Narrow Results

Nanocrystalline silicon (nc-Si) thin film transistors (TFTs) have potential for high-performance applications in large area electronics, such as next generation of flat panel displays and medical x-ray imagers, for pixel drivers, readout circuits, as well as complementary channel logic circuits for system-on-panel integration. This potential stems from reduced threshold voltage shift and higher transconductance, compared to amorphous silicon counterpart. In this paper, we discuss various TFT structures, their associated design and performance considerations, including leakage current and threshold voltage stability mechanisms.

A thin film transistor (TFT) characteristics measuring and bias stress applying system using an alternating current (AC) pulse sequence similar to a real driving pulse was developed to study the properties of hydrogenated amorphous-silicon (a-Si:H) TFTs under real operating conditions. Using this system, the application of a gate bias stress and the measurement of source-to-drain current were performed successfully. Degradation of the TFT transfer curve depended on the ratio of on time to off time for a fixed on time; a longer off time made the shift of threshold voltage V_TH smaller. In addition, degradation of transfer curves depended on the frequency of the driving pulse; a higher frequency pulse produced a larger degradation. These results could originate from the dependence of the direction of V_TH shift on the polarity of the gate bias, and the differences of injection barrier height and the mobility of the electron and hole. Using the AC driving pulse and the transient measurement system proposed in this study may be useful in understanding the response of TFTs under real operating conditions.

Channel mobility in the p-type polycrystalline silicon thin film transistors (poly-Si TFTs) is extracted using Hoffman method, linear region transconductance method and multi-frequency C-V method. Due to the non-negligible errors when neglecting the dependence of gate-source voltage on the effective mobility, the extracted mobility results are overestimated using linear region transconductance method and Hoffman method, especially in the lower gate-source voltage region. By considering of the distribution of localized states in the band-gap, the frequency independent capacitance due to localized charges in the sub-gap states and due to channel free electron charges in the conduction band were extracted using multi-frequency C-V method. Therefore, channel mobility was extracted accurately based on the charge transport theory. In addition, the effect of electrical field dependent mobility degradation was also considered in the higher gate-source voltage region. In the end, the extracted mobility results in the poly-Si TFTs using these three methods are compared and analyzed.

Low frequency noises in the p-type polycrystalline silicon thin film transistors are investigated. It shows a pure 1/f${}^{\gamma }$ (with $\gamma$ near one) noise behavior which can be explained by emission and trapping processes of carriers between trapping states. Subsequently, the gate voltage-dependent drain current noise power spectral densities closely follow the mobility fluctuation model, and the average Hooge’s parameter is then extracted. By considering traditional tunneling processes, the flat-band voltage spectral density is extracted and the concentration of traps in the grain boundary is calculated to be $7.17\times 10^{20}{\mathrm{\text{cm}}}^{-3}{\mathrm{\text{eV}}}^{-1}$. By converting the frequency to tunneling depth of carriers in the gate oxide, the spatial distribution of gate oxide trapped charges are obtained. Finally, the distribution of localized states in the energy band is extracted. The experimental results show an exponential deep states and tail states distribution in the band gap while $N_{\mathrm{\text{DD}}}$ is about $1.5\times 10^{20}{\mathrm{\text{cm}}}^{-3}{\mathrm{\text{eV}}}^{-1}$, $T_{\mathrm{\text{DD}}}$ is $\sim$617 K, $N_{\mathrm{\text{TD}}}$ is $\sim 3.6\times 10^{21}{\mathrm{\text{cm}}}^{-3}{\mathrm{\text{eV}}}^{-1}$ and $T_{\mathrm{\text{TD}}}$ is $\sim$265 K.

The transfer and low frequency noise characteristics of hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) were measured in the temperature range of 230–430 K. The variation of threshold voltage, field effect mobility and sub-threshold swing with increasing temperatures were then extracted and analyzed. Moreover, the shifts of low frequency noise in the a-Si:H TFT under various temperatures are reported for the first time. The variation of flatband voltage noise power spectral density with temperature is also calculated and discussed.

Graphene is a promising alternative to indium tin oxide for use in transparent conducting electrodes. We review recent progress in production methods of graphene and its applications in optoelectronic devices such as touch panel screens, organic photovoltaic cells, organic light emitting diodes and thin film transistors. In addition, we discuss important criteria such as optical transmittance, electrical conductivity and work function, which are critical considerations in the integration of graphene conductive films with optoelectronic devices.

In this work, (n)InSe/(p)ZnSe and (n)InSe/(p)ZnSe/(n)InSe heterojunction thin film transistor (TFT) devices are produced by the thermal evaporation technique. They are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion X-ray spectroscopy and optical spectroscopy techniques. While the InSe films are found to be amorphous, the ZnSe and InSe/ZnSe films exhibited polycrystalline nature of crystallization. The optical analysis has shown that these devices exhibit a conduction band offsets of 0.47 and valence band offsets of 0.67 and 0.74eV, respectively. In addition, while the dielectric spectra of the InSe and ZnSe displayed resonance peaks at 416 and 528THz, the dielectric spectra of InSe/ZnSe and InSe/ZnSe/InSe layers indicated two additional peaks at 305 and 350THz, respectively. On the other hand, the optical conductivity analysis and modeling in the light of free carrier absorption theory reflected low values of drift mobilities associated with incident alternating electric fields at terahertz frequencies. The drift mobility of the charge carrier particles at femtoseconds scattering times increased as a result of the ZnSe sandwiching between two InSe layers. The valence band offsets, the dielectric resonance at 305 and 350THz and the optical conductivity values nominate TFT devices for use in optoelectronics.

Nanocrystalline silicon (nc-Si) thin film transistors (TFTs) have potential for high-performance applications in large area electronics, such as next generation of flat panel displays and medical x-ray imagers, for pixel drivers, readout circuits, as well as complementary channel logic circuits for system-on-panel integration. This potential stems from reduced threshold voltage shift and higher transconductance, compared to amorphous silicon counterpart. In this paper, we discuss various TFT structures, their associated design and performance considerations, including leakage current and threshold voltage stability mechanisms.