Zero Capacitor RAM or Z-RAM is a new type of computer memory that is being developed at a company called Innovative Silicon. The RAM functions using the principle of floating body effect of silicon on insulator. AMD has purchased licenses to the use of Z-RAM and may use it in its future microprocessors. Z-RAM is almost similar to SRAM in terms of access speeds. However, it is much more compact–SRAM uses 6 transistors while Z-RAM uses only 1. Due to this, RAM chips with much higher capacities can be produced.
Z-RAM is based on the floating body effect. In silicon on insulator process technology, transistors are placed in isolated tubs. The voltage supplied to the transistors floats with respect to the wafer substrate below the tubs and due to this a variable capacitance appears between the tub bottom and the substrate. This used to be a problem with earlier circuit designs. However, the same effect allows a DRAM cell to be constructed without the use of a separate capacitor as the floating body capacitance can be used as the main capacitor. Since the capacitor is located below the transistor, it can be interpreted as another connotation of the name as it extends in the Z-axis.
Due to smaller cell sizes, Z-RAM is significantly faster than SRAM especially when used in large blocks. Due to smaller size, the physical distance over which data has to travel is reduced, which makes Z-RAM faster. When used in large sized caches like those around the microprocessor, the speed offered by Z-RAM is comparable to that of SRAM. However, Z-RAM is much smaller and at the same time inexpensive. Response times of 3 Nano-seconds or less have been observed with Z-RAM.
Silicon on Insulator i.e. SOI technology is costlier when compared to conventional technology. However, it is aimed at high performance computer market segment. Z-RAM offers the possibility of producing cheaper on-chip memory without hampering performance.
Z-RAM technology has already been adopted by market leaders such as AMD and Hynix. AMD has acquired licenses to use Z-RAM in its future microprocessors. Hynix has acquired licenses to use it to produce DRAM chips in the future.