For Information
February 29, 1996

NTT Develops High-Speed LSI on Solar Battery Power
-Reduces power consumption of LSI to 1/100 for portable communications equipment-

Nippon Telegraph and Telephone Corporation (NTT) has developed a CMOS (*1) LSI that operates on a 0.4V power supply at high speed that is expected to pave the way toward significantly extending the battery life of portable communications equipment as well as toward the realization of a direct supply voltage by solar battery-driven portable equipment. (A 0.4V operating voltage is similar to that generated by a single solar battery cell.)

With the features of NTT's unique SIMOX (*2) substrate, the new LSI was realized by developing variable threshold transistors for controlling the supply current at which CMOS logic function unit consists of low-threshold voltage transistors. (Voltage at which transistor currents start to run.) The new LSI reduces power consumption to about 1/100 the level required by conventional LSIs operation on 3.3V. When 1V, equivalent to single nickel-cadmium battery cell, is supplied to the circuit, 1V outperforms the conventional CMOS LSI and achieves an operational speed of 300 pico (1/1 trillion) per second with about 1/15 the power consumption.


I. Development Background

The advancement of cellular phones and information equipment increases the necessity for more energy-efficient LSIs. Lower power consumption means longer battery life. If the operational time is the same, lower power consumption leads to a smaller battery size and increases the portability of such equipment.

In 1993, NTT developed a high-speed LSI that operated on 1V. The 1V LSI is based on the MT (multi-threshold) CMOS circuit technique for integrating, on a single chip, both low-threshold high-speed CMOS transistors and high-threshold transistors, which eliminate the leakage current of the low-threshold transistors in standby mode. As a result, since 1993, NTT has conducted research in the application of MTCMOS circuit technique to NTT's unique SIMOX substrate toward the realization of an LSI that can operate on a lower supply voltage.

Operating CMOS transistors on a lower supply voltage effectively reduces power consumption. However, speed characteristics deteriorate as the supply voltage approaches the threshold voltage that determines the transistor's on and off states. Reducing power consumption while maintaining the operational characteristics requires lowering the threshold voltage simultaneously. In the past, the conventional problem was that lowered threshold voltage increased the transistor's leakage current in standby mode and did not result in a reduction of total power consumption.

II. Technological Points

1. MTCMOS circuit technique
MTCMOS circuit's power consumption is significantly influenced by the power switch (equivalent to a cork of supply current) CMOS transistor's threshold voltage. Even if the logic-function low-threshold transistor's threshold voltage is lowered further, the logic function unit does not operate at high speed because the threshold voltage of the high-threshold CMOS transistors, which functions as the power switch, is high. With this in mind, NTT uses a variable threshold voltage structure for the power switch CMOS transistors to make it possible to prevent leakage current by raising the threshold voltage when the logic function unit is in standby mode and smoothly supplies current at low voltage when the logic function unit is in operation mode. The structure allows for a further reduction in power consumption without adversely affecting the high-speed operation of the MTCMOS circuit.

NTT's unique SIMOX substrate technique is used for the power switch CMOS transistors to realize the variable threshold voltage structure.

2. SIMOX substrate technique
The SIMOX substrate technique insulates elements on the silicon substrate with an oxide layer to reduce the capacity and increase the operational speed of the elements. The technique also insulates the CMOS transistor's body from the silicon substrate, making it possible to apply control voltage to the body from an external source. Because the CMOS transistor's threshold voltage is quite sensitive to the voltage applied to the body, the threshold voltage can be controlled by varying the voltage.

The developed 0.25 micron CMOX LSI test device, made using these techniques, achieves an operational speed of about 1.1 nano (1/1 billion) per second and features about 1/100 the power consumption of the conventional device operated on 3.3V. When operating on 1V, equivalent to the voltage supplied by a single cell of the nickel-cadmium battery, which is widely used in portable communications equipment, the test device achieves a circuit operation speed of 300 pico per second and outperforms conventional 0.5 micron CMOX LSIs. Therefore, the new device can easily replace conventional CMOS LSIs and reduce power consumption to 1/15 the current level.

III. Future Plan

NTT will apply the techniques to commercially develop LSIs for portable communications equipment, which currently uses CMOS LSIs operated on 3.3V, and conduct more research in an attempt to realize energy-efficient portable communications equipment that may directly supply voltage with a single solar battery cell.

IV. Terms

*1) CMOS: Complementary Metal Oxide Semiconductor
CMOS is the most elementary structure constituting the current generation of LSIs and features low power consumption.

*2) SIMOX: Separation by Implanted Oxygen
One of the SOI structure (*) formation techniques developed by NTT with which high-energy oxygen ions are implanted into a silicon substrate to form a buried oxide layer inside the wafer.

*SOI (Silicon On Insulator) structure: the structure in which a silicon layer for forming elements is placed on an insulation layer, such as oxide film.