To most people, integrated circuits (ICs) are the little black squares in electronic products that make the product work. How they are made or what’s in these black squares is a mystery to most people. But it’s no mystery that when it comes time to build a system, the process used for fabricating an IC makes a big difference in resulting power consumption rates. Most digital ICs are made on silicon wafers. Transistors are formed in the silicon wafers to create circuits. The various process technologies are usually described in terms of minimum transistor geometries, such as 250 nanometer (nm), 120nm or 90nm. To provide some scale, a typical human hair is 1000 times larger than the transistor dimensions, which is why so many transistors can fit onto an IC no bigger than a thumbnail. The more transistors that can be squeezed onto a die, the more functionality can be provided, which normally allows IC manufacturers to charge more money for their products. So, IC manufacturers continue to invest in shrinking process geometries in order to make more money. Currently, the cutting edge of process geometries is between 45nm and 32nm.
The big trade-off
But as always, there are trade-offs. As the saying goes, “there is no free lunch.” As the process geometries shrink, the device characteristics must be tailored to meet the performance requirements of the end products. As the width of transistors shrink, the gate oxide, or insulator, must be made thinner in order to maintain control of the current flow through the channel of the transistor. Since the gate oxide thickness is reduced, the maximum voltage that can be used on these transistors must be reduced to prevent damaging the devices. A lowered device voltage is good for active power since active power is proportional to the square of the voltage. However, this typically means that the threshold voltage of the CMOS transistor is reduced in order to maintain transistor performance at a lower voltage. The lowered threshold voltage of these small geometry devices allows more current to leak through the channel even when the device is off. The increase in leakage current is not linearly proportional to the shrinking device size. The increase in leakage is exponential. So, when going from a 130nm process to a 65nm process, the leakage can increase by 1000x! If not controlled, the leakage can be hundreds of milliamps (mA).
Years ago, leakage was ignored in most processes. That is no longer the case. Since the leakage is increasing so quickly, the Freescale design engineers work hard designing transistors that maintain the required performance while keeping the leakage under control. One way to do this is to create several flavors of transistors that can be used in the most appropriate ways, such as low performance, low leakage transistors for circuits that don’t need high performance but need low leakage. Or building high performance, high leakage transistors for circuits where performance is critical and the additional leakage can be supported.
Choosing the right components
So what does this mean when it comes time to choose the components for a low power system? The answer depends on the type of system you are developing.
A: If your system is running all the time, such as a network server, then selecting components that can operate at the lowest voltage is probably the best, since it minimizes active power consumption. Typically, components made in the newer technologies support the lower voltage supplies.
B: If your product is in a low-power state most of the time, such as a smoke detector, a low standby current component is probably the best. Typically, components made in the older process technologies have lower standby or leakage currents.
C: If your product is sometimes active and sometimes in standby, such as a mobile phone, a compromise is required to balance a component’s active power and standby power consumption. Components specifically designed to optimize both active and standby power should be chosen. Typically, components made in the newer technologies that also incorporate leakage control techniques meet the critical demands of the end products.
These guidelines are just general rules. Your product’s specifications will dictate the exact requirements for component selection. If more discussion is required, please send in your comments.
— Chris
