increases, the electrons gain energy (heat is absorbed by the silicon crystal), which causes
some of the valence electrons to break free and move to a conducting energy level, Ec.
Figure 2.11 shows the movement of an electron from the valence band to the conduction
band. Note that there aren't any allowable energies between Ev and Ec in the silicon
crystal structure (if the atom were by itself, that is, not in a crystal structure this exact
limitation isn't present). Further note that when the electron moves from the valence
energy band to the conduction energy band, a hole is left in the valence band. Having an
electron in the conduction band increases the material's conductivity (the electron can
move around easily in the semiconductor material because it's not tightly coupled to an
atom's nucleus). At the same time a hole in the valence band increases the material's
conductivity (electrons in the valence band can move around more easily by simply
falling into the open hole). The key point is that increasing the number of electrons or
holes increases the materials conductivity. Since the hole is more tightly coupled to the
atom's nucleus (actually the electrons in the valence band), its mobility (ability to move
around) is lower than the electron's mobility in the conduction band. This point is
fundamentally important. The fact that the mobility of a hole is lower than the mobility
of an electron (in silicon) results in, among other things, the size of PMOS devices being
larger than the size of NMOS devices (when designing circuits) in order for each device
to have the same drive strength.
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