8/7/2019 Mps Lect 02 Why Silicon

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Why Silicon?

Technological evolution toward integrated circuits began with development of an

understanding of diode action and the invention of the transistor in the late 1940s. At that

time the semiconductor of greatest interest was germanium. Experiments withgermanium produced important knowledge about the growth of large single crystals

having chemical purity and crystalline perfection that were previously unachievable.

Gennanium is an element that crystallizes in a diamond-like lattice structure, in whicheach atom forms covalent bonds with its four nearest neighbors. The crystal structure is

shown in Figure 1.

Figure 1 The diamond-crystal lattice characterized by four covalently bonded atoms. The lattice constant, denoted by ao,

is 0.356, 0.543 and 0.565 nm for diamond, silicon, and germanium, respectively. Nearest neighbors are spaced (\I3ao/4)units apart. Of the 18 atoms shown in the figure, only 8 belong to the volume a03. Because the 8 corner atoms are eachshared by 8 cubes, they contribute a total of1 atom; the 6 face atoms are each shared by 2 cubes and thus contribute 3atoms, and there are 4 atoms inside the cube. The atomic density is therefore 8/a03, which corresponds to 17.7, 5.00, and4.43 x 1022 cm-3, respectively. (After W. Shockley: Electrons and Holes in Semiconductors, Van Nostrand, Princeton, N.J.,1950.)

Limitations of Germanium:

Germanium has a band gap of 0.67 eV and an intrinsic carrier density equal to 2.5

X 1013cm-3at 300 K. Because of the relatively small band gap in germanium, its intrinsic-

carrier density increases rapidly with increasing temperature, growing roughly to 1015 cm-

3at 400 K (see Figure 2).

Figure 2 Electron concentration versus temperature for two n-type doped semiconductors: (a) Silicon doped with 1.15 x

1016arsenic atoms cm

-3[1]. (b) Germanium doped with 7.5 x 1015arsenic atoms cm-3

[2].

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8/7/2019 Mps Lect 02 Why Silicon

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Because most devices are no longer useful when the intrinsic-carrierconcentration becomes comparable to the dopant density, germanium devices are limited

to operating temperatures below about 70C (343 K).

As early as 1950, the temperature limitations of germanium devices motivatedresearch on several other semiconductors that crystallize with similar lattice structures

but can be used at higher temperatures. In the intervening decades technological

development for integrated circuits has focused on the elemental semiconductor silicon(Eg= 1.12 eV) and the compound semiconductor gallium arsenide (Eg= 1.42 eV). Silicon

is used in the overwhelming majority of integrated circuits, while compound

semiconductors, such as gallium arsenide, find application in specialized, high-

performance circuits. Compound semiconductors are especially useful in optical devicesthat rely on the efficient light emission from direct band gap compound semiconductors.

Advantages of Silicon:

In addition to the good semi-conducting properties of silicon, the major reason for

its widespread use is the ability to form on it a stable, controllable oxide film (silicon

dioxide Si02) that has excellent insulating properties. This capability, which is notmatched by any other semiconductor-insulator combination, makes it possible to

introduce controlled amounts of dopant impurities into small, selected areas of a siliconsample while the oxide blocks the impurities from the remainder of the silicon. Theability to dope small regions of the silicon is the key to producing dense arrays of devices

in integrated circuits.

Two chemical properties of the Si-Si02 system are of basic importance to silicon

technology. First, selective etchingis possible using liquid or gaseous etchants that attackonly one of the two materials. For example, hydrofluoric acid dissolves silicon dioxide

but not silicon. Second, silicon dioxide can be used to shield an underlying silicon crystal

from dopant impurity atoms brought to the surface either by high-energy ion beams orfrom a high-temperature gaseous diffusion source.

Using these features, dopant atoms can be introduced into areas on the silicon that

are not shielded by thick silicon dioxide. Proper sequencing and repetition of theoxidation, patterning, and dopant-addition operations can be used to introduce p- and n-

type dopant atoms selectively into regions having dimensions ranging down to the few

hundred nanometer range.

In addition to providing a means of limiting the area of dopant introduction, awell formed oxide on silicon improves the electrical properties at the surface of the

silicon substrate. Because of the termination of the crystal lattice of the silicon substrate,

uncompleted or dangling bonds exist at an ideal free surface. These broken bonds canintroduce allowed states into the energy gap of the silicon substrate at its surface and

degrade the electrical behavior of device regions near the surface. However, a well-

formed silicondioxide layer on the silicon surface electrically passivates almost all of

these surface states, allowing nearly ideal behavior of the surface region of the silicon.

Although the area density of bonds at the silicon surface is about 10 15cm-2

, the number of

electrically active bonds can be reduced to less than 1011 cm-2

by properly growing a

silicon-dioxide layer on the surface. The ability to remove virtually all the electrically

active states at the silicon surface allows the successful operation of the ubiquitous

silicon metal-oxide-semiconductor (MOS) transistor, which is the basis of most large-scale integrated circuits today.

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