Diode

Definition: A diode is a semiconductor component that allows current to flow easily in one direction while blocking it in the other.

How It Works

It’s built from a single p-n junction: a p-type region joined to an n-type region within one semiconductor crystal
Current flows when the p-side (anode) is more positive than the n-side (cathode), called forward bias
In forward bias, the junction’s depletion region narrows and majority carriers cross freely once a threshold voltage is exceeded
Current is blocked when the polarity is reversed, called reverse bias, since the depletion region widens and carriers can’t cross
Beyond a small forward voltage drop, typically 0.6-0.7V for silicon and about 0.2-0.3V for germanium, the diode conducts with very low resistance
LEDs (light-emitting diodes) release energy as photons instead of heat when carriers recombine at the junction, with color set by the semiconductor’s bandgap
Zener diodes are deliberately operated in a controlled reverse-breakdown region to hold a fixed reference voltage
Schottky diodes use a metal-semiconductor junction instead of a p-n junction, giving a lower forward voltage drop and faster switching
Photodiodes work in reverse: light striking the junction generates carriers, producing a small current or voltage proportional to light intensity
Varactor (varicap) diodes exploit the depletion region’s capacitance, which changes with reverse voltage, letting them act as voltage-tunable capacitors in radio circuits
Reverse recovery time describes how quickly a diode stops conducting after forward current is removed, an important spec in fast-switching power supplies
Tunnel diodes exploit a quantum-mechanical effect to produce a region of negative resistance, useful in high-frequency oscillators, though now a niche application

Illustration

Under the Hood

The Shockley diode equation:

I = Is × (e^(V/(n×VT)) - 1)

Is: reverse saturation current (very small, often nanoamps)
VT: thermal voltage, about 0.026V at room temperature
n: ideality factor, typically 1-2

Simplified practical model for hand calculations, treating the diode as a fixed 0.7V drop when conducting:

Iforward = (Vsupply - 0.7V) / Rseries

Worked Problem 1: Current-limiting resistor for an LED Given: A 9V supply drives a red LED with a 2V forward drop, target current 15 mA (0.015A). Step 1: Voltage across the resistor = 9 - 2 = 7V Step 2: R = V / I = 7 / 0.015 ≈ 466.7Ω Answer: A 470Ω resistor (nearest standard value) limits the LED current to approximately 14.9 mA.

Worked Problem 2: Diode drop in a simple circuit Given: A 5V supply feeds a silicon diode (0.7V drop) in series with a 220Ω resistor. Step 1: Voltage across the resistor = 5 - 0.7 = 4.3V Step 2: I = 4.3 / 220 ≈ 0.01955 A Answer: The circuit draws about 19.5 mA through the diode and resistor.

Worked Problem 3: Zener regulator check Given: A 5.1V Zener diode is fed from 9V through a 100Ω series resistor, with a 10 mA load. Step 1: Voltage across the resistor = 9 - 5.1 = 3.9V Step 2: Total current through the resistor = 3.9 / 100 = 0.039 A (39 mA) Answer: Since load draws 10 mA, the Zener itself carries 39 - 10 = 29 mA to maintain regulation, well within a typical 500 mW Zener’s rating at 5.1V (about 98 mA max).

Worked Problem 4: Full-wave bridge rectifier output Given: A bridge rectifier fed from a 12V RMS AC transformer secondary, with two diode drops in the current path at any instant (0.7V each). Step 1: Peak AC voltage = Vrms × √2 = 12 × 1.414 ≈ 16.97V Step 2: Peak rectified output = 16.97 - (2 × 0.7) = 16.97 - 1.4 ≈ 15.57V Answer: Before filtering, the rectifier produces pulsing DC peaking around 15.6V, which a smoothing capacitor then flattens.

Why It Matters

Diodes are used to rectify AC to DC, the essential first step in nearly every DC power supply
They protect circuits from reverse voltage, such as a battery inserted backward or inductive kickback from a relay coil
They set fixed reference voltages (Zener diodes) for voltage regulation and overvoltage protection
LEDs built from diode junctions have replaced incandescent bulbs across lighting, displays, and indicators due to their efficiency and long lifespan
Photodiodes and phototransistors form the sensing half of countless optical systems, from fiber-optic receivers to remote control detectors
Diodes enable logic-level voltage clamping and ESD protection on sensitive IC input pins, quietly preventing damage from static discharge or overvoltage spikes
Solar cells are essentially large-area photodiodes optimized to convert sunlight into usable current rather than to sense small light changes

Common Pitfalls

Reversing a diode’s polarity in a circuit, which either blocks current entirely or, in the case of exceeding reverse breakdown voltage, destroys the diode
Forgetting a flyback (freewheeling) diode across an inductive load like a relay or motor, letting the collapsing magnetic field generate a damaging voltage spike
Omitting a current-limiting resistor for an LED, since an LED’s exponential current-voltage curve means a tiny overvoltage causes a huge overcurrent
Assuming all diodes have the same forward voltage drop, when silicon, germanium, Schottky, and LED diodes differ significantly
Exceeding a diode’s maximum reverse voltage (breakdown voltage) or forward current rating, causing permanent damage
Using a standard diode where a fast-recovery or Schottky diode was needed, causing switching losses or slow response in high-frequency circuits
Overlooking a diode’s temperature sensitivity, forward voltage drop decreases as temperature rises, which matters in precision reference and thermal-compensation designs
Assuming a diode blocks reverse current perfectly, when a small reverse leakage current always flows and increases with temperature
Soldering a heat-sensitive diode without a heat sink clip, risking thermal damage to the junction during assembly

Comparison

Type	Forward Drop	Key Trait	Common Use
Silicon rectifier	~0.6-0.7V	General purpose, robust	AC-DC rectification
Schottky	~0.2-0.4V	Fast switching, lower drop	High-frequency, efficient power supplies
Zener	Set by breakdown voltage	Operates in controlled reverse breakdown	Voltage regulation, reference
LED	~1.8-3.3V (varies by color)	Emits light	Indicators, lighting, displays
Germanium	~0.2-0.3V	Low drop, more temperature-sensitive	Vintage radios, some RF detectors
Photodiode	Varies (reverse-biased)	Generates current from light	Optical sensors, fiber-optic receivers

Example

A bridge rectifier made of four diodes converts AC wall power into pulsing DC, which a capacitor then smooths and a regulator stabilizes, the basic front end of most phone chargers and wall-wart power supplies.

History

Early diodes were vacuum tube devices, invented by John Ambrose Fleming in 1904 as the “Fleming valve,” used to detect radio signals.
“Cat’s whisker” crystal detectors, using a fine wire touching a galena crystal, exploited a primitive semiconductor rectifying effect in early 20th-century radios.
The modern semiconductor p-n junction diode became practical alongside transistor development at Bell Labs in the late 1940s.
Nick Holonyak Jr. created the first practical visible-spectrum LED in 1962, starting the path toward today’s efficient solid-state lighting.
Shuji Nakamura’s development of practical blue LEDs in the early 1990s completed the RGB set needed for white LED lighting and displays, earning him a share of the 2014 Nobel Prize in Physics.

FAQ

Why do diodes have a “forward voltage drop” instead of zero resistance when conducting? The junction requires a minimum energy to let carriers cross the depletion region, this shows up electrically as a threshold voltage. Above that threshold, the diode’s effective resistance is low but never exactly zero.

What happens if you exceed a diode’s reverse breakdown voltage? Ordinary diodes are damaged or destroyed by excessive reverse current once breakdown occurs. Zener and avalanche diodes are specifically designed to survive and regulate voltage in that same regime.

Why does an LED need a series resistor but an incandescent bulb doesn’t? An LED’s current rises exponentially with only a small increase in voltage past its forward drop, so an unregulated supply can destroy it almost instantly. An incandescent bulb’s filament resistance rises with temperature, which naturally self-limits its current.

Can a diode be used as a simple voltage reference without a resistor? No, a Zener or reference diode still needs a series resistor to limit current from the supply, the diode alone would let excessive current flow and burn out.

Why do LEDs of different colors have different forward voltages? LED color is set by the semiconductor material’s bandgap energy, and forward voltage roughly tracks that bandgap. Blue and white LEDs use wider-bandgap materials and need higher forward voltage than red or infrared LEDs.

Is a diode the same as a rectifier? A rectifier is a circuit function, converting AC to DC, usually built from one or more diodes. “Diode” refers to the component itself, which can be used for rectification or many other purposes like clamping, protection, or reference voltage.

What does the band printed on a diode’s body mean? It marks the cathode lead, the terminal current exits when the diode is forward biased. Connecting it backward in a circuit that expects forward conduction will simply block current rather than pass it.

Why do some diodes get warm even though the forward drop is small? Power dissipation in a diode is forward voltage times forward current, and in high-current applications even a small 0.5-0.7V drop multiplied by several amps adds up to real heat, which is why rectifier diodes in power supplies often need heat sinking.

Why do some circuits use a diode purely for protection, with no other apparent purpose? A reverse-biased diode placed across a supply input, or in the “wrong” orientation relative to normal current flow, conducts only during a fault condition like reversed battery polarity, silently protecting the rest of the circuit without affecting normal operation.

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