In this paper, we show that by suppressing the background carrier concentration, the breakdown voltage (BV) of Ga2O3 heterojunction PN diode (HJ-PND) can be essentially enhanced. Meanwhile, through combing with high-level hole injection induced conductivity modulation effect, the specific on-resistance (Ron, sp) can be significantly reduced. Even without a carefully designed edge termination, a high power figure of merit (P-$\text{FOM}=\text{BV}^{2}/\mathrm{R}_{\text{on},\text{sp}})$ of $(3.75\ \text{kV})^{2}/(2.5\ \mathrm{m}\Omega\cdot \text{cm}^{2})=5.6\ \text{GW}/\text{cm}^{2}$ can be achieved. Ga2O3 HJ PN diodes with suppressed background carrier concentration for BV enhancement and high-level hole injection for Ron, sp reduction verify their great promise for future high voltage and high power applications.