Laser diodes are composed of layered semiconductor materials, typically forming a p–i–n junction with a double-heterostructure and quantum well active region for efficient light emission.

Semiconductor Materials

Laser diodes are made from direct bandgap semiconductors, which allow electrons and holes to recombine and emit photons efficiently. Common materials include GaAs, InGaAs, AlGaInP, and GaN, depending on the desired emission wavelength, ranging from ultraviolet to infrared . Direct bandgap materials are essential because indirect bandgap semiconductors like silicon or germanium primarily convert recombination energy into heat rather than light .

Layered Structure

A typical laser diode consists of several layers grown epitaxially on a substrate:

• N-doped substrate: Provides electrons and forms the base of the diode.
• Intrinsic (I) active layer: The region where electrons and holes recombine to produce light. This layer often contains quantum wells to lower threshold current and increase efficiency .
• P-doped cladding layer: Confines carriers and photons to the active region, enhancing recombination and optical gain.
• Contact layers: Metalized layers for electrical connections to the P and N regions . This double-heterostructure design confines both carriers and photons, improving efficiency and reducing threshold current .

Optical and Electrical Features

Laser diodes are electrically a PIN diode, where carriers are injected into the intrinsic region. The device also includes a resonator or optical cavity, typically formed by cleaved or coated facets of the semiconductor wafer, which provides feedback for stimulated emission . Some laser diodes incorporate a monitor photodiode in the package to regulate output power and maintain stable operation .

Advanced Structures

Modern laser diodes may use quantum dots or multiple quantum wells in the active region to further enhance performance, including higher output power, lower threshold currents, and improved beam quality . Surface-emitting laser diodes emit perpendicular to the wafer surface, while edge-emitting types emit from the cleaved edges of the wafer.

Summary

In essence, a laser diode is a semiconductor device with a layered p–i–n structure, using direct bandgap materials and a double-heterostructure with quantum wells to efficiently convert electrical energy into coherent, monochromatic, and directional light . The combination of material choice, layer design, and optical cavity ensures high efficiency and precise wavelength control.