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.

Halls of Academia

Kroemer was happy to move from industry to academia. Things at Fairchild had not gone well, because the company was dedicated to silicon technology and Kroemer's interests had long been elsewhere. He looked forward to the research freedom and also to teaching.

But he became dissatisfied. "We had hoped to set up a good solid-state engineering graduate program at Boulder, " he says. During the Vietnam War, many students went on to graduate school to reduce their chances of getting drafted 10. Stanford Uni­versity typically recruited the academically top 5 percent of graduate students interested in solid-state research, and Boul­der drew on the next 5 percent, who were still extremely good. But when graduate enrollments fell after the war's end, that source dried up. "Our ambitious graduate program would not fly - it was clear to me that I would be professionally dead if I stayed there," Kroemer recalls.

Word went out 11 that he was open to a change, and in the fall of 1975, the University of California at Santa Barbara, in the person of Edward Stear, then head of its electrical engineering and computer sciences department, came calling. Santa Bar­bara at the time didn't have a very good academic reputation; what it did have was a well-equipped semiconductor device teaching laboratory.

"So, Herb, you know about our laboratory," Stear opened. "What would you do with it?" Kroemer momentarily forgot that this visit was actually a job interview. "Sure as hell not what you're doing!"

"It was a rather unfriendly and hostile ," Kroemer recalls. He fig­ured he had blown any chance of being hired 12. But then Stear told him, "I'm looking for someone to rock the boat; it looks like you're my man."

Kroemer, Stear tells, "speaks very directly. He is honest, but can be sharp with people, too. He is intense and demanding. He can be a difficult person at times to work with, but people have ended up loving him." In any case, Stear knew that Kroemer could build the kind of program that Santa Barbara needed, and Kroemer was hired.

By the Sea

Kroemer left for Santa Barbara in the summer of 1976. He had persuaded Stear not to compete with Stanford, Berkeley, and other top engineering schools in silicon technology, but instead to focus on compound semiconductors such as GaAs. He gave Santa Barbara even odds for making an im­pact in that technology.

"You want to be first-rate or absent," Kroemer says.

Kroemer convinced a few former colleagues that they should join him at Santa Barbara, and he also convinced the U.S. Army it should buy him a molecular beam epitaxy machine. He said at the time that he wanted it for making tran­sistors with a gallium phosphide emitter on a silicon base, a crazy project if there ever was one. It was not enough to put Santa Barbara's engineering school on the map.

In the mid-1980s, however, the chancellor of the university, Bob Huttenback, decided to put all available money into improving the College of Engineering. A new dean was hired, and 15 faculty were added. Kroemer says, "Today we have one of the best materials departments in the country - and we still don't have any silicon technology."

At 73, Kroemer remains a full-time member of the faculty. One problem he is working on concerns the influence of high electric fields on electron transport in semiconductor superlattices (alternating thin layers of two or more materials with different band gaps but similar crystal structures and lat­tice constants). More specifically, he is focused on a concept, called a Bloch oscillator, which can in theory generate oscil­lations up into the terahertz range, potentially opening up that frequency range for numerous applications. So far, it has never been satisfactorily demonstrated as a continuously run­ning device. "I have some ideas, which may or may not be cor­rect, of what to do about it," Kroemer tells .

He is also looking at the phenomenon of induced super­conductivity in semiconductors, created when supercon­ducting materials are deposited on semiconductors and oper­ated at low temperatures.

Date: 2015-12-24; view: 808