Physics · Ch 9 — Semiconductor Electronics
Energy Band Diagram of Solids
Energy Band Diagram of Solids
Defines the valence band (the band formed from the filled valence orbitals) and the conduction band (formed from the empty orbitals electrons jump into once energised), separated by the forbidden energy gap , in which no electron can exist. The classification that follows (9.1.2) hinges entirely on how wide this gap is, and on whether the valence and conduction bands overlap at all -- a wide, non-overlapping gap gives an insulator, an overlapping pair of bands gives a conductor, and a narrow, non-overlapping gap gives a semiconductor. The kinetic energy of an electron increases moving away from the nucleus (outer orbitals), while its potential energy decreases, which is why electrons closest to the nucleus are the most tightly bound and hardest to excite, whil …
What this figure shows. Panel (a) is the master diagram: a lower shaded region labelled Valence band, an upper shaded region labelled Conduction band, and the gap between them labelled Forbidden energy gap, with marking the top (maximum energy) of the valence band and marking the bottom (minimum energy) of the conduction band -- a single electron is shown unable to sit anywhere inside the gap. Panels (b), (c) and (d) redraw this same CB/VB pair for the three material classes side by side: (b) an insulator, where the gap is drawn very wide ( eV) with CB and VB far apart; (c) a conductor, where the CB and VB bands are drawn overlapping, so free electrons and holes coexist with no gap to cross at all; and (d) a semiconductor, where the gap is drawn narrow, comparable to eV, small enough that thermal energy alone can lift a modest population of electrons across it, shown as 'Free electrons' in the CB and 'Holes' left behind in the VB. Placed together, the three panels make the gap width -- not the presence of electrons as such -- t …
Worked out. Because the energy levels of orbiting electrons in atoms and solids are so small on the joule scale, they are conventionally measured in electron volts (eV) rather than joules; one eV is the kinetic energy gained by an electron accelerated through a potential difference of one volt, J. Every band-gap value quoted in this unit -- 1.1 eV for silicon, 0.7 eV for germanium, roughly 6 eV for a typical insulator -- uses this conve …