Keithley 4200A-SCS Reference Manual page 1166

Model 4200A-SCS Parameter Analyzer Reference Manual Appendix D: Using a Model 82 C-V System
For example, the complete doping profile is derived from data taken in the depletion region of the
curve by using a derivative calculation. As the data point spacing decreases, the vertical point
spacing is increasingly caused by noise rather than changes in the desired signal. Consequently,
choosing too many points in the sweep will result in increased noise rather than an increased
resolution in C-V measurement. It also takes more time to perform a C-V sweep.
Many calculations depend on good measurements in the depletion region, and too few data points in
this region will give poor results. A good compromise results from choosing parameters that will yield
a capacitance change per step of approximately ten times the error in the signal.
Sweep Direction
For C-V sweeps, you can sweep either from accumulation to inversion, or from inversion to
accumulation. Sweeping from accumulation to inversion will allow you to achieve deep depletion,
profiling deeper into the semiconductor than you otherwise would obtain by maintaining equilibrium.
When sweeping from inversion to accumulation, you should use a light pulse to achieve equilibrium
more rapidly before the sweep begins.
Delay Time
For accurate measurement, delay time must be carefully chosen to ensure that the device remains in
equilibrium in the inversion region during a sweep. With too fast a sweep, the device will remain in
non-equilibrium, affecting Q/t, shown in the following figure, and also resulting in skewed C-V curves.
Figure 601: Leakage current Q/t through device
4200A-901-01 Rev. C / February 2017 D-25