According to this theory, the width and the height of the potential barrier determines the relation of the electric current and bias. A sharp i-n junction may narrow the potential barrier. This, in turn, can result in an enhancement of the electric field in the i-n junction so that the electron can easily tunnel the barrier. The sharp i-n interface is maybe one of the reasons of the low operational voltage. This is maybe the second reason of the low operational voltage.
More investigation is needed to understand the reason.
The i-GaN was grown by doping with Zn. The growth conditions and the diode structure have not been optimized. The authors would like to express thanks to professor M. K. Lee of Sun Yat-Sen Univ. This work is supported by Chinese High Technology Development Project.