Die Beweglichkeit “heisser” elektronen und ihr einfluss auf die Anstiegszeit der impulse von Halbleiterzählern aus n-silizium

Abstract

The rise time of the pulses in semiconductor counters excited by alpha particles is calculated and compared with measurements on counters fabricated on high resistance n-silicon. The calculation is effected including the strong dependence of the mobility of charge carriers on the high electric field in a semiconductor counter. The case of “overvoltage” on a fully depleted counter has been taken into account, too. Calculated and measured rise times agreed reasonably well, using a fixed set of constants for all counters. For low electric field strenghts of the order of 1000 V/cm the governing time constant is the “plasma time”. Its value for alpha particles was empirically determined to be 1.9 × 10 −2 sec ß V 2/cm 2. For an average field strength the rise time is strongly influenced by integration of the pulses by the resistance of the counter base together with the input capacitance of the measuring equipment, providing the depletion layer is not extended up to the back contact. Only in a fully depleted counter the rise time is proportional to the transient time of the charge carriers. This transient time evem in a not fully depleted counter will depend on the electric field strength in as much as the mobility itself is field dependent. For field strengths of about 30 000 V/cm the transient times and thus the drift velocity are constant, the mobility therefore inversely proportional to the field strength. The value determined for this maximum drift velocity ν ∞ for conduction electrons amounts to 7.4 × 10 6 cm/sec, for defect electrons to 4.8 × 10 6 cm/sec.