In many semiconductor technologies, including GaN, the lack of p-channel devices is a major obstacle for complementary operations. Here, we demonstrate high-performance polycrystalline diamond p-channel transistors on GaN-on-Si. Following the optimization of the microwave-plasma chemical-vapor-deposition of diamond on GaN, the polycrystalline layer was hydrogenated to form a 2D hole-gas at the surface, acting as p-channel. Relying on a rather simple fabrication process, these devices exhibited excellent electrical and thermal performances with on-off ratio of 10^{9, breakdown voltage of 400 V, specific on-resistance of 84 mΩ·cm2, and thermal conductivities higher than 900 W/m·K. The presented hetero-integration technology provides a promising platform for future complementary logic operations, gate drivers, complementary power switch applications such as integrated power inverters and converters, simultaneously serving as a very efficient thermal management solution in high power density applications.}