Thermal stability of MISFET with low-temp molecular-beam epitaxy-grown GaAs and Al0.3Ga0.7As gate Ins

Abstract

GaAs and Al0.3Ga0.7As epilayers grown at LT (low-temperature) by MBE (molecular beam epitaxy) were used as insulators in the fabrication of MISFET (metal insulator semiconductor field-effect transistor) devices. Parametric changes were used to evaluate the thermal stability of MISFET, to identify failure mechanisms and validate the reliability of these devices. A similar MESFET was used as the reference in evaluating the MISFET performance. The LT-Al0.3Ga0.7As MISFET showed superior thermal stability. The degradation in the performance of MISFET with 1000 angstrom thick LT-GaAs gate insulator was worse than those of the MESFET. On the other hand, MISFET with 250 angstrom thick LT-GaAs gate insulators exhibited stable characteristics with thermal stressing. LF (low frequency) noise studies on the TLM structures of MISFET layers exhibited 1/f noise in the LT-Al0.3Ga0.7As samples and 250 angstrom LT-GaAs samples; whereas the 1000 angstrom thick LT-GaAs samples exhibited 1/f3/2 noise, which was attributed to: i) the thermal noise generated at the interface of the insulator, and ii) the active layer due to the outdiffused metallic arsenic. Under thermal stress, this metallic arsenic contributed to composition changes at the interface in MISFET with thicker LT-GaAs gate insulators. To corroborate our claims, reverse gate-drain current degradation experiments were carried out at 120°C, 160°C, 200°C, and 240°C. The activation energy obtained from these experiments for 1000 angstrom thick LT-GaAs samples was 0.94 eV, which indicated composition changes at the interface of insulator and active layer. For further confirmation, transconductance frequency dispersion studies were carried out before and after thermal stress on these MISFET. The transconductance of MISFET with 1000 angstrom LT-GaAs gate insulators was degraded by 40% at 100 kHz after thermal stress. The rest of the samples exhibited stable characteristics. These results indicate that composition changes had occurred at the interface in thicker LT-GaAs MISFET structures. Our studies showed that thinner LT-layers are ideal for achieving higher transconductance and better thermal stability without sacrificing the power capability of MISFET.