Traditionally method of determining of extracting capacitance of active deices is to use s parameters , then convert to y parameters and change bias points . This will give a CV curve . Of course capacitances of RF devices will be very small , for example Cds .
In the case of a HBT , current mirrors are typically used . If the capacitance of the HBT current mirror reference device can be actively measured for capacitance , it may be possible to create a active linearizer based on the capacitance swing of the reference device in the current mirror which follows the AC load line of the larger RF transistor in a transistor power amplifier .
The capacitance swing of the large RF power transistor creates the AM to PM distortion from capacitance swing , but this capacitance swing is limited by the AC load line . Potentially minimizing distortion into higher VSWR operation .
The problem of using current mirrors is bandwidth considerations in relation to the baseband modulation on the RF signal . To overcome this alternative techniques of RF pre-distortion may need Look up tables .
Please answer what are minimum capacitance levels you are measuring ?
Bet regards
Tim Aust
Dear Tim,
as you correctly stated, the methods to extract the parameters of the small-signal equivalent circuit of active devices rely on S-parameter measurements at different bias points. In my experience, for what concern bipolar devices and in particular HBTs for RF applications, most of the time measurements on a single device in different operating conditions are enough without resorting to measurements of current-mirror topologies.
To answer your question, the capacitance levels for sub-micron InP/InGaAs DHBTs are in the fF order of magnitude and are commonly measured as a function of collector current. The frequency range of measurements with a VNA could be up to 140 GHz or higher.
I hope this answers your question and in case I'm open for further discussion.
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