Optical and Transport Properties of Semiconductors

How a semiconductor absorbs light and how its carriers drift and diffuse under fields determine whether it makes a good detector, emitter, or transistor, and these properties follow from its band structure and scattering.

Definition

The transport properties of a semiconductor describe how electrons and holes move under electric fields and concentration gradients, characterized by mobility, conductivity, and diffusion; the optical properties describe how the material absorbs and emits light across its band gap, set by the band structure and the directness of the gap.

Scope

This topic covers the electrical transport and optical response of semiconductors: carrier drift and mobility, the scattering mechanisms (phonon and impurity) that limit it, diffusion and the Einstein relation, the Hall effect, and recombination. On the optical side it covers band-edge absorption, the distinction between direct and indirect gaps for light emission, excitons, and photoconductivity. It connects the band structure and carrier statistics of the area to measurable device-relevant properties.

Core questions

• What determines carrier mobility, and which scattering mechanisms limit it?
• How are drift and diffusion related through the Einstein relation?
• Why does the directness of the band gap control whether a semiconductor emits light efficiently?
• What are excitons and photoconductivity, and how do they shape optical response?

Key concepts

• Carrier drift, mobility, and conductivity
• Phonon and impurity scattering
• Diffusion and the Einstein relation
• Direct versus indirect optical transitions
• Excitons and photoconductivity

Clinical relevance

Transport and optical properties decide device performance: mobility sets transistor speed, the direct or indirect gap determines whether a material can make efficient LEDs and lasers (as in gallium arsenide versus silicon), and absorption governs photodetectors and solar cells.

History

The Hall effect (1879) provided an early means to measure carrier sign and density; the quantum theory of band-edge absorption and excitons developed in the 1930s, and the recognition that direct-gap compounds like gallium arsenide emit light efficiently underpinned the optoelectronics that emerged from the mid-twentieth century.

Key figures

• Edwin Hall
• Albert Einstein
• Gregory Wannier

Seminal works

• ashcroft1976
• sze2007

Frequently asked questions

Why does silicon make poor light-emitting devices?

Silicon has an indirect band gap, so an electron and hole recombining across the gap must also involve a phonon to conserve momentum; this makes radiative recombination inefficient, which is why direct-gap materials like gallium arsenide are used for LEDs and lasers.

What limits how fast carriers move in a semiconductor?

Carriers are scattered by lattice vibrations (phonons) and by ionized impurities; these collisions cap the mobility, with phonon scattering dominating at high temperature and impurity scattering at low temperature and heavy doping.