Is air an insulator for electricity?

Conductors, semiconductors and insulators¶

Whether an electric current can flow in a substance depends on the number of freely moving charge carriers within the substance.

Ladder¶

Electrical conductors are called materials in which charges can move easily. The best-known conductors are metals, graphite, acids, alkalis and salt solutions.

In metals, the individual atoms are arranged in a lattice structure, whereby the outermost electrons ("valence electrons") can move freely along the entire lattice ("metal bond", "electron gas"). If there is only one unbound electron per atom, there is an enormous number of charge carriers - for example Electrons per cubic millimeter! - freely available for cargo transport. [1]

At higher temperatures, the atomic cores of a metal lattice carry out ever stronger heat oscillations around their positions of rest. This restricts the mobility of the free electrons, which leads to a higher electrical resistance. Since the increase in resistance is, to a good approximation, linear with heating, the resistance value of a conductor can be calculated using the following formula:

Here there the resistance value at room temperature at, corresponds to the temperature difference to this reference value (also in ). The so-called "temperature coefficient" depends on the material and is used in specified.

At low temperatures, the heat oscillations of the atomic cores and the electrical resistance decrease accordingly; In many metals, the electrical resistance even disappears completely from a material-specific temperature ("superconductivity").

Electrically charged atoms and molecules (“ions”) can occur in liquids as a result of dissolution processes. Although these are significantly heavier than electrons, they can still conduct electricity as free charge carriers.

Semiconductor¶

In pure semiconductor materials - in contrast to electrical conductors - (almost) no free charge carriers are available in the ground state. For example, in what is probably the most typical semiconductor element silicon (and also in germanium), all four valence electrons form covalent bonds with the four neighboring atoms. Charge transport in a pure semiconductor can only be achieved by temporarily releasing the valence electrons using light, heat or sufficient electrical voltage.

Elementary semiconductors are in the fourth main group of the periodic table of the elements and have a chemical valency of four. According to the same bonding principle, so-called compound semiconductors are also possible, in which the average chemical valency four is:

  • With gallium arsenide , a III-V semiconductor, each gallium atom contributes three valence electrons and each arsenic atom five valence electrons to the covalent bonds.
  • With cadmium sulfide , a II-VI semiconductor, each cadmium atom contributes two and each sulfur atom six valence electrons to the covalent bonds. [2]

At higher temperatures, electrons break loose from the covalent bonds more often; The electrical resistance of a semiconductor is therefore - unlike metals - initially lower when it is heated. With even greater heating, however, the current-inhibiting effect of the atomic oscillations prevails again, which in turn increases the electrical resistance sharply.

The conductivity of a semiconductor can be influenced by the targeted introduction of foreign atoms.

The --Crossing¶

If you connect one - with a -doped semiconductor crystal, a neutral transition area is formed along the connecting line, in which the additional electrons of the -doped layer the holes of the -fill doped layer. The pairwise cancellation of electrons and holes is called “recombination”, the resulting space area is called “space charge zone” or “boundary layer”.

A semiconductor crystal is the same within the space charge zone -- Transition to a semiconductor without doping, i.e. the electrical resistance of the neutral boundary layer is significantly increased compared to the remaining doped areas.

The thickness of the neutral boundary layer is limited by the fact that the drifting of the electrons in the (originally neutral) -doped layer creates a positive accumulation of electrical charges; in the (originally also neutral) -doped layer, a negative charge accumulation builds up due to the additional electrons along the boundary layer. [3] As the charge separation creates an electrical voltage between the two layers, there is ultimately an equilibrium between the (thermally induced) drift of the electrons and the opposing “drift voltage”.

In silicon crystals the drift voltage is around , in germanium crystals round . With typical doping, the thickness of the space charge zone is of the order of magnitude .

Pass-through direction and cut-off direction

If you put it on a semiconductor crystal --Transition from the outside to an electrical voltage, the thickness of its boundary layer and thus also its electrical conductivity can be influenced in a targeted manner.

  • If you connect the -doped layer with the plus and the -doped layer with the minus pole of a current source ("reverse direction"), the electrons and holes are drawn away from the boundary layer or to the edge of the semiconductor crystal. The neutral space charge zone is thereby enlarged, the electrical resistance rises sharply.
  • If you connect the -doped layer with the minus and the -doped layer with the plus pole of a current source ("forward direction"), so are at the -Layer continuously replenished electrons. If the applied voltage is greater than the drift voltage in the crystal, the additional electrons can spread in the crystal. The electrons and holes are distributed, similar to dissolved ions in water, over the entire crystal and contribute equally to the charge transport.

Be in electronics --Transitions used in semiconductor components. A single one --Transition fulfills the function of a diode. Double or triple --Transitions are used in transistors and triacs.

Isolators¶

Insulators are substances in which charges can (almost) not move at all. Well-known examples are rubber, porcelain, glass, diamond, distilled water and air. All of these substances have in common that neither ions nor weakly bound electrons are available for charge transport; all electrons are firmly anchored in chemical bonds.

At extremely high voltages (a few kilo- to megavolts), electrons can be snatched from the material bonds for a short time. The result is a “breakthrough” current (for example lightning) in which a great deal of energy is transported in a very short time. Since the damage to people, animals, plants and electronic devices can be correspondingly large, high-altitude lines and buildings are equipped with surge arresters or lightning arresters, for example.

vacuum

Usually electrons are bound in molecules or metal solids. However, electrons can leave these bonds under different conditions:

  • If sufficient high-energy light hits a solid, electrons can be released from the solid-body bonds through the action of light (“photo effect”).
  • At sufficiently high temperatures, electrons are released from metals ("glow emission").

The light or heat energy supplied to the electrons must be large enough to overcome the binding energy of the electrons; the rest of the supplied energy remains with the electrons as kinetic energy.

In a vacuum, free electrons do not collide with other objects. If you accelerate them by applying an electrical voltage, they can be steered in a certain direction and accelerated to high speed values. Electron tubes, for example, are based on this principle.

Excursus: drift speed of electrons¶

If you press a light switch, the lamp lights up almost "immediately". The voltage initially only applied to the conductor ends of the switch that is still open must therefore also be applied to the lamp immediately after the switch is closed.

In fact, electric fields propagate at the speed of light. You can imagine this in a simple comparison like a hose that is already completely filled with water: If you add water at one end, it immediately overflows at the other end. To do this, however, the newly flowing water does not have to travel all the way through the hose.

So how fast do electrons move in a conductor? One can for an estimate of the so-called electron density run out of the respective conductor material; this indicates how many free electrons there are in a material per unit of space. For copper, for example, this value is .

Each electron carries exactly one elementary charge . The following applies to the movable amount of charge in a section of ladder:

The volume of the conductor section can in turn as a product of its cross-sectional area and its length to be written. The following applies:

An electric current corresponds to a time-moving amount of charge, so it is . If the conductor piece is made of the same material at all points and has a constant cross-sectional area, then the length is the only variable over time. A change in a route over time, however, is nothing more than a speed; so one can write:

The drift speed indicates how quickly a certain volume of electrons, which one could imagine colored, for example, “pushes through” the conductor. Solve the above equation on, one obtains:

In addition to the natural constants, this equation contains and the material constant only the cross-sectional area of the conductor as well as the flowing amperage as easily measurable quantities.

Example:

  • What is the drift speed of the electrons in an ordinary copper line with a cross-sectional area of when a current of occurs in the circuit?

    For copper, the electron density is ; thus one obtains according to the formula (1) for the drift speed:

    The unity arises because of .

The drift speed of the electrons in a metallic conductor is therefore only very low compared to the propagation speed of the electric field (speed of light).


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