Improved geometrical design of the circular transmission line model ohmic contact test structure

This paper proposes a method to determine the design of the Circular Transmission Line Model (CTLM) in order to ensure accurate results are obtained. The CTLM is used to measure the specific contact resistance of a metalsemiconductor barrier. Through analytical modelling it has been shown that the accuracy of the measurements obtained using a particular CTLM pattern, depends on the geometry chosen. By determining which geometries will yield the most sensitive measurement will ensure an accurate result when compared to the sheet resistance and specific contact resistance of an ohmic contact sample. Analysis of the equations reveals that for any given sample a smaller geometry is preferable. This is determined by comparing the differential of specific contact resistance of the sample with the contact end resistance of the test structure. It has been found that for confident results to be obtained then the annular (centre) ring of the structure should be as narrow as is possible within testing constraints