Peyman Servati

Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic

This paper presents design considerations along with measurement results pertinent to hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) drive circuits for active matrix organic light emitting diode (AMOLED) displays. We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the OLED layer is integrated directly above the TFT circuit layer, to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Various drive circuits on glass and plastic were fabricated and tested. Integration of on-panel gate drivers is also discussed where we present the architecture of an a-Si:H based gate de-multiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.

Above-threshold parameter extraction and modeling for amorphous silicon thin-film transistors

This paper presents modeling and parameter extraction of the above-threshold characteristics of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) in both linear and saturation regions of operation. A bias- and geometry-independent definition for field effect mobility considering the ratio of free-to-trapped carriers is introduced, which conveys the properties of the active semiconducting layer. A method for extraction of model parameters such as threshold voltage, effective mobility, band-tail slope, and contact resistance from the measurement results is presented. This not only provides insight to the device properties, which are highly fabrication-dependent, but also enables accurate and reliable TFT circuit simulation. The techniques presented here form the basis for extraction of physical parameters for other TFTs with similar gap properties, such as organic and polymer TFTs.

Effects of inter-tube distance and alignment on tunnelling resistance and strain sensitivity of nanotube/polymer composite films

A model for carbon nanotube (CNT)/polymer composite conductivity is developed, considering the effect of inter-tube tunnelling through the polymer. The statistical effects of inter-tube distance and alignment on the tunnelling are investigated through numerical modelling, to highlight their role in the conductance and piezoresistance of the composite film. The impact of critical parameters, including the concentration, alignment and aspect ratio of the CNTs and the tunnelling barrier height of the polymer is statistically evaluated using a large number of randomly generated CNT/polymer composite films. A numerical model is presented for the tunnelling resistance as a function of CNT concentration and polymer properties, which provides good agreement with the reported conductance in the literature. In particular, for a low concentration of CNTs close to the percolation threshold, we demonstrate how tunnelling dominates the conductance properties and leads to significant increase in the piezoresistance of the composite. This is important for gaining insight into the optimum concentration and alignment of the CNTs in the composite film for applications such as strain sensors, anisotropic conductive films, transparent electrodes and flexible electronics.

Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress

We investigate the time-dependent shift in the threshold voltage of amorphous silicon thin-film transistor stressed with constant drain current. We observe a nonsaturating power-law time dependence, which is in contrast to the conventional stretched exponential that saturates at prolonged stress time. The result is consistent with the carrier-induced defect creation model and corroborates the nonlinear dependence of the rate of defect creation on the band-tail carrier density.

P-25: A New Driving Method for a-Si AMOLED Displays Based on Voltage Feedback

We present a new driving technique for active-matrix organic light-emitting diode displays using amorphous silicon backplanes. The technique uses voltage feedback to compensate for threshold voltage shift of TFTs. Measurement results show less than 3.5% change in OLED current over 2700 hours of bias stress.

Amorphous silicon driver circuits for organic light-emitting diode displays

This article presents design considerations pertinent to amorphous silicon (a-Si:H) pixel drive circuits for mobile display applications. We describe both pixel architectures and circuit topologies that are amenable for vertically stacked organic light-emitting diode (OLED) pixels in a-Si:H technology. Here, a dual-gate transistor structure is used to minimize the parasitic coupling between the OLED and the transistor layers. We consider both the two-transistor (2-T) voltage-programmed drive circuit and the five-transistor (5-T) current-programmed drive circuit. The latter provides compensation for shifts in device characteristics by virtue of metastable shifts in the threshold voltage of the thin-film transistor (TFT). Implementation of the 5-T drive circuit using dual-gate TFTs that enables high aperture ratio (∼100%), low leakage current, and surface emissive OLED pixels that are independent of scaling is also presented, along with simulation results of transfer characteristics.

Effective mobility and photocurrent in carbon nanotube–polymer composite photovoltaic cells

We examine the dark and the illuminated current?voltage (J?V) characteristics of poly(3-octylthiophene) (P3OT)/single-wall carbon nanotube (SWNT) composite photovoltaic cells as a function of SWNT concentration. Using an exponential band tail model, the influence of SWNT concentration on the J?V characteristics of the cells is analysed in terms of corresponding parameters such as effective hole mobility, short-circuit current, and open-circuit voltage. For the device with optimum 1% SWNT concentration, the increased photoresponse (~500 times) as compared to the pristine P3OT cell can be attributed partly to the increase (~50 times) in effective hole mobility, due to the reduction of localized states of the pristine P3OT matrix, and partly to the enhanced exciton extraction at the polymer/nanotube junctions.

Thermal and chemical vapor deposition of Si nanowires: Shape control, dispersion, and electrical properties

We investigate and compare complementary approaches to SiNW production in terms of yield, morphology control, and electrical properties. Vapor-phase techniques are considered, including chemical vapor deposition (with or without the assistance of a plasma) and thermal evaporation. We report Au-catalyzed nucleation of SiNWs at temperatures as low as 300°C using SiH4 as precursor. We get yields up to several milligrams by metal-free condensation of SiO powders. For all processes, we control the final nanostructure morphology. We then report concentrated and stable dispersions of SiNWs in solvents compatible with semiconducting organic polymers. Finally, we investigate the electrical response of intrinsic SiNWs grown by different methods. All our SiNWs exhibit p-type behavior and comparable performance, though in some cases ambipolar devices are observed. Thus, processing and morphology, rather than the growth technique, are key to achieve optimal samples for applications.

Thin film imaging technology on glass and plastic

Hydrogenated amorphous silicon (a-Si:H) technology offers a viable technological alternative for improved imaging of optical signals and high energy radiation. This paper reviews X-ray imaging technology in terms of detector operating principles, including optoelectronic characteristics, and fabrication process issues related to pixel (Schottky diode detector plus thin film transistor) integration. Recent results which describe the extension of the current fabrication processes to low (/spl sim/120/spl deg/C) temperature are also presented. The low temperature processing enables fabrication of thin electronics on flexible (polymer) substrates.

Modeling of the static and dynamic behavior of hydrogenated amorphous silicon thin-film transistors

This article reports on physically based models for hydrogenated amorphous silicon (a-Si:H) inverted staggered thin-film transistors (TFT), which accurately predict both the static and dynamic characteristics of the TFT. The model is implemented in VerilogA hardware description language, which comes as a standard feature in most circuit simulation environments. The static model includes both forward and reverse regimes of operation. The model for leakage current takes into account the physical mechanisms responsible for the source of the reverse current, viz., the formation of the conducting channels at the back and front a-Si:H/a-SiNx:H interfaces and their relative dominance at different bias conditions. The dynamic model includes the different charge components associated with the tail states, deep states, interfaces, and traps and their associated time constants. Good agreement between modeling and experimental results is obtained.