The effect of passivation on the performance of AlGaN/GaN heterostructure field-effect transistors

Four different passivations are applied on AlGaN/GaN heterostructure field-effect transistors ( HFETs) and their performance is compared. SiO2 and SiN layers of different thicknesses and deposition temperatures, which induced different stresses, are used for passivation. The sheet charge density and the saturated drain current increased up to 27% and 37%, respectively, with increasing stress from compressive (- 150 MPa) to tensile ( 50 MPa). The change of the stress-induced sheet charge density is 1.5 x 10(11) cm(-2) for 100 MPa. For non-stressed conditions, a passivation- induced sheet charge density of similar to 1.3 x 10(12) cm(-2) is extrapolated. This indicates that the passivation- induced stress is only a partial effect of the HFET passivation, compared to the surface states reduction. The current collapse evaluation, by consecutive I - V sweeps with 20 ms and 416 mu s integration time, shows that the devices with 30 nm thick SiN passivation exhibited the best performance. However, two processes with different time constants need to be considered. Trapping processes at the GaN/passivation interface as well as in the AlGaN( GaN) barrier layer are supposed to be responsible for the observed behaviour.