  Fabrication of low-RF loss GaN-on-Si HEMT stacks is critical to enable
competitive front-end-modules for 5G and 6G applications. The main contribution
to RF losses is the interface between the III-N layer and the HR Si wafer, more
specifically the AlN/Si interface. At this interface, a parasitic surface
conduction layer exists in Si, which decreases the substrate effective
resistivity sensed by overlying circuitry below the nominal Si resistivity.
However, a clear understanding of this interface with control of the parasitic
channel is lacking. In this letter, a detailed physical and electrical
description of MOCVD-grown AlN/Si structures is presented. The presence of a
$\text{SiC}_\text{x}\text{N}_\text{y}$ interfacial layer is revealed and its
importance for RF losses is shown. Through C-V and I-V characterisation, an
increase in the C concentration of this interfacial layer is linked to the
formation of negative charge at the AlN/Si interface, which counteracts the
positive charge present in the 0-predose limit. The variation of TMAl predose
is shown to allow precise tuning of the C composition and, consequently, the
resulting interface charge. Notably, a linear relationship between predose and
net interface charge is observed and confirmed by the fabrication of an AlN/Si
sample with close to zero net charge. In addition, a higher $D_{it}$ ($\sim
2\times 10^{12}$ cm$^\text{-2}$) for such compensated samples is observed and
can contribute to low RF loss. An exceptionally high effective resistivity of
above 8 k$\Omega\cdot$cm is achieved, corresponding to an RF loss below 0.3
dB/mm at 10 GHz.
