Page 47 - A Practical Guide from Design Planning to Manufacturing

The Evolution of the Microprocessor 23

Scaling the supply voltage by the same amount as the channel length
and oxide thickness keeps all the electrical fields in the device constant.
This concept is called constant field scaling and was proposed by Robert
6
Dennard in 1974. Constant field scaling is an easy way to address prob-
lems such as subthreshold leakage and dielectric breakdown, but a
higher supply voltage provides for better performance. As a result, the
industry has scaled voltages as slowly as possible, allowing fields in
the channel and the oxide to increase significantly with each device
generation. This has required many process adjustments to tolerate the
higher fields. The concentration of dopants in the source, drain, and
channel is precisely controlled to create a three-dimensional profile that
minimizes subthreshold leakage and hot electron effects. Still, even the
very gradual scaling of supply voltages increases delay and hurts per-
formance. This penalty increases dramatically when the supply voltage
becomes less than about three times the threshold voltage.
It is possible to design integrated circuits that operate with supply
voltages less than the threshold voltages of the devices. These designs
operate using only subthreshold leakage currents and as a result are
incredibly power efficient. However, because the currents being used are
orders of magnitude smaller than full ON currents, the delays involved
are orders of magnitude larger. This is a good trade-off for a chip to go
into a digital watch but not acceptable for a desktop computer. To main-
tain reasonable performance a processor must use a supply voltage sev-
eral times larger than the threshold voltage. To gain performance at
lower supply voltages the channel doping can be reduced to lower the
threshold voltage.
Lowering the threshold voltage immediately provides for more on
current but increases subthreshold current much more rapidly. The
rate at which subthreshold currents increase with reduced threshold
voltage is called the subthreshold slope and a typical value is 100
mV/decade. This means a 100-mV drop in threshold will increase sub-
threshold leakage by a factor of 10. The need to maintain several orders
of magnitude difference between the on and off current of a device there-
fore limits how much the threshold voltage can be reduced. Because the
increase in subthreshold current was the first problem encountered
when scaling the channel length, we have come full circle to the origi-
nal problem. In the end there is no easy solution and process engineers
are continuing to look for new materials and structures that will allow
them to reduce delay while controlling leakage currents and reliability
(Fig. 1-11).

6
Dennard, “MOSFETs with Small Dimensions.”