Radiation detectors based on the deposition of a 10 to 30 μm thick hydrogenated amorphous silicon (a-Si:H) sensor directly on top of integrated circuits have been developed. The performance of this detector technology has been assessed for the first time in the context of particle detectors. Three different circuits were designed in a quarter micron CMOS technology for these studies. The so-called TFA (Thin-Film on ASIC) detectors obtained after deposition of a-Si:H sensors on the developed circuits are presented. High internal electric fields (104 to 105 V/cm) can be built in the a-Si:H sensor and overcome the low mobility of electrons and holes in this amorphous material. However, the deposited sensor's leakage current at such fields turns out to be an important parameter which limits the performance of a TFA detector. Its detailed study is presented as well as the detector's pixel segmentation. Signal induction by generated free carrier motion in the a-Si:H sensor has been characterized using a 660 nm pulsed laser. Results obtained with a TFA detector based on an ASIC integrating 5 ns peaking time pre-amplifiers are presented. Direct detection of 10 to 50 keV electrons and 5.9 keV X-rays with the detectors are then shown to understand the potential and the limitations of this technology for radiation detection. © 2008 IEEE.