Consider the newest precharged floating-gate MOSFETs from Advanced Linear Devices as very fast alternatives for normally closed relays that consume virtually zero power, and you won't go far wrong. (Typical turn-on and turn-off delay is 10 ns.) These dual and quad n-channel depletion-mode devices are attractive for fail-safe circuits in alarms, battery backup circuits, energy harvesting, alternative energy, and many other applications (see the figure).
These MOSFETs have extremely low gate-threshold voltages (VTH) of -0.4 V, thanks to those floating gates, which are programmed in manufacture. The dual ALD114904 and quad ALD114804 both boast threshold matching of ±0.020 V for the A-suffix grade and ±0.04 V for the standard grade. Nominal on resistance with 0 V on the gate is 4.5 kΩ. Each drain can sink 20 µA.
These devices control the gate in a fashion similar to the company's enhancement-mode MOSFETs, except that the threshold voltage is shifted by a fixed negative amount. Otherwise, they work like ordinary MOSFETs.
Pricing for the ALD114904 and ALD114804 starts at $0.91 in quantities of 1000. The ICs also are available from stock at Mouser Electronics (www.mouser.com).
Advanced Linear Devices