Thank you for these comments; I was not able to reply more promptly. Unfortunately the test equipment was built by somebody else so I am not intimately acquainted with the details of its construction.
The key question is what could cause a diurnal variation. Yes, lots of things could be wrong but what could be diurnally wrong? The diodes have opaque covers and are inside a closed miniature oven so we can eliminate light. Likewise moisture in the atmosphere inside the chamber will not not, surely, vary in a diurnal manner. If you recall the characteristics of a diode you will remember that the reverse bias current is not very sensitive to voltage so even if the voltage were badly controlled (It is set to -10V) it should not have very much effect. Reading are taken once every two minutes so I don't see that it is a software problem. The temperature is maintained via a PID controller but unless that has some odd firmware I see no reason why it should develop a daily rhythm. My laboratory is on the outskirts of a residential area, becoming rural, with no unusual vibrations. However I have just found some evidence to suggest my data may be sensitive to the room temperature.
There is some work going on elsewhere in the building today and the outside door down the corridor is open. The leakage currents I am measuring have been dropping steadily from the time I arrived and opened the door to the room where the measurements are taking place; this door is otherwise closed. This provides the best evidence to date that there is some residual sensitivity to room temperature as the room has been cooling since I arrived. I measure and control the temperature in the device chamber with a thermocouple using electronic cold junction compensation so perhaps that gives me some small residual sensitivity to the ambient room temperature? And at this time of year there is no control of the room temperature (neither air conditioning nor heating).
I am in a single storey building with a pitched roof. The roof warms up during the day but the insulation between the roof and the loft space and then between the loft space and the rooms below might mean that the interior temperature of the building varies out of phase with the external temperature of the roof. So perhaps my daily cyclic variation is just a reflection of a small variation in the ambient room temperature and a flaw in my equipment causes the actual controlled temperature to be slightly sensitive to room temperature?
To test this hypothesis I need to set up more equipment to log the room temperature and, if possible, get a more accurate reading of the temperature in the chamber.
