|
Semiconductors are the foundation of today's information age. They enable
and underlie virtually every system involved in the manipulation and movement
of information, from laptop computers to satellite-based global communications
networks. Moreover, as the manipulation and movement of information rapidly
becomes as vital to world economic growth as were once the generation and
movement of food (the agricultural age) and the development and powering of
machine tools (the industrial age), so semiconductor materials are becoming as
(or more) vital to world economic growth as were once other materials such as
stone and wood, metals and coal.
This page describes some of the unique characteristics of semiconductors
that have led to their importance to the information age, with a particular
focus on compound semiconductors.
Characteristics of Semiconductor Materials
A primary characteristic, from which they derive their name, is that they are
"lukewarm" electrical conductors. Neither highly insulating nor highly
conducting, they were not initially believed to be particularly useful.
However, because their conductivity is not only lukewarm, but also extremely
sensitive to the introduction of impurities and the application of electric
fields, they can easily be precision engineered into the basic building blocks
of microelectronics: current or voltage controlled switches and amplifiers.
A secondary (but also crucial) characteristic is that their electrical
conductivity is based on two "effective" carrier types, negatively charged
electrons and positively charged holes. The interaction between these carrier
types is very strong, and in many cases is either caused by the absorption, or
results in the emission, of light (photons). Then, by precision engineering of
the interaction between electrons and holes in compact devices, they can be
used to form the basic building blocks of photonics: light absorbing and
emitting devices such as detectors, lasers and light-emitting diodes
(LEDs).
Families of Semiconductor Materials
The dominant semiconductors are inorganic, and can be divided into three major
families, according to whether they are composed of elements from column IV,
compounds between elements in columns III and V, or compounds between elements
in columns II and VI of the periodic table. Although they all have the two
unique characteristics described above, all their other physical and chemical
properties vary widely both from family to family as well as within each
family. For example, at one extreme C in its diamond form has a very small
lattice constant, is transparent into the ultraviolet, and is mechanically hard
with a high thermal conductivity. At the other extreme, HgCdTe has a very large
lattice constant, is transparent
only in the far infrared, and is mechanically soft with a low thermal
conductivity.
CSRL
Sandia's Center for CSRL focuses on compounds
derived from the column III-V family of elements. These compounds are
permutations of the column III elements Al, Ga and In and the column V elements
N, P, As and Sb. They are characterized by their excellent optoelectronic
(efficient light emission and absorption) and electronic (high carrier
mobilities) properties.
The optoelectronics applications (e.g., optical communications, displays,
sensors) of the various families of III-V materials are determined in large
part by the wavelength ranges within which they emit and absorb light
efficiently:
- GaAs-related materials: 0.8-1.0 µm
- InP-related aterials: 1.3-1.7 µm
- GaN-related materials: 0.3-0.6 µm
- InSb-related materials: 2-10 µm
- GaP-related materials: 0.5-0.7 µm
The electronic applications (e.g., wireless communications based on
high-frequency RF or microwave carriers, radars, and magnetic-field sensors) of
the various families of III-V materials are determined by trade-offs between
performance and material robustness during device manufacture and operation. In
practice, GaAs-related materials are the most common, but InP-related materials
and InSb-related materials also have important applications.
One particularly useful aspect of III-V materials
is their richness and variety, as illustrated by this band-gap versus lattice
constant "road-map" to III-V materials. This richness enables high-performance
"band-gap engineered" heterostructures and devices with optical and electronic
properties difficult to achieve in other materials.
|
