The metamorphic HEMT and its applications in remote sensing

The metamorphic high electron mobility transistor (mHEMT) concept exploits the superior high speed and low noise performance of In-rich channels in InAlAs/InGaAs hetero-structures, and combines this with the advantageous properties of a GaAs substrate in terms of ease of handling and cost efficiency. The electrical performance of the mHEMT is found to be comparable to InP-based HEMT transistors, and the technology trades the mechanical advantages of a GaAs substrate for the thermal advantages of an InP substrate and the lower complexity of the required epitaxial layer sequence. The Fraunhofer IAF is developing high performance mHEMT technology and its accompanying monolithic millimetre-wave integrated circuit (MMIC) processes. Currently, the active devices realized with gate lengths down to 35 nm achieve cutoff frequencies in excess of 500 GHz (f ind t) and 700 GHz (f ind max), allowing for the realization of active electronics covering the entire millimetre-wave frequency range. High-speed electronics have been demonstrated up to 310 GHz, and even higher operating frequencies in the sub-millimeter-wave range are expected to be achievable with the current technology. Since, in a field-effect transistor, essentially the same physical properties are responsible for the speed and the noise performance (e.g. carrier mobility and velocity), a device offering gain at high frequencies will at the same time offer low-noise properties also at low frequencies. Many applications in remote sensing are particularly striving for high receiver sensitivity, and are thereby imposing stringent demands on the noise figure of pre-amplification stages. Typical applications in remote sensing along with their respective operating frequencies are indicated in Figure 1. Even at moderate frequencies in the microwave range (3 to 30 GHz), e.g. in intermediate frequency (IF) and radio frequency (RF) amplifiers in radio astronomy, lowest possible noise figures are desirable. Although many semiconductor technologies are able to cover this frequency range, the InP HEMT and the GaAs mHEMT are today's best performing technologies in terms of noise figure, especially so if cooled down to cryogenic temperatures.