Lecture 1: Introduction, Properties and Structure
MSE 6001, Semiconductor Materials Lectures Fall 2006
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1.1
Semiconductor Materials and Devices
Introduction and overview
Semiconductor devices make modern information technology possible, and they are beginning to
play a much larger role in energy and lighting. The semiconductor industry is now larger than
the steel industry, with sales of more than $250B this year. We are probably still in the early
stages of the microelectronics age. The control of semiconductor electrical and optical properties
make these materials useful for electronic and photonic devices. The properties include, for example, electrical resistivity ρ and optical absorption α, which are related to one another by the
semiconductor electronic structure. Properties depend on the molecular-scale structure, the atom
positions and the electron states around these atoms. These electronic states and their occupation
with electrons is the electronic structure. The numerical values of the properties may be calculated
with quantum mechanics, and in turn, semiconductor properties are increasingly engineered using
quantum mechanical design at the nanometer scale.
The most prominent device is the silicon metal-oxide-semiconductor field-effect transistor
(MOSFET). The electron concentration at the silicon-silicon dioxide interface is controlled by
the gate voltage, which controls the MOSFET conductivity, allowing the MOSFET to be turned on
and off. (Figs. 1 and 2)
ns vs. surface potential
MOSFET cross section
physical gate length
L
gate
oxide,
tox
n+ source
+
n drain
depletion
p-Si
MOSFET 2D electron
concentration controlled
by surface potential
1e+13
ns, electron concentration (cm–2)
poly-Si gate
1e+12
1e+11
Na = 4x1015 cm–3
T = 300 K
1e+10
• 2D electron sheet, ~ 2 nm
1e+09
• surface potential (V)
0
• ns, 2DEG concentration (cm-2)
0.5
1
surface potential (V)
F IGURE 1: (left) Schematic cross sectional view of a MOSFET. (right) Calculated surface electron concentration in an MOS structure. The surface potential controls the surface electron concentration ns through
the field effect in a MOSFET.
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gate
5 µm
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NMOSFET
W/L = 30 µm/5 µm
J. Jackson and
M. Sorenson, U Utah
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F IGURE 2: Drain-source characteristics of a MOSFET fabricated at the UU microfabrication lab. The gate
voltage Vg controls the surface potential, which controls the drain-source current Ids
1.2
Semiconductor Materials (Inorganic)
Both organic and inorganic materials can be semiconductors; and both crystalline and amorphous
materials can be semiconductors. Many oxides also have semiconducting properties. Most of
our physical models, understanding, and applications come from the study of crystalline inorganic
semiconductors.
Elemental semiconductors Elemental semiconductors include Si, Ge, and even C in the form
of diamond and carbon nanowires. These column-IV materials are generally used for microelectronics, and Si and Ge are used in solar cells and photodetectors.
Compound semiconductors Prominent examples of compound semiconductors include members of the III-V semiconductor family, which includes GaAs and InP. The optical properties of
III-V semiconductors makes them useful for optoelectronic devices, such as light emitting diodes
(LEDs) and communication lasers for fiber optic networks, and these materials are also used for
very high frequency microelectronics, at 40 GHz and above. Other important families of compound
semiconductors include the II-VI’s, such ad ZnSe and HgTe. The bonding in the column-IV, III-V,
and II-VI materials is tetrahedral sp3 , corresponding to the diamond lattice (Fig. 4).
An important feature of compound semiconductors is that the properties may be adjusted by
using alloys. For example, GaAs is alloyed with AlAs to form a semiconductor with intermediate
properties, used in TV remote controls and other applications.
Doping Semiconductors are useful in devices because the free carrier concentration, the concentration of electrons and holes, may be in part controlled by doping with chemical impurities.
Dopants in inorganic semiconductors are covalently-bonded, substitutional impurities having ei1-2
IIB
III
IV
V
VI
B
C
N
O
Al
Si
P
S
Zn Ga Ge As Se
Cd
In
Sn Sb
Hg
Tl
Bi
Te
F IGURE 3: Portion of the periodic table from which most of the major inorganic semiconductors are taken.
109°
F IGURE 4: Bond angles in the diamond lattice.
ther one extra valence electron, which can be donated to the crystal, or one fewer valence electron,
which can accept an electron from the crystal to leave behind a hole.
1.3
Semiconductor Devices
Perhaps over one hundred semiconductor devices have been invented, with applications ranging
from megawatt switches in the power distribution grid to communication lasers to nano-scale memory elements. The devices may be broadly categorized as electronic or photonic, with photonic
devices also often referred to a optoelectronic devices.
Electronic devices Transistors are three-terminal devices used for switching and amplification.
The MOSFET is an example of a field effect transistor, with other examples including junction
field effect transistors (JFETs) and metal-semiconductor field effect transistors (MESFETs). Field
effect transistors are unipolar devices, conducting electrical current with a single carrier type, either
electrons or holes, and the current is a drift current driven by an electric field. MOSFETs are
used for the largest and integrated circuits, such CPUs (central processing units) in computers and
memories, which can have over a billion transistors on a chip.
Bipolar junction transistors (BJTs) operate using both carrier types, and the currents are diffu1-3
ge, showing the SRAM
photo is shown in Fig-
n
ing 65nm CMOS techprocessors with excelformance, along with
high performance, high
has been successfully
process features. This
h volume manufactur-
Figure 2: Transistor size trend for technology nodes.
L = 35 nm
h. Dig., 2004.
, pp. 197-200, 2003.
AspectRatio
Transmission electron
micrograph of MOSFET,
Intel 65 nm process
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Figure 3: TEM cross section of 35nm NMOS
F IGURE 5: P. Bai, et al., 65 nm Logic Technology Featuring 35 nm Gate Lengths, Enhanced Channel
Strain, 8 Cu Interconnect Layers, Low-k ILD, and 0.57 µm2 SRAM Cell (IEDM Proceedings, 2004)
sion currents driven by carrier concentration gradients. BJTs are used for higher power and higher
frequency circuits, such as for the radio transmitters in cell phones.
spect ratio
2
ll Area
2 years
1
65nm
2004
SRAM Cell Area (um )
10
Photonic devices Photonic devices emit or detect light. Solar cells are simple semiconductor
diodes that convert optical power to electrical power. LEDs convert electrical power into optical
power. In the coming decade or so, most lighting will be done by LEDs rather than incandescent or
florescent bulbs because of the much greater efficiency of LEDs. Communication lasers are based
on LED-like structures with mirrors and optical waveguides added to provide a cavity to produce
coherent light.
PMOS
Figure 4: TEM cross section of 35nm PMOS
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2006
nd SRAM area trends
F IGURE 6: Micrograph of 256 Mb DRAM die.
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