MSM has gain, which may be due to the presence of
dislocations or deep level defects in the epilayers [38]. It is
also possible that there exist some trap levels in the AlN layer
which help to transport electrons through the AlN layer via
deep-level assisted tunneling [39]. Zhou et al. [41] reported
that the maximum responsivities of the AlN/GaN hetero-
structures photodetector can be achieved at 360 nm, which
is much higher than those of a GaN monolayer photodetector
[40]. They suggest that the main reason is that a strong
polarization field was formed in the AlN/GaN heterostructure
interface [40]. Since AlN has the largest energy band gap
( 6.1 eV) among nitride semiconductors and offers the
ability for band gap engineering through the use of alloying
and heterostructure design, our results indicate good quality
of AlN/GaN/AlN heterostructures on silicon substrate, and
the potential for MSM application for these AlN/GaN/AlN
heterostructures on silicon substrate.