4. Consider an intrinsic semiconductor. Let & be the energy of an electron. Let D(e) be the density of states in the conduction band and D, (s) be the density of states in the valence band (see figure below). Assume &- >> k,I,&, - Ey >>k I and De(s)=C₁(8-8) Dy (8)-C₁₂(8-8) 0.5 where represents the energy of the bottom of the conduction band and &, represents the top of the valence band. The Fermi energy is sp. De(s) EC Dy (ε) D(ε) b. Find an expression for p-the concentration of valence holes in terms of k.T.C,,&,,& and a dimensionless definite integral of the form Jexp(-x)√xdx (hint: make a substitution of the form x=1 εν ε kBI