Grown-junction transistor

Grown-junction transistor

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The grown-junction transistor was the first type of bipolar junction transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 made. It was invented by William Shockley
William Shockley
William Bradford Shockley Jr. was an American physicist and inventor. Along with John Bardeen and Walter Houser Brattain, Shockley co-invented the transistor, for which all three were awarded the 1956 Nobel Prize in Physics.Shockley's attempts to commercialize a new transistor design in the 1950s...

 at Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

 on June 23, 1948. The patent was filed on June 26, 1948. The first germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

 prototypes were made in 1949. Bell Labs announced Shockley’s grown-junction transistor on July 4, 1951.

An NPN grown-junction transistor is made of a single crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 of semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 material which has two PN junctions grown into it. During the growth process, a seed crystal
Seed crystal
A seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...

 is slowly pulled from a bath of molten semiconductor, which then grows into a rod-shaped crystal (boule
Boule (crystal)
A boule is a single-crystal ingot produced by synthetic means. A boule of silicon is the starting material for most of the integrated circuits used today....

). The molten semiconductor is doped
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...

 N-type at the start. At a predetermined moment in the growth process a small pellet of a P-type dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...

 is added, almost immediately followed by a somewhat larger pellet of an N-type dopant. These dopants dissolve in the molten semiconductor changing the type of semiconductor subsequently grown. The resulting crystal has a thin layer of P-type material sandwiched between sections of N-type material. This P-type layer may be as little as a thousandth of an inch thick. The crystal is sliced, leaving the thin P-type layer in the center of the slice, then cut into bars. Each bar is made into a transistor by solder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

ing its N-type ends to supporting and conducting leads, then welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

 a very fine gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 lead to the central P-type layer, and finally encasing in a hermetically sealed can
Hermetic seal
A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.-Etymology :...

. A similar process, using the opposite dopants, makes a PNP grown-junction transistor.

The most difficult part of this process is welding the gold wire to the base layer, as the wire may have a larger diameter than the thickness of the base. To facilitate this operation, the gold wire is pointed or flattened until the end is thinner than the base layer. The tip of the gold wire is slid along the bar until electrical resistance measurement shows it is in contact with the base layer. At this time a pulse of current is applied, welding the wire in place. Unfortunately sometimes the weld is too large or slightly off center in the base layer. To avoid shorting the transistor, the gold wire is alloyed with a small amount of the same type dopant as used in the base. This causes the base layer to become slightly thicker at the point of the weld.

Grown-junction transistors rarely operated at frequencies above the audio range, due to their relatively thick base layers. Growing thin base layers was very hard to control and welding the wire to the base became harder the thinner it got. Higher-frequency operation could be obtained by welding a second wire on the opposite side of the base, making a tetrode transistor
Tetrode transistor
-Early tetrode transistors:There were two types of tetrode transistor developed in the early 1950s as an improvement over the point-contact transistor and the later grown junction transistor and alloy junction transistor. Both offered much higher speed than earlier transistors.*Point-contact...

, and using special biasing on this second base connection.

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