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/cm
²
. 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.