Method for plotting frequency cutoff measurements for GaAs varactor diodes

Frequency cutoff for GaAs varactor diodes of diffused-P
⁺
-n-epitaxial n
⁺
-substrate construction can be plotted so as to conveniently show dependence of diode cutoff as measured at 10 GHz on diode n-region thickness and substrate resistivity. The measured data fit a model for the diode where the zero bias junction capacitance
C_{j0}
is in series with a junction resistance R
j
that is proportional to
1/C_{j0}
, and a resistance R
X
that is constant. R
j
is the resistance of the diode n-region and the p
⁺
-diffused region, and is primarily dependent on the n-region thickness. R
X
is dependent on diode spreading resistance, and varies with substrate resistivity. R
X
is about 1 Ω for usual substrate material.