Circuit for standby supply.

Hi.
I am looking for a way of supplying standby power at 1.8V - 3.3V to a
microcontroller during shutdown - ideally a circuit that doesn't itlelf
consume power. I proposed the circuit below. This is only for shutdown
mode. For normal operation there is a 3.3V regulator which isn't shown.
The theory is when the capacitor voltage drops below ~3V, the FET will
conduct until bias reduces to Vgs(cutoff) and then conduction will
cease. The weakness of this design is that Vgs(cutoff) varies
considerably from FET to FET, so it is unlikely that a capacitor voltage
of 1.8V - 3.3V can be repeatably achieved. Other than using a voltage
regulator (which consumes power) can anyone suggest a solution to
maintain ~3V on C1?
Suggestions appreciated.


solar panel 12V -------|-----------
|
|
|
|--
-------->|
| |--
| | ~3V
| |-------------|
| | |
| | |-------
| | | Vcc
| ----- C1 |
| ----- 0.5F |
| | | uC
| | |
| | |-------
| | |
| | |
--- --- ---