Transmission line measurement
Encyclopedia
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.
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
to determine the contact resistance
Ohmic contact
An ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage curve of the device is linear and symmetric. If the I-V characteristic is non-linear and asymmetric, the contact is not ohmic, but is a blocking or Schottky contact...
between a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
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
Metal-semiconductor junction
In solid-state physics, a metal–semiconductor junction is a type of junction in which a metal comes in close contact with a semiconductor material...
, 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
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
. The resistance measured is a linear combination (sum) of the contact resistance
Contact resistance
The term contact resistance refers to the contribution to the total resistance of a material which comes from the electrical leads and connections as opposed to the intrinsic resistance, which is an inherent property, independent of the measurement method...
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
Graph of a function
In mathematics, the graph of a function f is the collection of all ordered pairs . In particular, if x is a real number, graph means the graphical representation of this collection, in the form of a curve on a Cartesian plane, together with Cartesian axes, etc. Graphing on a Cartesian plane is...
of resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
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
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
, with the slope
Slope
In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....
of the line
Line (mathematics)
The notion of line or straight line was introduced by the ancient mathematicians to represent straight objects with negligible width and depth. Lines are an idealization of such objects...
being the sheet resistance
Sheet resistance
Sheet resistance is a measure of resistance of thin films that are namely uniform in thickness. It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing, and glass coating. Examples of these processes are: doped semiconductor regions...
. The intercept
Y-intercept
In coordinate geometry, using the common convention that the horizontal axis represents a variable x and the vertical axis represents a variable y, a y-intercept is a point where the graph of a function or relation intersects with the y-axis of the coordinate system...
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.