Photodiode forward bias to reduce temporal effects in a-Si based flat panel detectors

Lag and sensitivity modulation are well known temporal artifacts of a-Si photodiode based flat panel detectors. Both effects are caused by charge carriers being trapped in the semiconductor. Trapping and releasing of these carriers is a statistical process with time constants much longer than the frame time of flat panel detectors. One way to reduce these temporal artifacts is to keep the traps filled by applying a pulse of light over the entire detector area every frame before the x-ray exposure. This paper describes an alternative method, forward biasing the a-Si photodiodes and supplying free carriers to fill the traps. The array photodiodes are forward biased and then reversed biased again every frame between the panel readout and x-ray exposure. The method requires no change to the mechanical construction of the detector, only minor modifications of the detector electronics and no image post processing. An existing flat panel detector was modified and evaluated for lag and sensitivity modulation. The required changes of the panel configuration, readout scheme and readout timing are presented in this paper. The results of applying the new technique are presented and compared to the standard mode of operation. The improvements are better than an order of magnitude for both sensitivity modulation and lag; lowering their values to levels comparable to the scintillator afterglow. To differentiate the contribution of the a-Si array, from that of the scintillator, a large area light source was used. Possible implementations and applications of the method are discussed.