All we need for this is equation (1.12), which gives the energies of the bound states for a particle of mass m in an infinitely deep well of width a. h2x2n2 n = (Q1.1) 2ma2 with m = mom and the appropriate effective mass me. The first few energy levels for each case are listed in table Q1.1. The infinitely deep well might give a reasonable approximation for the energy of an electron in the lowest state of a 10 nm well, because this energy level is far below the top of the real, finite well. It is doubtfulfor the next state, whose 225 me V energy is not far below the 300 meV depth of the real well. The approximation is useless for the 4 nm well because it predicts that the energy of the lowest state is greater than the depth of the real well, and we shall see in section 4.2 that a one- dimensional square well always has at least one bound state. Its validity is similar for light holes, but the greater mass of heavy holes reduces their energy levels so that the infinitely deep well works reasonably well even for the narrower 4 nm well