The curvematch program

The curvematch program quantifies the «similarity» of two curve sets. Consider the following figure with the \(U_1\), \(I_1\), and \(U_2\) data measured from two different VFET power transistors. The data from the two transistors are represented by the red and blue surfaces in a 3-dimensional plot instead of the conventional curves plot:

The «similarity» between the two data sets is defined as the RMS value of the vertical difference between the two surfaces (i.e., the RMS difference along the \(U_2\) axis).

The procedure implemented in the curvematch program to calculate and report the similarity between the curve sets is as follows:

1. For current-controlled DUTs (BJTs), the \(U_2\) value is converted to the base current \(I_{\rm B}\). For voltage controlled DUTs (FETs, tubes), the \(U_2\) voltage is the gate/grid voltage \(V_{\rm G}\).

2. The \(I_{\rm B}\) or \(V_{\rm G}\) data of both DUTs are interpolated to a rectangular grid of \(U_1\) and \(I_1\) values.

3. At each grid point \((i,j)\) the differences \(D_{ij}\) of the \(I_{\rm B}\) or \(V_{\rm G}\) values are calculated.

4. The RMS value of the \(D_{ij}\) values is reported. A low RMS value indicates that the two curve data sets are similar to each other (i.e., the surfaces in the 3-dimensional plot are almost identical).

5. The arithmetic mean \(D_0\) of all \(D_{ij}\) values is calculated, and the RMS value of \(D_{ij}-D_0\) is reported. A low RMS value indicates that the curves of the two DUTs tend to be parallel to each other, but may exhibit a constant \(V_{\rm G}\) or \(I_{\rm B}\) offset (i.e., the surfaces in the 3-dimensional plot are parallel, but may be offset along the \(U_2\) axis).

Optionally, the calculation of these RMS values can be restricted with the curvematch program to data points that lie within a certain range of interest of the \(U_1\) and \(I_1\) data.

The curvematch documentation can be accessed from the curvematch program directly:

curvematch --help