ASIPET chip: Low noise optoelectronic integrated readout with n-i-p a-Si:H photodiode array for Positron Emission Tomography

The scope of this work is to present a novel technology to detect the scintillation light from the LSO crystals used in Positron Emission Tomography (PET): it is based on vertically integrating a hydrogenated amorphous silicon (a-Si:H) film on a pixel readout chip. The article describes the design and the test of the new ASIPET chip, designed in 0.25 μm technology. This prototype chip includes two separate structures of 3×3 and 4×4 macro-pixels. The pixel width is respectively 660 μm and 495 μm for the two structures. The readout channel has a low noise preamplifier and a shaper stage per each pixel. The section I describes a novel method to achieve sufficiently high open loop gain in a charge sensitive amplifier (CSA) using a regulated cascode structure with active feedback [1]. The section II shows the noise analysis and simulations [2]. An additional amplifier is used to reconstruct the original signal proportional to the incoming light respectively for the 3×3 and 4×4 pixel structures. The excellent test results confirm the simulations: the total gain is 800 mV/fC, and the ENC is 40e- with a total input capacitance of the 6 pF. To conclude, section IV shows the quantum efficiency (QE) measurements of a-Si:H photodiodes optimized for the wavelength of 420 nm.