Influence of trap states on dynamic properties of single grain silicon thin film transistors

Abstract

The transient properties of single grain-thin film transistors ͑SG-TFTs͒ with high electron mobility have been studied. Overshoot current induced by trap states has been observed in most of the devices. A method of ac measurements has been used to investigate the trap processes. Both transient and ac measurements show that the response of some SG-TFTs with high field effect mobility is dominated by a single trap level. Bias stressing on SG-TFT can induce more trap states and thus change the ac response of the device. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2193049͔Excimer-laser crystallization of amorphous silicon films is a well-established method for producing large-grain polycrystalline silicon thin film transistors ͑poly-Si TFTs͒ on glass substrates, which makes it possible to achieve the system-on-panel active matrix liquid crystal display. 1 The poly-Si TFTs have much higher field-effect mobility, typically about 100 cm 2 / V s, compared to that of amorphous silicon TFTs. However, it is still much lower than that of metal-oxide-silicon ͑MOS͒ transistors formed on bulk Si wafers. Recently, a development in TFT technology is the fabrication of location controlled single grain-thin film transistor ͑SG-TFT͒ by the method referred to as microCzochralski or grain-filter process. [2][3][4] Since there are few or no twin boundaries in the active region of SG-TFTs, higher mobilities of SG-TFTs ͑400 cm 2 /V s͒ than standard lasercrystallized poly-Si TFT are obtained. While the static characteristics of SG-TFTs have been studied before, little work has been devoted to the dynamic properties. The dynamic properties of TFTs are very important for circuit operation. Furthermore dynamic measurements are also a good method to get the information on trap parameters, 5,6 which are needed for device simulation. In this letter, we report on the dynamic characteristics of SG-TFTs. Two methods are used: pulsed transient current measurement 5,7 and small signal ac analysis. 8 N-channel SG-TFTs employed here were fabricated inside location-controlled grains as described before. 2 The gate size is width/length= 3.21 m / 2.88 m. The gate oxide is a 162 nm thick SiO 2 deposited by the low temperature oxide ͑LTO͒ process. TFT characteristics of devices fabricate at same condition have big variation, which can be attributed to the presence of twin boundaries with different numbers and different configurations in the channel. 4 We found the field effect mobility of these devices varied from 500 cm 2 /V s down to 200 cm 2 /V s.The transient current measurement was described before. 5,7 A train of pulses was applied to the gate by an Agilent 33250A wave form generator. The source was grounded and a constant voltage ͑V DS = 0.1 V͒ was applied to the drain by a Keithley 230 voltage source. The transient current was amplified by a Keithley 428 current voltage converter and detected by a LC584AL digital oscilloscope. The output signal was averaged over 1000 periods to reduce the noise. The pulse rise time was set to 20 ns, ch...

Highly photosensitive thin film transistors based on a composite of poly(3-hexylthiophene) and titania nanoparticles

“…Since the trap states have different capture cross sections and energy levels, the recovery process will show a broad distribution of relaxation time. 26 We consider that electrons accumulated in the TiO 2 nanoparticles will decrease the electrostatic potential of the active layer ͑P3HT/TiO 2 composite film͒ and thus change the threshold voltage of the device, as shown in Fig. 7.…”

Organic phototransistor based on poly(3-hexylthiophene)/TiO2 nanoparticle composite

“…Electrons trapped in TiO 2 nanopartilces will increase the potential of the channel which can be regarded as a floating body effect of the transistor. 18,19 Therefore the more electrons trapped in the channel, the bigger shift of the threshold voltage can be induced.…”

An experimental study of band gap states electrical properties in poly‐Si TFTs by the analysis of the transient currents

“…In both cases, AC and transient, the currents have been correlated to the trapping and emission of carriers in these states [3][4][5]. Regarding the drain current transients, the overshoot method has been employed for the determination of hole generation lifetime [6], while recently the existence of the undershoot effect has been reported when the device gate is switched from strong to weak or moderate inversion [7].…”

“…From the experimental point of view, several methods have been employed based on the dispersion of AC (trans)conductance [3,4] or the transient response of the drain current [4,5] monitored after an electrical excitation. In both cases, AC and transient, the currents have been correlated to the trapping and emission of carriers in these states [3][4][5].…”