1. Field of the Invention
The present invention relates to memory devices, and more specifically, to random access memory devices having vertical access transistors.
2. Description of the Related Art
Semiconductor memory devices are widely used for storing data in systems such as computer systems. Random access memory devices include dynamic random access memory (DRAM), magnetic random access memory (MRAM) and programmable conductor random access memory (PCRAM) memory cells that are being developed for non-volatile data storage in computers and other information processing devices.
A DRAM cell typically includes an access device, such as a field effect transistor (FET), which is coupled to a storage device, such as a capacitor. The access device allows the transfer of charges to and from the storage capacitor, thereby facilitating read and write operations in the memory cell. The memory cells typically are arranged in a number of rows and columns to provide a memory array. DRAM cells require continual refreshing.
Recent developments in memory include MRAM and PCRAM devices. MRAM memory cells are vertical stacks of thin films with certain magnetic and conductive interactive properties. An array of these cells forms a row and column addressable memory array. Bitwise information is stored in each cell based on the relative orientation of magnetic moments in thin films of the cell. Magnetic tunnel junctions are examples of the type of thin film structures used in magnetic memory cells.
A PCRAM memory cell utilizes silver electromigration through a glass to form a contact when an electrical potential is applied across the plates of the cell. The contact may be broken by reversing the polarity of the potential and allowing reverse current to flow until the silver migrates back and breaks the connection.
MRAM and PCRAM do not require dynamic refreshing as does DRAM, and can be implemented without using a cell access transistor. Accordingly, MRAM arrays can be produced more efficiently (more bits per unit area) than DRAM. The lack of an access transistor, however, exposes MRAM cells to “sneak” currents through unselected cells, which requires compensation by sense circuitry in order to isolate the selected MRAM cell.
Memory cells, such as MRAM and PCRAM cells, would benefit from having an access transistor that would allow array efficiency to be preserved, while providing current isolation of the cells.
With the constantly increasing demand for higher data storage capacity, memory arrays are becoming denser. Memory density typically is limited by current processing technologies used for fabricating the memory arrays. Thus, it also is desirable to increase memory density.