GaN-based light emitting diodes (LEDs) with InGaN as the capping layer was designed in our experiment. The forward voltage at the typical driving current of 20mA was obviously changed by adjusting the thickness of the InGaN layer. We were concerned with the effect of polarization and solved the concentration and the tunneling probability of the two dimensional hole in the triangular potential well at the surface InGaN/GaN interface and obtained the minimal forward voltage. The calculation results were consistent with the experimental data.