8.7. DESIGN ISSUES IN HFETS 409
Figure 8.36: a) Effect of the insertion of an ultra-thin layer of InGaN in the conduction band
diagram of a GaN buffer. Due to the extremely thin InGaN layer, the conduction band discon-
tinuity,ΔEc, of one side of the heterostructure is canceled by theΔECin the other side and
it can be neglected, resulting in an effective band discontinuity equal toΔEp. In the figure,
the polarization-induced sheet charges at the heterointerfaces are also shown. b) Schematic and
conduction band diagram of the basic InGaN back-barrier sample used in this work.
In conclusion, in this section we have reviewed several of the problems limiting the perfor-
mance of real HEMTs as well as some of the solutions normally adopted to overcome them.
Some solutions, like to keep a good gate aspect ratio, are common to every semiconductor fam-
ily; others, like the use of InGaN back-barriers, are specific to some materials. Therefore, to
fabricate high performance transistors is fundamental not only to understand the physics of the
device but also to know the particularities of each material system, its limitations and advanced
properties.