Dr. Mimura invented the High Electron Mobility Transistor (HEMT) with a new structure, in which two layered semiconductors are stacked. He revealed that HEMT has excellent high-frequency characteristics because of its high mobility nature of electrons. This invention has led to significant advancements both in information and communications technology and in physics studies of electrons confined in ultrathin conductive layers.
In 1979-80, Dr. Takashi Mimura invented a new transistor, called High Electron Mobility Transistor (HEMT), in which two different semiconductors were stacked (1, 2). He realized that if one semiconductor with a wider gap is doped with donor impurities, electrons with high mobility would be accumulated in the other along the interface of the two. He succeeded in the first operation of the HEMT by controlling the number of electrons with a voltage applied to its gate and noted that high mobility electrons in the HEMT would yield excellent high-speed performances. Then he played leading roles in the development of HEMTs as high-frequency devices and promoted their applications to microwave receivers for radio astronomy and receivers for broadcasting satellite (BS) system, contributing a great deal to the progress of information and communications technology (3). Moreover, since electrons confined in the ultrathin layer of HEMTs can move freely only along the interface and behave as two-dimensional electrons with very high mobility, the HEMT has immensely contributed to physics studies of electrons with reduced dimensions (4).
In 1970, a decade prior to the HEMT invention, Esaki and Tsu proposed a man-made superlattice (SL) in which two kinds of ultrathin semiconductor layers of about 10 nm in thickness were alternately stacked (5). Stimulated by this work, SLs of GaAs and AlGaAs layers were studied to show the electron confinement in GaAs layers. It was discovered in 1978 that if such a SL structure was formed by putting positively-charged donor impurities only into AlGaAs layers, electrons, confined in GaAs layers, would show high mobility, as they were separated from impurities (6). Dr. Mimura, inspired by this discovery, noticed that, if a single AlGaAs layer with donor impurities is deposited onto an undoped GaAs, it will then similarly induce high-mobility electrons along the interface. The HEMT was invented by using this concept.
Because of their superior high-frequency characteristics, HEMTs are widely used in such areas as receivers for BS and GPS systems, mobile phones and their base stations, and millimeter-wave car-borne radars for collision avoidance; HEMTs now serve as one of the core high-speed devices, on which the information and communications society is built. Initially, HEMTs were fabricated by using mainly the AlGaAs-GaAs pair, but the choice of materials was expanded to the InAlAs-InGaAs pair. Moreover, a psudomorphic system of an ultrathin InGaAs layer embedded in lattice-mismatched GaAs has been widely used to fabricate excellent high speed low noise HEMTs, operating in the microwave/millimeter wave region (7). AlGaN-GaN HEMTs have been also developed (8) and now widely used as high-frequency power devices in base stations of mobile phone systems and also as high-voltage power devices in switching power-supply systems.
As two-dimensional electrons confined in the ultrathin conductive layer along the interface of HEMTs are nearly free from influences of impurities and interface roughness, HEMT structures have greatly contributed to the progress of physics studies of low-dimensional electrons.
As mentioned above, the invention of HEMTs and related works by Dr. Mimura are indeed a Kyoto Prize worthy achievements, as they have made great contribution to the progress of information and communications technology and to the advancement in physics studies of low-dimensional electrons.
(1)Mimura T, et al. (1980) A new field-effect transistor with selectively doped GaAs/n-AlxGa1-xAs heterojunctions. J. J. Appl. Phys. 19: L225-L227.
(2)Mimura T (1987) Japan Patent 1409643.
(3)Suzuki S, et al. (1986) Detection of the interstellar C6H radical. Publ. Astron. Soc. Japan 38: 911-917.
(4)Tsui DC, et al. (1982) Two-dimensional magnetotransport in the extreme quantum limit. Phys. Rev. Lett. 48: 1559-1562.
(5)Esaki L & Tsu R (1970) Superlattice and negative differential conductivity in semiconductors. IBM J. Res. Dev. 14: 61-65.
(6)Dingle R, et al. (1978) Electron mobilities in modulation-doped semiconductor heterojunction superlattices. Appl. Phys. Lett. 33: 665-667.
(7)Yamashita Y, et al. (2002) Pseudomorphic In0.52Al0.48As/In0.7Ga0.3As HEMTs with an ultrahigh fT of 562 GHz. IEEE Electron Device Lett. 23: 573-575.
(8)Khan A, et al. (1993) High electron mobility transistor based on a GaN-AlxGa1-xN heterojunction. Appl. Phys. Lett. 63: 1214-1215.