High Sensitivity Readout of 2D a-Si Image Sensors

Abstract

A highly sensitive charge-sensitive amplifier IC was used to read out large-area two-dimensional arrays of amorphous silicon (a-Si) p-i-n photodiodes addressed by a-Si thin film transistors (TFTs). At the highest sensitivity mode, the random noise of the sensor was equivalent to an input charge of 1500 electrons rms. Feedthrough charge injection associated with TFT switching had to be avoided at the expense of the signal charge. Image lag caused by charge trapping in a-Si was suppressed by limiting the input light intensity. A simple model was developed for the noise contribution from a pixel and compared with measurements.

Photon Detectors

“…[60] From Fujieda [60] and referring to Fig. During readout, one TFT is turned ON and its associated photodiode is connected to the data line with a TFT ON resistance of about 1 MΩ.…”

“…This resistive component in the circuit generates thermal noise. [60] The amplifier noise can be modeled as having a fixed noise component N amp0 in addition to an input-capacitance-dependent component: [12] (12) However, because even the photocurrent level is very low in these diagnostic X-ray imaging systems (~1 fA to 10 pA), 1/f and shot noise sources may be neglected as a first-order approximation.…”

Application of a-Si:H radiation detectors in medical imaging

“…[29] As the dark current in the photodetector is very low ( less than 0.1 nA/cm2), the shot and l/f noise can be neglected and the dominant noise source would be the thermal noise due t o TFT ON-state resistance. This thermal noise is proportional to the capacitance of the photodetector and the temperature, [37] so it can be reduced either by making the pixel size smaller or reducing the operating temperature, which is also needed for long-term charge storage in the storage capacitor. At room temperature, the reported mean value of the noise with similar pixel size as the one used in the simulation was -1.6 fC/pixel.…”

“…where &si is the dielectric constant of a-Si:H, Tt represents the characteristic temperature of the exponential distribution of traps and Jd is the space charge limited dark current density which is given by [34][35][36][37] (5-15) where C is a constant which depends on the material properties.…”

Effect of depth-dependent modulation transfer function and K-fluorescence reabsorption on the detective quantum efficiency of indirect-conversion flat-panel x-ray imaging systems using CsI

“…In the indirect method the phosphor layer is used to absorb x rays and the resultant light photons are detected by an active matrix array with a single photo-diode and thin film transistor TFT switch at each pixel. 4,5,6,7 The photodiode absorbs the light given off by the phosphor in proportion to the x-ray energy absorbed. The TFTs are used to readout the stored charge to produce a digital image.…”