Negative capacitance on HP4194A lcr meter

[iheb]

I am trying to measure a photodetector's internal Series and parallel capacitance using HP 4194A, however i am encountering this issue that the capacitance is negative ?
can anyone help and explain what am i doing wrong or what can i do to get the right capacitance and series resistance. 
thank you

[RolandK]

negative capacitance means an inductive behaviour. So your

Xc = 1 / (2 * Pi * f * C)

As Xc is negative:

XL = 2 * Pi * f * L

with XL = -Xc you get

L = - 1 / (2 * Pi * f)^2 / f

If you reduce your measurement-frequency, the Xc increases and the XL decreases.

You have both at the same time, but you only see the difference on the instrument.

[RolandK]

The other point is, you should calibrate the 4194A four your individual probe adapter. If you don't know how, read the f... manual.
Or: Measure the capacitance of your setup with removed photodiode (which should be more negative) and with photodiode and calculate the difference.

For measuring the diode you should know your required bias and explore how to apply it: you must decoupple the power supply for the used frequency (use incutors and or high resistors) and you must protect your instrument from the powersupply (use capacitors) Theare is a difference in capacitance depending on the actual voltage (vorward, none, reverse).

A nice introduction is keysight's impedance measurement handbook. You should know what to do before killing your instrument.