Abstract :
Heterojunctions which have been used for years in optoelectronic devices, are now entering in microwave applications. The heterojunction bipolar transistor, proposed since 1957 in not matured in its technology. The new born selectively-doped heterojunction (1978) is gaining in interest in both aspects, physics of the two-dimensional electron-gas (2DEG) and electronic applications. This paper will first recall the transport properties of the 2DEG accumulated at the interface of undoped GaAs-n doped A1GaAs heterunctions. In particular, extremely high mobility and high electron velocity were observed. Growth of these selectively doped heterojunctions by MBE and MOCVD is then described. The best results were obtained by the former technique. Low-noise two-dimensional electron gas FETs (TEGFETs) are presented. With gate length of ~ 0.5¿m noise figure as low as 1.26, 1.4 and 2.3 dB were measured at 10, 12 and 17.5 GHz respectively, with associated gain of 12, 11 and 7.1 dB. The TEGFET have been shown to be the fastest semiconductor device at 300 K with 12.2ps (gate length ~1¿m). They also present low power dissipation (5 times less than conventional GaAs FETs). TEGFET frequency dividers operate up to 5.5 GHz (gate length ~l¿m).
