In order to reduce the high electric field peak near the gate edge and optimize the non-uniform surface electric field distribution of conventional AlGaN/GaN high electron mobility transistor (HEMT), a novel AlGaN/GaN HEMT with a partial GaN cap layer is proposed in this paper. The partial GaN cap layer is introduced at the top of the AlGaN barrier layer and is located from the gate to the drain drift region. A negative polarization charge at the upper hetero-junction interface is induced, owing to the polarization effect at the GaN cap layer and AlGaN barrier layer interface. Hence, the two dimensional electron gas (2DEG) density is reduced. The low-density 2DEG region near the gate edge is formed, which turns the uniform distribution into a gradient distribution. The concentration distribution of 2DEG is modified. Therefore, the surface electric field distribution of AlGaN/GaN HEMT is modulated. By the electric field modulation effect, a new electric field peak is produced and the high electric field peak near the gate edge of the drain side is effectively reduced. The surface electric field of AlGaN/GaN HEMT is more uniformly redistributed in the drift region. In virtue of ISE-TCAD simulation software, the equipotential and the surface electric field distribution of AlGaN/GaN HEMT are obtained. For the novel AlGaN/GaN HEMT employing a partial GaN cap layer, the 2DEG is completely depleted from the gate to the drain electrodes, arising from the low-density 2DEG near the gate edge, while the 2DEG is partly depleted for the conventional AlGaN/GaN HEMT. The surface electric field distribution of the conventional structure is compared with the one of the novel structures with partial GaN cap layers of different lengths at a fixed thickness of 228 nm. With increasing length, the new electric field peak increases and shifts toward the drain electrode, and the high electric field peak on the drain side of the gate edge is reduced. Moreover, the breakdown voltage dependence on the length and thickness of the partial GaN cap layer is achieved. The simulation results exhibit that the breakdown voltage can be improved to 960 V compared with 427 V of the conventional AlGaN/GaN HEMT under the optimum conditions. The threshold voltage of AlGaN/GaN HEMT remains unchanged. The maximum output current of AlGaN/GaN HEMT is reduced by 9.2% and the specific on-resistance is increased by 11% due to a 2DEG density reduction. The cut-off frequency keeps constant and the maximum oscillation frequency shows an improvement of 12% resulting from the increased output resistance. The results demonstrate that the proposed AlGaN/GaN HEMT is an attractive candidate in realizing the high-voltage operation of GaN-based power device.