Postal Service

In 1952, when Kroemer received his Ph.D., an academic career was out of the question. The lines of succession4 at existing Ger­man universities were long, and no new ones were being established. So he joined the Telecommunications Research Laboratory of the German Postal Service in Darmstadt.

The postal serviceran the telephone system and had a small semiconductor research group - some 10 scientists - in its telecommunications labora­tory. That group hired Kroemer to answer any theoretical ques­tions that arose, to give talks on any subject he thought rele­vant - and to keep his hands off the research equipment.

"I enjoyed this thoroughly," he recalls. For one, he had liked the role of teacher since high school, when his physics teacher asked him to prepare and deliver a lecture to the class. For another, being at the researchers' beck and call5 presented him with a wide variety of problems in diverse subjects.

In solving one of those problems, he went against the con­ventional wisdom of the time. Researchers were developing pn junctions of indium and germanium. They did this by deposit­ing a layer of indium on a layer of germanium, then heating the structure to form the pn junction. Kroemer was trying to understand how exactly the junction formed.

Obviously the molten indium dissolved some of the ger­manium, and the belief was that it diffused into the germanium beyond the layer in which the germanium dissolved. But Kroe­mer concluded that the process was one of recrystallization - the heated indium dissolves some of the germanium, and then upon cooling the germanium precipitates out and recrystallizes, incorporating some of the indium atoms, which replace some of the germanium atoms in the lattice.

What he didn't know was that researchers in the United States, at General Electric Co. and RCA Corp., had simulta­neously reached the same conclusion.

But what he did know was that to be at the research fore­front, he needed to leave the German Postal Service and get to the United States. He started looking for a way to get there.

Researchers from other countries occasionally visited the lab in which he worked, curious about this small semiconduc­tor research group. In 1953 one visitor was William Shockley, then at Bell Telephone Laboratories. "I spent about two hours with him," Kroemer said. "We were having a marvelous time. I told him about the work that I'd done for my Ph.D. disser­tation, and about some of my ideas of how to make transis­tors fast by putting an electric field into the base. He seemed intrigued by that."

Kroemer asked him about coming to Bell Labs, but Shockley, as an official visitor, told Kroemer that he would have to go through official channels, starting with informing Postal Ser­vice management of his intentions to apply for a job in the United States. The young researcher was so grateful for the job he had at the Postal Service that he was "terribly squeamish6 about telling my management that I wanted to leave."

Later in 1953, the Darmstadt lab had another U.S. visitor: Ed Herold from RCA. Kroemer asked him whether RCA was working on npn transistors (back then pnp transistors domi­nated). Herold was careful in his responses; but Kroemer guessed out loud what the RCA researchers were doing, what alloys they were using (lead-antimony), the percentage of the antimony, and the alloy temperatures. His guesses proved quite dose to RCA's experiments, and the impressed Herold didn't hesitate to offer him a job. (All the same, it took a year for Kroemer to obtain a visa, even with RCA's help.)

At RCA in Princeton, N.J., Kroemer did theoretical research on an impurity diffusion process for building transistors. In the diffusion process, the doping of the base region was delib­erately graded from a high value at the emitter to a lower value at the collector. Because this gradient introduced a built-in electric drift field into the base, the result was called a drift transistor. The first commercial product to come out of that research - the 2N247 - had a high-frequency performance far beyond that of other commercially available transistors of its time. Its power gain cutoff frequency 7 of 132 MHz made it suit­able for use in FM radios.

While Kroemer was theorizing about how a drift field could make transistors switch faster, he had an idea about grading the basic semiconductor itself. If an alloy of two semiconduc­tors replaced the single semiconductor, it could be given a continually varying composition to change its band gap, which is a measure of the amount of energy required to move an elec­tron from a semiconductor's valence band to its conduction band. This varying band gap would be another way to intro­duce a drift field into the base, again in order to improve tran­sistor frequency performance.

He had mentioned varying a material's band gap in a paper while still in Germany, but expanded the idea and in 1957 pub­lished two papers about it, one in the RCA Review, another in the Proceedings of the IEEE.

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