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Wednesday, June 22, 2005
 NEC Low-Power LSIs for Mobile Devices
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Message Text: NEC Electronics Corp. has developed a circuit technology to lower power consumption by controlling the LSI threshold voltage and the power source voltage.

According to the company, LSI voltage is regulated so that power consumption remains at the lowest level, even in the presence of ambient changes such as changes in temperature or in load. The LSI threshold voltage control is achieved by increasing or decreasing the substrate bias voltage of an LSI circuit. Details on the technology were presented at the 2005 Symposium on VLSI Circuits.

The company is currently working on how to apply this technology, codenamed LongRun2, to low-power LSIs for mobile devices. LongRun2 is a high-performance LSI-oriented substrate bias controlling technology, licensed from Transmeta Corp. By combining High-k materials and the circuit technology now under study, the company plans to optimize power consumption technology for use with mobile devices.

Although not much information is available about the LonRun2 technology, we can say that it generally controls transistor leakage power. When the 0.18?m process was used, power leakage could be considered as negligible. Currently, as semiconductor technology scales to 90nm and below, the transistor leakage is becoming a bigger problem, and total power must now be considered as the sum of active and leakage power.

According to LongRun2, the concept is based on controlling the LSI threshold voltage. When the LSI circuit is operating, threshold voltage is controlled so that the ratio of transistor leak current (Ileak) and switching current (Isw) stays at a certain level. When the total of Ileak and Isw is at a minimum, the Ileak/Isw value remains constant in the presence of changes in temperature or load.

Similarly, with the LSI circuit in standby, the threshold voltage is controlled to maintain a constant ratio of the subthreshold current (Ioff) running between source and drain and the current (Isub) such as the gate leak current or GIDL disseminating into the substrate. Each of the currents is measured by a dummy transistor group formed on the LSI circuit.
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