Surface-Barrier Transistor
The surface-barrier transistor is a high-frequency germanium transistor developed at Philco and announced in 1953, in which the active region is made not by melting metal into the crystal but by electrochemically etching a thin base from a germanium wafer with two opposed jets of electrolyte and then plating indium onto the thinned surfaces to form the emitter and collector. Because the base could be made far thinner than in grown or alloy devices, it worked at tens of megahertz, and it became the transistor of choice for early fast computers.
The key step is jet etching, which thins the wafer from both sides at once. A small slab of n-type germanium, lapped to a modest thickness, is held between two fine nozzles that squirt an electrolyte, typically an indium salt solution, at opposite faces. A current passed through the jets makes the germanium dissolve, and as a pit deepens on each side the remaining germanium web between them becomes very thin, a few micrometres or less; a light beam or an electrical measurement monitors its thickness so the etching can be halted at the right moment. The current is then reversed, so that the same jets now deposit indium from the solution, plating a small spot of metal into each pit. The two spots become the emitter and collector contacts, and the thin germanium between them is the base.
The speed comes from the very thin base and the small junction area. Carriers cross a base a few micrometres thick quickly, and the tiny plated spots have little capacitance, so surface-barrier transistors reached cutoff frequencies in the tens of megahertz, far above the audio-oriented alloy-junction parts of the same years. They were the first widely available transistors that could switch fast enough for serious digital work. The machine at MIT known as TX-0, completed in 1956, used thousands of Philco surface-barrier transistors in its logic, and the type was a natural fit for early military and scientific computers, where speed mattered more than price.
The method's drawbacks were fragility and difficulty of manufacture. The thin base, a fraction of a thousandth of an inch, was mechanically delicate; a slight overshoot in etching could punch a hole through, and the structure tolerated little heat or mechanical shock. Breakdown voltages were low, the base was thin enough for punch-through to be a design limit, and the manufacture required precise optical or electrical monitoring and careful control of the electrolyte. It also remained a germanium device, with germanium's temperature sensitivity and leakage. Philco turned to an improved structure, the microalloy-diffused-base transistor of 1957, which kept the jet-etched approach but added a graded base, and then silicon planar devices from other firms overtook the whole family.
A common point of confusion is the name. The surface-barrier transistor is not a surface-barrier diode and is not the same as an alloy-junction device; the junctions come from plated metal on a freshly etched surface rather than from a melted dot, and the term barrier refers to the thin region at the metal-semiconductor interface that blocks and controls current. Another mistake is to think it was a modern Schottky transistor. It belongs to the 1950s germanium family, even though the idea of a metal contact that controls a semiconductor surface returns in later Schottky diodes and in the metal contacts of every modern chip.
What remains is the lesson that thinness equals speed. The same logic drives the base width of today's microwave bipolar devices and the gate length of modern field-effect transistors, and electrochemical etching and plating are still core tools in microfabrication, from copper interconnect to micro-machined sensors. Philco's jet-etch machine was an early demonstration that a chemical process could shape a semiconductor to a precision of micrometres on a production line.
Read the surface-barrier transistor as the first transistor built for speed by chemistry: a jet of electrolyte sculpted an ultra-thin base and plated its own contacts, buying tens of megahertz at the cost of fragility, and giving the early fast computers their switching devices.