Anthony R. Franklin

Quality factor in a‐Si:H nip and pin diodes

We analyze the forward characteristics of a‐Si:H nip and pin diodes. At low bias, a well‐defined exponential region exists, described by a noninteger quality factor n between 1.2 and 1.7. With increasing temperature, the quality factor decreases. This behavior can be understood with a model based on electron and hole recombination in the i layer, which relates the temperature dependence of the quality factor to the distribution of localized states in the amorphous silicon. The predictions of the model are supported by numerical calculations in which the diode device equations are solved for a given distribution of localized states. The different ideality factors are due to different energy dependencies of the density of deep states in the i layer.

Analysis of lead oxide (PbO) layers for direct conversion X-ray detection

Lead oxide (PbO) is a candidate direct conversion material for medical X-ray applications. We produced various samples and detectors with thick PbO layers. X-ray performance data such as dark current, charge generation yield and temporal behavior were evaluated on small samples. The influence of the metal contacts was studied in detail. We also covered large a-Si thin-film transistor (TFT)-plates with PbO. Imaging results from a large detector with an active area of 18 cm /spl times/ 20 cm are presented. The detector has 960 /spl times/ 1080 pixels with a pixel pitch of 184 /spl mu/m. The modulation transfer function at the Nyquist frequency of 2.72 linepairs/mm is 50%. Finally, a full size X-ray image is presented.

PbO as direct conversion X-ray detector material

A flat X-ray detector with lead oxide (PbO) as direct conversion material has been developed. The material lead oxide, which has a very high X-ray absorption, was analysed in detail including Raman spectroscopy and electron microscopy. X-ray performance data such as dark current, charge yield and temporal behaviour were evaluated on small functional samples. A process to cover a-Si TFT-plates with PbO has been developed. We present imaging results from a large detector with an active area of 18 × 20 cm2. The detector has 1080 × 960 pixels with a pixel pitch of 184 μm. The linearity of detector response was verified. The NPS was determined with a total dark noise as low as 1800 electrons/pixel. The MTF was measured with two different methods: first with the analysis of a square wave phantom and second with a narrow slit. The MTF at the Nyquist frequency of 2.72 lp/mm was 50 %. We calculated first DQE values of our prototype detector plates. Full size images of anatomic and technical phantoms are shown.

Amorphous Silicon Photodiode-Thin Film Transistor Image Sensor with Diode on Top Structure

We have developed a new amorphous silicon image sensor technology using a matrix array of amorphous silicon thin film transistors and photodiodes, where the amorphous silicon nip photodiode is fabricated on top of a thick insulating layer, on top of the thin film transistor array. We call this ‘diode on top’ technology or DOTTY. The active diode area can be as high as 93%, compared to 50% for our conventional photodiode-TFT technology. This leads to a higher signal to noise performance, which is important for medical X-ray applications.

Flat detector with integrated dose sensing

Integrated dose sensing in Flat Detectors allows a during pulse control of the X-ray illumination without the need for external dose sensing devices. Standard designs of Flat Detectors do not allow during pulse dose sensing since the information is collected from the pixels only in the read-out phase after the X-ray illumination. This paper introduces a special detector plate design for obtaining dose sensing information directly from the X-ray detector while the X-ray pulse is being applied. This dose sensing information is read at a lower spatial resolution than the actual X-ray image but with a sub-millisecond temporal resolution. The dose sensing operates without any additional radiation burden on the patient and without attenuation of the image information. Experimental results from a small area (4x4 cm2) detector are presented, including an analysis of noise, linearity and cross-talk.