Influence of surface layers on the RF-performance of AlInAs-GaInAs HFETs

The influence of thickness and doping level of the GaInAs cap layer in AlInAs-GaInAs-InP HFET structures on the DC and RF performance is systematically investigated. The authors compare three different approaches, the undoped cap layer, the highly doped thick cap layer, and, as a new approach, the thin doped and therefore surface depleted cap layer. HFET devices with 0.3 mu m gates have been processed. While all devices demonstrate f/sub T/-values around 80 GHz, distinct differences are observed for the f/sub max//f/sub T/ ratios from 1 (highly doped cap) over 1.3 (undoped cap) to 2.7 (surface depleted cap). The best f/sub max/ of 240 GHz is achieved for the new cap layer approach. A systematic investigation of the influence of the g/sub m//g/sub d/ and C/sub gs//C/sub ds/ ratios demonstrates the strong influence of a proper layout of the cap layer at the drain side of the gate region.<>