Intrinsic Conduction

At T= 300 K, the electric conductivity of conductors is 10⁴-10⁶ S cm^-1 (resistivity is 10^-4 – 10^-6 Ω∙cm). The figure for dielectrics is less than 10^-10 S cm^-1(more then 10¹⁰ Ω∙cm), and for semiconductors it ranges from 10^-9 to 10⁴ S cm^-1(10⁹ – 10^-4 Ω∙cm).

Semiconductor diodes and transistors de­pend for their operation on the fact that two types of electric conduction exist in semicon­ductor materials. Like metals, they show elec­tron conductivity, that is, the flow of current due to the motion of conduction electrons. The other type of conduction displayed by semiconductors but nonexistent in metals is hole conductivity. Since it is a distinction of semiconductors.

The vacancy left in the valence band of a semiconductor due to an electron being lost from the band by, say, thermal excitation has come to be known as a hole. Holes behave like positive charge carriers (Fig.1.3).

Fig.1.3. Electron-hole pair generation

Fig.1.4. Pattern of energy levels in a semiconductor

Electrons and holes that can move and bring about electric conduction are called charge carriers or simply carriers. It is customary to say that thermal excitation results in the generation of electron-hole pairs. An electron moves from the valence band into the conduction band.

Both conduction electrons and holes move in a random fashion, and this is responsible for a process which is the reverse of electron-hole pair generation. This oc­currence is referred to as electron-hole pair recombination. During this process, an electron moves from the conduction band into the valence band (Fig.1.4).

A semiconductor free from impurities is called an intrinsic, or i-type, semiconductor. In an intrinsic semiconductor:

n_i = p_i .

One cubic centimetre of a metal or semi­conductor specimen has a number N of atoms of the order of 10²². At a temperature close to 20°C, the (approximate) carrier concentration for pure germanium is:

n_i = p_i = 2,3·10¹³ cm^-3

and for silicon,

n_i = p_i = 1,5·10¹⁰ cm^-3.

Thus, at room temperature the ratio of mobile carriers to the total number of atoms in an intrinsic semiconductor is about 10^-7 % for germanium and about 10^-10 % for silicon. In metals, the number of conduction electrons is no less than that of atoms (n ≥ N). For example, the resistivity at room temperature is 1,7·10^-6 Ω cm for copper, about 45 Ω cm for germanium, and about 230 000 Ω cm for silicon.

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