Herbert Kroemer - Scientists, Family and Childhood

Herbert Kroemer was born on August 25, 1928, in Weimar, which after the Second World War became a part of German Democratic Republic (East Germany). None of his parents attended high school. His father worked for city administration and his mother was a typical German homemaker.

Herbert is the eldest of his parent’s three children. Though little educated, his parents were determined to give their children the best education. His mother was especially keen on it and relentlessly pushed him to score the highest mark in class.

He was a brilliant student, especially fond of physics, chemistry and mathematics. However, just like many other brilliant students, young Herbert frequently felt very bored and entertained himself by disturbing his classmates. It was his academic excellence, which prevented him from being expelled.

In 1947, he passed out from gymnasium and entered the University of Jena with physics. Unfortunately, by the following year, the University came under state scrutiny and students began to disappear; some escaped to West Germany, others were arrested and sent to work in mines.

In 1948, Kroemer moved to Berlin as a summer student at the Siemens Company. As the ‘Berlin airlift’ began he decided to move to West Germany via one of the empty airlift return flights. Before going over, he wrote to several West German universities for admission, but did not receive any reply.

On reaching West Germany, he remembered one of his Jena professors, who had asked him to convey his greetings to Professor König in Göttingen. Therefore, he met König, who told him the admission to physics department was closed.

Nonetheless, he asked him to meet Professor Richard Becker and Dr. Günther Leibfried for a friendly chat. They in turn asked him to meet Wolfgang Paul and Robert Pohl. Soon he realized that the professors were actually trying to evaluate him.

Whatever was the purpose behind these ‘friendly chats’, he ultimately obtained admission at the University of Göttingen and found the environment there “wonderfully stimulating”. However, life was not easy for him.

As his father could no longer support him because of currency difference he had to work in an aluminum factory in night shifts and study during the day. In time, he started working under Fritz Sauter on what later began to be known as band offsets at heterojunctions for his diploma (MS) thesis.

Soon, he became engrossed in heterojunctions. Sometime now, he also gave a talk on John Bardeen and Walter Brattain's paper ‘Physical Principles Involved in Transistor Action’ and made some suggestions about a few points raised by the authors.

Sauter was highly excited about it and asked him to stop working on diploma thesis and submit his paper immediately. Although he objected to this, but did what he was told. Thus in 1951, he received his diploma in theoretical physics.

Sauter now wanted Kroemer to concentrate on his PhD thesis and following his advice Kroemer began working on hot-electron effects, in the collector space-charge layer of the then-new transistor. Eventually he obtained his PhD in 1952.

Soon after receiving his PhD in 1952, Herbert Kroemer began his career as a ‘House Theorists’ at the Central Telecommunications Laboratory (FTZ) of the German postal service. His duty was to answer any theoretical question that any group member might have and give weekly talk on any appropriate subject.

Later, he began working on the frequency limitations of the new transistors, which led to his work on heterostructures. In 1954, he wrote a paper outlining his first ideas about the heterostructure bipolar transistor.

Later in the same year, he shifted to the United States of America and joined the RCA Laboratories in Princeton, New Jersey as research scientist. Here he resumed his work on heterostructures and published two important papers.

In one of them, published in 1957, he spelled out the concept of quasielectric fields, which he considered to be the fundamental design principle for all heterostructures. Unfortunately, it attracted little attraction.

Soon he began to feel homesick and in 1957 went back to Germany. There he joined Philips Semiconductor Research Group in Hamburg as the Group Leader. However, he was not fully satisfied with the work.

Therefore on getting a call from Varian Associates in Palo Alto, California, he returned to America in 1959 and joined the institute as Senior Scientist. It was while working here in 1963 that he proposed the theory that would bring him the Nobel Prize many years later.

This was also the year, when he concluded that it was possible to achieve a steady-state population inversion in semiconductor diode laser even at room temperature if the outer regions are provided with a wider energy gap.

Unfortunately, the paper was ignored. He was also refused funds for carrying on further experimentation. Therefore for next ten years, he worked on Gun effect, which concerns the high-frequency oscillation of electrical current flowing through certain semiconducting solids.

In 1966, he left Varian Associate and joined Semiconductor Research and Development Laboratory as Head of the New Phenomena Section. Two years later, he made another move and joined the University of Colorado at Boulder as Professor of Electrical Engineering in 1968.

It was at Colorado University that he returned to heterostructure. The early 1970s was spent in working on the theory of band offsets, which refers to the relative alignment of the energy bands at a semiconductor heterojunction.

In 1976, he moved to University of California, Santa Barbara, where he remained until his retirement in 2012. Here he continued his work on band offsets and developed an experimental method to determine these energy bands.

In the late 1970s, he returned to heterostructure bipolar transistor (HBT), a work he had started in early 1950s. He soon realized that the technology developments, which enabled the invention of the DH laser, was also appropriate for development of the HBT and began to advocate that strongly.

Professor Kroemer retired in 2012. However, he remains attached to the University of California as Professor Emeritus and continues to work on various semiconductor topics.

Herbert Kroemer is best known for his work on heterostructure bipolar transistors (HBP). In 1957, he published a paper, in which he described his idea of quasielectric fields, which turned out to be the fundamental design for all heterostructures.

Later in 1963, he carried out the calculation and theoretically showed that heterostructure transistor would be superior to a conventional transistor, especially for certain high-frequency uses. This is because unlike most computer chips and semiconductor components heterostructures are made of different materials.

In 2000, Herbert Kroemer was jointly awarded the Nobel Prize in Physics "for developing semiconductor heterostructures used in high-speed- and opto-electronics". His co-recipients were Zhores I. Alferov and Jack S. Kilby. Kroemer and Alferov got one-fourth of prize share each, while Kilby got the other half "for his part in the invention of the integrated circuit".

In addition, he has received numerous other awards, among which J J Ebers Award (1973) and IEEE Medal of Honor (2002) are most significant.

He was elected a member of National Academy of Engineering (Foreign Associate) in 1997 and National Academy of Sciences (Foreign Associate) in 2003.

Professor Kroemer is married to Marie Louise Kroemer, whom he met as a student at the University of Göttingen. The couple has five children.

Asteroid 24751, discovered on 21 September 1992 has been named as Asteroid Kroemer in his honor.

When Kroemer decided to pursue higher studies with physics, his father, not knowing anything about the subject, merely asked him if he could make a decent living with it. When Kroemer assured him that he could always become a teacher at a gymnasium, he was contented.

Although he was appointed as the Professor of Electrical Engineering at the University of Colorado and later at the University of California his degree in Engineering is honorary.