In the case of conventional memory devices, in particular conventional semiconductor memory devices, it is sometimes common to differentiate between functional memory devices (e.g., PLAs, PALs, etc.) and table memory devices. For example, some table memory devices include ROM devices (Read Only Memory) such as PROMs, EPROMs, EEPROMs, flash memories, etc., and RAM devices (Random Access Memory or read-write memory) such as DRAMs and SRAMs.
In the case of SRAMs (Static Random Access Memory), individual memory cells consist of several, for example, six, transistors configured as a cross-coupled latch. In the case of DRAMs (Dynamic Random Access Memory), generally only one single, correspondingly controlled capacitive element (e.g., the gate-source capacitance of a MOSFET) is employed, wherein charge may be stored in the capacitance. The charge in a DRAM, however, remains for only a short time, and a periodic refresh must be performed, to maintain a data state. In contrast to the DRAM, the SRAM requires no refresh, and the data stored in the memory cell remains stored as long as an appropriate supply voltage is fed to the SRAM. Both SRAMs and DRAMs are considered volatile memories, wherein a data state is only retained as long as power is supplied thereto.
In contrast to volatile memory, non-volatile memory devices (NVMs), e.g., EPROMs, EEPROMs, and flash memories, exhibit a different property, wherein the stored data is retained even when the supply voltage associated therewith is switched off. This type of memory has several advantages for various types of mobile communications devices such as, for example, in an electronic rolodex on cell phones, wherein the data therein is retained even when the cell phone is turned off.
One type of non-volatile memory that has recently been developed is called resistive or resistively switched memory devices. In such a resistive memory, a memory material positioned between two appropriate electrodes (i.e., an anode and a cathode) is placed, by appropriate switching processes, in a more or less conductive state, wherein the more conductive state corresponds to a logic “1”, and the less conductive state corresponds to a logic “0”, for example (or vice versa). Suitable resistive memories can be, for example, perovskite memory, as described in W. W. Zhuamg et al, “Novell Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM 2002, resistive switching in binary oxides (OxRAM), as, for example, described in I. G. Baek et.al., “Multi-layer crosspoint binary oxide resistive memory (OxRAM) for post-NAND storage application”, IEDM 2005, or phase change memory.
In the case of phase change memory, an appropriate chalcogenide compound (e.g., a GeSbTe or an AgInSbTe compound) may, for instance, be used as the active material that is positioned between the two corresponding electrodes. The chalcogenide compound material can be placed in an amorphous, i.e., relatively weakly conductive, or a crystalline, i.e., relatively strongly conductive state by means of appropriate switching processes, and thus behaves like a variable resistance element, which as highlighted above, may be exploited as differing data states.
In order to achieve a change in the phase change material from an amorphous state to a crystalline state, an appropriate heating current can be applied to the electrodes, wherein the current heats the phase change material beyond the crystallization temperature thereof. This operation is sometimes called a SET operation. Similarly, a change of state from a crystalline state to an amorphous state may be achieved by application of an appropriate heating current pulse, wherein the phase change material is heated beyond the melting temperature thereof, and the amorphous state is obtained during the rapid cooling process thereof. This operation is sometimes called a RESET operation. The combination of SET and RESET operations is one means by which data can be written to a phase change memory cell.
Conventionally, phase change memory devices were organized in one or more arrays of phase change cells in a core area of the device, wherein each phase change memory cell was composed of a phase change memory element coupled to a selection switching device. One conventional arrangement is illustrated in FIG. 1, wherein a phase change element 10 is coupled between a bit line 12 and a bipolar select transistor 14. A word line 16 is coupled to the base terminal of the transistor 14. By properly addressing the bit line 12 and word line 16 associated therewith, data may be written thereto and read therefrom. An array of phase change memory cells configured in the manner described above is sometimes called a NOR type memory array.
It is often desirable to increase the density in a memory, that is, to increase the amount of data that can be stored within a given area. In some instances, attempts have been made to increase memory density by storing multiple bits of data within a single memory cell. Such a solution in phase change memory cells, however, disadvantageously results in an increase in power consumption when reading data from or writing data to the cell. Consequently, another solution to increase the memory density is to reduce the physical size of an individual memory cell.
One attempt to reduce the size of an individual phase change memory cell employed a cross-point array-like structure, where a bit line and a word line are used to select a phase change cell using an additional two-port element that was similar to a diode. This solution, however, suffers from a variety of disadvantages such a signal integrity, and difficulties arise in the integration of the diode-like structure into a standard process flow. Therefore there is a need in the art for improved memory cell architectures for phase change memory devices.