Theory into Practice

While Kroemer trusted his theory, he didn't know how to build actual semiconductors using his principles. Building them would require either a base region consisting of a graded mix of different semiconductor materials with vary­ing band gaps or else one material in the base but a differ­ent material in the emitter.

He tried to build a transistor with germanium-silicon alloy as the emitter on a germanium base. To this end, a gold-sili­con blended mixture was alloyed onto germanium at 600 °C, hot enough for the melted mixture to begin eating up germa­nium, precipitating the germanium-silicon alloy emitter on cooling. Unfortunately, during the cooling, most of the devices cracked. "It was one of those technological blind alleys⁸ where you're not exactly embarrassed that you have tried it, but you're not surprised it didn't work," he says.

At the end of 1957, Kroemer decided to get out of transis­tor research. He had no interest in traditional transistors, and heterostructure transistors, with existing material technology, could not be built.

"I promised myself," he says, "that if a new technology for building heterostructures arose, I'd get back into it."

Kroemer left RCA in 1957 and returned to Germany; he, and more especially, his wife, were homesick. Becoming head of a semiconductor group at Philips Research Laboratory in Hamburg, he pushed for work on gallium arsenide, looking at what happens when you apply large electric fields to gallium arsenide semiconductors. "I thought GaAs was going to be an important material, so it was worthwhile studying it."

Kroemer feels he did little significant work at Philips and, since his wife quickly concluded she preferred the United States after all, in 1959 he went to Varian Associates (Palo Alto, Ca.), where he did a little research on tunnel diodes before turning to other problems.

Back in the Heterostructure Game

Then Kroemer's ideas about heterostructure devices, shelved for half a dozen years⁹, came back to his attention.

It was March 1963. The previous summer, Kroemer and a Varian colleague, Sol Miller, had attended the Annual Device Research Conference, at which GaAs lasers had been intro­duced. Miller was interested and at Varian’s weekly collo­quium, he gave a talk about the new lasers. Though scientifi­cally fascinating, he said, the devices could only work at very low temperatures and only for very short pulses, and so would never be truly practical. Asked why, Miller explained that the problem was the lack of charge-carrier confinement: at normal temperatures, electrons would diffuse out of one side of the device as quickly as they were supplied from the other side, as would the holes; therefore the electron-hole pair concentration would never become high enough to cause laser action by stimulated emission. Low temperatures suppressed the effect, but only for brief periods of time.

Kroemer disagreed. Based on his work in heterostructures, the solution, to him, seemed obvious - you just vary the device's band gap, putting a narrower gap in the center and a wider gap in the outer regions, so that the electrons and holes would con­centrate in the center [see "Heterostructures Explained"].

Kroemer wanted to start working on the creation of room-temperature lasers at once, but his superiors at Varian told him that such a device would never have any applications.

"This is the classic mistake - judging something not by what applications it might create, but by how it could fit into applications we've already thought of," Kroemer says. The applications it was useful for turned out to include fiber-optic communications, CD and DVD players, LED traffic lights, and laser pointers - none of which were around at the time.

Though Kroemer wasn't pleased by Varian’s decision, the Gunn effect, which had just been discovered, interested him. This is a phenomenon in which microwave oscillations are produced when a certain voltage is applied to opposite faces of a semiconductor. For the next decade and more, Kroemer explored theories of why this occurred, three of those years at Varian, two at Fairchild Semiconductor Corp. (Palo Alto, Ca.), and nearly eight at the University of Colorado in Boulder.