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Low voltage CMOS — Having your cake and eating it?
MicroelectronicsJoumal, 23 (1992) 245-247
Low Voltage CMOS- Having Your Cake and Eating It? by Roy Szweda
A t the moment, the computer industry is having hard times and is desperate for winning products. The only sector which seems to be enjoying the successes of old is portable computing and everyone, device suppliers and O E M s alike, is hanging his hat on it. It's not surprising therefore to see a profusion of coincident offerings of lower voltage ICs aimed specifically at this marketplace. Lower voltage has more to it than meets the eye, however; with it you get not only lower power consumption but also better peoCormance. here has long been a distinct
trend running through the T IC industry towards ever lower
supply voltages. Just recently, at the ISSCC in San Francisco, quite a number of companies presented work on lower voltage logic including microprocessors and memories. T h e prediction made then that these developments would shortly break into the commercial world is already coming true. Over the last few months many leading suppliers - National,
Philips, Motorola, IDT and Performance Semiconductor, have released offerings in anticipation of huge take-up by makers of battery-powered equipment, lap-top and notebook computers in particular. Intriguingly, the move to lower voltage has come about because of the coincidence of two causative factors. Firstly, and perhaps most importantly, the computer industry had created a market pull for lower power devices for portable computing. Secondly, there is a technology push from the device engineers who were running into physics problems when trying to run their sub-half micron devices on the standard 5V line. They w a n t e d to o v e r c o m e these problems by reducing the supply voltage.
Cutting Down is Good For You just cutting a couple of volts off the supply voltage sounds easy enough but it can cause prob-
0026-2692•92•55.00 © 1992, Elsevier Science Publishers Ltd.
lems of incompatibility in a mixed voltage level circuit. Incidentally, device suppliers are making efforts to overcome these problems by offering dual level supply rails. For example, they employ a 5V ring which can be stepped up or down by 2V to supply other parts of the circuit. In this way some power savings can be made even in a mixed circuit technology. The performance advantages of lower supply voltages make such a move certainly worth it - if you can pull it off. T h e principal advantage is simple: the logic chips in your lap-top consume less power, and so you get more computing power, longer battery life etc. etc. There is, however, a more fundamental reason to reduce supply voltage. The trend towards narrower gate lengths means that the electric field increases for a given supply voltage. N o w that g e o m e t r i e s are d o w n
245
Low Voltage CMOS
around 0.8 microns or less, the electric fields have increased to the point where such undesirable effects as electron injection and gate rupture occur. T h e most significant effect, however, is that of electro-migration in the metal interconnects under the higher field. W h e n you go sub-0.5 micron the metal tracks need to be narrower and if you are to avoid electro-migration of the metal then the current density must be reduced - the best way to do this is to reduce the supply voltage. So as you get around these physics and technological problems, the lower voltage allows you to move to the smaller geometry and thus gain considerable speed improvement into the bargain. It is here though that Sodd's Law intervenes; this speed increase is not quite as dramatic as simple dimensional scaling because some speed is lost due to the lower supply voltage. Before we get carried away with the story, I must say here that this does not mean that C M O S will continue to win out indefinitely. C M O S is still the best solution for absolutely lowest power and it will continue to be so for a very long time. Other parts o f the circuit may have other types which can handle the speed but you can bet that a C M O S solution will always be sought w h e r e v e r possible.
246
So while it is a salutary lesson that the physics problems which C M O S has now run into can sometimes
be
overcome,
C M O S still does not have the capability for 100MHz plus performance. W h e n considering such speed then an alternative technology such as bipolar ECL, or better still gallium arsenide, must remain the stronger contender (see "GaAs Cries Wolf' in the March issue).
Bipolar Relics and CMOS Champions The industry standard of 5V is a relic of bipolar days when this was the threshold limit for TTL and other devices. In my formative years, transistor circuits were almost exclusively bipolar and portable application circuits were based on 9V largely because of the convenience o f the standard PP3 battery size. When thinking o f this, one wished that 4.5V could not have been chosen instead of 5V; it's half of 9V and MN1500 type dry cell batteries are based around 1.5V, so you 'could use 3xMN1500s. N o w things are getting sorted out as the industry standard seems to be moving to 3V and that means you can use two MN1500s. The move to lower voltages also reflects the predominance o f CMOS-type logic in modern circuits. C M O S is already a low power technology and now its much lower sensitivity to threshold voltage is cementing its advantage over bipolar.
I would be surprised if the supply voltage issue stopped here. At ISSCC results from Hitachi's 1.5V B i C M O S logic were presented and recently Philips' HLL series claimed an operating range between 3.6V down to 1.2V. The evolution o f supply voltage will continue as long as designers see a performance advantage for the taking.
Assault and Battery A key part o f the evolutionary process towards lower power, more compact electronic equipment is the synergy between battery and circuit techn o l o g y . It s e e m s for t h e m o m e n t , however, that the s e m i c o n d u c t o r suppliers are making the running and the battery manufacturers must follow. It seems that the two technologies are b e c o m i n g inseparable and soon we shall see portable computers which can run for a complete working week without a pause for recharging from a battery pack w h i c h isn't bigger than the computer itself. Batteries are as much an enabling factor in electronics as the devices even though we may take them for granted most o f the time. What use is it to have a camcorder only to have to lug around a briefcase-sized battery pack? But this was the case only a decade ago. These gadgets have reached the point where they are limited in total size by the videotape cassette dimen-
Microelectronics Journal, VoL 23, No. 4
sions, not the electronics or the battery.
More Than You Could Expect Just as the portable computer is seen as one o f the ways for the
computer industry to get itself out o f the doldrums, lower voltage ICs will surely be the device successes o f the mid-90s. Lower voltage has more going for it than anyone can really expect from any technology: not only lower power consumption but
also b e t t e r p e r f o r m a n c e . It seems that the device suppliers, the designers and the OEMs will soon be able to have their cake and eat it too.
Roy Szweda
247
Low Voltage CMOS- Having Your Cake and Eating It? by Roy Szweda
A t the moment, the computer industry is having hard times and is desperate for winning products. The only sector which seems to be enjoying the successes of old is portable computing and everyone, device suppliers and O E M s alike, is hanging his hat on it. It's not surprising therefore to see a profusion of coincident offerings of lower voltage ICs aimed specifically at this marketplace. Lower voltage has more to it than meets the eye, however; with it you get not only lower power consumption but also better peoCormance. here has long been a distinct
trend running through the T IC industry towards ever lower
supply voltages. Just recently, at the ISSCC in San Francisco, quite a number of companies presented work on lower voltage logic including microprocessors and memories. T h e prediction made then that these developments would shortly break into the commercial world is already coming true. Over the last few months many leading suppliers - National,
Philips, Motorola, IDT and Performance Semiconductor, have released offerings in anticipation of huge take-up by makers of battery-powered equipment, lap-top and notebook computers in particular. Intriguingly, the move to lower voltage has come about because of the coincidence of two causative factors. Firstly, and perhaps most importantly, the computer industry had created a market pull for lower power devices for portable computing. Secondly, there is a technology push from the device engineers who were running into physics problems when trying to run their sub-half micron devices on the standard 5V line. They w a n t e d to o v e r c o m e these problems by reducing the supply voltage.
Cutting Down is Good For You just cutting a couple of volts off the supply voltage sounds easy enough but it can cause prob-
0026-2692•92•55.00 © 1992, Elsevier Science Publishers Ltd.
lems of incompatibility in a mixed voltage level circuit. Incidentally, device suppliers are making efforts to overcome these problems by offering dual level supply rails. For example, they employ a 5V ring which can be stepped up or down by 2V to supply other parts of the circuit. In this way some power savings can be made even in a mixed circuit technology. The performance advantages of lower supply voltages make such a move certainly worth it - if you can pull it off. T h e principal advantage is simple: the logic chips in your lap-top consume less power, and so you get more computing power, longer battery life etc. etc. There is, however, a more fundamental reason to reduce supply voltage. The trend towards narrower gate lengths means that the electric field increases for a given supply voltage. N o w that g e o m e t r i e s are d o w n
245
Low Voltage CMOS
around 0.8 microns or less, the electric fields have increased to the point where such undesirable effects as electron injection and gate rupture occur. T h e most significant effect, however, is that of electro-migration in the metal interconnects under the higher field. W h e n you go sub-0.5 micron the metal tracks need to be narrower and if you are to avoid electro-migration of the metal then the current density must be reduced - the best way to do this is to reduce the supply voltage. So as you get around these physics and technological problems, the lower voltage allows you to move to the smaller geometry and thus gain considerable speed improvement into the bargain. It is here though that Sodd's Law intervenes; this speed increase is not quite as dramatic as simple dimensional scaling because some speed is lost due to the lower supply voltage. Before we get carried away with the story, I must say here that this does not mean that C M O S will continue to win out indefinitely. C M O S is still the best solution for absolutely lowest power and it will continue to be so for a very long time. Other parts o f the circuit may have other types which can handle the speed but you can bet that a C M O S solution will always be sought w h e r e v e r possible.
246
So while it is a salutary lesson that the physics problems which C M O S has now run into can sometimes
be
overcome,
C M O S still does not have the capability for 100MHz plus performance. W h e n considering such speed then an alternative technology such as bipolar ECL, or better still gallium arsenide, must remain the stronger contender (see "GaAs Cries Wolf' in the March issue).
Bipolar Relics and CMOS Champions The industry standard of 5V is a relic of bipolar days when this was the threshold limit for TTL and other devices. In my formative years, transistor circuits were almost exclusively bipolar and portable application circuits were based on 9V largely because of the convenience o f the standard PP3 battery size. When thinking o f this, one wished that 4.5V could not have been chosen instead of 5V; it's half of 9V and MN1500 type dry cell batteries are based around 1.5V, so you 'could use 3xMN1500s. N o w things are getting sorted out as the industry standard seems to be moving to 3V and that means you can use two MN1500s. The move to lower voltages also reflects the predominance o f CMOS-type logic in modern circuits. C M O S is already a low power technology and now its much lower sensitivity to threshold voltage is cementing its advantage over bipolar.
I would be surprised if the supply voltage issue stopped here. At ISSCC results from Hitachi's 1.5V B i C M O S logic were presented and recently Philips' HLL series claimed an operating range between 3.6V down to 1.2V. The evolution o f supply voltage will continue as long as designers see a performance advantage for the taking.
Assault and Battery A key part o f the evolutionary process towards lower power, more compact electronic equipment is the synergy between battery and circuit techn o l o g y . It s e e m s for t h e m o m e n t , however, that the s e m i c o n d u c t o r suppliers are making the running and the battery manufacturers must follow. It seems that the two technologies are b e c o m i n g inseparable and soon we shall see portable computers which can run for a complete working week without a pause for recharging from a battery pack w h i c h isn't bigger than the computer itself. Batteries are as much an enabling factor in electronics as the devices even though we may take them for granted most o f the time. What use is it to have a camcorder only to have to lug around a briefcase-sized battery pack? But this was the case only a decade ago. These gadgets have reached the point where they are limited in total size by the videotape cassette dimen-
Microelectronics Journal, VoL 23, No. 4
sions, not the electronics or the battery.
More Than You Could Expect Just as the portable computer is seen as one o f the ways for the
computer industry to get itself out o f the doldrums, lower voltage ICs will surely be the device successes o f the mid-90s. Lower voltage has more going for it than anyone can really expect from any technology: not only lower power consumption but
also b e t t e r p e r f o r m a n c e . It seems that the device suppliers, the designers and the OEMs will soon be able to have their cake and eat it too.
Roy Szweda
247