Influence of quantum-well width on device performance of Al/sub 0.30/Ga/sub 0.70/As/In/sub 0.25/Ga/sub 0.75/As (on GaAs) MODFETs

An experimental study in which the quantum well width (W) is varied from 45 to 200 AA is discussed. Optimum device performance was observed at a well width of 120 AA. The 0.2- mu m*130- mu m devices with 120-AA quantum-well width typically exhibit a maximum channel current density of 550 mA/mm, peak transconductance of 550 mS/mm, and peak current gain cutoff frequency (f/sub T/) of 122 GHz. These results have been further improved in subsequent fabrications employing a trilevel-resist mushroom-gate process. The 0.2- mu m*50- mu m devices with mushroom gate exhibit a peak transconductance of 640 mS/mm, peak f/sub T/ of 100 GHz, and best power gains cutoff frequency in excess of 200 GHz. These results are among the best ever reported for GaAs-based FETs and are attributed to the high two-dimensional electron gas (2DEG) sheet density, good low-field mobility, low ohmic contact, and the optimized mushroom gate process. >