Why are the electron generation rate and recombination rate equal in thermal equilibrium? $2.$ Defi ne the excess carrier recombination rate in terms of excess carrier concentration and lifetime. $3.$ Explain how the density of holes, for example, can change as a result of a change in the fl ux of particles. $4.$ Why is the general ambipolar transport equation nonlinear? $5.$ Explain qualitatively why a pulse of excess electrons and holes would move together in the presence of an applied electric fi eld. $6.$ Explain qualitatively why the excess carrier lifetime reduces to that of the minority carrier under low injection. $7.$ What is the time dependence of the density of excess carriers when the generation rate becomes zero? $8.$ In the presence of an external force, why doesn$$t the density of excess carriers continue to increase with time? $9.$ When a concentration of one type of excess carrier is suddenly created in a semiconductor, what is the mechanism by which the net charge density quickly becomes zero? $10.$ State the defi nition of the quasi$-$Fermi level for electrons. Repeat for holes. $11.$ Explain why the presence of traps in a semiconductor increases the recombination rate of excess carriers. $12.$ Why, in general, is the concentration of excess carriers less at the surface of a semiconductor than in the bulk?