As in GaN, p-type conductivity in InN has proven to be difficult to achieve. Even though the valence band edge lies 1.6 eV below the Fermi level stabilization energy , the low position of the conduction band edge makes efficient p-type doping very difficult. Another issue is the pinning of the surface Fermi level above the conduction band edge, due to native donor defects, which leads to a n-type accumulation layer at the surface [237,238]. Any study of p-type bulk material has to isolate the effects of this accumulation layer. This was achieved by Jones et al., who provided the first indirect evidence of a net concentration of acceptors, but who were however unable to verify the presence of free holes . Later works were not yet able to demonstrate net p-type conductivity , but an activation energy for the Mg acceptor of about 61 meV was extracted by photoluminescence measurements . Using the same value for the activation energy Wang et al.  calculated a hole mobility in the range of 1736 cm/Vs for a hole concentration of about (1.43.0)10 cm. However, they used a suggested effective hole mass value (0.42m) , which was not experimentally confirmed. The same value was adopted by Fujiwara et al. , who reported mobilities of 2570 cm/Vs. Recent works agree , that free holes can be detected only for moderate Mg contents. Most of the evidences of electrical conductance related to free electrons are yet to be confirmed.