power loss and noise in switching circuits. Once the stored charge is gone, the diode blocks current properly again.

Schottky diodes, being majority carrier devices, don’t have this significant charge storage problem, which is why they are much faster than p-n diodes for switching applications.

Special diodes like step recovery diodes are designed to make this reverse current stop extremely abruptly. This sudden stop can generate very fast voltage pulses, useful in certain high-frequency circuits.

Lots of Different Semiconductor Diodes!#

Engineers have developed many types of semiconductor diodes, each optimized for specific tasks:

Avalanche Diodes: Designed to handle breakdown in the reverse direction safely due to the “avalanche effect” (charge carriers bumping into atoms and creating more charge carriers). Used for voltage regulation and protection, similar to Zener diodes but typically for higher breakdown voltages (> 6.2V).
Zener Diodes: Designed to operate reliably in the reverse breakdown region (the “Zener effect”). They maintain a nearly constant voltage across them once the reverse voltage reaches the Zener voltage. Excellent for voltage reference and regulation circuits. Typically have breakdown voltages below 6.2V, although the term is often used for higher voltage breakdown diodes (which might technically be avalanche).
Light-Emitting Diodes (LEDs): Emit light when current flows forward. Made from direct band-gap semiconductors like gallium arsenide. The color depends on the material. Used for indicators, lighting, displays.
Laser Diodes: Similar to LEDs but designed to produce a coherent, focused beam of light (a laser). Used in CD/DVD players, laser pointers, fiber optic communication.
Photodiodes: Detect light. When photons (light particles) hit the junction, they create electron-hole pairs, which causes a current to flow (or changes the diode’s conductivity). Used in light sensors, solar cells, optical communication receivers. Often PIN diodes (see below).
PIN Diodes: Have an undoped (Intrinsic) layer between the p and n regions (P-Intrinsic-N). This intrinsic layer makes them suitable for high-voltage applications (power electronics) and also makes them good photodetectors and radio frequency switches/attenuators.
Varactor Diodes (or Varicap Diodes): Act like capacitors whose capacitance changes based on the reverse voltage applied across them. Useful for tuning circuits in radios, TVs, and voltage-controlled oscillators (VCOs).
Tunnel Diodes (or Esaki Diodes): Show a strange property called “negative differential resistance” due to quantum tunneling. This means over a certain voltage range, the current actually decreases as the voltage increases. Used in very high-speed switching circuits and oscillators, even at low temperatures or in high radiation.
Gunn Diodes: Also exhibit negative resistance, typically used for generating microwave signals.
Constant-Current Diodes (CLDs): Actually a type of JFET (junction field-effect transistor) connected to act like a two-terminal device. They allow current up to a certain level, then hold it constant. Act like a current-limiting equivalent of a Zener diode.
Super Barrier Diodes: Try to combine the low forward drop of a Schottky diode with the lower reverse leakage of a p-n diode.
Gold-Doped Diodes: Gold is added as an impurity to help charge carriers recombine faster. This makes the diode faster than a normal p-n diode, allowing it to work at higher frequencies, but increases the forward voltage drop.
Stabistors: Designed to have a very stable forward voltage drop over a wide range of currents and temperatures. Used for low-voltage reference.
Transient Voltage Suppression Diodes (TVS Diodes): Specialized avalanche diodes with a large cross-sectional area to absorb large voltage spikes (transients) safely, protecting sensitive circuits.

How to Read Diode Symbols#

In circuit diagrams, different symbols are used to represent diodes. The basic symbol for a standard diode is a triangle with a line. The triangle points in the direction of conventional current flow (from positive to negative, which is opposite the direction of electron flow).

The triangle side is called the Anode (positive side, where current enters in forward bias).
The line side is called the Cathode (negative side, where current exits in forward bias).

Special symbols add extra lines or squiggles to indicate specific types:

A standard diode is just the triangle and line.
A Zener diode has bent lines on the cathode bar.
A Schottky diode has bent lines on the cathode bar pointing the other way.
An LED has arrows pointing away from the diode symbol (indicating light emitted).
A photodiode has arrows pointing towards the diode symbol (indicating light received).
A Varactor diode has a capacitor symbol attached to the cathode.

You’ll usually see diodes labeled with a ‘D’ or sometimes ‘CR’ (for Crystal Rectifier) in circuit diagrams.

How Diodes Get Their Names (Numbering)#

Manufacturers use different systems to name diodes. Some common ones are:

EIA/JEDEC (US Standard): Uses the “1N” prefix. For example, 1N4148 is a common small signal diode, and 1N4001 through 1N4007 are common power rectifier diodes.
Pro Electron (European Standard): Uses a two-letter prefix followed by numbers. The first letter indicates the material (A=Germanium, B=Silicon). The second letter indicates the function (A=low power, Y=Rectifier, Z=Zener). Examples: AA119 (Germanium signal), BY127 (Silicon rectifier), BZY88 (Silicon Zener).
JIS (Japanese Standard): Uses “1S” for semiconductor diodes.

Knowing these helps you identify what kind of diode you’re looking at when reading datasheets or component lists.

Diodes are part of a larger family of semiconductor devices that control current flow. Some related components include:

Rectifier: Often refers to a specific diode designed for power conversion, or an entire circuit that uses diodes for rectification.
Transistor: A three-terminal device that can amplify or switch electronic signals. Can be thought of as two back-to-back junctions.
Thyristor / Silicon Controlled Rectifier (SCR): A four-layer semiconductor device that acts like a diode but can be switched on (allowed to conduct) by a signal on a third terminal (the gate). Once on, it stays on until the current drops below a certain level or the voltage is reversed. Used for high-power switching and control.
TRIAC / DIAC: Devices related to thyristors, used for controlling AC power.
Varistor: A device whose resistance changes significantly with voltage, primarily used for surge protection.

In the world of optics, an equivalent concept exists called an optical isolator or optical diode, which allows light to pass through in one direction only.

Where Diodes Are Used: Real-World Applications#

Diodes are everywhere in electronic circuits. Here are some key ways they are used:

Radio Demodulation: In old radios (and some modern circuits), a diode acts as a simple rectifier to separate the audio information (the “envelope”) from the high-frequency radio wave.
Power Conversion (Rectification): This is huge. Almost every electronic device that plugs into AC wall power uses diodes to convert that AC into the DC voltage needed to run the internal circuits. This can be done with a single diode (half-wave rectification) or multiple diodes (full-wave rectification, often using a diode bridge). Diodes are also used in circuits that multiply AC voltages to get higher DC voltages (Cockcroft–Walton multipliers). Car alternators use diodes to convert the AC voltage they generate into DC to charge the battery and power the car’s electrical system.
Protecting Circuits:
- Reverse Polarity Protection: A diode placed in series with the power input can prevent damage if the power supply is accidentally connected backward. Current just won’t flow.
- Over-voltage Protection: Diodes can steer damaging voltage spikes away from sensitive parts. For example, flyback diodes are used across relay coils or motors. When the coil is suddenly switched off, it generates a large voltage spike. The flyback diode provides a path for this current to flow harmlessly, preventing the spike from damaging the switch (like a transistor). Specialized TVS diodes are designed specifically for this job.
Simple Logic Gates: Diodes, combined with resistors, can form basic logic gates like AND and OR. While more complex logic uses transistors, diodes are still fundamental in understanding logic circuits.
Detecting Radiation: Certain semiconductor diodes, especially PIN diodes with thick intrinsic layers, can be used to detect high-energy particles or radiation. When radiation hits the semiconductor material, it creates electron-hole pairs, which are then collected as a signal.
Temperature Sensing: The small voltage drop across a forward-biased diode changes predictably with temperature (usually dropping by about 2 mV for every degree Celsius increase for silicon diodes). This effect is used in some temperature sensor circuits.
Steering Current: Diodes ensure current flows only where you want it to go.
- Battery Charging/Backup: In systems with multiple power sources (like a main power adapter and a backup battery), diodes can prevent the battery from trying to power the adapter, or prevent one battery from discharging into another, ensuring power is drawn only from the active source or the battery when needed (like in an uninterruptible power supply, UPS).
- Keyboard Matrices: In electronic keyboards (musical or computer), diodes are placed with each key switch. This prevents “ghosting” or “phantom keys,” where pressing multiple keys at once could trick the circuit into thinking other keys were pressed due to current flowing backward through the grid of wires. Pinball machines use a similar setup for their switch matrix.
Waveform Shaping:
- Clipping: Diodes can be used to “clip” or limit the maximum positive or negative voltage of a signal to a specific level.
- Clamping: Diode clamp circuits can shift the entire AC signal up or down so that either the positive or negative peaks sit at a certain DC voltage level.
Analog Computation: The exponential current-voltage relationship of a diode can be used in analog circuits (often with operational amplifiers) to perform mathematical operations like computing the exponential or logarithm of a voltage signal.

Shorthand for Diodes#

When you look at the markings on a circuit board (PCB), diodes are usually labeled with the letter ‘D’. Sometimes, you might see ‘CR’, which stands for Crystal Rectifier, a holdover from older terminology.

So, that’s the diode! A seemingly simple two-terminal device, but incredibly fundamental and versatile in the world of electrical engineering, acting as a critical one-way street for current in countless applications.