Tezzaron's 3T-iRAM® memory technology, based on the time-tested DRAM model, offers DRAM-like
advantages in cost and density; however, its novel current-sensing process achieves
SRAM-like performance. The name "3T-iRAM" comes from the three-transistor cell
design plus the letter "i" -- the engineering symbol for electric current -- to
represent the current-sensing technology employed.
3T-iRAM Speed
The dramatic speed of 3T-iRAM was achieved by re-designing the DRAM read mechanism to
react to changes in current rather than changes in voltage. In memory
circuits, the current changes more quickly than the voltage does, so changes in current can
be detected more quickly than changes in voltage. By exploiting this difference, 3T-iRAM
technology can provide better-than-SRAM speed at a much lower cost.
3T-iRAM chips incorporate another speed advantage. In many memory devices, some idle time
is required in “turnaround” – that is, when a read command is followed by a write command,
or vice versa, the memory “bus” is idle for one or more cycles. 3T-iRAM chips require no
idle time at all on turnaround; this contributes to their speed.
3T-iRAM Density
3T-iRAM provides twice the capacity of standard SRAM in exactly the same footprint. The
reason for this is simple: standard SRAM technology requires six transistors for each memory
cell, but each 3T-iRAM cell needs only three transistors.
3T-iRAM Soft Error Resistance
Because 3T-iRAM is built on DRAM technology, it shares DRAM’s reliability; like DRAM, it is
more resistant to soft errors than SRAM.
Q: Is 3T-iRAM a kind of SRAM, or is it DRAM?
A: 3T-iRAM is closer to DRAM, but it has some unique properties. It is dynamic
in that refresh is required, but 3T-iRAM reads are nondestructive (unlike DRAM reads).
Whereas DRAM uses voltage deflection on a bit-line pair, 3T-iRAM provides a real positive
current drive. This positive drive reduces noise sensitivity and increases speed.
Q: Is 3T-iRAM faster than SRAM?
A: Yes, it generally is. The higher speed is mostly due to 3T-iRAM's smaller cell size,
lower bit-line capacitance, and better bit-line drive from the memory cells. Even with the
addition of hidden refresh, 3T-iRAM will perform at least as fast as SRAM, in a much
smaller area, and using less power.
Q: How does 3T-iRAM compare to 1T SRAM?
A: 1T SRAM provides a cell size similar to that of 3T-iRAM, but 3T-iRAM provides better
speed - in fact, the speed of 1T SRAM is comparable to that of ordinary DRAM of the same
array size.
Q: Is 3T-iRAM a good replacement for all types of SRAMs?
A: Different types of SRAMs require different replacements. The first wave of stand-alone
3T-iRAM parts, built in a standard CMOS logic process, can replace high-speed SRAMs.
Another version of 3T-iRAM, already under development, is optimized for incorporation in
an embedded DRAM process; this version will be better for replacing large, low-power
SRAMs. The latter version will achieve lower power and higher density than is available
with any DRAM or SRAM structure.
Q: Can I run 3T-iRAM slower than 1GHz?
A: Yes. We will offer two types of 3T-iRAM parts: Type 1 is an SRAM drop-in replacement, and Type 2
uses an externally controlled refresh. Type 1 (SRAM replacement) is designed to operate
anywhere from DC to full rated speed. Type 2 (external refresh) may be run at any speed
up to the full rated speed, provided that the refresh requirements are met.
Q: What processes have been used with the 3T-iRAM technology?
A: We have run 3T-iRAM components in 0.5µm, 0.35µm, 0.25µm, 0.13µm, and 90nm processes; 0.13µm
parts are in production.
Q: How long has 3T-iRAM been in production?
A: Proprietary memory blocks that incorporate 3T-iRAM technology have been in volume
production since 1999. The first stand-alone memory prototype was produced
in 2003. Commercial parts began production in 2008.
Q: Can this technology be built in any fab?
A: Yes. Tezzaron has produced 3T-iRAM components in no fewer than 5 different foundries and 6
different processes.
Q: Is any special processing required?
A: No. All 3T-iRAM parts to date have used standard single poly CMOS processing. Tezzaron is in
discussions with some manufacturers about higher-performance parts; if these are developed, they
could require unique processing.
Q: Can 3T-iRAM be built in a DRAM Fab?
A: Yes. While Tezzaron has so far concentrated on maximum portability, 3T-iRAM can easily be
altered to run in a DRAM fabrication process.
Q: What voltage does 3T-iRAM use?
A: 3T-iRAM operating voltage depends on the process in which it is made. The 90nm prototype was
designed for 1.2V operation, whereas the 0.5µm part previously created ran at 5V. In general,
the lowest possible operating voltage is 2.5 to 3 times the threshold voltage (Vt).
Q: What is array efficiency, and how does 3T-iRAM measure up?
A: Array efficiency is a measurement of how much area in a memory block is used by memory cells
compared to the area used by drivers, sense amps, and other required support circuitry. The
highest performance 3T-iRAM has array efficiencies in the 75% range. Other versions of 3T-iRAM
(with somewhat slower performance) can reach efficiencies of 90%.
Q: What is the bit cell size?
A: In a 90nm process, the 3T-iRAM bit-cell size is 0.59 square µm. For a rough estimate in any
process, a 3T-iRAM bit-cell is just under 50% the size of an SRAM bit-cell.
Q: How sensitive to soft errors are 3T-iRAM parts?
A: 3T-iRAM parts built in a standard CMOS process are less sensitive than standard SRAMs but
more sensitive than DRAMs. Soft error sensitivity has a lot to do with the internal capacitance
of the bit-cell: higher capacitance means lower susceptibility. 3T-iRAM in a standard CMOS
process has 2 to 4 times the storage node capacitance of a standard SRAM in the same process.
Tezzaron is working on a version of 3T-iRAM to be built in an embedded DRAM process. Among other
significant benefits, this version should have a lower soft error rate than competitive standard
DRAM solutions.
Q: Can I obtain 3T-iRAM for SOC integration?
A: Yes. Direct your inquiries to Memory@tezzaron.com.
Q: Can 3T-iRAM be extended to dual port, multi-port, CAM, or other specialty memories?
A: Yes. 3T-iRAM is easily adapted to these and other memory architectures.
Q: How often do 3T-iRAM memory cells need to be refreshed?
A: This depends on a number of factors such as process, density and temperature range for
operation. Typically, the refresh period is tens to hundreds of microseconds in a standard CMOS
process. The "SRAM-replacement" 3T-iRAM parts feature completely hidden refresh.
Q: Is the standard 3T-iRAM bit-cell single-port or dual-port memory?
A: Neither: 3T-iRAM is best described as "port and half" memory. Each 3T
bit-cell has independent read and write bit-lines and control signals, allowing some overlapping
of functions. A standard 3T-iRAM bit-cell has more overlap capability than a typical single-port
bit-cell, but not as much as a true dual-port bit-cell. Hence "port and a half."
Q: What densities are planned?
A: Current devices are 72Mb; Tezzaron plans to produce up to 144Mb. Larger sizes are possible
and may be produced based on market demand.
Q: Are there currently any second sources for 3T-iRAM parts?
A: No, but Tezzaron plans to provide one or more in the future.
