230 CHAPTER 5. SEMICONDUCTOR JUNCTIONS
barrier,theelectronscantunnelthroughthebarrierwithease, as shown in figure 5.7. The quality
of an ohmic contact is usually defined through the resistanceRof the contact over a certain area
A. The normalized resistance is called the specific contact resistancercand is given by
rc=R·A (5.4.2)Under conditions of heavy doping where the transport is by tunneling, the specific contact
resistance has the following dependence for tunneling, probabilityT, through a triangular bar-
rier):
ln (rc)∝1
n(T)∝
(Vbi)^3 /^2
F(5.4.3)
where the field is
E=Vbi
W∝ (Vbi)^1 /^2 (Nd)^1 /^2 (5.4.4)Thus,
n(rc) ∝ Vbi∝1
√
Nd(5.4.5)
TheresistancecanbereducedbyusingalowSchottkybarrierheightanddopingasheavily
aspossible. The predicted dependence of the contact resistance on the doping density is, indeed,
observed experimentally. It is observed from experiments that it is usually more difficult to obtain
contacts withp−type semiconductors with low resistance. This is due to the difficulty inp-
doping. It is also due to the fact that in many materials the relatively high effective mass of holes,
leads to reduced tunneling currents. Also, in the case of many wide bandgap semiconductors
such as GaN, the barrier heights between available metals and the valence band is much greater
than that of the conduction band.
5.5 INSULATOR-SEMICONDUCTOR JUNCTIONS .................
In chapter 2 we have called materials with large bandgaps insulators. Usually these materials
don’t have high crystalline quality and are difficult to dope. These materials have very high
resistivity and are used to isolate regions to prevent current flow. Most insulator-semiconductor
combinations involve structures that are not lattice-matched. In most cases the insulator and
the semiconductor do not even share the same basic lattice type. In this section we will briefly
review a few such combinations. Important issues in these junctions are listed in figure 5.8. The
key issues here revolve around producing an interface with very low density of trapping states
and low interface leakage.
5.5.1 Insulator-Silicon ..............................
The most important junction in solid state electronics is the SiO 2 -Si system. In spite of the
severe mismatch between SiO 2 structure and Si structure, the interface quality is quite good.