Electronic state density  at an indentical energy can be looked as  the electron number proportion per unit energy(occupy possibility ) occupied at this energy . Yuo can refer to some books on quantum mechanics and semiconductor physics.

The density of states for an electron in a crystal is explained in most Solid State Physics texbooks (e.g. C. Kittel "Introduction to Solid State Physics", Wiley, 2005).  For a three dimensional free electron gas, E is proportional to kx^2 + ky^2 + kz^2, but only the states at the surface of the sphere can contribute to electrical conduction. A is proportional to kf^2.  In a two dimensional solid E goes like kx^2 + ky^2, and only states at the edge of the circle can contribute.  A is proportional to kf.  Thus 2D solids have many more opportunities to modulate the conductivity and my be used in future logic devices.  If you need large drive currents, however, 3D materials will rule.

I sudied it in "Introduction to Solid State Physics", Charles Kittel.