Influence of the geometrical layout of the silicon microstrip detectors on electrical characteristics

The silicon microstrip detectors have to fulfill special requirements due to high radiation environment where they are usually used. Because of the radiation damages in n-type silicon bulk, the high bias voltage that has to be applied in order to obtain efficient charge collection is limited by the breakdown process onset. Geometrical parameters as microstrip and readout pitch, the distance between p+ multiguardrings, or between the last guardring and sensor border, influence the electrical parameters. We used computer simulations to enable the design of multiguardring region and sensor border. The work consists in studying the right configuration between the last p+ guardring and the sensor border n+ implanted, in order to obtain the highest breakdown threshold for the bias voltage. We obtained the potential and the electric field distribution and current characteristics at different values of bias voltage applied to the p+ n junction. The region of maximum electric field is situated at the p+ implant corner and is influenced by two main factors: - the presence of the n+ implant in vicinity and, - the electron accumulation layer under the silicon oxide. In DESSIS-ISE main program two general models were used: - the drift-diffusion model, which solves Poisson equation together with electron and hole current continuity equations, and - the thermodynamic model, which simulates the effects of self-heating on the temperature distribution and the effects of the non-uniform temperature distribution on the electrical characteristics. Such a model extends the drift-diffusion approach taking into account electrothermal effects, under the assumption that the charge carriers are in thermal equilibrium with the lattice. Also the particular model of the generation-recombination was used, with terms for avalanche process and Shockley-Read-Hall recombination (which depends on doping concentration), Auger recombination, tunneling recombination (for high electric fields) and surface recombination. The simulation results reveals that for small distances between p+ and n+ implants the punch-through is the main process that appears, while for higher distances avalanche multiplication prevails. The use of the n+ at the sensor edge is necessary to protect the active area from the outside electron currents. This should be no closer that 100 - 150 microns (this value depends slightly on the bulk doping concentration). It might be concluded that, roughly speaking, after 150 microns, the influence of n+ implant on the breakdown voltage value of the diode is very small. The presence of the outward field plates over the p+ implant is beneficent, because compensates the damaging influence of the electron accumulation layer and the result is a higher curvature radius of the iso-surface of the electric field in the critical region. This might be seen from the fact that for higher distances W, i.e. the distance between the diode border p+ and the n+ implant border, the breakdown process starts for higher bias voltage than the computed values. From simulation results it is also seen that for D, i.e. the distance between the diode border and the cut edge of the silicon crystal, higher than 400 microns, the breakdown voltage is not influenced by the distance between p+ implant and the cut.. (authors)