Alloy-Junction Transistor
An alloy-junction transistor is a bipolar junction transistor made by melting small dots of a metal, usually indium, into opposite faces of a thin wafer of germanium so that each dot dissolves some of the wafer and, on cooling, regrows as a heavily doped layer of the opposite type. It was developed at General Electric in 1951, with the work of John Saby often cited, and RCA and others took it up in 1952; it was far easier and cheaper to make than the grown-junction device and became the workhorse of early consumer transistor electronics.
The process starts with a thin n-type germanium wafer and two small pellets of indium. The wafer is cut from a grown crystal, lapped and etched to a controlled thickness, which will become the base, with the wafer only a fraction of a millimetre thick. A larger indium pellet is placed on one face, to be the collector, and a smaller one is placed in line with it on the opposite face, to be the emitter, all held in a graphite jig. The assembly is heated in a furnace with an inert or reducing atmosphere to a temperature at which the indium melts and dissolves germanium beneath it. On cooling, the dissolved germanium recrystallises on the wafer, now carrying indium as a dopant, and forms a p-type region ending at a sharp junction with the unchanged n-type base. The result is a p-n-p transistor whose two junctions were formed in one firing.
Its great advantage was cost and repeatability. Many wafers could be loaded in a boat and fired together, base thickness was set by wafer thinning rather than by an uncertain crystal-growth schedule, and a given recipe gave reasonably consistent junctions. Because the alloying used a modest amount of germanium per device and little was wasted in sawing, prices fell quickly through the mid-1950s. That made alloy p-n-p parts the standard for audio and low-frequency work; the GE 2N107 became a hobbyist staple, and the tiny amplifiers in hearing aids and early portable radios were typically made this way. The shift from point-contact and grown devices to alloy devices is visible in the first mass-market radios, which relied on a handful of cheap junction transistors rather than costly early types.
The weaknesses followed directly from how the junctions were made. Melting a dot into germanium produces a junction that is rather rough and curved, and the depth to which the indium penetrates is hard to control precisely, so a base thinner than a few thousandths of an inch was difficult to guarantee, and a thick base is slow to cross. The collector and emitter dots also have large areas, and the resulting junction capacitance, along with the base resistance, limited gain at higher frequency. Alloy devices were therefore good at audio and the lower radio frequencies but poor beyond roughly a few megahertz. Pushing the dots too deep could even cause punch-through, where the emitter and collector regions meet through the base. Germanium itself added limits on temperature and leakage.
Common mistakes involve confusing the alloy method with its neighbours. It is not a diffusion process, because the junction forms by recrystallisation from a liquid, not by atoms migrating through a solid; and it is not a grown junction, because the structure is made on a finished wafer rather than inside a crystal. Another frequent error is to assume that all early transistor radios used grown-junction devices. Several used germanium types of other kinds, and the alloy types in particular supplied much of the low-cost market. Later hybrid refinements, such as microalloy and diffused-base devices, kept the alloy idea but added a graded base to improve speed.
Its modern relevance is mostly as a precedent. Alloying survives in power-device and die-attach metallurgy, where metal is melted into silicon to make contacts and bonds, and the failure modes that engineers still watch for, such as uneven wetting and penetration spikes, were first met in alloy transistors. For historians, the device marks the moment the transistor became cheap enough for consumer products.
Read the alloy-junction transistor as the transistor made by brute simplicity: two metal dots melted into a germanium wafer gave a dependable, inexpensive p-n-p part that put transistors in hearing aids and radios, at the price of a thick base that held it to audio and low radio frequencies.