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This page discusses of the differences between five categories of materials according to their conductivities:
Insulators
Semi-insulators
Semiconductors
Conductors
Superconductors
This page also explains the concept of sheet resistance and conductivity.
The distinctions below are in common usage but may be seen as arbitrary since there is no industry standard that can be applied here (perhaps IEEE will someday come up with one). You can also check out our page on conducting materials.
Insulators
This category is VERY resistive (duh). Bulk resistivity is within the range of 1010 to 1022 ohm-centimeter to be considered an insulator. Any "good" dielectric material is an insulator. Insulators include glass, plastic, rubber, silicon dioxide, and silicon nitride
Insulating materials have tightly bound electrons (usually eight) in the outer shell that are happy to remain there even in the presence of high voltage electric fields.
Semi-insulators
Semi-insulators have bulk resistivities between 103 to 1010 ohm centimeters. Cut fresh from the boule, GaAs wafers are said to be semi-insulating.
Semiconductors:
Semiconductors have bulk resistivity in the range of 10-4 ohm-cm (heavily doped) to 103 ohm-cm (undoped, or intrinsic). That's seven orders of magnitude! Semiconducting elements include silicon and germanium, it is no coincidence they are both from group 4 of the period table. Semiconducting compounds include gallium arsenide, indium phosphide, and gallium nitride, from groups 3/5 or 2/6 of the period table.
Semiconductor materials have 4 electrons in their outer shell (it's half filled). When bonded together in a crystal lattice, atoms share electrons such that they each have eight electrons in the outer shell. Electrons are somewhat loosely bound so they can become carriers in the presence of an electric field.
Conductors:
To be considered a conductor, a material must have a bulk resistivity within the range of 10-6 to 10-4 ohm-cm. Conductor materials have loosely bound electrons (one or two) in the outer (valence) shell that can move easily under the influence of a voltage to form current. Conductors include elemental metals such as copper, gold, silver, aluminum. Heavily-doped semiconductors can also be considered conductors.
Superconductors:
Content moved to a new superconductor page.
Sheet resistance and sheet conductivity
This concept is further explained on our page on resistor mathematics.
Bulk resistivity is a the property which is independent of frequency and geometry. In microwaves, often we are dealing with thin films of conductors, which have been applied at a controlled thickness. A more convenient property to deal with in this case is sheet resistance. The sheet resistance of a metal film is often expressed in ohms/square. What's a square? Exactly that. Who's on first? I Don't Know's on second.
Recall the equation for calculating resistance from bulk resistivity:
Remember, the resistance calculated this way does not account for skin depth effects. It is accurate if your conductor thickness is small compared to a skin depth.
If you consider the quantity L/w, it is unitless. It can be considered as a measure of how many squares of area your conductor or resistor has. For example, a thin-film resistor with length 30 mils and width 10 mils is three squares. A smaller resistor of 3 microns length and 1 microns width also has three squares (thanks Jack!) If they both have the same thickness and bulk resistivity, they both have the same value in ohms. They will have far different power ratings, and the smaller resistor will have a higher usable frequency response. Be careful not to mix up length and width, a resistor with 10 microns length and 30 microns width measures 1/3 square, not three squares!
Sheet resistance, Rsh, is equal to bulk resistivity divided by thickness. It can be used to conveniently calculate resistance values from number of squares, as follows:
As in all engineering, you will need to keep units consistent in order to make the calculation correctly (if rho is in ohm centimeters, the thickness must also be in centimeters). One last thing to consider: sheet conductivity is the inverse of sheet resistivity. When is sheet conductivity useful? When you have more than one metal layer. The sheet conductivities of the layers can be added, because the conduction paths are in parallel.