Physics · Ch 9 — Semiconductor Electronics
Intrinsic Semiconductors
Intrinsic Semiconductors
A semiconductor in its pure form, without any added impurity, is an intrinsic semiconductor. Every atom in the crystal lattice -- silicon has four valence electrons -- is covalently bonded to its four nearest neighbours, so at absolute zero every valence electron is locked into a bond and the material behaves like an insulator. A small rise in temperature is enough to break some of these covalent bonds and free the corresponding electrons into the conduction band; each broken bond leaves behind an empty state in the valence band called a hole, which -- being a deficiency of a (negative) electron -- is treated as carrying an effective positive charge. Because every thermally freed electron leaves behind exactly one hole, an intrinsic semiconductor always has EQUAL numbers of conduction-band electrons and valence-band holes; this …
What this figure shows. Panel (a) draws a flat grid of silicon atoms (each labelled Si), every atom joined to its four neighbours by a pair of shared 'valence electron' dots forming a covalent bond -- a schematic, two-dimensional stand-in for silicon's real three-dimensional diamond lattice, showing every bond intact and every electron accounted for. Panel (b) redraws the same lattice after one covalent bond has broken: a 'free electron' is shown drifting away from its original site, leaving behind a 'broken covalent bond' and, at the vacated position, a 'hole'. Together the two panels give the microscopic picture behind : thermal energy occasionally supplies enough energy to snap one bond, releasing exactly one free electron and …
What this figure shows. Panel (a) shows the valence band (bounded above by ) and conduction band (bounded below by ) of an intrinsic semiconductor with the band gap marked between them, drawn completely empty in the conduction band and completely full in the valence band, representing the material at 0 K. Panel (b) redraws the same two bands at room temperature: a few electrons now sit inside the conduction band, labelled 'thermally generated electrons', while the valence band shows the corresponding 'vacancy (hole) created due to the release of an electron' directly below. The pairing of one dot in the CB with one empty circle in the VB in panel (b) is the band-diagram equivalent of Figure 9.3(b): every thermally generated electron in the conduction band is matched …
Worked out. When an electron is thermally excited out of a covalent bond, the bond can no longer satisfy the octet rule -- each excited electron leaves behind a vacancy needed to complete that bonding arrangement, and this 'deficiency' of an electron is what is termed a hole. A hole is not a real particle; it is a convenient bookkeeping device for the fact that a missing negative charge at a fixed lattice site behaves, for every practical purpose (motion under a field, contribution to current), exactly as though a positive charge sat there instead. Crucially, electrons in the valence band that have NOT been excited are still bound electrons that cannot move and hence cannot themselves contribute to conduction -- it is only the hole (i.e. the pattern of a neighbouring bound electron hopping into the vacancy, …