Hot-Electron Degradation of Gallium Arsenide Metal-Semiconductor Field-Effect Transistors.

The physical mechanism of gradual degradation of GaAs MESFETs during RF overdrive is investigated in detail. A hot-electron effect was found responsible for this so-called "power slump" problem. Hot electrons produced by a large drain-gate voltage swing, tunnel from the MESFET channel and get trapped in SiN. These trapped electrons (i) increase surface depletion, hence reduce maximum channel current, transconductance and transistor gain, (ii) increase knee voltage through an increase in series channel resistance, (iii) relax gate-drain field distribution, thereby suppressing avalanche breakdown, (iv) decrease gate-drain capacitance, hence rm S_{22} under open-channel condition, and (v) increase surface leakage through trap hopping in SiN. The damage to SiN can only be partially recovered by deep UV illumination or 200^circrm C anneal. The evidence supports that trapping occurs in the bulk SiN, instead of at the GaAs/SiN interface. The possible chemical reaction responsible for this trap formation is breaking of the Si-H bond in SiN. An analytical theory of hot-electron effects, which combines hot-electron trapping with gate-drain breakdown and pinched-channel electro-luminescence, was developed and verified using experimental data and numerical simulations. Based on this theory, the rate of hot electron trapping was obtained and the threshold energy for trap formation was determined. The square-root time dependence given by the theory and the threshold energy of 1.9 eV were found consistent with gate current and electro-luminescence measurements. Numerical analysis was consistent with a trap density of the order of 5times10^{12}/rm cm^2 over a distance of approximately 0.1 murm m from the gate toward the drain, and it predicted the experimentally observed open-channel current reduction and gate-drain field relaxation. The spatial distribution of trapped electrons was directly observed by a novel high-voltage electron-beam-induced -current imaging technique. It confirmed the model's prediction. These results can be incorporated into large-signal transistor models for computer-aided circuit design. Such models would quantify trade-off between performance and reliability. An accelerated qualification procedure for the hot-electron-induced degradation trend is devised. This is based on the high-frequency waveform probing and high -sensitivity electro-luminescence measurements. Hot-electron-induced degradation was also found to take place in pseudomorphic high-electron-mobility transistors (PHEMT). The basic signatures of PHEMT degradation are similar to those of MESFETs, however some differences exist due to the structural differences between them. For PHEMT, in addition to SiN surface passivation, hot-electron traps may be formed in the AlGaAs layer under the gate. In addition, various temperature-activated degradation modes are more strongly coupled in the case of PHEMT, which requires analyzing them separately from the field-activated mode.