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
            |          |         |
            |          |         |-------
	    |	       |             |
	    |          |             |
           ---	      ---           ---
	

.