Page 29 - A Practical Guide from Design Planning to Manufacturing

The Evolution of the Microprocessor 5

spacing of atoms is nonuniform so the available quantum states vary
through the material, but in a crystal the spacing of atoms is uniform
so that continuous bands of allowed energy states are created. In a con-
ductor the electrons do not completely fill these bands so that any energy
at all will allow them to move through the crystal.
In an insulator the lowest energy band, the valence band, is com-
pletely filled with electrons and there is a large energy gap to the next
band, the conduction band, which is completely empty. No current
flows, because there are no open spaces for electrons in the valence
band to move to, and they do not have enough energy to reach the con-
duction band. The conduction band has open spaces but has no electrons
to carry current.
Semiconductors have a very small band gap between the valence and
conduction band. This means that at room temperature a small number
of electrons from the valence band will have enough energy to reach the
conduction band. These electrons in the conduction band and the holes
they leave behind in the valence band are now free to move under the
influence of an electric field and carry current.
In a pure semiconductor there are very few free carriers, but their
number is greatly increased by adding impurities. On the periodic table
of elements, shown in Fig. 1-2, silicon is a column 4 element which
means it has 4 outer electrons that can bond to neighboring atoms. If a

I VIII
1 2
H II III IV V VI VII He
3 4 5 6 7 8 9 10
Li Be B C N O F Ne
11 12 13 14 15 16 17 18
Na Mg Al Si P S Cl Ar
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
Cs Ba La ∗ Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
87 88 89 104 105 106 107 108 109
Fr Ra Ac † Rf Ha Sg Ns Hs Mt

∗ 58 59 60 61 62 63 64 65 66 67 68 69 70 71
Lanthanide series
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
† Actinide series 90 91 92 93 94 95 96 97 98 99 100 101 102 103
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
Figure 1-2 Periodic table.