The DDC114 is a 4-channel current input ADC. Featuring 20-bits, the data sheet claims the ability resolve fA's. It is expensive, costing $18 in 1000 unit quantities. Applications include CT scanners, photodiode sensors, infrared pyrometers and liquid/gas chromatography. It is protected by US5841310.
Vanadium dioxide is a semiconductor which undergoes a first-order metal-insulator phase transition (MIT) at 340K. First observed in 1959 by Morin, the increase in conductivity has been observed to be as much as five orders of magnitude. At temperatures above the critical temperature, each vanadium atom is located at the center of an oxygen tetrahedron. At lower temperatures, the structure is distorted as vanadium atoms pair up and are alternately displaced from the center of oxygens. The transition mechanism is still not well understood. See [1,2] for more detailed information.
 N.F. Mott, Metal-insulator transitions, London ; New York: Taylor & Francis, 1990, pp. 185-9.
 S. Luryi, J. Xu and A. Zaslavsky, Future trends in microelectronics : the nano millennium, New York: IEEE : Wiley-Interscience, 2002.