SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

70 CHAPTER 2. ELECTRONIC LEVELS IN SEMICONDUCTORS

(a)

(b)

Surface Donors

GaN

2DEG

0 200 400 600 800 1000 1200

-4

-3

-2

-1

0

1

2

Energy (eV)

Distance (Angstrom)

Ec

Ev

Ef

Ec

Ev

Ef

AlXGa1-XN

ΔP

SP

+ P

PE

ΔP

SP

+ P

PE

Figure 2.27: Mobile 2-dimensional sheet of electrons induced by polarization fields in an Al-
GaN/GaN heterostructure. (a) Charge distribution and (b) band diagram for the structure.

As can be seen from the discussion of the previous section the strain tensor is diagonal for growth
along (001) direction. As a result there is no piezoelectric effect. However for other orientations,
notably for (111) growth there is a strong piezoelectric effect.