In this chapter, first the concept of crystal structures and energy bands is explained in brief. The density of states for 0D, 1D, and 2D systems as well as Somerfield’s free electron theory are explained in detail. The Kronig–Penney model of electrons and the concept of band structure of nanoparticles are discussed. The idea of flat band voltage is then utilized to model the subthreshold surface potential for nanodevices. This idea can be utilized by researchers to obtain mathematical expressions of subthreshold drain current, threshold voltage, transconductance, and other characteristic parameters for short-channel MOSFETs.