Transmission line measurement


Transmission line measurement or Transfer Length Measurement is a technique used in semiconductor physics and engineering to determine the contact resistance between a metal and a semiconductor. The technique involves making a series of metal-semiconductor contacts separated by various distances.

General description

Probes are applied to pairs of contacts, and the resistance between them is measured by applying a voltage across the contacts and measuring the resulting current. The current flows from the first probe, into the metal contact, across the metal-semiconductor junction, through the sheet of semiconductor, across the metal-semiconductor junction again, into the second contact, and from there into the second probe and into the external circuit to be measured by an ammeter. The resistance measured is a linear combination of the contact resistance of the first contact, the contact resistance of the second contact, and the sheet resistance of the semiconductor in-between the contacts.
When several such measurements are made between pairs of contacts that are separated by different distances, a plot of resistance versus contact separation can be obtained. The contact separation can then be expressed in terms of the ratio L/W - where L and W are the length and width of the area between the contacts - such a plot should be linear, with the slope of the line being the sheet resistance. The intercept of the line with the y-axis, is two times the contact resistance. Thus the sheet resistance as well as the contact resistance can be determined from this technique.