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Computer memory types

Twin Transistor RAM (TTRAM) is a new type of computer memory in development by Renesas.

TTRAM is similar to conventional one-transistor, one-capacitor DRAM in concept, but eliminates the capacitor by relying on the floating body effect inherent in a silicon on insulator (SOI) manufacturing process. This effect causes capacitance to build up between the transistors and the underlying substrate, originally considered a nuisance, but here used to replace a part outright. Since a transistor created using the SOI process is somewhat smaller than a capacitor, TTRAM offers somewhat higher densities than conventional DRAM. Since prices are strongly related to density, TTRAM is theoretically less expensive. However the requirement to be built on SOI fab lines, which are currently the “leading edge”, makes the cost somewhat unpredictable at this point.

In the TTRAM memory cell, two transistors are serially connected on an SOI substrate. One is an access transistor, while the other is used as a storage transistor and fulfils the same function as the capacitor in a conventional DRAM cell. Data reads and writes are performed according to the conduction state of the access transistor and the floating-body potential state of the storage transistor. The fact that TTRAM memory cell operations don't require a step-up voltage or negative voltage, as DRAM cells do, makes the new cell design suitable for use with future finer processes and lower operating voltages.

With the Renesas TTRAM, a read signal from a memory cell appears as a difference in the transistor on-current. A current-mirror type sense amplifier detects this difference at high speed, using a reference memory cell that allows reliable identification of the 0 and 1 data levels. This reading method significantly decreases power consumption by eliminating the charging and discharging of bit lines, operations required for reading DRAM memory cells.

A similar technology is Z-RAM, which uses only a single transistor and is thus even higher density than TTRAM. Like TTRAM, Z-RAM relies on the floating body effect of SOI, and presumably has a similar manufacturing process. Z-RAM also claims to be faster, as fast as SRAM used in cache, which makes it particularly interesting for CPU designs which are being built on SOI lines anyway.

TTRAM should not be confused with TT-RAM, a type of memory found in Atari TT030 and Falcon030 personal computers. TT-RAM is not used for memory-mapped I/O therefore does not suffer from the bus-contention that slows down the traditional ST-RAM.

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