Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1 - xN/GaN heterostructures on Si(111) containing an AIN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5-2.7 nm thick amorphous SiNx layer due to the high growth temperature of T-AIN = 1040 degrees C. Therefore, a grain-like GaN/AlN region extending 40-60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be epsilon(r) = 0.3+/-0.6% by Fourier filtering of HRTEM images and a moire fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)= +23.4% which seems to be supported by the SiNx layer. Weak beam imaging and plan view TEM show typical threading dislocations in the epilayer with a density of 3x10(9) cm(-2) extending along < 0001 > which sometimes form grain boundaries. An AlxGa1 - xN/GaN interface roughness of 3 monolayers is estimated and a small AlxGa1 - xN surface roughness of 1.5 nm is obtained by HRTEM and atomic force microscopy investigations which correspond to two-dimensional growth. C-V and Hall measurements reveal two-dimensional electron gas at the Al32Ga68N/GaN interface that has a sheet carrier concentration of 4x10(12) cm(-2). The electron mobility of 820 cm(2)/Vs measured at room temperature is applicable for a HEMT grown on Si(111).