Tetsuya Kawamura

Hot Carrier Effects in Low-Temperature Polysilicon Thin-Film Transistors

We have studied the reliability of a low-temperature polysilicon (poly-Si) thin-film transistor (TFT). The drain avalanche hot electron effect was characterized by changing the stress gate and drain voltage dependence. Generation of hot carriers was confirmed using an emission microscope. It was found that the degradation was improved by the lightly doped drain (LDD) structure. A degradation model was proposed and analyzed along with a two-dimensional device simulator. Reasonable agreement with the experimental results was successfully obtained. It was found that the density of state (DOS) of poly-Si was increased by the hot carrier effect locally around the drain region.

Reliability of low temperature poly-silicon TFTs under inverter operation

We have studied the reliability of low-temperature polycrystalline-silicon thin-film-transistors (TFTs) under dynamic bias stress using a CMOS inverter circuit. A remarkable decrease in the mobility and the ON-current was observed in n-channel TFTs under dynamic stress. The degradation depends strongly on the falling edge of the voltage pulse and the number of pulses. Observation by emission microscopy revealed that hot electrons were generated around the edge of the drain region in the n-channel TFT. We also confirmed that TFTs with lightly doped drain were less degraded. Based on these experimental results, a new degradation model was proposed. The model suggests that upon the gate voltage drop, electrons move rapidly to the drain, thus, becoming hot and creating electron traps in the grain boundaries around the drain. Consequently, the ON-current is decreased.

Reliability of High-Frequency Operation of Low-Temperature Polysilicon Thin Film Transistors under Dynamic Stress

The reliability of low-temperature poly-Si under dynamic stress was evaluated. A decrease in the mobility and the ON current was observed under the dynamic stress. We found that the degradation depends strongly on falling time and frequency. Based on these findings, degradation was found to be induced by high electric field during pulse fall duration. This degradation is dominated by hot electrons and can be improved by adopting a lightly doped drain (LDD) structure.

Polycrystalline silicon recrystallized with excimer laser irradiation and impurity doping using ion doping method

Polycrystalline silicon (poly‐Si) thin films having large grain size have been obtained by XeCl excimer laser annealing of a‐Si deposited by plasma enhanced chemical vapor deposition on SiNx. The grain size of poly‐Si reaches to about 300 nm with excimer laser irradiation around 500 mJ/cm2. For excimer laser irradiation from 500 to 600 mJ/cm2, instead of further enlargement of grains, amorphous regions appear and increase in the grains. Most of the Si layer becomes an amorphous state at pulse energy density over 600 mJ/cm2. On the other hand, impurity implantation to a‐Si layers using non‐mass‐separated ion doping equipment and a subsequent activation step with excimer laser irradiation enable us to obtain recrystallized poly‐Si layers having low resistivity. By using ion doping and excimer laser annealing, we have obtained excellent n‐channel thin film transistors, the field effect mobility in excess of 40 cm2/V s and the ON/OFF current ratio of more than 106, respectively.

Effect of Residual Phosphorus on Amorphous Silicon Thin Film Transistors

Effects of residual phosphorus in the channel region of amorphous silicon thin film transistors(a-Si TFTs) on the TFT characteristics were quantitatively investigated. Concentration and the depth profile of the residual phosphorus were measured by high resolution secondary ion mass spectroscopy(SIMS). The OFF characteristics of a-Si TFTs were also measured. The SIMS data showed that the phosphorus exists about 100nm deep into intrinsic a-Si(i-a-Si), but the OFF characteristics showed that the activity of the residual phosphorus is 4 order of magnitude lower than that of heavily phosphorus doped a-Si( + -a-Si). The residual phosphorus is found to be inactive and stable, and has little effect on a-Si TFT characteristics. These results enabled us to fabricate inverted staggered a-Si TFTs by the simplest process using only 2 photo-mask steps and 1 self-aligned exposure.