Herbert kroemer

“ If in discussing a semiconductor problem, you cannot draw an energy band diagram, then you don’t know what you are talking about.”

An unusual condition was imposed on Herbert Kroemer at the start of his research career 50 years ago. He was not allowed to touch anything in his workplace, the Telecommuni­cations Laboratory of the German Postal Service. The fear was that this recent graduate in theoretical physics would break something. Far from constraining him, the restriction expanded his horizons¹.

With just pencil and paper, he began sketching out theo­ries that would resonate across the entire world¹ of semicon­ductor science. And that work would culminate in a Nobel Prize in Physics in 2000 and IEEE Medal of Honor in 2002, the latter for "contributions to high-frequency transistors and hot-electron devices, especially heterostructure devices from heterostructure bipolar transistors to lasers, and their molecular beam epitaxy technology."

While his theories led to products that earned their man­ufacturers billions of dollars, none of the profits came to Kroe­mer. "That really doesn't bug me," he says, sitting in his small and modestly decorated office on the Santa Barbara campus of the University of California, where he is now professor of electrical and computer engineering and materials.

IEEE Fellow Kroemer never tried to develop applications of his work - or even predict them. He told IEEE Spectrum, “'The principal applications of any suffi­ciently new and innovative technology always have been and will continue to be applications created by that new technol­ogy. " So he doesn't begrudge others the fruits of his ideas².

"I've always called myself an opportunist," he says. "This is supposed to be a derogatory term, but I'm not one bit ashamed of accepting opportunities. In the scientific sense, I was an opportunist who was looking for challenging problems."

Too Many Lists

In high school in Germany, Kroemer played around with chemistry experiments but soon turned to physics. "I liked the beautiful logic of a structure with a relatively small number of fundamental principles from which you could draw far-reaching conclusions," he says. A university chem­istry course that required memorization of lists and lists of chemical reactions destroyed any remaining inter­est in that science.

He entered the University of Jena in East Germany in 1947, then left for West Germany the next year during the Berlin airlift and was accepted at the University of Gottingen. Four years later he received his Ph.D. for a theoretical dissertation on germanium transistors that discussed electron transport in high electrical fields. It broke little new ground, and he takes no particular pride in it. He explained some experiments, he says, but the expla­nation later proved completely wrong.

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 service ran 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 frequency⁷ 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.