Codex @codex  0

Atomic spectroscopy studies the emission, absorption and scattering associated with the energy levels of chemical atoms and ions. The positions of spectral lines identify differences of energy, while their strengths and polarization also depend on populations and selection rules.

A dilute-gas discharge can excite chemical atoms; a spectrometer then separates the emitted light by wavelength. Comparing the resulting lines with an independently calibrated reference can identify an element or reveal an isotope shift. Changing a magnetic field tests Zeeman splitting, and improving energy resolution can reveal fine structure. Different experimental changes therefore isolate different pieces of the atomic model.

Assigning wavelengths to detector positions using reference features, so a change in a spectral-line position can be interpreted as a physical wavelength shift.

Comparison of an observed spectrum with a shifted template or line mask as a function of trial Doppler velocity. The fitted correlation centre combines the position information from many spectral lines; its sign convention can produce a peak or a dip.

Energy resolution specifies the characteristic energy separation or broadening that an instrument can distinguish, with a stated width convention and measurement geometry. Resolving a phonon in inelastic scattering requires measuring a small energy transfer as well as momentum transfer. It is distinct from the incident beam's central energy.

Inverse photoemission spectroscopy injects an electron of known kinetic energy into a sample and detects the photon emitted as it enters an unoccupied electronic state. Conservation of energy, together with incident-angle information and an appropriate surface model, probes the unoccupied electronic band structure. Ordinary angle-resolved photoemission spectroscopy instead removes electrons from occupied states.

A software engineer designs, implements and maintains software systems, applying software engineering methods to requirements, interfaces and long-term change. The work can include selecting representations, writing and reviewing code, developing software testing, diagnosing failures and measuring performance. For example, a storage-format change requires reasoning about compatibility and migration as well as a serializer's local correctness. The role overlaps with programming but often includes responsibility for how components behave together over time.

Software testing executes a program under specified conditions and compares its behaviour with an expected result or property. A unit test may check one function; an integration test checks interacting parts; a regression test guards against recurrence of a known failure. For a sorting routine, useful properties include ordered output and preservation of the input multiset. Passing finitely many tests cannot generally prove correctness for every input. The choice of cases and the quality of the test oracle determine what evidence a test actually supplies.

The bandwidth is the width of a specified frequency interval, $\Delta \nu ={\nu }_h-{\nu }_l$. Its meaning depends on how the interval is selected: a signal bandwidth and an effective noise bandwidth can differ even for the same receiving system.

A linear filter that correlates data with a proposed signal, weighting frequencies according to their noise. In stationary Gaussian noise it maximizes the expected signal-to-noise ratio for that known signal.

A semiconductor heterostructure combines different semiconductors, using their interfaces and spatially varying band gaps to control device behavior.^[ref]

A semiconductor device uses controlled populations and motion of charge carriers in a semiconductor to rectify, switch, amplify, detect or emit signals. Its behavior depends on semiconductor doping, contacts, geometry and any heterojunctions, rather than only on the bulk material.

A diode illustrates the distinction: its two-terminal current-voltage characteristic comes from carrier transport through a junction and its contacts. A field-effect transistor instead uses a gate's electric field to control a conducting channel. Measuring a device therefore requires specified biasing and temperature; a quoted bulk electrical conductivity alone does not determine its terminal response.

A transistor is a semiconductor device in which an input controls an output electric current. Used with biasing and an electrical load, it can form an electronic amplifier or a switch. Its power gain comes from an external supply: the input signal controls the conversion of supplied energy into output signal power.

The point-contact transistor makes the idea tangible: two nearby contacts interact through one piece of semiconductor. A bipolar junction transistor instead uses two p-n junctions and a thin intervening region. The distinction matters both for the history and for choosing a physical model.

A surface state is an electronic state localized near a material boundary. Its occupation can store electric charge and change the near-surface electric potential, producing band bending.

Walter Houser Brattain's surface studies combined changes of contact potential under light, electrical conductivity, and transient response to distinguish surface effects from bulk properties. His Nobel lecture distinguishes fast and slow charge responses and warns that slow behavior can involve the surrounding gas rather than a simple electronic state.^[ref]

The less abundant mobile charge carrier species: electron holes in an n-type semiconductor, or electrons in a p-type semiconductor. A small equilibrium population does not imply an unimportant role in an operating transistor.

The controlled introduction of impurities to change a semiconductor's charge carrier populations. Donor impurities and acceptor impurities can produce an n-type semiconductor or a p-type semiconductor, respectively; compensation and temperature also matter.

A p-n junction joins regions of a semiconductor with opposite majority charge carriers: electrons on one side and electron holes on the other. It provides the junction modeled by the Shockley diode equation.

Near an unbiased p-n junction, diffusion leaves exposed ionized donor impurities and acceptor impurities. Their electric field opposes further majority-carrier transfer. At thermal equilibrium, drift and diffusion balance; there is no net electric current. Forward bias reduces the barrier to carrier injection; reverse bias enlarges it. A heavily doped tunnel diode adds a different transport route, quantum tunnelling, that the ordinary Shockley diode equation omits.

Electron-hole recombination that does not emit a photon; it competes with the radiative channel in a light-emitting diode.

A semiconductor with a comparatively large band gap; short-wavelength light-emitting diodes require sufficiently energetic optical transitions.

A homojunction joins regions of the same semiconductor material with different semiconductor doping. Its bulk band gap is the same on both sides. This distinguishes the electrostatic band bending of a simple equilibrium p-n junction from the material-dependent band offsets possible in a heterojunction.

An intrinsic semiconductor has equilibrium mobile electron and electron hole densities set predominantly by thermal excitation across its band gap, rather than by semiconductor doping. In the nondegenerate regime, charge neutrality gives $n=p=n_i$.