Physics · Ch 14 — Semiconductors
Intrinsic Semiconductor
Intrinsic Semiconductor
A pure semiconductor -- pure silicon or pure germanium, with no deliberately added impurity -- is called an INTRINSIC semiconductor. Silicon has atomic number 14 and electronic configuration , giving it a valence of four: every silicon atom forms four covalent bonds with its neighbours, and each silicon atom sits surrounded by four neighbouring silicon atoms placed at the corners of a regular tetrahedron (Fig. 14.6).
At absolute zero temperature, every valence electron is tightly bound within a complete covalent bond, so no electrons are free to conduct electricity through the crystal -- none of them can gain enough energy to reach a higher (conduction-band) energy level. At room temperature, however, thermal agitation is enough to break a small number of these covalent bonds, and the valence electrons freed this way gain enough energy to move into the conduction band. Breaking a bond this way leaves behind a VACANCY in the valence band at the position the electron departed from (Fig. 14.7); these vacancies are called HOLES -- an absence of an electron in the valence band, which behaves as if it carries an effective POSITIVE charge.
Because every thermally broken bond produces exactly one free electron and exactly one hole together, an intrinsic semiconductor always has EQUAL numbers of electrons and holes per unit volume: if is the hole density and the free-electron density, then for an intrinsic semiconductor.
Conduction in an intrinsic semiconductor is genuinely interesting because it involves TWO distinct types of charge carrier working together: the electron, and the hole (the absence of an electron). When a semiconductor is connected in a circuit, the negatively-charged free electrons drift toward the positive terminal of the battery, while the effectively positively-charged holes drift toward the negative terminal -- so the total current through the semiconductor is carried by two carrier types moving in OPPOSITE directions at once (Fig. 14.8), a conduction mechanism unique to semiconductors that underlies almost all of the devices built from them. …
What this figure shows. A schematic of the tetrahedral covalent-bonding arrangement in a pure silicon crystal. One central silicon atom is shown surrounded by exactly FOUR neighbouring silicon atoms, positioned at the four corners of a regular tetrahedron around it, with a covalent bond (a shared electron-pair bond) drawn along each of the four lines connecting the central atom to its four neighbours. This illustrates that every silicon atom (atomic number 14, electronic configuration 1s^2 2s^2 2p^6 3s^2 3p^2, valence 4) forms exactly four covalent bonds with its nearest neighbours, and that at absolute zero temperature every one of these bonds throughout the crystal i …
What this figure shows. A diagram illustrating what happens at room temperature (as opposed to absolute zero) in the same tetrahedral silicon bonding arrangement of Fig. 14.6: thermal agitation breaks one of the four covalent bonds around a silicon atom, and the electron that was part of that bond is shown being promoted (with an arrow) out of the bond and up into the conduction band, leaving behind an empty bond position -- a VACANCY -- at the location it departed from, within the valence band/bonding structure. This vacancy left by the departed electron is the figure's visual definition of a hole: an absence of an electron at a position in the valence band, shown alo …
What this figure shows. A schematic circuit diagram of a slab of intrinsic semiconductor material connected across a battery. Inside the semiconductor, electron symbols are drawn with arrows showing them drifting toward the POSITIVE terminal of the battery (since electrons are negatively charged), while hole symbols are drawn with arrows showing them drifting in the OPPOSITE direction, toward the NEGATIVE terminal of the battery (holes behaving as effective positive charges). Both sets of arrows point along the same overall current-carrying path but in opposite physical directions, illustrating that the two carrier types move oppositely yet both contribute current in the same conventional-curre …