SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

5.5. INSULATOR-SEMICONDUCTOR JUNCTIONS 231

Insulator

Semiconductor

Interface defects

Interface roughness

Substrate

{

Figure 5.8: Insulator-semiconductor junctions are dominated by interface quality and defect
levels in insulators.

Midgap interface density as low as 1010 eV−^1 cm−^2 can be readily obtained. The ability to
produce such high-quality interfaces is responsible for the remarkable success of the metal-
oxide-silicon (MOS) devices. Due to the low interface densities, there is very little trapping of
electrons (holes) at the interface so that high-speed switching can be predictably used. In has to
be recognized though that the interface is still rough with islands with a height of 5A over lateral ̊
extents of∼ 50 A. Typical electron mobility in Si MOSFETs is ̊ ∼ 600 cm^2 /(V·s)compared to
a mobility of∼ 1100 cm^2 /(V·s)(300 K) for bulk pure Si. We will discuss the MOS structure
in detail in chapter 9.
Silicon nitride (Si 3 N 4 ) is another important film that forms modest-quality junctions with Si.
Silicon nitride can be used in a metal-insulator-semiconductor device in Si technology, but its
applications are limited. The film is used more as a mask for oxidation of the Si film. It also
makes a good material for passivation of finished devices. Silicon oxy-nitride on the other hand
forms high-quality interfaces with silicon and can be used in FETs.
Although not an insulator or a metal, we include polycrystalline silicon (“poly”) in this chapter
because of its importance in Si technology. Polysilicon can be deposited by the pyrolysis (heat-
induced decomposition) of silane:

SiH 4 −→Si + 2H 2 (5.5.1)

Depending upon the deposition temperature, micro crystallites of different grain sizes are
produced. Typical grain size is∼ 0. 1 μm.
Polyfilmscanbedopedtolowresistivitytoproduceusefulconductorsforanumberof
applications.PolyisoftenusedasagateofanMOStransistor,asaresistor,orasalinkbetween
ametalandtheSisubstratetoensureanohmiccontact.