Semiconductor

Everyday term

In plain words

A material whose ability to carry electricity can be switched on and off, for example by an applied voltage. That switching is what every in a computer chip relies on. Silicon is the classic example; several , such as MoS₂, are semiconductors too.

Going deeper

The same semiconductor three ways. Pure, it has only the few electrons and holes that heat creates across the gap. Donor impurities just below the conduction band hand it extra electrons (n-type); acceptors just above the valence band take electrons out of it, leaving mobile holes (p-type). The Fermi level follows, moving towards whichever band holds the extra carriers.

Between a metal and an insulator

A semiconductor has a large enough that, when pure and cold, it barely conducts, but small enough – of the order of an electronvolt – that its carriers can be supplied in controlled amounts. Silicon, with a gap of 1.12 eV, holds only about 10¹⁰ free electrons per cubic centimetre at room temperature, a few parts in 10¹³ of its atoms. Adding impurities at the level of parts per million raises that by a factor of a million or more. That enormous, controllable range is what makes semiconductors useful: a transistor is a device for changing the number of carriers in a small region, quickly and on command.

Electrons, holes and doping

Current in a semiconductor is carried by two kinds of charge. Electrons lifted into the conduction band move freely; the they leave in the – move too, behaving like positive charges. chooses which dominates. Donor atoms with one electron more than the host give that electron up easily and make the material n-type; acceptor atoms with one electron fewer capture electrons from the valence band and make it p-type. Joining n- and p-type regions creates the junctions behind diodes, , LEDs and transistors.

What is different in 2D semiconductors

In an atomically thin semiconductor there is no bulk in which to hide dopant atoms: every substitutional impurity sits at a surface, perturbs the crystal strongly and is hard to place reproducibly. Carrier density is therefore more often set by from adsorbed molecules or neighbouring layers, or electrostatically by a gate. Thinness brings a decisive advantage in return. Because a gate controls the whole thickness of the channel, transistors made from 2D semiconductors keep their switching even when the channel is only a few nanometres long – the reason they are candidates for transistors beyond the limits of silicon. The difficulty that remains is the contact: metals tend to pin the , giving high .

For specialists

A material with a band gap of a few electronvolts or less, whose conductivity can be tuned over many orders of magnitude by doping, , temperature or light. 2D semiconductors attract interest because their sub-nanometre thickness preserves electrostatic gate control in very .

Where this comes from

Single-layer MoS₂ transistors Radisavljevic et al. · Nature Nanotechnology 6, 147 (2011) cited by 14,996

Point defect

A flaw in a crystal that involves a single atomic site: an atom missing (a vacancy), an atom of the wrong kind in a site (a substitution, or an antisite when the crystal’s own two kinds of atom swap places), or an extra atom squeezed in between (an interstitial). Every real crystal has some. In a sheet a few atoms thick each one sits at the surface, so a handful of them can change how the material conducts, glows or reacts far more than in a thick crystal.

A defect confined to one or a few lattice sites – vacancy, interstitial, substitutional impurity, antisite or adatom – or a small complex of them. What it does is set by the levels it introduces: shallow levels dope, deep levels trap carriers and act as non-radiative recombination centres, and an isolated deep level in a wide gap can emit single photons. Formation energies and charge states depend on the chemical conditions of growth and on the Fermi level. In 2D materials every defect is a surface defect, less screened and more exposed to the environment, and densities of 10¹² to 10¹³ per cm² are typical of TMDC monolayers.

Full explanation

Electron hole

Usually just called a hole: the empty place an electron leaves behind in an otherwise full band. It behaves like a particle of its own with a positive charge. When a neighbouring electron steps into the gap, the gap moves the other way – like the empty space in a queue of cars, which travels backwards as each car edges forward. Many materials conduct mainly with holes, and chips need both kinds of carrier.

An unoccupied state in an otherwise filled band, usually near the valence-band maximum, treated as a quasiparticle with positive charge and a positive effective mass set by the band curvature. Holes carry the current in p-type semiconductors and give a positive Hall coefficient. In 2D semiconductors good p-type transport is harder to obtain than n-type, largely because metal contacts pin the Fermi level nearer the conduction band.

Full explanation

Gating

Changing a material by putting a voltage on a nearby electrode – a gate – separated from it by an insulator. The voltage pulls electrons into the material or pushes them out, as in every transistor. Because a 2D material is so thin, the gate reaches all of it, so the number of electrons can be dialled up and down continuously: a single device can be turned from insulator to metal, its magnetism strengthened, or superconductivity switched on. Two gates, above and below, can also apply an electric field across the layer.

Electrostatic control of carrier density and electric field through a gate coupled capacitively across a dielectric: the induced density equals the gate capacitance times the voltage beyond threshold, divided by the electron charge, reaching a few 10¹³ cm⁻² with oxide or hBN gates before breakdown. Dual gating sets density and perpendicular displacement field independently, opening the gap of bilayer graphene and tuning moiré flat bands. Ionic-liquid and solid-electrolyte gating form an electric double layer about a nanometre thick and reach 10¹⁴–10¹⁵ cm⁻², enough to induce superconductivity in MoS₂ or raise the Curie temperature of Fe₃GeTe₂, but can intercalate or react electrochemically and work only while the ions are mobile. Hysteresis from traps, contact effects and quantum capacitance are the usual caveats.

Full explanation

Complementary metal–oxide–semiconductor (CMOS)

The way almost every digital chip is built. Each logic gate pairs a transistor that conducts with electrons with one that conducts with holes, so one of the two is always off and the gate draws almost no current except while it switches. A 2D material becomes useful for logic only if both kinds of transistor can be made from it, or if it can be added to a silicon chip without harming it.

Logic in which n- and p-channel field-effect transistors are wired in series between supply and ground, so that in either stable state one of them is off and static power is set by leakage alone. It demands n- and p-type devices of comparable drive, threshold voltages held within tens of millivolts across a wafer, and a process compatible with the silicon line – about 400 °C at most for back-end-of-line integration. For 2D channels the p-type half is the harder one: most TMDCs conduct n-type with common contact metals, and WSe₂ is the leading p-type candidate.

Full explanation