TTRAM
Encyclopedia
Twin Transistor RAM 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, Atari's name for a special bank of DRAM in Atari TT030
personal computers.
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
in development by Renesas.
TTRAM is similar to conventional one-transistor, one-capacitor DRAM
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...
in concept, but eliminates the capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
by relying on the floating body effect
Floating body effect
The floating body effect is the effect of dependence of the body potential of a transistor realized by the silicon on insulator technology on the history of its biasing and the carrier recombination processes. The transistor's body forms a capacitor against the insulated substrate...
inherent in a silicon on insulator
Silicon on insulator
Silicon on insulator technology refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance and thereby improving performance...
(SOI) manufacturing process. This effect causes capacitance to build up between the transistors and the underlying substrate
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
, 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
Z-RAM
Zero-capacitor is a novel DRAM computer memory technology developed by Innovative Silicon based on the floating body effect of silicon on insulator process technology. Z-RAM has been licensed by Advanced Micro Devices for possible use in future microprocessors...
, 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
Static random access memory
Static random-access memory is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM , it does not need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit...
used in cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
, which makes it particularly interesting for CPU designs which are being built on SOI lines anyway.
TTRAM should not be confused with TT-RAM, Atari's name for a special bank of DRAM in Atari TT030
Atari TT030
-History:Atari Corporation realized that to remain competitive, they needed to begin taking steps to exploit the power offered by other processors in the Motorola 68000 series. At that time, the best option was the 68020. It was the first true "thirty-two bit bus/thirty-two bit instruction" chip...
personal computers.