IEEE Spectrum October, 2007 - 20

++++++++

V+

Gate oxide

n+
Source

Channel

+ +
+ + +
+

n+
Drain

-------------- + + + +
+
+ + + +

ran out of atoms, designers had devised some tricks to throttle
back on the power without losing speed. But without a way to
stanch the unwanted flow of electrons through that sliver of
insulation, the battle to make ever more powerful processors
would soon be lost.
To understand why, you need a quick lesson (or refresher)
in semiconductor basics. The type of transistor that is chained
together by the hundreds of millions to make up today's microprocessors, memory, and other chips is called a metal-oxidesemiconductor field effect transistor, or MOSFET. Basically, it
is a switch. A voltage on one terminal, known as the gate, turns
on or off a flow of current between the two other terminals, the
source and the drain [see illustration, "The Transistor"].
MOSFETs come in two varieties: N (for n-type) MOS and
P (for p-type) MOS. The difference is in the chemical makeup
of the source, drain, and gate. Integrated circuits contain both
NMOS and PMOS transistors. The transistors are formed on
single-crystal silicon wafers; the source and drain are built by
doping the silicon with impurities such as arsenic, phosphorus,
or boron. Doping with boron adds positive charge carriers, called
holes, to the silicon crystal, making it p-type, while doping with
arsenic or phosphorus adds electrons, making it n-type.
Taking an NMOS transistor as an example, the shallow
source and drain regions are made of highly doped n-type silicon. Between them lies a lightly doped p-type region, called the
transistor channel-where current flows. On top of the channel
lies that thin layer of SiO 2 insulation, usually just called the
gate oxide, which is the cause of the chip industry's most recent
technological headaches.
Overlying that oxide layer is the gate electrode, which is
made of partially ordered, or polycrystalline, silicon. In the case
of an NMOS device it is also n-type. (The silicon gates replaced
aluminum gates-the metal in "metal-oxide semiconductor"-
in work described in the 1969 IEEE Spectrum article. But the
"MOS" acronym has nevertheless lived on.)
The NMOS transistor works like this: a positive voltage
on the gate sets up an electric field across the oxide layer. The
electric field repels the holes and attracts electrons to form an
electron-conducting channel between the source and the drain.
A PMOS transistor is just the complement of NMOS. The
source and drain are p-type; the channel, n-type; and the gate,
p-type. It works in the opposite manner as well: a positive voltage on the gate (as measured between the gate and source) cuts
off the flow of current.
In logic devices, PMOS and NMOS transistors are arranged so
that their actions complement each other, hence the term CMOS
for complementary metal-oxide semiconductor. The arrangement of CMOS circuits is such that they are designed to draw
power only when the transistors are switching on or off. That's
the idea, anyway.

Channel

Thick
Gate
gate oxide

THE TRANSISTOR: A positive voltage on the gate of an NMOS transistor drives
positive charge in the channel away from the insulating gate oxide and attracts
electrons, forming a path for electrons to flow.

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Channel

1.2-nm
gate oxide

Gate

1000
100
High-k

SiO2

10
1

High-k
1

0.1

Relative gate leakage

Energy

Gate-oxide thickness (nm)

Gate

Energy

V+

0.01
350

180

Technology generation (nm)
RUNNING OUT OF ATOMS: The transistor's gate oxide thinned with each new
technology generation until it reached just 5 atoms (1.2 nm) thick. At that scale,
the wave describing the probable location of an electron [red curve, top] is
broader than the gate oxide, and the electron can simply appear on the other side
of the gate oxide, having tunneled through the insulation. This so‑-called gate
leakage increased 100‑-fold in the last three generations of transistors. A switch
to a new gate oxide, a high‑-k dielectric, was needed to plug the leak.

Although the basic features and materials of the MOS transistor have stayed pretty much the same since the late 1960s, the
dimensions have scaled dramatically. The transistor's minimum
layout dimensions were about 10 micrometers 40 years ago, and
are less than 50 nm now, smaller by a factor of more than 200.
Suppose a 1960s transistor was as big as a three-bedroom house
and that it shrank by the same factor. You could hold the house
in the palm of your hand today.
In the Penryn processors that we recently began fabricating, most of their transistors' features measure around 45 nm,
though one is as small as 35 nm. It's the first commercial microprocessor to have features this small; all other top-of-the-line
microprocessors in production as this article is being written have 65-nm features. In other words, Penryn is the first
of the 45-nm generation of microprocessors. Many more will
soon follow.
The thickness of the SiO2 insulation on the transistor's gate
has scaled from about 100 nm down to 1.2 nm on state-of-theart microprocessors. The rate at which the thickness decreased
was steady for years but started to slow at the 90-nm generation,
which went into production in 2003. It was then that the oxide
hit its five-atom limit. The insulator thickness shrank no further
from the 90-nm to the 65-nm generation still common today.
The reason the gate oxide was shrunk no further is that it
began to leak current [see illustration, "Running Out of Atoms"].
This leakage arises from quantum effects. At 1.2 nm, the quantum nature of particles starts to play a big role. We're used to
thinking of electrons in terms of classical physics, and we like
to imagine an electron as a ball and the insulation as a tall and
narrow hill. The height of the hill represents how much energy
you'd need to provide the electron to get it to the other side. Give
it a sufficient push and-sure enough-you could get it over the
hill, busting through the insulation in the process.

October 2007 | IEEE Spectrum | NA 31

http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum October, 2007