Carrier Concentrations
Pure silicon is often called intrinsic silicon. As the temperature of the silicon crystal is
increased it absorbs heat. Some of the electrons in the valence band gain enough energy
to jump the bandgap energy of silicon, Eg (see also Eq. (23.21)), as seen in Fig. 2.11. This
movement of an electron from the valence band to the conduction band is called
generation. When the electron loses energy and falls back into the valence band, it is
called recombination. The time the electron spends in the conduction band, before it
recombines (drops back to the valence energy band), is random and often characterized by
the carrier lifetime, Tr(a root-mean-square, RMS, value of the random times the electrons
spend in the conduction band of the silicon crystal). While discussing the actual processes
involved with generation-recombination (GR) is outside the scope of this book, the
carrier lifetime is a practical important parameter for circuit design. Another important
parameter is the number of electrons in the conduction band (and thus the number of
holes in the valence band) at a given time (again a random number). These carriers are
called intrinsic carriers, nt. At room temperature