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
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.