Junction Diode

Part of Semiconductors

The p-n junction diode — structure, operation, and practical applications as a one-way current valve.

Why This Matters

The junction diode is the simplest semiconductor device and the foundation for all others. A piece of p-type semiconductor bonded to a piece of n-type semiconductor creates a junction that passes current freely in one direction and blocks it in the other. This asymmetric behavior — rectification — is useful for converting AC to DC, detecting radio signals, protecting circuits from reverse polarity, and generating reference voltages.

For a rebuilding civilization, the diode is likely the first semiconductor device fabricated and tested. Its two-terminal simplicity makes it easier to make and test than a transistor. A successful batch of diodes demonstrates that your semiconductor material is adequate, your junction formation technique works, and your purity is sufficient. It is a milestone that validates the entire semiconductor program before moving on to the more demanding three-terminal devices.

Understanding the junction diode deeply — not just the empirical rule that it conducts one way — provides the conceptual foundation for understanding transistors, solar cells, LEDs, and all p-n junction-based devices. The same physical mechanisms (depletion zone, built-in field, minority carrier injection) appear in every device.

P-N Junction Formation and Built-In Potential

When p-type and n-type semiconductor are brought into contact (or created adjacent in the same crystal), carriers diffuse across the boundary. Electrons from the n-side diffuse to the p-side; holes from the p-side diffuse to the n-side. This diffusion leaves behind ionized donor atoms (positive, in n-side) and ionized acceptor atoms (negative, in p-side) near the junction — a region depleted of mobile carriers called the depletion zone.

The ionized atoms create an electric field pointing from n to p (from positive to negative charges). This field opposes further diffusion: carriers trying to cross the junction must work against it. Equilibrium is reached when the diffusion tendency exactly balances the field repulsion. The potential difference across the depletion zone is the built-in potential (V_bi): about 0.3V for germanium and 0.7V for silicon.

This built-in field is the key to diode operation. No external current flows at equilibrium because diffusion current and drift current exactly cancel. The depletion zone is a thin insulating barrier at the junction — about 0.1-1 µm wide depending on doping.

Forward and Reverse Bias Operation

Forward bias: Apply external voltage with positive terminal to p-side, negative to n-side. This voltage opposes the built-in potential. The effective barrier across the depletion zone is reduced from V_bi to (V_bi - V_applied). The depletion zone narrows. Carriers can now cross the junction more easily. Exponential current increase results — doubling roughly every 60 mV for silicon (ideality n=1).

Once forward voltage exceeds ~0.7V for silicon (0.3V for germanium), the barrier is effectively overcome and current is limited only by external resistance. The diode voltage clamps at this value regardless of current — useful as a voltage reference.

Reverse bias: Apply voltage with positive terminal to n-side, negative to p-side. This reinforces the built-in potential. The barrier increases; the depletion zone widens. Majority carriers cannot cross. Only thermally generated minority carriers (electrons on p-side, holes on n-side) can be swept across by the strong field. This reverse saturation current I_s is very small — nanoamps for silicon at room temperature, microamps for germanium (larger due to smaller band gap generating more thermal carriers).

I_s doubles approximately every 10°C rise — the exponential temperature dependence of intrinsic carrier generation. This is why germanium diodes become leaky when warm.

Breakdown: At high reverse voltage, one of two mechanisms causes sharp current increase. Avalanche breakdown: carriers crossing the depletion zone gain enough energy to ionize lattice atoms, creating more carriers in a chain reaction. Zener breakdown: quantum tunneling directly across the thin depletion zone (dominant for heavily doped junctions, reverse voltages below ~5V). Both are exploited in Zener diode voltage references.

Diode Fabrication Methods

Alloy junction: Melt a small dot of p-type metal (indium for n-type germanium base) into the semiconductor surface. On cooling, the recrystallized alloy region is heavily p-type. The germanium adjacent to it forms a p-n junction. Simple to make; junction depth controlled by temperature and time. The first practical production method.

Diffusion: Heat semiconductor in atmosphere of dopant vapor. Dopant atoms diffuse into surface to a controlled depth. A p-type surface layer on n-type bulk creates a planar junction. Requires temperature-controlled furnace and dopant source (boron tribromide vapor for p-type silicon).

Grown junction: Change dopant type during crystal growth (Czochralski method). For diodes, simpler to use alloy or diffusion.

Point-contact: A fine wire pressed against semiconductor surface. The pressure and a forming current pulse create a small p-n junction at the contact. Crude but functionally adequate for signal detection. The original “cat’s whisker” detector.

Practical Applications and Circuit Use

Rectifier (AC to DC): Full-wave bridge rectifier uses four diodes. Each half-cycle of AC, two diodes conduct in series; the other two are reverse biased. Output is pulsating DC, smoothed by a filter capacitor. For a 12V AC transformer output with silicon diodes, peak DC voltage ≈ 16.9V minus two diode drops ≈ 15.5V before filter capacitor.

Clipping and clamping: Diodes clip signal peaks when they conduct. A diode from signal to supply rail clips positive peaks at supply voltage plus diode drop. Useful in audio limiters and logic level converters.

Protection: A diode across an inductive load (relay, motor) in reverse polarity clamps the voltage spike generated when current is suddenly cut off. Without this “flyback diode,” the spike destroys the driving transistor.

Signal detection (envelope detection): A diode in series with an RC network demodulates AM radio signals. The diode passes the positive envelope of the carrier; RC smoothing removes the carrier frequency, leaving the audio signal. The crystal radio and early semiconductor radios used this principle.

Zener reference: A Zener diode reverse biased through a resistor maintains constant voltage across it regardless of current variations. A 5.6V Zener with 470Ω series resistor from 9V supply gives a stable ~5.6V reference — useful as a voltage regulator for logic circuits.

Forward voltage reference: Silicon diodes have well-defined forward voltage temperature coefficients (approximately -2 mV/°C). Two diodes in series give 1.4V reference with -4 mV/°C coefficient, useful as a temperature sensor or as the bias for bipolar transistor temperature compensation.

Testing and Quality Assessment

Test each fabricated diode before use:

1. Continuity check: Low resistance one way (~10-1000 Ω), high resistance reverse (~1 MΩ minimum). Any device with low resistance both ways is shorted. Any device with high resistance both ways is open.

2. Forward voltage: At 1 mA, silicon should be 0.55-0.75V, germanium 0.15-0.35V. Outside these ranges indicates wrong material identification or heavily contaminated junction.

3. Leakage current: At rated reverse voltage, measure reverse current. Silicon: <1 µA acceptable. Germanium: <10 µA acceptable, <50 µA marginal, >100 µA reject.

4. Breakdown voltage: Gradually increase reverse voltage until sharp current increase. Record this voltage. Must exceed maximum circuit reverse voltage by at least 20% margin.

5. Temperature test: Warm the diode in your hand (~35°C) and retest leakage. Germanium leakage increasing by 2-5× over 10°C rise is normal. Greater than 10× increase indicates marginal junction quality.

Sort diodes by measured forward voltage at 1 mA (within 20 mV lots) for use in matched pairs in bridge rectifiers and differential circuits. Sort by leakage for temperature-sensitive applications. Keep a logbook with each batch’s statistics.