Special Purpose Diodes

14.8 Special-Purpose p-n Junction Diodes

Beyond rectification, the p-n junction can be engineered to perform a variety of specialised functions. By tuning the doping levels, the band gap of the semiconductor and the device geometry, we get four important "special diodes":

14.8.1 Zener Diode — Voltage Regulator

A Zener diode is a heavily doped p-n junction designed to operate in the breakdown region under reverse bias. Two distinct breakdown mechanisms can occur:

• Zener breakdown (heavy doping, V_br < 6 V): the very thin depletion region produces a huge electric field that rips electrons out of covalent bonds. Tunnelling current rises sharply.
• Avalanche breakdown (lighter doping, V_br > 6 V): high-energy minority carriers ionise lattice atoms by impact, multiplying the carrier population.

Both produce the same effect: once V exceeds V_z, the diode current can change by orders of magnitude while V across the diode remains nearly constant at V_z. This is the property exploited for voltage regulation.

How regulation works

If V_in increases, the extra voltage drops across R_s while V across Zener stays at V_z. If V_in decreases below V_z, the Zener stops conducting and acts like an open circuit — so V_in must always exceed V_z. The Zener current adjusts itself to keep V_out fixed.

Worked Example — Designing a regulator

14.8.2 Optoelectronic Devices — Photodiode and LED

Both rely on the relationship: energy of a photon = E_g of the semiconductor. If hν ≥ E_g, a photon striking the depletion region creates an electron-hole pair (light → current). Conversely, an electron-hole pair recombining releases a photon of energy ≈ E_g (current → light).

Photodiode

A photodiode is a junction diode operated under reverse bias and exposed to light. The transparent window above the junction lets photons reach the depletion region. Each absorbed photon creates an e-h pair which the field separates, producing additional reverse current proportional to the light intensity.

Why operate under reverse bias and not no-bias? Because reverse bias gives a wider depletion region, and the change in fractional current is much larger and more easily measured. Photodiodes are used in TV remote receivers, light meters, optical fibre detectors and barcode scanners.

Light-Emitting Diode (LED)

An LED is a heavily doped p-n junction operated under forward bias. Injected minority carriers recombine with majority carriers near the junction; the released energy emerges as a photon of wavelength λ ≈ hc/E_g.

The colour of an LED is set by the band gap of the semiconductor:

LEDs have several advantages over incandescent bulbs: low operating voltage, low power, long life (~10⁵ h), fast switching, no warm-up time, ruggedness. They have replaced filament bulbs in nearly every general-lighting application.

14.8.3 Solar Cell

A solar cell is a large-area p-n junction that converts sunlight directly into a DC voltage without any external bias. Sunlight (with photon energies above E_g) creates electron-hole pairs in the depletion region; the built-in field separates them — electrons drift to n-side, holes to p-side. This separation makes the n-side the (−) terminal and the p-side the (+) terminal, generating an EMF.

Design considerations:

• The semiconductor must have a band gap matched to the solar spectrum (~1.0-1.8 eV is optimal). Si (1.12 eV) and GaAs (1.43 eV) are common.
• The top junction must be very near the illuminated surface so photons reach the depletion region.
• Anti-reflection coatings reduce surface losses; metal contact grids let light in while collecting current.

Comparison Table — All Four Special Diodes

Worked Example — LED wavelength

Interactive — Zener Voltage Regulator