Chapter 5
SEMICONDUCTOR JUNCTIONS
5.1 INTRODUCTION .................................
The discussions in chapter 4 suggest that when two different materials form a junction (e.g.
n−type andp−type semiconductors) interacting electrical effects arise. We have seen how
thep-ndiode has nonlinearI-Vcharacteristics and tunableC-Vcharacteristics. We can form
junctions between metals and semiconductors, between semiconductors with different gaps etc.
These junctions also have special properties useful for devices. Metals by themselves are neces-
sary to connect the semiconductors to the “outside world” of voltage sources and circuits. They
are also able to produce rectifying junctions. Insulators are also an integral part of electronics.
These materials provide an isolation between two regions of a device, can be used for bandstruc-
ture tailoring, can be used as capacitors, etc. In this chapter we will examine some important
properties of a variety of junctions.
5.2 METAL INTERCONNECTS ...........................
Metals form an important part of semiconductor technology. As shown in figure 5.1, they are
used as interconnects (i.e. low resistance conductors), they form Schottky barriers and Ohmic
contacts, and they form gates in field effect transistors. We have discussed in section 2.7 that
due to the high density of mobile electrons, the resistivity of metals is very low. In table 5.1 we
show the resistivities of some important metals used in electronics. In semiconductor circuits,
interconnects provide pathways through which charge travels from one point to another. While
these interconnects are obviously passive elements of the circuit they are extremely important
and play a role in circuit performance. The metal strips making up the interconnect must be able
to carry adequate current and make good contact with the devices. Interconnects are deposited
on insulators and touch the active devices only through windows that are opened at select points.
Aluminum is a commonly used interconnect material. In bulk, Al is a good conductor, with
resistivity of 2.7× 10 −^6 Ω−cm. In thin-film form, the resistivity can be up to a factor of 20