Parameter Extraction of Short-Channel a-Si:H TFT Including Self-Heating Effect and Drain Current Nonsaturation

We present an extraction procedure of the above-threshold parameters of a modified level-15 model of hydrogenated amorphous-silicon thin-film transistors (a-Si:H TFTs). This procedure is useful for model parameter extraction for short-channel devices, including the self-heating effect (SHE) and the nonsaturating drain current effect via the channel length modulation (CLM). The drain current formula of the AIM-Spice level-15 model was modified to include the nonsaturating drain current and SHE in the model. This procedure is also useful for devices in which the source–drain contact resistance $R_{sd}$ is comparable to, or greater than, the channel resistance, which is common in short-channel a-Si:H TFTs with a channel length of 10 $\mu\hbox{m}$ or less. We propose a modified integral function method for the extraction of contact resistance and threshold voltage. This method includes features of the ratio method and the integral function method. Using this modified integral function method, we extract both the threshold voltage $V_{t}$ and the series contact resistance $R_{sd}$ at the very beginning of the extraction process. When simulated data were used for extraction, $V_{t}$ and $R_{sd}$ extracted by our proposed method agreed with the true parameter values better than the parameters extracted by the integral function method or the ratio method. For a short-channel device with a significant SHE, the field-effect mobility $\mu_{ \rm FE}$ parameters were separately extracted for the linear and saturation regions, because $\mu_{\rm FE}$ was higher in the saturation region than that in the linear region, which is probably caused by the SHE-induced rise of channel temperature. The calculated $I$–$V$ characteristics based on the extracted parameters fit the experimental data well in both the short- and long-channel devices. This suggests that the modified drain current model including the SHE and the nonsaturating drain current is valid; the proposed parameter extraction procedure is valid and may be implemented in the circuit simulator, such as AIM-SPICE, with some further improvement in the field-effect mobility formula when SHE is present.