Simple Circuits Suppose there is a hole at site as shown in Fig. . (a). The movement of holes can be visualised as shown in Fig.
. (b). An electron from the covalent bond at site may jump to the vacant site (hole). Thus, after such a jump, the hole is at site and the site has now an electron.
Therefore, apparently, the hole has moved from site to site . Note that the electron originally set free [Fig. . (a)] is not involved in this process of hole motion.
The free electron moves completely independently as conduction electron and gives rise to an electron current, I e under an applied electric field. Remember that the motion of hole is only a convenient way of describing the actual motion of bound electrons, whenever there is an empty bond anywhere in the crystal. Under the action of an electric field, these holes move towards negative potential giving the hole current, I h . The total current, I is thus the sum of the electron current I e and the hole current I h : I = I e + I h ( .
) It may be noted that apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers. The recombination occurs due to an electron colliding with a hole. FIGURE .
Schematic two-dimensional representation of Si or Ge structure showing covalent bonds at low temperature (all bonds intact). + symbol indicates inner cores of Si or Ge. FIGURE . (a) Schematic model of generation of hole at site and conduction electron due to thermal energy at moderate temperatures.
(b) Simplified representation of possible thermal motion of a hole. The electron from the lower left hand covalent bond (site ) goes to the earlier hole site1, leaving a hole at its site indicating an apparent movement of the hole from site to site . E XAMPLE . An intrinsic semiconductor will behave like an insulator at T = K as shown in Fig.
. (a). It is the thermal energy at higher temperatures ( T > 0K), which excites some electrons from the valence band to the conduction band. These thermally excited electrons at T > K, partially occupy the conduction band.
Therefore, the energy-band diagram of an intrinsic semiconductor will be as shown in Fig. . (b). Here, some electrons are shown in the conduction band.
These have come from the valence band leaving equal number of holes there. Example . C, Si and Ge have same lattice structure. Why is C insulator while Si and Ge intrinsic semiconductors?
Solution The bonding electrons of C, Si or Ge lie, respectively, in the second, third and fourth orbit. Hence, energy required to take out an electron from these atoms (i.e., ionisation energy E g ) will be least for Ge, followed by Si and highest for C. Hence, number of free electrons for conduction in Ge and Si are significant but negligibly small for C.