  High-conductivity undoped GaN/AlN 2D hole gases (2DHGs), the p-type dual of
the AlGaN/GaN 2D electron gases (2DEGs), have offered valuable insights into
hole transport in GaN and enabled the first GaN GHz RF p-channel FETs. They are
an important step towards high-speed and high-power complementary electronics
with wide-bandgap semiconductors. These technologically and scientifically
relevant 2D hole gases are perceived to be not as robust as the 2DEGs because
structurally similar heterostructures exhibit wide variations of the hole
density over $\Delta p_s >$ 7 x 10$^{13}$ cm$^{-2}$, and low mobilities. In
this work, we uncover that the variations are tied to undesired dopant
impurities such as Silicon and Oxygen floating up from the nucleation
interface. By introducing impurity blocking layers (IBLs) in the AlN buffer
layer, we eliminate the variability in 2D hole gas densities and transport
properties, resulting in a much tighter-control over the 2DHG density
variations to $\Delta p_s \leq$ 1 x 10$^{13}$ cm$^{-2}$ across growths, and a
3x boost in the Hall mobilities. These changes result in a 2-3x increase in
hole conductivity when compared to GaN/AlN structures without IBLs.
