Walter Houser Brattain (February 10, 1902–October 13, 1987) was an American physicist at Bell Labs who, along with John Bardeen and William Shockley, invented the transistor. They shared the 1956 Nobel Prize in Physics for their invention. He devoted much of his life to research on surface states.
Brattain's concerns at Bell Laboratories in the years before World War II were first in the surface physics of tungsten and later in the surfaces of the semiconductors cuprous oxide and silicon. During World War II Brattain devoted his time to developing methods of submarine detection under a contract with the National Defense Research Council at Columbia University.
Crystals of pure semiconductors (such as silicon or germanium) are very poor conductors at ambient temperatures because the energy that an electron must have in order to occupy a conduction energy level is considerably greater than the thermal energy available to an electron in such a crystal. Heating a semiconductor can excite electrons into conduction states, but it is more practical to increase conductivity by adding impurities to the crystal. A crystal may be doped with a small amount of an element having more electrons than the semiconductor, and those excess electrons will be free to move through the crystal; such a crystal is an n-type semiconductor. One may also add to the crystal a small amount of an element having fewer electrons than the semiconductor, and the electron vacancies, or holes, so introduced will be free to move through the crystal like positively-charged electrons; such a doped crystal is a p-type semiconductor.
At the surface of a semiconductor the level of the conduction band can be altered, which will increase or decrease the conductivity of the crystal. Junctions between metals and n-type or p-type semiconductors, or between the two types of semiconductors, have asymmetric conduction properties, and semiconductor junctions can therefore be used to rectify electrical currents. In a rectifier, a voltage bias that produces a current flow in the low-resistance direction is a forward bias, while a bias in the opposite direction is a reverse bias.
Semiconductor rectifiers were familiar devices by the end of World War II, and Shockley hoped to produce a new device that would have a variable resistance and hence could be used as an amplifier. He proposed a design in which an electric field was applied across the thickness of a thin slab of a semiconductor. The conductivity of the semiconductor changed only by a small fraction of the expected amount when the field was applied, which John Bardeen (another member of Shockley's division) suggested was due to the existence of energy states for electrons on the surface of the semiconductor.