A thin-film x-ray imager is typically constructed by first forming a TFT (thin film transistor) array 160 on a substrate 170. A conductive layer 120 is then deposited on the substrate to form charge-collecting electrodes 125. Above the charge collecting electrodes 125 the semiconductor layer 110 is formed. In one prior art method, the semiconductor layer 110 is a composite material that is formed by combining a powder of loose polycrystalline semiconductor material with an organic binder. The polycrystalline semiconductor material can be amorphous selenium, lead oxide, polycrystalline silicon, thallium bromide, cadmium telluride, cadmium sulfide, mercuric iodide, and lead iodide. One prior method of forming the semiconductor layer 110 is by spraying or painting the organic binder on a purified polycrystalline semiconductor powder that has not yet been combined with any other material. Another prior art method of forming the semiconductor layer 110 is by forming a slurry of a polycrystalline semiconductor powder and an organic binder and then depositing this material on a substrate. The organic binder can be a polymer having a long chemical chain such as polyvinyl acrylate. The binder fills in all voids between the many crystals of the polycrystalline semiconductor powder. In the prior art, after the semiconductor layer 110 has been formed, an insulator layer 150 can be deposited above the semiconductor layer 110 to form a discreet insulator layer 150 that does not permeate the semiconductor layer 110. Above the semiconductor layer 110 the bias electrode 130 is formed by depositing a conductive material.
In use, a positive or negative bias voltage is applied to the electrode layer 130 relative to electrodes 125. When x-ray radiation 140 is made incident on the bias electrode 130 and then through the semiconductor material 110, electron-hole pairs form and drift apart within the semiconductor material 110 under the influence of the bias voltage across that region. If voltage is being continuously applied across the semiconductor layer 110, the electrons and holes will tend to separate, thereby creating a current flowing through the semiconductor layer 110. The magnitude of the current produced in the semiconductor layer 110 is related to the magnitude of the incident x-ray radiation 140 received. The current is collected, amplified and quantified to a digital code for a corresponding pixel. After removal of the incident x-ray radiation 140, the charges (electrons and holes) remain for a finite period of time until they can be recombined. When no radiation is present, a small current referred to as a “dark” current can be measured. Dark current is a problem, even in small quantities, because it can lead to a distortion of the values actually measured. This is because there is a linear contribution by the dark current to the measurement of the current flowing through the semiconductor material during the integration time. A significant amount of dark current could distort the end value measured for the pixel currents.