胡淑芬個人小檔案
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The Nanoscale
■ 10-10 m = 1 Ångstrom
■ 10-9 m = 1 Nanometer
■ 10-6 m = 1 Micrometer
■ 10-3 m = 1 Millimeter
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Transistor Scaling
3
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Moores’ Law
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Silicon Nanotechnology
50 nm transistor diameter is ~ 2000x smaller than
diameter of human hair
Transistor for 90 nm Process
Source: Intel
Influenza virus
Source: CDC
Gate dielectric thickness = 1.2 nm
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Nano Technology
Intel 90 nm node device
1.2 nm SiO2
Gate oxide is less than 5 atomic layers
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Transistor Nanotechnology
7
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Single-electron tunneling devices are
promising candidates for future devices:
-- low power consumption
-- high integration density
Silicon – The most promising material for application to SLIs
1. Si-SETs can be used
jointly with conventional
CMOS circuits
2. Advanced fabrication
technologies for Subquarter- micron CMOS
LSIs can be used to
make small silicon structures.
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Comparison of the conventional MOSFET (left column) and the quantum dot
transistor (right column) in structure, band diagram, and ID - VG characteristics.
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SET Operation Principle
Q<e/2
Q>e/2
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Single Electron Transistor
Coulomb oscillations in a SET transistor
Stability plot for the SET transistor
The shaded regions are stable regions
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Solar Cell Structure
•
•
A solar cell or photovoltaic cell is a device that converts solar energy into
electricity by the photovoltaic effect.
Light shining on the solar cell produces both a current and a voltage to
generate electric power
The basic steps in the operation of a solar cell are:
• The generation of light-generated carriers;
• The collection of the light-generated carries to generate a current
• The generation of a large voltage across the solar cell
• The dissipation of power in the load and in parasitic resistances
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When a photon hits a piece of silicon,
one of three things can happen:
1. the photon can pass straight through the silicon —
this (generally) happens for lower energy photons,
2. the photon can reflect off the surface,
3. the photon can be absorbed by the silicon which either:
Generates heat, OR
Generates electron-hole pairs, if the photon energy
is higher than the silicon band gap value.
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Light Generated Current
1. The absorption of a photo creates an
electron-hole pair
2. Ideally the minority carrier (in this case,
a hole) make it cross the junction and
becomes a majority carrier
3. After passing through the load, the
electron meets up with a hole and
complete the circuit
If the light-generated minority carrier reaches the p-n junction, it is swept across
the junction by the electric field at the junction, where it is now a majority carrier.
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Solar Cell Parameters
 The IV curve of a solar cell is the superposition of the IV curve of the solar cell diode
in the dark with the light-generated current
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Current-Voltage Measurements
B
A
 The efficiency of a solar cell is determined as the fraction of incident power
which is converted to electricity and is defined as:
Voc is the open-circuit voltage
Isc is the short-circuit current
FF is the fill factor
η is the efficiency.
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Quantum Efficiency
 The "quantum efficiency" (Q.E.) is the ratio of the number of carriers collected
by the solar cell to the number of photons of a given energy incident on the solar cell
 It is an accurate measurement of the device's electrical sensitivity to light
 If the quantum efficiency is integrated (summed) over the whole solar spectrum,
one can evaluate the current that a cell will produce when exposed to white light
(the short-circuit current)
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Light-absorbing materials
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Solar Cells
Moving to 2nd & 3rd Generation Solar Cells
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Why Quantum Dots?
Quantum dot solar cells have the potential to increase the
maximum attainable thermodynamic conversion efficiency
of solar photon conversion up to about 66% by utilizing hot
photogenerated carriers to produce higher photovoltages
or higher photocurrents.
The former effect is based on miniband transport and
collection of hot carriers in QD array photoelectrodes
before they relax to the band edges through phonon
emission. The latter effect is based on utilizing hot
carriers in QD solar cells to generate and collect
additional electron–hole pairs through enhanced impact
ionization processes.
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Novel materials consisting of silicon (Si) nanocrystals embedded in a
dielectric matrix have attracted considerable interest in the field of silicon
optoelectronics and third generation Photovoltaics.
When Si nanocrystals are made very small (<∼7 nm in diameter), they
behave as quantum dots (QDs) due to the three-dimensional quantum
confinement of Carriers.
Indirect bandgap
Quasi-directbandgap
the band gaps can be adjusted specifically
to convert also longer- wave light
Enhanced electron–hole pair
(exciton) multiplication in quantum
dots that could lead to enhanced
solar photon conversion efficiency
in QD solar cells.
MRS BULLETIN • VOLUME 32 • MARCH 2007 •
www.mrs.org/bulletin
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Band Gap Diagram
Buck device
Quantum dot device
Thermal energy
C.B.
C.B.
Si3N4
hν < Eg
hν
hν >> Eg
light
Si3N4
Band gap
V.B.
V.B.
Si quantum dot
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Proceedings of ECTI-CON 2008
Quantum Dots:
E(eV)= 1.56+2.40/a2
(a is the dot size)
Quantum Well:
E(eV)= 1.6+0.72/d2
(d is the well width)

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1-4244-0016-3/06/$20.00 ©2006 IEEE
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APPLIED PHYSICS LETTERS 91, 163503 2007
Solar cells based on quantum dots may carry extra benefits of increased
radiation hardness and improved collection efficiency.
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2008
The cell with the 3 nm QDs had the highest efficiency, with an open-circuit voltage
(Voc) of 556 mV, a short-circuit current (Jsc) of 29.8 mA cm−2,a fill factor (FF) of
63.8%, and conversion efficiency of 10.6%.
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