A storage device is a hardware device that holds data. There are two types of the storage device, a non-volatile storage device and a volatile storage device. The non-volatile storage device (e.g., a hard drive) may hold data even when power is turned off. The volatile storage device (e.g., a static random access memory (a SRAM), a dynamic random access memory (a DRAM), etc.) may lose data when power is turned off.
A transistor (e.g., a simple electronic switch, either preventing or allowing current to flow through) is a solid state semiconductor device which may be used in the storage device. A storage cell (e.g., a physical unit of the storage device that may store exactly one bit) may be created from a number of transistors arranged in such a way as to allow data to be at least temporarily stored. For example, each bit in the SRAM may be stored on four transistors that form two cross-coupled inverters. The storage cell in the SRAM may have two stable states which may used to denote 0 and 1. Two additional access transistors may serve to control access to the storage cell during read and write operations. Thus, it may take six transistors (e.g., CMOS SRAM) to store one memory bit in the storage device.
As application programs demand more storage space and as computing power becomes faster, there is an increasing demand for denser and quicker versions of the storage device. In addition, widespread acceptance and adoption of portable computing devices (e.g., laptop computers, personal digital assistants, etc.) have created a demand for versions of the storage device that use less power.
In order to meet this demand, engineers have reduced physical size of the storage cell to be able to achieve desired properties of lower power, faster speed, and/or higher density of the storage device. In recent years, it has become increasingly difficult to achieve higher densities in the storage device because of leakage (e.g., thinner gate oxides of the transistor may create larger interference between a source and a drain), geometry constraints (e.g., patterning smaller features of the storage cell on a wafer is increasingly difficult and may be expensive), mask step complexity (e.g., it may take nineteen or more mask steps to make each storage cell), and photolithography challenges (e.g., shorter wavelengths are difficult to achieve and may be needed to define smaller features of the storage cell).