The junction 110 of FIGS. 2 and 3 are depicted as a double heterostructure junction comprising a first junction layer 116 of an n- or p-type semiconductor material such as for example either AlGaN, an intermediate layer 118 for example of InGaN, and a second junction layer 120 of opposite doping polarity (i.e. p- or n-type respectively) to that of the first junction layer in accordance with common