  Mostly optical spectroscopies are used to investigate existence of excitons,
Mott transitions and other exquisite excitonic condensed phases of matter. On
the other hand, electrical signatures of excitons are hardly explored. Here we
examine steady state, small signal, electrical response of AlGaInP based
multi-quantum well laser diodes to identify and investigate the presence of
excitons in electroluminescence. This frequency dependent response shows bias
activated capacitance following a phenomenological rate equation. Dynamic
dependence of this response on frequency reverses after light emission. This
results in negative activation energy which we explain with the formation of a
stable, intermediate transition state whose average energy matches well with
excitonic binding energy of these III-V semiconductors. Hence we identify the
presence of excitons which are also responsible for observed
electroluminescence at low charge injections by electrical measurements alone.
Further increase in charge injection suppresses the electrical signature of
excitons representing their Mott transition into electron-hole plasma which is
supported by standard optical measurements. This kind of correlation between
electrical and optical properties of excitons was not systematically
investigated in the past. Therefore, this study demonstrates a fresh
experimental approach to electrically probe the rich and complex physics of
excitons.
