High-Speed AlInN/GaN HEMTs on SiC and (111) HR-Silicon (INVITED)
AlGaN/GaN high electron mobility transistors (HEMTs) have greatly matured after more than a decade and a half of frantically paced development. The current interest in extending GaN -based HEMTs to millimeter-wave frequencies requires thinner top barriers to maintain a favorable channel aspect ratio LG/d. Unfortunately, AlGaN/GaN two-dimensional electron gases (2DEGs) are subject to surface depletion when the top barrier thickness is thinner than ~15 nm . This issue can be circumvented by using AlN thin top barriers to boost channel sheet densities . An alternative solution relies on nearly lattice-matched AlInN/GaN heterostructures, as originally proposed by Kuzmík . This material system potentially offers a solution to strain-related device reliability concerns associated with conventional AlGaN/GaN heterostructures. Furthermore, surface depletion effects should be far weaker for the AlInN/GaN system, which should maintain excellent channel aspect ratios down to very short gate lengths. The feasibility of ultrathin barrier AlInN/GaN HEMTs was indeed verified down to 3 nm thick AlInN barriers .
In the present paper, we review the progress [5-6] in extending the bandwidth of AlInN/GaN HEMTs well into the millimeter-wave régime for AlInN/GaN HEMTs grown on semi-insulating SiC and on (111) high-resistivity silicon (HR-Si) substrates. At the time of writing, these devices offer the highest bandwidths available for nitride transistors. 55 nm gate devices on SiC feature cutoff frequencies as high as fT = 205 GHz with a simultaneous fMAX = 191 GHz, while 80 nm gate HEMTs on (111) HR-Si show fT = 143 GHz with a simultaneous fMAX = 178 GHz. These numbers represent the highest bandwidths attained by GaN HEMTs on silicon substrates as well.
The performance of our AlInN/GaN HEMTs on HR-Si will be contrasted to state-of-the-art results achieved in our lab with recessed gate AlGaN/GaN HEMTs on HR-Si. Some of the above mentioned world-records will fall with new announcements from competing groups at IEDM, and we hope to have new results to show at MANTECH.