Rvvvj. Rao et al., Thermal stability of MISFET with low-temp molecular-beam epitaxy-grown GaAs and Al0.3Ga0.7As gate ins., IEEE RELIAB, 49(2), 2000, pp. 147-152
GaAs and Al0.3Ga0.7As epilayers grown at LT (low-temperature) by MBE (molec
ular beam epitaxy) were used as insulators in the fabrication of MISFET (me
tal insulator semiconductor field-effect transistor) devices. Parametric ch
anges were used to evaluate the thermal stability of MISFET, to identify fa
ilure mechanisms and validate the reliability of these devices. A similar M
ESFET was used as the reference in evaluating the MISFET performance. The L
T-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 Angs
trom thick LT-GaAs gate insulators exhibited stable characteristics with th
ermal 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 An
gstrom LT-GaAs samples; whereas the 1000 Angstrom thick LT-GaAs samples exh
ibited 1/f(3/2) noise, which was attributed to: i) the thermal noise genera
ted at the interface of the insulator, and ii) the active layer due to the
outdiffused metallic arsenic. Under thermal stress, this metallic arsenic c
ontributed to composition changes at the interface in MISFET with thicker L
T-GaAs gate insulators, To corroborate our claims, reverse gate-drain curre
nt degradation experiments were carried out at 120 degreesC, 160 degreesC,
200 degreesC, and 240 degreesC. The activation energy obtained from these e
xperiments for 1000 Angstrom thick LT-GaAs samples was 0.94 eV, which indic
ated composition changes at the interface of insulator and active layer, Fo
r further confirmation, transconductance frequency dispersion studies were
carried out before and after thermal stress on these MISFET. The transcondu
ctance 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 sta
ble characteristics.
These results indicate that composition changes had occurred at the interfa
ce 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.