Department of Electrical and Electronics Engineering
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Item “Effects of grain size on the mobility and transfer characteristics of polysilicon thin-film transistors(NISCAIR, 2004-07) Gupta, NavneetThe effect of the grain size on the effective carrier mobility (μ eff) and transfer characteristics of a polycrystalline silicon thin-film transistor (poly-Si TFT) have been theoretically investigated by developing an analytical model. The dependence of μ eff is studied as a function of doping concentrations and gate voltage for different values of grain size. It is observed that at low as well as at high doping concentrations, the effective carrier mobility (μ eff) increases with increase in grain size, whereas the observed dip at the intermediate doping concentration is getting confirmed. The effect of the grain size on transfer characteristics of poly-Si TFT in its linear region is also presented. It is found that at low gate voltages, μ eff and I D increase rapidly with the increase in V G for all values of grain sizes due to the grain boundary barrier lowering effect. At high gate voltage the grain boundary barrier lowering effect becomes insignificant and causes the saturation of μ eff and I D . The model was found to account correctly for the experimentally observed mobility variation and yield a reasonably good agreement.Item Physica Scripta On-Current Modeling of Polycrystalline Silicon Thin-Film Transistors(IOP, 2005) Gupta, NavneetWe propose an on-current (above threshold voltage) model of polycrystalline silicon thin-film transistors (poly-Si TFTs). The model includes the study of the effect of trap state density, poly-Si inversion layer thickness and temperature on the TFT characteristics. Effective carrier mobility and I-V characteristics are described by considering the mechanism of capture and release of carriers at grain boundary trap states and the thermionic emission theory. It is found that at low as well as at high doping concentrations, the effective carrier mobility (µeff) increases with increasing temperature whereas a dip is observed at intermediate doping concentration. At very high and very low doping concentration the effect of temperature on the mobility is found to be almost negligible. Calculations reveal that effective carrier mobility and drain current increase as the gate bias increases and are larger for a lower trap state density. The calculated value of activation energy decreases as the gate bias increases and is larger for a larger poly-Si inversion layer thickness. A comparison between the present predictions and the experimental results shows reasonably good agreement.Item Effect of Inversion Layer Thickness on the Activation Energy and Turn-On Characteristics of Polysilicon Thin-Film Transistors(World Scientific, 2005) Gupta, NavneetThe influence of inversion layer thickness on the activation energy and turn-on characteristics of a polycrystalline silicon thin-film transistor (poly-Si TFT) have been investigated theoretically by developing an analytical model. It is observed that activation energy decreases as the gate bias increases and is larger for a larger poly-Si inversion layer thickness. It is also observed that effective carrier mobility and drain current increase rapidly with the increase in gate voltage for all values of inversion layer thickness and are larger for lower inversion layer thickness. The computed results are in reasonable agreement with the experimental observations.Item Physica Scripta An Analytical Model of the Influence of Grain Size on the Mobility and Transfer Characteristics of Polysilicon Thin-Film Transistors (TFTs)(IOP, 2005) Gupta, NavneetInfluence of the grain size on the effective carrier mobility (μeff) and transfer characteristics of a polycrystalline silicon thin-film transistor (poly-Si TFT) has been theoretically investigated by developing an analytical model. The dependence of μeff is studied as function of doping concentration and gate voltage for different values of grain size. It is observed that at low as well as at high doping concentrations, the effective carrier mobility (μeff) increases with increasing grain size, whereas the observed dip at the intermediate doping concentration is confirmed. The effect of the grain size on transfer characteristics of poly-Si TFT in its linear region is also presented. It is found that at low gate voltages, μeff and ID increase rapidly with the increase in VG for all grain sizes due to the grain boundary barrier lowering effect. At high gate voltage the grain boundary barrier lowering effect becomes insignificant and causes the saturation of μeff and ID. The model was found to account correctly for the experimentally observed mobility variation and yield a reasonably good agreement.Item Analytical modeling of carrier transport through transverse and longitudinal grain boundaries in polysilicon thin-film transistors(The National Institute of Science Communication, 2006) Gupta, NavneetCarrier transport through transverse and longitudinal grain boundaries (GBs) in polysilicon thin film transistors (poly-Si TFTs) has been studied. The model considers an array of square grains in the channel of poly-Si TFT in which current flows along the longitudinal GBs and through the grains and the transverse GBs. The variation of field-effect mobility (HFE) and drain current (ID) is computed for different values of grain size. This study reveals that at low gate voltage the longitudinal GBs are seen to influence the field-effect mobility and drain current. As gate voltage increases, the effect of transverse GBs is found to account for experimental results. This is attributed to the fact that at low gate voltage, the carriers moving through longitudinal GBs have more opportunities to be trapped at the trapping sites and as gate voltage increases the carriers have sufficient energy to bypass the longitudinal GBs and obstructed by transverse GBs alone. This may be the reason that the calculated effects of longitudinal GBs do not appear in the experimental results at high gate voltageItem On the mobility, turn-on characteristics and activation energy of polycrystalline silicon thin-film transistors(Elsevier, 2006-05) Gupta, NavneetIn the present paper we propose a turn-on current model of polycrystalline silicon thin-film transistors (poly-Si TFTs). It is found that at low as well as at high doping concentrations, effective carrier mobility (μeff) increases with increase in temperature whereas a dip is observed at intermediate doping concentration. At very high and very low doping concentration the effect of temperature on the mobility is found to be almost negligible. Calculations reveal that effective carrier mobility and drain current increases as the gate bias increases and are larger for a lower trap state density. The calculated value of activation energy decreases as the gate bias increases and is larger for a larger poly-Si inversion layer thickness. A fair agreement is observed between the present predictions and the experimental results.Item Effects of grain boundary width on transfer characteristics of polysilicon thin-film transistor(World Scientific, 2006) Gupta, NavneetThe grain boundary scattering effects on carrier transport were studied analytically by considering the grains and grain boundaries that act as the series resistance in the channel of a polycrystalline silicon (poly-Si) thin-film transistor (TFT). Effective carrier mobility (μ*) and drain current (ID) variations were analyzed using the model by changing the grain boundary width (DGB) in the channel as a function of the gate voltage, in the linear region of the poly-Si TFT characteristic at room temperature. μ* and ID were computed for DGB ranging from 1 nm to 10 nm. It was found that for different values of the gate voltage, μ* and ID increased with a decrease in grain boundary width (DGB). A remarkable improvement was observed in device characteristics as DGB was decreased below 2 nm. The predicted results using the present model are in a reasonably good agreement with experimental results, hence confirming the validity of the model.Item Threshold voltage modelling and gate oxide thickness effect on polycrystalline silicon thin-film transistors(IOP, 2007-10) Gupta, NavneetThis paper presents an analytical model for calculating the threshold voltage in polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with large grains. In the present study, it is assumed that the oxide-silicon interface traps are uniformly distributed and the channel of the device contains only a single grain boundary. Further, the effect of gate oxide thickness on threshold voltage and hence on transfer characteristics has also been incorporated in this paper. It is observed that scaling down of the oxide thickness is an efficient way to reduce the threshold voltage and hence to improve the poly-Si TFT characteristics at different temperatures and trap densities. The results so obtained are compared with the available experimental data which show a satisfactory match thus justifying the validity of the model.Item Effect of gate oxide thickness on polycrystalline silicon thin-filmtransistors(Trade Science Inc, 2007) Gupta, NavneetThis work presents the study of the effect of gate oxide thickness on the performance of lightly doped polycrystalline silicon thin-filmtransistors with large grains. It is observed that scaling down of the oxide thickness is an efficient way to reduce the threshold voltage and hence to improve the poly- Si TFT characteristics. A reasonably good fitting between the analytical results and the experimental data support the validity of this model.Item Modeling of different shaped microcantilevers for sensing applications(Trade Science Inc, 2008) Gupta, NavneetWe present the comparison of frequency response of different shaped microcantilever beams (namely rectangular, T-shaped andV-shaped) which are commonly used for sensing applications. Performance was measured on the basis of both resonant frequency and geometrical parameters. It was found that the overall cantilever design that produced optimum sensitivity for sensing applications is the rectangular shaped microcantilever. After optimizing the shape ofmicrocantilever we have performed the analysis based on material properties. It is observed that out of all possible candidates silicon carbide possesses highest value of resonant frequency