4.1.1 — Diodes

Diodes are the simplest semiconductor devices and form the building blocks of all modern electronics. This section covers the physics of semiconductor materials, how PN junctions work, and the wide variety of diode types used in aircraft electronic systems — from simple rectifiers to Zener voltage regulators, LEDs for cockpit lighting, and protective varistors.

Semiconductor Materials and Electron Configuration

All materials can be classified by their electrical conductivity into three groups:

Category	Resistivity	Examples	Valence Electrons
Conductors	Very low	Copper, aluminium, silver	1–2 (loosely held)
Semiconductors	Between conductors and insulators	Silicon (Si), germanium (Ge)	4
Insulators	Very high	Glass, rubber, ceramic	7–8 (tightly held)

Silicon is the most widely used semiconductor in modern electronics. Each silicon atom has 4 valence electrons in its outer shell and forms covalent bonds with 4 neighbouring atoms, creating a crystal lattice structure. In its pure (intrinsic) state, silicon is a poor conductor because almost all electrons are locked in bonds.

The key property that makes semiconductors useful is that their conductivity can be precisely controlled by adding tiny amounts of impurity atoms — a process called doping.

Energy Bands

In any solid, electrons occupy energy levels grouped into bands:

• Valence band — the highest energy band that is fully occupied at absolute zero. Electrons here are bound to atoms.
• Conduction band — the next higher band. Electrons here are free to move and carry current.
• Band gap — the energy gap between the valence and conduction bands. In semiconductors, this gap is small enough (about 1.1 eV for silicon) that thermal energy or an applied voltage can push electrons across it.

In conductors, the valence and conduction bands overlap (no gap). In insulators, the band gap is very large (5+ eV). In semiconductors, the small band gap gives them their unique, controllable behaviour.

P-Type and N-Type Materials

N-Type Semiconductor

When a pentavalent (5 valence electrons) impurity such as phosphorus (P), arsenic (As), or antimony (Sb) is added to silicon, four of its electrons bond with surrounding silicon atoms. The fifth electron is free to move and conduct current.

• The impurity atom is called a donor (it donates a free electron)
• Majority carriers: electrons (negative)
• Minority carriers: holes
• The material is electrically neutral overall (the donor atom has an equal positive charge in its nucleus)

P-Type Semiconductor

When a trivalent (3 valence electrons) impurity such as boron (B), gallium (Ga), or indium (In) is added to silicon, it can only form three bonds. The missing bond creates a hole — an absence of an electron that behaves as a positive charge carrier.

• The impurity atom is called an acceptor (it accepts an electron to fill the hole)
• Majority carriers: holes (positive)
• Minority carriers: electrons
• The material is electrically neutral overall

The PN Junction

When P-type and N-type materials are joined, a PN junction is formed — this is the heart of every diode. At the junction, free electrons from the N-side diffuse across and fill holes on the P-side. This creates a thin region with no free carriers called the depletion region (or depletion zone).

The exposed fixed ions on each side of the junction create a small potential barrier (about 0.6 V for silicon, 0.3 V for germanium). This barrier opposes further diffusion and establishes equilibrium. No current flows through an unbiased PN junction.

Forward and Reverse Bias

Forward Bias

Connect the positive terminal of a battery to the P-side and the negative terminal to the N-side. This pushes holes toward the junction from the P-side and electrons toward the junction from the N-side, narrowing the depletion region. Once the applied voltage exceeds the barrier potential (~0.6 V for Si), current flows freely through the junction.

Reverse Bias

Connect the positive terminal to the N-side and the negative terminal to the P-side. This pulls carriers away from the junction, widening the depletion region. Only a tiny leakage current (due to minority carriers) flows. The junction effectively blocks current.

If the reverse voltage exceeds the breakdown voltage, the junction conducts heavily in reverse — this is destructive in normal diodes but is exploited in Zener diodes.

Diode Characteristics and Parameters

Important Diode Parameters

Parameter	Description
Forward voltage drop (VF)	Voltage across the diode when conducting: ~0.6 V for Si, ~0.3 V for Ge, ~0.2 V for Schottky
Peak Inverse Voltage (PIV)	Maximum reverse voltage the diode can withstand without breakdown
Maximum forward current (IF max)	Maximum continuous current in forward direction before damage
Leakage current (IR)	Small current that flows in reverse bias (due to minority carriers); increases with temperature
Power dissipation	\( P = V_F \times I_F \). Must not exceed the diode's thermal rating.
Reverse recovery time	Time to switch from forward conduction to reverse blocking. Important at high frequencies.
Temperature sensitivity	VF decreases ~2 mV/°C; leakage current roughly doubles for every 10°C rise

Diode Symbols

In all diode symbols, the triangle (arrow) points from anode (A) to cathode (K), showing the direction of conventional current flow. The cathode is the bar (line). On a physical diode, the cathode is typically marked with a band.

Diodes in Rectifier Circuits

The most common application of diodes is rectification — converting alternating current (AC) to direct current (DC). This is essential in aircraft power supplies, battery chargers, and electronic systems.

Half-Wave Rectifier

A single diode passes only the positive half-cycles of the AC input and blocks the negative half-cycles. The output is pulsating DC with significant ripple.

Disadvantages: Only 50% of the input energy is used. The ripple frequency equals the supply frequency. Not efficient for most applications.

Full-Wave Rectifier (Centre-Tapped Transformer)

Uses two diodes and a centre-tapped transformer. Each diode conducts on alternate half-cycles, so both halves of the AC input contribute to the output. The ripple frequency is double the supply frequency, making filtering easier.

Bridge Rectifier

The most common full-wave rectifier uses four diodes in a bridge arrangement. It does not require a centre-tapped transformer and is the standard circuit in most power supplies.

During the positive half-cycle, D1 and D4 conduct. During the negative half-cycle, D2 and D3 conduct. The load always receives current in the same direction.

Clippers, Clampers, and Voltage Multipliers

Clippers (Limiters)

A clipper circuit removes (clips) a portion of a waveform above or below a certain level. A diode conducts when the signal exceeds the threshold, diverting the excess away from the load.

• Positive clipper — removes the positive portion above a set voltage
• Negative clipper — removes the negative portion below a set voltage
• Biased clipper — a DC voltage in series with the diode sets the clipping level

Clippers are used for waveform shaping and protecting sensitive circuits from voltage spikes.

Clampers (DC Restorers)

A clamper shifts the entire waveform up or down by adding a DC offset, without changing its shape. It consists of a capacitor and a diode. The capacitor charges to the peak value, then adds this as a DC level.

• Positive clamper — shifts the waveform upward so its negative peak sits at 0 V (or another reference)
• Negative clamper — shifts the waveform downward

Voltage Doublers and Triplers

Voltage multiplier circuits use combinations of diodes and capacitors to produce a DC output that is a multiple of the AC peak input voltage. A voltage doubler produces approximately 2 × V_peak; a voltage tripler produces approximately 3 × V_peak. They are used where high DC voltages are needed from a low AC supply, at relatively low current.

Special Diode Types

Zener Diode

A Zener diode is designed to operate in reverse breakdown at a specific, stable voltage (the Zener voltage, V_Z). When reverse voltage reaches V_Z, the diode conducts freely in reverse, maintaining a near-constant voltage across itself regardless of current variations.

Primary use: Voltage regulation. A Zener diode in parallel with a load provides a stable reference voltage even when the input voltage or load current fluctuates. Common Zener voltages: 3.3 V, 5.1 V, 6.8 V, 12 V, 15 V, 24 V.

Silicon Controlled Rectifier (SCR / Thyristor)

An SCR is a four-layer (PNPN) device with three terminals: anode, cathode, and gate. It acts as a latching switch:

• It remains OFF (non-conducting) until a small current pulse is applied to the gate
• Once triggered, it latches ON and conducts like a normal diode (gate loses control)
• It turns OFF only when the anode current falls below a minimum value (the holding current) — typically at the zero crossing of an AC waveform

Uses: Power control, motor speed control, lighting dimmers, overvoltage protection (crowbar circuits). In aviation, SCRs are used in TRUs and generator control units (GCUs).

Light Emitting Diode (LED)

An LED emits light when forward-biased. Electrons recombining with holes release energy as photons. The colour depends on the semiconductor material:

Colour	Material	Forward Voltage
Red	GaAsP	~1.8 V
Green	GaP	~2.2 V
Blue	InGaN	~3.3 V
White	Blue LED + phosphor	~3.3 V

Aviation uses: Cockpit indicator lights, annunciator panels, position lights (navigation lights), landing lights on modern aircraft, panel backlighting. LEDs offer long life, low heat, and high reliability compared to incandescent bulbs.

Schottky Diode

Uses a metal-semiconductor junction instead of a PN junction. Key advantages:

• Very low forward voltage drop (~0.2–0.3 V vs 0.6 V for standard silicon)
• Very fast switching speed (no minority carrier storage)
• Used in high-frequency circuits, switching power supplies, and as clamping/protection diodes

Photodiode

A photodiode is operated in reverse bias. When light falls on the junction, it generates electron-hole pairs, increasing the reverse (leakage) current in proportion to light intensity. Used in optical sensors, smoke detectors, fibre-optic receivers, and light meters.

Varactor (Varicap) Diode

A varactor is operated in reverse bias. The width of the depletion region (and therefore its capacitance) changes with the applied reverse voltage. Used as a voltage-controlled capacitor in automatic frequency control (AFC) circuits, FM modulators, tuners, and phase-locked loops (PLLs).

Varistor (MOV — Metal Oxide Varistor)

A varistor is a voltage-dependent resistor. At normal voltages, its resistance is very high (effectively open circuit). When the voltage exceeds a threshold (the clamping voltage), its resistance drops sharply and it conducts, diverting the surge energy safely to ground.

Use: Transient voltage suppression. Protects sensitive electronic circuits from voltage spikes caused by lightning, switching, or static discharge. Essential in aircraft avionics protection.

Diodes in Series and Parallel

Series Connection

Diodes connected in series increase the total reverse voltage capability: the PIV rating adds up. However, the forward voltage drops also add up. Used when the reverse voltage exceeds the rating of a single diode.

Parallel Connection

Diodes connected in parallel increase the total current-carrying capability. However, because no two diodes have exactly the same V_F, current will not divide equally. Small series resistors (ballast resistors) are added to ensure equal current sharing.

Functional Testing of Diodes

A diode can be tested with a multimeter set to the diode test mode:

Test	Connection	Good Diode Reading
Forward bias	Red lead → Anode, Black lead → Cathode	0.5–0.7 V (silicon) or 0.2–0.3 V (germanium/Schottky)
Reverse bias	Red lead → Cathode, Black lead → Anode	"OL" (open line / infinite resistance)

Fault indications:

• Short circuit: Low reading (near 0 V) in both directions
• Open circuit: "OL" reading in both directions
• Leaky: Some reading in reverse direction (should be OL)