HEMT's Design for Applications beyond 100GHz

摘要

Electronics up to 100 GHz have applications in atmospheric sensing, radio astronomy, passive imaging applications, wide-band communication systems. Millimeter Wave (MMW) analog and numerical circuits have to be developed. HEMTs on InP substrate are largely used in D-band (110-150 GHz) and G-band (140-220GHz) circuits. Improvement of frequency operation has been obtained by reduction of gate length to nanometer values and higher Indium content up to 80%. Another field of application is induced by the demand of higher bit-rate communication, which is rapidly growing. 40Gbit/s system has been recently developed [9] and intensive research on 80Gbit/s and 160Gbit/s is being done. Analog and numerical circuits used in such optical transmission systems can also be realized with nanometer gate length InP-based HEMTs. With InP-based HEMTs, it is possible to reach f_T higher than 472GHz with 30 nanometer gate length. To obtain that good value, gate recess undercut has been optimized. Cutoff frequency f_T is an important parameter in particularly for numerical circuits. However reduction of gate length will involve an increase of short channel effects. This point will limit the maximum oscillation frequency f_(max) and microwave performance of analog circuits. To avoid this effect, layer structure has to be correctly designed for sub-100 nanometer gate length HEMTs. In this paper, we present an optimized InAlAs/InGaAs/InP layer structure for sub-100 nanometer gate length HEMTs using a scaling down rule. HEMTs have been fabricated on such layer structure and compared with devices fabricated on standard structure usually used for 100 nanometer gate length HEMTs. Same gate lithography of 70 nanometer length has been achieved on both layer structures. DC and microwave characteristics are compared. Degradation of f_(max) in short channel HEMTs can be overcome by the use of transferred-substrate technological process. This technique offers the possibility to realize insulating buffer HEMTs or by the addition of a second gate under the channel, (double gate HEMTs). Technological process and electrical results of TS-HEMTs will be presented in this paper. Finally passive elements and specially transmission lines will be described and presented. These elements are also a limiting factor for the frequency raise of mm-wave circuits. Indeed these passive structures have to present low loss, high characteristic impedance range. Moreover for high speed mixed-mode circuits, these transmission lines have to be blinded to avoid any clock cross-talk phenomena.