| Citation | Distinguished for his work on negative differential resistance (NDR), instabilities and hot-electron transport in semiconductors, he was a joint discoverer of the electron-transfer mechanism (Ridley-Watkins-Hilsum effect) which underlies microwave generation in Gunn diodes, and he was the first to discover the impurity-barrier mechanism for NDR and to demonstrate its existence in germanium. He was also the first to describe the consequences of NDR instabilities in terms of propagating dipole domains and current filaments, and the existence of these non-linear entities has been amply verified in a wide variety of solids. His work on acoustoelectric instabilities led to his invention of the microsonic analogue of the laser. Believing that simple models should always complement numerical simulations he developed a highly successful "lucky-drift" theory of impact ionization whch allowed him to predict the existence of field striations in insulators, analagous to those in gaseous plasmas. Throughout his research he has made original contributions to the theory of electron transitions in solids, particularly impurity scattering and multiphonon processes, and this work is embodied in his monograph "Quantum Processes in Semiconductors", widely used as a reference text. Recently he has pioneered a study of high-field transport in low-dimensional semiconductors which has led to the prediction of novel NDR mechanisms, and to some original theories of well-capture and intersubband transitions involving confined photons. His interest in popularizing physics is manifested in his book "Time, Space and Things" which haas been much translated and widely quoted. He has held Distinguished Visiting Professorial appointments at Cornell and the Danish Technical University, and has also held research appointments at Princeton and Stanford. |