  We investigate the nature of a class of probability density functions, say
G(a), with a the shape parameter, which generalizes the Gumbel distribution.
These functions appear in a model of charge transport, when applied to a
metal-insulator-metal structure, where the insulator is constituted by a
monolayer of bacteriorhodopsin. Current shows a sharp increase above about 3 V,
interpreted as the cross-over between direct and injection sequential-tunneling
regimes.
  In particular, we show that, changing the bias value, the probability density
function changes its look from bimodal to unimodal. Actually, the bimodal
distributions can be resolved in at least a couple of $G(a)$ functions with
different values of the shape parameter.
