Nikolas P. Papadopoulos

Blue-Light-Sensitive Phototransistor for Indirect X-Ray Image Sensors

A thin-film phototransistor with a peak wavelength sensitivity of 420 nm was fabricated by integrating a bilayer hydrogenated-amorphous-silicon absorber with a nanocrystalline-silicon channel layer. The phototransistor response at λ = 420 nm was about 92 mA/W for an incident power density of 0.16 mW/cm2. A readout pixel circuit having a single transistor, acting as both the sensing and switching elements, is proposed to demonstrate the device potential for high-resolution pixelated X-ray imaging arrays. Simulation results reveal less than 150 μs to read out the pixel voltage with a storage capacitor of 1 pF. Thus, a frame rate of about 75 ms is predicted for a 500 × 500 pixel imaging array. Transient photocurrent measurements yield on (rise) and off (fall) times of 0.6 and 3 ms, respectively, verifying the requirements for a high frame rate.

A Novel Voltage-Programmed Pixel Circuit Utilizing

A novel voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diode (AMOLED) displays is proposed. The threshold voltage shift (ΔVT) of the drive TFT caused by electrical stress is compensated by an incremental gate-to-source voltage (ΔVGS) generated by utilizing the ΔVT-dependent charge transfer from the drive TFT to a TFT-based metal-insulator-semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of the drive TFT to improve the OLED drive current. The non-ideality of the ΔVT-compensation, TFT overlap capacitance, programming speed, and OLED degradation are discussed. The effectiveness of the proposed pixel circuit is verified by simulation results.

V_{T}

In this paper we analyze the effects of X-ray irradiation on a-Si:H TFTs. We have irradiated at low doses, up to 1 krad, and we have measured several transistors, obtaining coherent behaviors. The main effect is a shifting of the threshold voltage, as well as a change in the Ioff current and a change in the mobility. We discuss these effects, and find them to be around three orders of magnitude higher than in an equivalent bulk CMOS technology.

-Dependent Charge-Transfer to Improve Stability of AMOLED Display

Hybrid sensor/display pixels and their operating schemes are proposed, simulated, and characterized. The circuit design implemented a phototransistor connected to a drive thin-film transistor (TFT) and an organic light-emitting diode (OLED). The circuits were fabricated using hydrogenated amorphous-silicon (a-Si:H) TFTs as a hybrid photosensor/switching device. The light sensitivity of the phototransistors and response to incident illumination were modeled and verified experimentally for different white-light intensities. The pixel gray scale was determined by the pulse-height voltage modulation from the phototransistor, which was varied by changing the light intensity. The resulting circuits have a wide dynamic range with a light to dark output current ratio of 104 for TFTs having channel lengths of and light intensity up to ~ 8 mW/cm2. A 256-RGB-level imaging scale is possible with 5-nA steps for each grey scale in a pixel array matrix having a refresh rate of more than 200 Hz.

Low dose radiation effects on a-Si:H TFTs

A novel voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diode (AMOLED) displays is proposed. The threshold voltage shift (ΔVT) of the drive TFT due to electrical stress is compensated by the change of gate-to-source voltage (ΔVGS) generated by the ΔVT-dependent charge transfer from the drive TFT to a TFT-based Metal-Insulator-Semiconductor (MIS) capacitor. Another MIS capacitor is used to improve OLED drive currents. Measurement results verify the effectiveness and speed of the proposed pixel circuit.