or may lose adhesion when expanding at a different rate from the phase-change material. Phase-change memory is susceptible to a fundamental tradeoff of unintended vs. intended phase-change. This stems primarily from the fact that phase-change is a thermally driven process rather than an electronic process. Thermal conditions that allow for fast crystallization should not be too similar to standby conditions, e.g. room temperature, otherwise data retention cannot be sustained. With the proper activation energy for crystallization it is possible to have fast crystallization at programming conditions while having very slow crystallization at normal conditions. Probably the biggest challenge for phase-change memory is its long-term resistance and threshold voltage drift. The resistance of the amorphous state slowly increases according to a power law (~t0.1). This severely limits the ability for multilevel operation, since a lower intermediate state would be confused with a higher intermediate state at a later time, and could also jeopardize standard two-state operation if the threshold voltage increases beyond the design value. In April 2010, Numonyx released its Omneo line of parallel and serial interface 128 Mb NOR flash replacement PRAM chips. Although the NOR flash chips they intended to replace operated in the −40-85 °C range, the PRAM chips operated in the 0-70 °C range, indicating a smaller operating window compared to NOR flash. This is likely due to the use of highly temperature-sensitive p–n junctions to provide the high currents needed for programming. Timeline January 1955: Kolomiets and Gorunova revealed semiconducting properties of chalcogenide glasses. September 1966: Stanford Ovshinsky files first patent on phase-change technology January 1969: Charles H. Sie published a dissertation at Iowa State University on chalcogenide phase-change-memory device June 1969: US Patent 3,448,302 (Shanefield) licensed to Ovshinsky claims first reliable operation of PRAM device September 1970: Gordon Moore publishes research in Electronics Magazine June 1999: Ovonyx joint venture is formed to commercialize PRAM technology November 1999: Lockheed Martin works with Ovonyx on PRAM for space applications February 2000: Intel invests in Ovonyx, licenses technology December 2000: ST Microelectronics licenses PRAM technology from Ovonyx March 2002: Macronix files a patent application for transistor-less PRAM July 2003: Samsung begins work on PRAM technology 2003 through 2005: PRAM-related patent applications filed by Toshiba, Hitachi, Macronix, Renesas, Elpida, Sony, Matsushita, Mitsubishi, Infineon and more August 2004: Nanochip licenses PRAM technology from Ovonyx for use in MEMS probe storage August 2004: Samsung announces successful 64 Mbit PRAM array