HALL-MOBILITY PROFILING IN HIGH-ELECTRON-MOBILITY TRANSISTOR STRUCTURES

We present results on Hall mobility and carrier density profiles as a function of etched depth in pseudomorphic high electron mobility trans istor (HEMT) structures. The profiling technique involved the repetiti ve etching of a thin layer of semiconductor material by the process of anodic oxidation and oxide strip followed by a measurement of sheet r esistivity and Hall coefficient at both room temperature and 77 K. The work was carried out on a dedicated instrument, the HL5900 + Hall pro filer from Bio-Rad Microscience. The results show that anodic oxidatio n offers very good control on the etch step and hence good reproducibi lity of Hall profiles in any given HEMT structure. The structures inve stigated were conventional pseudomorphic HEMTs grown on GaAs substrate s. As expected, the sheet Hall mobility increased as the top layer was gradually removed, reaching a well defined maximum value before the t wo-dimensional electron gas (2DEG) layer was fully depleted. Peak valu es ranged from 4500 to 38 000 cm2 V-1 s-1 at 77 K and 2700 to 5900 CM2 V-1 s-1 at room temperature. The 2DEG sheet carrier densities yielded equal values at room temperature and 77 K that ranged from 1.2 to 2.3 x 10(12) cm-2. These results indicate that dual temperature Hall prof iling is a very useful tool for characterizing the 2DEG layer paramete rs in HEMT structures and could help determine the optimal depth for t he gate recess in real devices.