Charge Transport in Non-Irradiated and Irradiated Silicon Diodes

A model describing the transport of charge carriers generated in silicon detectors (standard planar float zone and MESA diodes) by ionizing particles is presented. The current pulse response induced by $\alpha$ and $\beta$ particles in non-irradiated detectors and detectors irradiated up to fluences $\Phi \approx 3 \cdot 10^{14}$ particles/cm$^2$ is reproduced through this model: i) by adding a small n-type region 15 $\mu$m deep on the $p^+$ side for the standard planar float zone detectors at fluences beyond the n to p-type inversion and ii) for the MESA detectors, by considering one dead layer 14 $\mu$m deep (observed experimentally) on each side, and introducing a second (delayed) component. For both types of detectors, the model gives mobilities decreasing linearily up to fluences of about $5 \cdot 10^{13}$ particles/cm$^2$ and converging, beyond, to saturation values of about 1000 cm$^2$/Vs and 455 cm$^2$/Vs for electrons and holes, respectively. At a fluence $\Phi \approx 10^{14}$ particles/cm$^2$, charge collection deficits of about 13\% for $\beta$ particles, 25\% for $\alpha$ particles incident on the front and 35\% for $\alpha$ particles incident on the back of the detector are found for both type of diodes.