Diffusion transistor

Diffusion transistor

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A diffusion transistor is any transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

 formed by diffusing 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...

s into a 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...

Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

. Diffusion transistors include some types of both 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...

s and field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...

s. The diffusion process was developed later than the alloy junction
Alloy-junction transistor
The germanium alloy-junction transistor, or alloy transistor, was an early type of bipolar junction transistor, developed at General Electric and RCA in 1951 as an improvement over the earlier grown-junction transistor....

 and grown junction
Grown-junction transistor
The grown-junction transistor was the first type of bipolar junction transistor made. It was invented by William Shockley at Bell Labs on June 23, 1948. The patent was filed on June 26, 1948. The first germanium prototypes were made in 1949...


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...

 developed the first prototype diffusion transistors in 1954.

Diffused-base transistor

The earliest diffusion transistors were diffused-base transistors. These transistors still had alloy emitters and sometimes alloy collectors like the earlier alloy-junction transistors. Only the base was diffused into the substrate. Sometimes the substrate formed the collector, but in transistors like Philco
Philco, the Philadelphia Storage Battery Company , was a pioneer in early battery, radio, and television production as well as former employer of Philo Farnsworth, inventor of cathode ray tube television...

's micro-alloy diffused transistors the substrate was the bulk of the base.

Double diffusion

At Bell Labs Calvin Souther Fuller
Calvin Souther Fuller
Calvin Souther Fuller was a physical chemist at AT&T Bell Laboratories where he worked for 37 years from 1930 to 1967. Fuller was part of a team in basic research that found answers to physical challenges...

 produced basic physical understanding of a means of directly forming the emitter, base and collector by double diffusion. The method was summarized in 1983 in a history of science at Bell:
Fuller had shown that acceptor
Acceptor (semiconductors)
In semiconductor physics, an acceptor is a dopant atom that when added to a semiconductor can form p-type regions.For example, when silicon , having four valence electrons, needs to be doped as a p-type semiconductor, elements from group III like boron or aluminium , having three valence...

s of low atomic weight
Atomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...

 diffuse more rapidly than donor
Donor (semiconductors)
In semiconductor physics, a donor is a dopant atom that, when added to a semiconductor, can form n-type regions.For example, when silicon , having four valence electrons, needs to be doped as an n-type semiconductor, elements from group V like phosphorus or arsenic can be used because they have...

s, which made possible n–p–n structures by simultaneous diffusion of donors and acceptors of appropriately different surface concentrations. The first n–layer (the emitter) was formed because of the greater surface concentration of the donor (for example, antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

). The base formed beyond it because of the more rapid diffusion of the acceptor (for example, aluminum). The inner (collector) boundary of the base appeared where the diffused aluminum no longer over-compensated the n–type background doping of the original silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

. The base layers of the resulting transistors were 4 μm thick. ... Resulting transitors had a cut-off frequency of 120 MHz.

Mesa transistor

The mesa transistor was developed at Texas Instruments
Texas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...

 in 1957. These transistors were the first to have both diffused bases and diffused emitters.

Unfortunately, like all earlier transistors, the edge of the collector–base junction was exposed, making it sensitive to contamination, thus requiring hermetic seal
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 :...

s or passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

 to prevent degradation of the transistor's characteristics over time.

Planar transistor

The planar transistor was developed by Dr. Jean Hoerni
Jean Hoerni
Jean Amédée Hoerni was a silicon transistor pioneer and a member of the Traitorous Eight. He was remembered for developing the planar process....

 at Fairchild Semiconductor
Fairchild Semiconductor
Fairchild Semiconductor International, Inc. is an American semiconductor company based in San Jose, California. Founded in 1957, it was a pioneer in transistor and integrated circuit manufacturing...

 in 1959. The planar process
Planar process
The planar process is a manufacturing process used in the semiconductor industry to build individual components of a transistor, and in turn, connect those transistors together. It is the primary process by which modern integrated circuits are built...

 used to make these transistors made mass produced monolithic integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s possible.

These transistors have a silica passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

 layer to protect the junction edges from contamination, making inexpensive plastic packaging possible without risking degradation of the transistor's characteristics over time.

The first planar transistors had much worse characteristics than alloy junction transistors of the period, but as they could be mass produced and alloy junction transistors could not, they cost much less and the characteristics of planar transistors improved very rapidly, quickly exceeding those of all earlier transistors and making earlier transistors obsolete.