Herbert Kroemer, a German-born American physicist who was awarded the Nobel Prize for his part in discoveries that paved the way for the development of many trappings of modern life, including high-speed internet communication, mobile phones and bar-code readers, died on March 8. He was 95.
The death was announced by the University of California, Santa Barbara, where he was an emeritus professor. No further details were provided in a statement.
Dr. Kroemer’s most important contributions were in the development of so-called heterostructures. They vastly enhance the speed, and therefore the power, of transistors and other types of semiconductors that are the building blocks of all electronic equipment.
The Nobel Committee’s recognition of Dr. Kroemer’s work was unusual, since his breakthrough was in applied science rather than in pure research, which is typically where the biggest advances in the understanding of physics occur. But by the time he received a share, with two other scientists, of the Nobel Prize in Physics in 2000, the impact of his work was so enormous, it could not be denied.
His most significant research was done entirely while he was employed in the private sector.
Dr. Kroemer, who had earned his Ph.D. from the University of Göttingen in Germany just before his 24th birthday — a young age for a theoretical physicist — went to work for the German postal service in 1952 because, he said in a 2008 interview with the Nobel Institute, there were no postdoctoral positions available at the time.
The postal service had created a small laboratory and research group to look into how to improve telecommunications, staffed with experts in designing experiments. But they needed a theoretician to help them understand what was happening. Dr. Kroemer’s job, as he explained it, was to poke his nose into everyone else’s business, so long as he did not touch any of the equipment.
At the time, the experimentalists were having trouble making use of transistors, which had been invented at Bell Laboratories in Murray Hill, N.J., five years earlier. It was clear that transistors, which consist of an electron emitter (electrons), a base and an electron collector (holes), were a great technological leap forward, but they were too slow for practical applications. They were inefficient — electrons going from the emitter to the base often flowed back to the emitter — and they could not handle high-frequency signals.
Dr. Kroemer’s first idea was to create a graded base so that the electrons would provide a greater charge, or more energy, as they went from the emitter to the collector, much as water does as it approaches a beach in waves that crash along the shore. The problem was that the technology did not exist at that time to build one. (It does now, and such graded bases are used in today’s transistors.)
A colleague at the postal service, Alfons Hähnlein, said that Dr. Kroemer’s idea was not possible, that the most that could be done was to build a transistor in which the emitter had a wider energy gap than the base.
But Dr. Kroemer thought that a wider energy gap could be created by either introducing impurities into the semiconductor materials, a process called doping, or by making the collectors and emitters out of different materials altogether, which is the common method used today.
The idea for the heterostructure had been born.
Dr. Kroemer published one paper about his ideas in 1954 and two more in 1957. It would take a couple of decades before the technology existed to build good heterostructure transistors. In the meantime, he moved on to other projects.
In 1963, Dr. Kroemer, then at Varian Associates, a company in Palo Alto, Calif., that made electromagnetic equipment, had a reason to revisit the idea. A colleague there, Sol Miller, gave a lecture on semiconductor lasers, which had been developed the year before. Dr. Miller said that the lasers had two drawbacks: They needed low temperatures, and the pulses they emitted would always be limited, meaning their energy would also be limited.
As soon as Dr. Miller finished speaking, Dr. Kroemer rose and said, “‘But that’s a pile of nonsense,’” he recounted in his Nobel lecture. “Actually, I used some stronger language.”
What Dr. Kroemer realized was that if a semiconductor laser was built from two different materials, each with heterostructure properties, it would overcome the problems that Dr. Miller had outlined.
Dr. Kroemer wrote up his idea and submitted it to the journal Applied Physics Letters, which rejected it. But he was persuaded to submit it to Proceedings of the IEEE, a journal primarily geared toward engineering, and it was accepted. He filed for a patent in 1967.
The idea eventually led to the development of laser diodes, which underlie many of today’s most widely used technologies, including fiber-optic cables, satellite communications and bar-code readers.
It was for this work that he and Zhores I. Alferov, a Russian scientist who had independently developed a similar technology, were jointly awarded half of the Nobel. The other half went to Jack S. Kilby, an American scientist, for the development of the integrated circuit.
Herbert Kroemer was born on Aug. 25, 1928, in the city of Weimar, Germany, the eldest of three brothers. His father was a civil servant and his mother took care of the home. Neither parent had finished high school, but they emphasized education for their children. (When Dr. Kroemer eventually decided to study physics, he recalled, his father asked what that was and whether he could make a living at it.)
The young Herbert displayed an immediate aptitude for math and physics, but he was also bored and disruptive. In math, he got into trouble by teaching some other students methods that they did not understand, whereupon the teacher made a deal with him: If he would refrain from disrupting the class, he did not have to turn in any work and would be guaranteed a top grade. He stuck to the deal.
After high school, he entered the University of Jena, about 15 miles southeast of Weimar. The entire region, which lay in East Germany, was by then under the jurisdiction of the Soviet Union, and Dr. Kroemer, like many students and professors, chafed under the restrictive government. After only a year, he decided to leave.
This was in 1948, during the Berlin Blockade, when the Allies were flying supplies into West Berlin after the Soviets had cut off railway, road and canal access. Dr. Kroemer, who had worked for the summer at Siemens, the technology company, stood in line for two days at the airport, then flew out on a British plane.
Before he left, he had written to several universities seeking admission. He eventually found a spot at the University of Göttingen, where he was tutored by Fritz Sauter, who specialized in solid-state physics. After Dr. Kroemer gave a colloquium on a new idea relating to transistors, Dr. Sauter suggested that he submit his paper for his master’s in theoretical physics. A year later, in 1952, Dr. Kroemer obtained his Ph.D.
After Varian Associates, he worked for Semiconductor Research and Development Laboratory in San Jose, Calif. In 1968, he joined the faculty of the University of Colorado as a professor of electrical engineering. He joined the University of California, Santa Barbara, again as a professor of electrical engineering, in 1976 and finished his career there in 2012. He spent a good deal of time during his academic work developing and refining heterostructures.
Dr. Kroemer and his wife, Marie Louise Kroemer, had met at Göttingen, where she was a student. They had five children. Information about his survivors was not immediately available.
Though Dr. Kroemer did much of his groundbreaking research while working in private industry, he noted somewhat ruefully in his Nobel lecture that he had not been able to develop laser diodes, for example, because the companies he worked for initially saw no value in the idea. The problem, he said, was that people often want immediate uses for new technology.
“It is totally pointless when it comes to a new research idea to ask, ‘Well, what is it good for?’” he said, “because very often the applications have to be created first.”
