Minority carrier recombination in heavily-doped silicon

Abstract

A review of our present understanding about the minority carrier recombination in silicon with dopant concentration in the range of 10 18–10 20 cm −3 is presented. After providing a short phenomenological description of carrier recombination processes and lifetime, the main theories of carrier recombination in a semiconductor are briefly reviewed and their expected contributions to carrier recombination in silicon at heavy doping are indicated. The various methods used for measuring the minority carrier lifetime in heavily doped silicon are described and critically examined. Four different mechanisms are examined to explain the available lifetime data. Two of these involve SRH-type phononic recombination (i) via deep level traps generated by dopant introduced defects and (ii) through shallow donor/acceptor states. The other two non-phononic mechanisms are: (iii) Band to band Auger recombination and (iv) trap assisted Auger recombination. Mechanism (i) can not explain the observed insensitivity of lifetime to processing conditions and the dopant atoms, and contribution of (ii) remains insignificant up to the heaviest doping. Phonon assisted band to band Auger recombination appears to explain the measured lifetimes satisfactorily in p-type silicon. However, for n-type silicon this mechanism predicts considerably higher values of lifetime than the measured results and it is likely that mechanism (iv) (and probably (i) also) competes with this process. Calculations indicate that the rate of trap-assisted Auger recombination through the dopant generated acceptor states in p-type silicon and through donor states in n-type silicon becomes large enough to compete with the band to band Auger process at heavy doping. In n-type silicon Auger recombination through crystal defects like vacancies may also become important. Perhaps all these processes contribute to the carrier recombination at heavy doping but which of these controls the lifetime in n-type silicon is not known.