232 CHAPTER 5. SEMICONDUCTOR JUNCTIONS
5.6 SEMICONDUCTOR HETEROJUNCTIONS .................
A growing number of modern devices are based on semiconductorheterojunctions, or junc-
tions formed between two different materials. Modern bipolar transistors employ ap-nhet-
erojunction in order to improve the emitter injection efficiency (see chapter 7), while in HFET
technology a heterojunction is used to form a high mobility channel (see chapter 8). In this
section we discuss the properties ofp-nheterojunctions. Specifically, we will focus on the junc-
tion formed between ann-type wide bandgap material (such as AlGaAs) and ap-type narrower
bandgap material (such as GaAs).
5.6.1 Abruptp-nheterojunction .........................
Electrostatics
To construct a band diagram for an abruptp-nheterojunction, we proceed in the same manner
as for thep-nhomojunction. We begin with two separate materials (figure 5.9a) and consider
what the equilibrium conditions must be when a junction is formed between them (figure 5.9b).
In figure 5.9a, the material on the left (material 1) isn-type and has a wide bandgap, while
the material on the right (material 2) isp-type and has a narrower bandgap. The doping in the
p-type material is much higher than that of then-type material (this is the typical emitter-base
structure in a III-Vnpnheterojunction bipolar transistor). The two materials have different
electron affinities (χ 1 andχ 2 ), bandgaps (Eg 1 andEg 2 ), and dielectric constants ( 1 and 2 ).
Figure 5.9b shows a band diagram of the system once a junction is formed between the two
materials. Since the materials have different bandgaps, there must exist a discontinuity in the
conduction band (ΔEc) and/or the valence band (ΔEv) at the interface. The difference in the
bandgap between the two materials is equal to the sum of the conduction band and valence band
discontinuities, or
ΔEg=Eg 1 −Eg 2 =ΔEc+ΔEv (5.6.1)
By examination of figure 5.9, it is tempting to assume thatΔEcis simply the difference in
the electron affinities of the two materials. However, there also exist dipole charges at the het-
erointerface which cause a shift in the relative band discontinuities. These dipole charges result
from the locally different atomic and electronic structures of the two materials at the heterointer-
face as compared to their bulk atomic structure. While electron affinity rules accurately predict
discontinuities in a limited number of material systems in which these dipole effects are small,
in most heterostructures these dipole charges are significant and must be accounted for. Band
line-ups for a number of materials were shown in figure 2.31.
Similar to the case of ap-nhomojunction, when thep-andn-type semiconductors are brought
together, a built-in voltage,Vbi, is produced between the two sides of the structure. The built-in
voltage is equal to the sum of the band bending on then-side (Vd 1 ) and the bend bending on the
p-side (Vd 2 ). By examination of figure 5.9, the built-in voltage can be shown to be
eVbi=eVd 1 +eVd 2 =Eg 2 −(EF−Ev)p−(Ec−EF)n+ΔEcwhere the subscriptsnandprefer to then-side andp-side of the device. Comparing this ex-
pression to that of thep-nhomojunction , we see that the only difference is the additionalΔEc