CONDUCTORS Applied Electronics Department Technical University of Cluj-Napoca Cluj-Napoca, Cluj, 400027, Romania Phone: +40-264-401412, E-mail: [email protected]

CONDUCTORS

Applied Electronics DepartmentTechnical University of Cluj-NapocaCluj-Napoca, Cluj, 400027, Romania

Phone: +40-264-401412, E-mail: [email protected]

ELECTRONIC MATERIALS Lecture 13


According to their electrical conductivity, materials are conventionally classified into three groups: conductors, semiconductors and insulators. Conductors, which include all metals, have high conductivities, semiconductors show intermediate conductivities and insulators have low conductivities.


The Basics

A conductor is a material which contains movable electric charges. In metallic conductors (such as copper, aluminum, gold etc), the movable charged particles are electrons. Positive charges may also be mobile in the form of ions, such as in the electrolyte.

All conductors contain electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on the material. This flow of charge (measured in amperes) is what is meant by electric current.


The essential parameters of interest for conductors are:

1. Specific resistivity ρ or specific conductivity σ = 1/ ρ.

σ = |q| · n · µ

With q = magnitude of the charge of the current carrying particles; n = concentration of current carrying particles (usually electrons in conductors); µ = mobility of the current carrying particles.

The units are:

[ρ] = Ωm [σ ] = (Ω m)–1 = S/m

Note that S = "Siemens" = 1/ Ω = A/V


The density and mobility of mobile charged carriers thus determines the conductivity. The carrier density is a function of bonding (metallic, covalent in semiconductor, etc.), defects (doping in semiconductors) and temperature in general. In metals, however, ne is nearly constant. The mobility is a function of collisions between carriers (e.g. electrons and holes) and/or between carriers and obstacles (e.g. phonons and crystal lattice defects).

In physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The study of phonons is an important part of solid state physics, because phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities. In particular, the properties of long-wavelength phonons give rise to sound in solids—hence the name phonon from the Greek φωνή (phonē) = voice.



Matthiesen's rule, stating that :

ρ = ρ Lattice(T) + ρ defect(N)

With N = some measure of defect density.

ρ is proportional to T for T >Tcrit

With Temperature coefficient δρ = 1/ρ · dρ / dT


The range of resistivity values (at room temperature) for metals is rather limited;

here are some values as well as a first and last reminder that σ and ρ, while closely related, are quite different parameters with a numerical value that depends on the choice of the units! Do not mix up cm and m!

Some Values


Pure metals are rarely used - in the real world one use alloys. It is not possible to produce an alloy with a resistivity smaller than one of its components.

Alloys (intermetallic compounds )


Conducting Polymers That polymers, usually associated with insulators, can be very good conductors was a quite unexpected discovery some 20 years ago (Noble prize 2001). They always need some "doping" with ionic components, however. The resistivity can be exceedingly low. e.g. for Iodine (J) doped poly-acethylene (pAc) we may have

ρ≤ 6,7 μΩcm

More typical, however, are resistivities around (10 .... 1000) μΩcm. The conduction mechanism is along –C=C–C=C–C= chains, it is not yet totally clear.The conductivity is strongly dependent on "doping" (in the % range!) with ions, and on many other parameters.A new object of hot contemporary research are now semiconducting polymers which have been discovered about 10 years ago.

Non-Metalic Conductors


Transparent conductors Indium Tin Oxide (ITO) (including some variations) is the only really usable transparent conductor with reasonable conductivity (around 1Ωcm)! It consists of SnO2 doped with In2O3. ITO is technically very important, especially for: - flat panel displays, e.g. LCD's - solar cells- research (e.g. for the electrical measurements of light-induced phenomena). ITO is one example of conducting oxides, others are TiO, NiO, or ZnO. The field is growing rapidly and known as "TCO" = Transparent Conducting Oxides


Ionic conductors Solid Ionic conductors are the materials behind "Ionics", including key technologies and products like:•Primary batteries. •Rechargeable (secondary) batteries. •Fuel cells •Sensors •High temperature processes, especially smelting, refining, reduction of raw materials (e.g. Al-production).

Documents

CONDUCTORS Applied Electronics Department Technical University of Cluj-Napoca Cluj-Napoca, Cluj, 400027, Romania Phone: +40-264-401412, E-mail: [email protected]