High-electron-mobility transistor
The applications of HEMTs include microwave and millimeter wave communications, imaging, radar, radio astronomy, and power switching.
[3] The invention of the high-electron-mobility transistor (HEMT) is usually attributed to physicist Takashi Mimura (三村 高志), while working at Fujitsu in Japan.
He conceived the HEMT in Spring 1979, when he read about a modulated-doped heterojunction superlattice developed at Bell Labs in the United States,[4] by Ray Dingle, Arthur Gossard and Horst Störmer who filed a patent in April 1978.
[4] Independently, Daniel Delagebeaudeuf and Tranc Linh Nuyen, while working at Thomson-CSF in France, filed a patent for a similar type of field-effect transistor in March 1979.
[4] One of the earliest mentions of a GaN-based HEMT is in the 1993 Applied Physics Letters article, by Khan et al.[8] Later, in 2004, P.D.
It used atomic layer deposition (ALD) aluminum oxide (Al2O3) film both as a gate dielectric and for surface passivation.
[9] Field effect transistors whose operation relies on the formation of a two-dimensional electron gas (2DEG) are known as HEMTs.
In HEMTS electric current flows between a drain and source element via the 2DEG, which is located at the interface between two layers of differing band gaps, termed the heterojunction.
[12] The advantages of HEMTs over other transistor architectures, like the bipolar junction transistor and the MOSFET, are the higher operating temperatures,[10] higher breakdown strengths, and lower specific on-state resistances,[3] all in the case of GaN-based HEMTs compared to Si-based MOSFETs.
In the case of GaAs HEMTs, they make use of high mobility electrons generated using the heterojunction of a highly doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case).
This diffusion of carriers leads to the accumulation of electrons along the boundary of the two regions inside the narrow band gap material.
The term "modulation doping" refers to the fact that the dopants are spatially in a different region from the current carrying electrons.
InP and GaN are starting to replace SiGe as the base material in MODFETs because of their better noise and power ratios.
In practice, the lattice constants are typically slightly different (e.g. AlGaAs on GaAs), resulting in crystal defects.
The buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel.
Due to the crystal orientation typically used for epitaxial growth ("gallium-faced") and the device geometry favorable for fabrication (gate on top), this charge sheet is positive, causing the 2D electron gas to be formed even if there is no doping.
By sufficient doping of the barrier with acceptors (e.g. Mg), the built-in charge can be compensated to restore the more customary eHEMT operation, however high-density p-doping of nitrides is technologically challenging due to dopant diffusion into the channel.
