Zener and Tunnel Diodes

Two diodes that are useful precisely because they do what an ordinary diode is not supposed to do.

Reverse breakdown, by two different mechanisms

Push the reverse voltage high enough and current rises sharply. Two distinct physical processes cause it, and the syllabus asks for both.

Zener breakdown dominates below about 5 V and requires heavy doping on both sides. Heavy doping makes the depletion region very thin — a few nanometres — so a modest reverse voltage produces an enormous field, of order $10^{8}$ V/m. That field tears electrons directly out of covalent bonds, or equivalently lets them tunnel across the thin barrier (Module 2). No collision is involved.

Avalanche breakdown dominates above about 7 V, in more lightly doped junctions with wider depletion regions. A minority carrier is accelerated by the field, gains enough kinetic energy to knock an electron out of a bond on impact, and the two carriers are then accelerated and knock out two more. The multiplication is the avalanche.

A distinguishing test: the temperature coefficient.

• Zener breakdown voltage decreases with temperature. Heating raises the energy of bonded electrons, so a slightly smaller field suffices to free them.
• Avalanche breakdown voltage increases with temperature. Heating increases lattice vibration, so carriers scatter more often and travel a shorter distance between collisions, gaining less energy — a larger field is needed to reach ionising energy.

The coefficients being opposite in sign gives a useful design trick: near 5–6 V both mechanisms operate and the coefficients partly cancel, so a diode in that range has the best voltage stability with temperature. This is why 5.6 V references are so common.

Note the naming is loose in practice: every breakdown diode is sold as a "Zener diode" whatever its actual mechanism, and a 12 V "Zener" almost certainly breaks down by avalanche.

The Zener characteristic and voltage regulation

Forward, it behaves as an ordinary diode. Reverse, it passes only $I_0$ until $V_Z$, where the current rises almost vertically.

That near-vertical region is the useful part: large current changes produce almost no voltage change. The diode holds $V_Z$ across itself over a wide current range.

To regulate, put a series resistor between supply and load and the Zener across the load, reverse biased. The resistor absorbs the difference:

If the supply rises, more current flows through the resistor and the extra is diverted through the Zener; the load voltage stays at $V_Z$. If the load draws more, the Zener gives up current to compensate.

Two conditions must hold. The Zener needs a minimum current (typically 5–10 mA) to stay in breakdown — below it, regulation collapses. And it must not exceed its power rating, $P = V_Z I_Z$; the worst case is maximum input voltage with the load disconnected, because then the Zener carries all the current.

Breakdown itself does not damage the diode. Excess power does.

The tunnel diode

Take a p-n junction and dope both sides so heavily that they are degenerate — the Fermi level lies inside the conduction band on the n side and inside the valence band on the p side. The depletion region becomes extremely thin, around 10 nm.

At that thickness electrons tunnel straight through the barrier rather than climbing it, and the result is a characteristic no ordinary diode has:

1. Small forward bias: filled n-side states align with empty p-side states at the same energy. Tunnelling current rises steeply. This happens near zero volts, far below the usual 0.7 V knee.
2. Peak at $V_P$, typically 50–60 mV.
3. Negative resistance region: increasing the voltage misaligns the bands, so fewer filled states face empty states. Tunnelling falls. Current decreases as voltage increases — the defining feature.
4. Valley at $V_V$, typically 350 mV.
5. Beyond the valley, normal diffusion current takes over and the curve rises like an ordinary diode.

Why negative resistance is valuable. A normal resistance dissipates energy; a negative one supplies it, cancelling losses in a tuned circuit. A tunnel diode can therefore sustain oscillation on its own — no transistor required. Since tunnelling is essentially instantaneous, it works at microwave frequencies where conventional devices of its era could not.

The peak-to-valley current ratio $I_P/I_V$ measures quality; typical values are 5–10.

Tunnel diodes are now largely historical, displaced by better transistors, but they remain the clearest example of quantum tunnelling visible on an ordinary oscilloscope.