Hidetoshi Koyama

Decrease in on-state gate current of AlGaN/GaN HEMTs by recombination-enhanced defect reaction of generated hot carriers investigated by TCAD simulation

Abstract Changes in the on-state gate current of AlGaN/GaN high-electron-mobility transistors (HEMTs) under various electrical and thermal stress conditions have been analyzed by technology computer-aided design (TCAD) simulation. A larger gate current is observed under on-state bias condition than that under off-state bias condition. The TCAD simulation indicates that on-state gate current flows from the heated gate electrode to the AlGaN layer by tunneling or hopping through the gate depletion layer when we apply some deep-donor-type traps under the gate in the AlGaN barrier layer. The gate current is caused by electrons that flow and is pulled away by the applied gate-to-drain voltage under a high channel temperature condition. The deep traps benefit both the on- and off-state gate current behavior. We found that the on-state gate current is effectively decreased by electrical stress under the on-state condition. Electroluminescence measurement indicates that a large number of hot carriers are generated under this condition. The results suggest that the process-induced crystal defects are annealed out by non-radiative recombination of the generated hot carriers by a recombination-enhanced defect reaction mechanism. The change in the on-state gate current in the TCAD simulation can be successfully explained by the decrease in the donor traps.

Temperature dependence of TDDB characteristics of thin SiO/sub 2/ film for flash memory

Using a large area capacitor, the temperature dependence of TDDB characteristics for various electric fields was investigated in detail. From the experimental results, it is found that the Qbd value is more dependent on temperature rather than on the electric field. It is also found that the initial breakdown region is divided into two parts, above 175/spl deg/C, the activation energy of the front part-of the initial breakdown region is greater than the intrinsic region. It should be possible to use this newly obtained result efficiently to estimate the extrinsic breakdown distribution within a short period during the burn-in test. Therefore, the findings should contribute to the improvement of the reliability of the actual device.

Oxide thickness dependence of nitridation effects on TDDB characteristics

Abstract Wet oxide thicknesses dependence of nitridation effects on electrical characteristics, charge trapping properties and TDDB (Time Dependent Dielectric Breakdown) characteristics have been investigated. It is found that the difference of conduction current between the wet and nitrided wet oxide increases with increasing oxide thickness both for negative and positive bias to the gate until constant current stress is applied. After the stress, with decreasing oxide thickness both in wet and nitrided wet oxide leakage current increases. Up to ∼ 60 A no difference was observed between the wet and nitrided wet oxide but at ∼ 50 A nitrided wet oxide has less increase of current comparing to the wet oxide for the same stress. In wet oxide with increasing stress current density initial hole trap decreases but electron trap increases whereas in nitrided wet oxide has less initial hole trap and also electron trap is less comparing to the wet oxide. Both in wet and nitrided wet oxide for negative bias stress, time to 50 % breakdown decreases with decreasing thickness but at ∼ 50 A a turn-around effect was observed due to nitridation i.e., the 50 % breakdown time is greater for nitrided wet oxide comparing to the wet oxide. On the contrary, for positive bias stress 50 % breakdown time increases with decreasing oxide thickness both in wet and nitrided wet oxide. For positive bias also a turn-around effect is observed at ∼ 50 A i.e., 50% breakdown time is less in nitrided wet oxide comparing to the wet oxide. The improved reliability of nitrided wet oxide at the thin region of ∼ 50 A seems to be due to the increase of more SiN bond to the interface of oxide and Si comparing to the thick oxide of above ∼ 60 A for the same nitridation conditions.