Semiconductors
are unique materials: solids whose electrical conductivity can be
changed deliberately, usually in a dynamic (reversible) fashion. They
are used to make semiconductor devices, which led to the Information Age
of the late 20th century. Today, these materials are everywhere and
continue to penetrate further into most people’s daily lives.
Devices
made with semiconductors include actuators and control systems in cars,
MP3 players, cell phones, and computers of all kinds. These materials
are arguably one of the most important technologies of the 20th century,
and they continue to be a central aspect of developed economies. The
most common are made of silicon, as it is relatively cheap to extract
from sand. The semiconductor industry sells several hundred billion US
Dollars of product per year.
The
first semiconductors were little detectors on radios popular around the
beginning of the 20th century. They were called “cat’s whiskers” and
the semiconducting element was lead sulfide. No one at the time really
understood how they worked; they just did. It was not until 1939 that
Richard Ohl, an inventor at Bell Labs who was also the first to patent
solar cells, discovered that certain crystals with small impurities have
conductivity that varies based on exposure to light. His work grew out
of an effort to find practical high-frequency amplifiers for
applications in radio.
Eight
years later, in 1947, other scientists at Bell Labs used semiconducting
materials to create a point-contact device which they called a
transistor. The material used was germanium. The whole device was
about half a foot tall, and required that the element be extremely
purified.
The
structure that underlies any transistor is the p-n junction. It has
two regions: a p region and an n region. The p region is “doped” with
small amounts of boron, causing the material to become filled with
numerous electron “holes,” which are an absence of electrons where
electrons should be. This happens because boron has a valence of three,
which causes it to absorb weakly-bound outer electrons from the
valence-four semiconductor atoms, leaving voids in its place. The n
region is doped with a material that has a valence of five, causing the
reverse effect, where the impurities donate their extra electron to the
material, causing an abundance of electrons.
This
relative abundance and absence of electrons is exploited in the
transistor. A series of two p-n junctions makes up the heart of the
device. By manipulating the junctions, charge flow can be regulated
precisely, allowing for complex electronics. Variations on the
transistor can be used to make LEDs and very delicate sensors, while
most computers have billions of transistors of several different types.
Although silicon is the most common transistor today, diamond, which
can be configured in a 3D matrix more easily than silicon transistors,
may be used in the future.