Special-Purpose Diodes: Schottky, Tunnel, LED, Laser, Photodiode, and Solar Cell Innovations

Schottky Diodes

Schottky diodes are metal‑to‑N junction devices, also known as hot‑carrier diodes. They offer exceptionally fast switching, a low reverse‑recovery time, a forward voltage drop of 0.25–0.4 V for a metal‑silicon junction, and minimal junction capacitance. The schematic symbol is shown below.

Advantages & Limitations

• Lower forward drop and reverse‑recovery time than standard rectifiers → ideal for high‑frequency circuits.
• Higher reverse‑breakdown voltage and current ratings are required for power applications; they are best suited for low‑power switching regulators.

Applications

• High‑speed computer logic and signal processing.
• Switching regulators operating at 100 kHz+ where silicon diodes’ 0.7 V drop would waste power.
• Low‑voltage supplies (e.g., 1 V) where a 0.7 V drop is unacceptable.

Tunnel (Esaki) Diodes

Tunnel diodes exploit resonant tunneling, producing a negative‑resistance region in their forward‑bias characteristic. When a small forward voltage is applied, current rises to a peak (IP), then falls to a valley (IV), and rises again. The forward voltages at the peak and valley are VP and VV, respectively.

Key Characteristics

• Extremely heavy doping (~1000× higher than rectifiers) → thin depletion region, low breakdown voltage, high capacitance.
• Negative‑resistance region is temperature‑stable, enabling fast switching beyond Schottky diodes.
• Small junction area is required to limit capacitance.

Typical Use Cases

• High‑frequency oscillators (up to 5 GHz) where the diode’s negative resistance drives an LC tank.
• Special microwave circuits and early satellite transmitters (1960s–70s).

Light‑Emitting Diodes (LEDs)

Principle of Emission

LEDs generate photons when electrons recombine with holes in a PN junction. The emitted wavelength is set by the semiconductor bandgap; common combinations include GaAsP (red), GaN (blue), and phosphor‑converted blue (white).

Electroluminescence & Forward Voltage

• Forward drop ranges from 1.6 V (infrared) to >3 V (violet/UV).
• Typical operating current: ~20 mA for standard LEDs; larger devices may use higher currents.
• Series resistors are required when driving from a supply higher than the LED’s Vf to limit current.

Example: With a 6 V supply and a 1.6 V LED, a 220 Ω resistor drops 4.4 V at 20 mA, dissipating 88 mW.

Multiple LEDs

• Parallel strings require individual current‑limiting resistors.
• Series strings share a single resistor but the supply must support the summed Vf.
• Series‑parallel configurations balance efficiency and brightness.

Reverse‑Bias Protection

LEDs have low reverse‑breakdown voltage (~5 V). When powered by AC, an anti‑parallel diode or LED protects the device. The AC half‑cycles then light two LEDs in opposite directions.

Specifications Table

LED	λ (nm)	Vf (from)	Vf (to)
Infrared	940	1.2	1.7
Red	660	1.5	2.4
Orange	602–620	2.1	2.2
Yellow, Green	560–595	1.7	2.8
White, Blue, Violet	-	3	4
Ultraviolet	370	4.2	4.8

LED vs. Incandescent

• LEDs deliver >3× the lumen/watt of incandescent lamps.
• Longer service life (100,000 h vs. 1,000 h).
• Fast switching enables optical data transmission.
• Monochromatic output can be blended into white LEDs via phosphor or RGB combinations.

Efficiency & Lifespan Comparison

Lamp type	Lumen/Watt	Life (h)	Notes
White LED	35	100,000	Costly
White LED, future	100	100,000	R&D target
Incandescent	12	1,000	Inexpensive
Halogen	15–17	2,000	High‑quality light
Compact fluorescent	50–100	10,000	Cost‑effective
Sodium vapor, low pressure	70–200	20,000	Outdoor
Mercury vapor	13–48	18,000	Outdoor

Laser Diodes

A laser diode builds on LED technology by incorporating an optical cavity, producing coherent, monochromatic light. The laser light remains highly focused over long distances because all emitted photons share the same frequency and phase.

Laser diodes are used in:

• Surveying and rangefinding (straight, non‑diffusing beam).
• Optical storage (CD/DVD/Blu‑ray).
• Fiber‑optic communication.

Operating modes:

• Continuous‑wave: requires a current‑regulating circuit within a narrow current window.
• Pulsed: high‑power short bursts, often for high‑output devices.

Photodiodes

Photodiodes are optimized to generate a current in response to incident light. Silicon is the most common material, though germanium and GaAs are also used. A shallow P‑layer allows most photons to reach the depletion region where electron‑hole pairs are created.

Key operating modes:

• Photovoltaic (PV) mode: small output, low noise below 100 kHz.
• Photocurrent (PC) mode: reverse bias or zero bias; current is linearly proportional to light intensity, ideal for high‑speed applications.

PIN Photodiodes

With an intrinsic layer between P and N regions, PIN diodes reduce capacitance, increase speed (up to tens of GHz), and enlarge the photosensitive volume.

Avalanche Photodiodes (APDs)

APDs apply high reverse bias (10–2,000 V) to amplify the photocurrent via impact ionization. They offer high gain but also higher noise and cost, making them suitable for niche high‑speed, low‑light applications such as single‑photon counting.

Solar Cells

Solar cells convert sunlight into electrical power, operating in photovoltaic mode. They come in several forms, each with trade‑offs in efficiency, cost, and application.

Monocrystalline Silicon Cells

Manufactured by pulling a single‑crystal boule from molten silicon, then slicing into wafers. The N‑type diffusion layer is formed on the wafer surface, and a textured front surface traps light. Back‑contact designs (e.g., SunPower’s 21.5 % cells) increase active area and efficiency.

Multi‑crystalline Silicon Cells

Molten silicon is cast into a block, solidifying into several large crystals. Grain boundaries reduce efficiency, but anti‑reflective coatings mitigate losses. Suitable for most terrestrial installations.

Triple‑Layer (Multi‑junction) Cells

Stacked or monolithic layers with bandgaps tuned to UV (GaInP, 1.8 eV), near‑IR (GaAs, 1.4 eV), and far‑IR (Ge, 0.7 eV) absorb light sequentially. Current‑matching yields voltages summing to ~2.7 V. Commercial space cells reach 32 % efficiency; terrestrial versions target 40.7 % with concentrators.

Process example: Metal‑organic CVD deposits GaInP, GaAs, and Ge layers onto a Ge substrate, each absorbing its designated spectral range.

Thin‑Film Alternatives

• Amorphous silicon: ~13 % max efficiency, 5–7 % after aging, better in low light, used in calculators.
• Cadmium telluride (CdTe): ~16 % efficiency, low‑cost substrates (glass or metal).
• Copper indium gallium selenide (CIGS): ~18 % efficiency, flexible polymer roll‑to‑roll fabrication.
• Organic polymer cells: ~4.5 % efficiency, research stage.

Performance Summary

Cell type	Max eff.	Practical eff.	Notes
Silicon single crystal (commercial)	24 %	14–17 %	$5–$10/peak W
Silicon single crystal (space)	25 %	—	High cost
Triple‑junction (space)	32 %	—	Preferred for space
Triple‑junction (terrestrial)	—	40.7 %	Concentrator‑based
CdTe (polycrystalline)	16 %	—	Glass or metal
CIGS (polycrystalline)	18 %	10 %	Flexible polymer
Amorphous Si	13 %	5–7 %	Degrades in sunlight

Key Takeaways

• Schottky diodes excel in high‑frequency, low‑power switching.
• Tunnel diodes provide ultra‑fast negative resistance for microwave oscillators.
• LEDs and laser diodes offer efficient, controllable light for illumination and communication.
• Photodiodes, especially PIN and APD types, enable sensitive light detection in optical links.
• Solar cell technologies span monocrystalline, multicrystalline, multi‑junction, and thin‑film, each balancing efficiency, cost, and application niche.

Further Reading

• Design Project: Telegraph System Worksheet
• Optoelectronic Worksheet
• Sources of Electricity Worksheet
• Special Diodes Worksheet