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. 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 (except this time in the other direction), 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 (sum) 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.
If 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. If the contact separation is 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.
Further Reading
- Gary Tuttle, Dept. of Electrical and Computer Engineering, Iowa State University. "Contact resistance and TLM measurements" (PDF).