394 CHAPTER 8. FIELD EFFECT TRANSISTORS
SS
G
D
Figure 8.23: Top view of an AlGaN/GaN HFET structure with 2 gate fingers. Pictured in the
inset is a close up of the 0.12μm T-gate. Picture courtesy of Ilesanmi Adesida.
8.6.2 PolarHFETStructures ...........................
Now that we have described how charge is distributed within polar materials, we are ready to
show how polarization fields can be used to generate a 2DEG in polar heterostructures. Consider
the AlGaN/GaN structure illustrated in figure 8.22. The charge density distribution is shown
along with the band diagram. For sufficiently thick AlGaN layers, the surface potentialeφsis
pinned by the surface donor and is approximately equal to the donor depthEDD. Due to the
lattice mismatch between AlGaN and GaN, the thin AlGaN cap is under tensile strain. Hence
the total polarization charge at the AlGaN surface−Qπ(AlGaN)is the sum of the spontaneous
and piezoelectric contributions from the AlGaN. In addition to the negative polarization charge,
there will also be a positive charge at the surfaceNDD+ resulting from the ionized surface donors.
At the AlGaN/GaN interface, the net polarization chargeQπ(net)is the sum of the polariza-
tion contributions from the AlGaN and the GaN, or
Qπ(net) =Qπ(AlGaN)−Qπ(GaN) (8.6.10)Qπ(net)is a positive number for Ga-face polarity because of the higher polarization in the
AlGaN relative to GaN. From the band diagram, we can see that there must also exist a dis-
tribution of electrons in the GaN near the AlGaN/GaN heterointerface. Again, we have drawn