Phase Change Random Access Memory

Phase-change random-access memory is a technology that uses a chalcogenide as the data storage material in the memory cell. The medium is similar to the switchable material used in rewritable CDs. It is being pioneered by Ovonyx, in association with Intel. A schematic diagram of an Ovonyx Unified Memory (OUM) is shown in Fig. 30.11 [30.16]. The OUM is an example of a phase-change random-access memory that operates much as the Flash Memory and MRAM, as far as the electrodes are concerned, except that the memory bit cell consists of a chalcogenide material that undergoes a phase change. Upon passing current from the word line to the bit line, the chalcogenide, a GeTeSb alloy, changes its phase reversibly between amorphous and crystalline states, depending on the temperature it is heated to and the rate of cooling. This change in phase is accompanied by a several orders of magnitude change in conductivity, which makes it possible to obtain a high reading signal-to-noise ratio. Similar to Flash Memory and MRAM, the OUM requires a protective shield from the environment for reliable operation over a long period. We shall come back to this issue later.

Fig. 30.11 A cross section of an OUM cell

Fig. 30.10 A diagram of a MRAM cell

Fig. 30.11 A cross section of an OUM cell

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