I have been thinking in somekind of thermoelectric geneator for years that maybe can be used for this... There is a temp difference from the surface down as you dig a hole. Maybe placing some bi-metal rods can havest some of the energy from the difference and it will be electrical energy already. If you go all CMOS and sleeping mode most of the time maybe you can use that energy to recharge something while sleeping or have two sets of batteies... Problem will be corrosion... but what if the rods are protected with vinyl or something like ceramic painting (like ovens)?. Maybe they produce enough power to supply the thing without a storage device. Remember spacecrafts use thermopiles (with a nuclear capsule for heating) to generate electricity. You don't need that much energy so you don't need the reactor. Multiple rods in parallel and series (like batteies) may meet both voltage and current requierements even with small differences in temp. Maybe... Testing is the poblem.
I also think semiconductors should last that long specially because there will be negligible amounts of heat generated.