Download Lecture 2

WAFER FABRICATION
Lecture 2
Taklimat UniMAP
Semiconductor Basic
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
What is a semiconductor ?
• Semiconductors are materials with electrical
conductivity between conductors and insulators.
• The most commonly
used semiconductor
Taklimat
UniMAP
materials are silicon, Si and germanium, Ge.
• Some compounds, such as gallium arsenate
(GaAs),
Silicon carbideMalaysia
(SiC) and silicon
Universiti
Perlis
germanium (SiGe).
• Most important property is its conductivity can be
controlled by adding certain impurities in the
process called doping.
Hasnizah Aris, 2008
WAFER FABRICATION
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Band Gap
• Atom is basic building block of
all materials
• Classical mechanics –every
atom has it own orbit structure.
• Electron orbits are called shells.
• The outermost shell is called
valence shell.
• When electron leaves the
valence shell, it becomes a free
electron and can conduct
electric current.
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
• When 2 or more identical atoms bond together
to form solid materials, their orbit overlap and
form so called energy bands. Can be
represented by the energy band diagram.
• The bottom of conduction band is called Ec,
and the top of the valence band is called Ev.
• Eg = Ec –Ev
Taklimat UniMAP
• Eg is defined as the energy required to break a
bond in semiconductor to free an e to cond
band and leave the hole in the valence band.
• Electrons in conduction band are free to move
and can conduct electric current.
• Electrons in the valence band are bonded with
nuclei and cannot move freely, therefore cannot
conduct electric current.
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Resistivity
• Resistivity is the capability of a material
resisting electric current.
Taklimathas
UniMAP
• A good conductor
a very low
resistivity and a good insulator has a
very high resistivity.
Universiti
Malaysia
Perlis
• Unit: Ohm.cm
Hasnizah Aris, 2008
WAFER FABRICATION
Resistivity and Band Gap
• For most metals, conduction and valence
bands almost overlap or very small band
gap. Electron can easily jump from valence
to conduction band. Therefore the
conductionTaklimat
band has a lot
of e.
UniMAP
• For insulators, the band gap is so large that
electrons cannot jump across it.
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Taklimat UniMAP
DOPING SEMICONDUCTOR
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Two types of Semiconductor Materials
1. Intrinsic Semiconductor
Taklimat
UniMAP
2. Extrinsic
Semiconductor
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Intrinsic Semiconductor
• Pure semiconductor materials with no
impurity atoms and no lattice defect.
• At T=0 K, all energy states in valence band
are filled with electrons, states in conduction
band are empty.
Taklimat UniMAP
• As the temperature increase above 0 K, a
few valence bond electrons may gain
enough thermal energy to break the bond
Universiti
Malaysia
Perlis
and jump
into the conduction
band.
• As temperature increase further, more
bonds broken, more electrons jump to the
conduction band and more “empty states
or holes” created in the valence band.
Hasnizah Aris, 2008
WAFER FABRICATION
• In intrinsic material, electrons and holes are
created in pairs by thermal energy. So the
number of electrons in conduction band is
equal to the number of holes in the valence
band
Taklimat UniMAP
• Electron concentration = hole
concentration, ni= pi and nipi= ni2 (MASS
ACTION
LAW) –the product
of n p Perlis
is always
Universiti
Malaysia
a constant for a given semiconductor
material at given temperature
Hasnizah Aris, 2008
WAFER FABRICATION
Extrinsic Semiconductor
• Extrinsic s/c is defined as a
semiconductor in which controlled
amounts of
specific dopant
or impurity
Taklimat
UniMAP
atoms have been added so that the
thermal equilibrium electron and hole
Universiti
Perlis
concentration
areMalaysia
different from
the
intrinsic carrier concentration.
Hasnizah Aris, 2008
WAFER FABRICATION
Doping of Semiconductors
• The purpose of doping is to alter the
conductivity of semiconductor
materials.
Taklimat UniMAP
• Two types of dopant; p-type (B), ntype (P, As)
• P-type
dopants provide
a holePerlis
in s/c
Universiti
Malaysia
materials, hence called acceptor.
• N-type dopants provide an electron in
s/c materials, hence called donors.
Hasnizah Aris, 2008
WAFER FABRICATION
n-type Dopant
• P and As have 5 electron valence
• When doped into Si, 4 electrons used to form the covalence
bond with Si.
• 1 extra electron is left in the outermost shell and will occupy a
new energy level called Donor Energy.
• Energy required to elevate donor electron is less than that for
electron involved in covalence bonding.
• With small thermal energy, donor electron is elevated to the
conduction band
• This process add electron to the conduction band without
creating holes in the valence band.
• The resulting material is referred as n-type semiconductor.
(draw the energy band and atom covalent bond)
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
p-type Dopant
• B have 3 electron valence
• When doped into Si, one empty state is created in the
covalence bond
• This empty state will occupy a new energy level called
Acceptor Energy.
• Some valence electron gain a small amount of energy to
move around the crystal lattice.
• This electron would occupy the “empty” position associated
with B atom.
• The vacated electron position is considered as holes.
• This process generate holes in the valence band without
creating electrons in the conduction band.
• The resulting material is referred as p-type semiconductor
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Dopant Concentration and Resistivity
- Pls verify this graph
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Basic Semiconductor Devices
Resistor
Taklimat UniMAP
Capacitor
Diode
Universiti Malaysia Perlis
Bipolar transistor
MOS transistor
Hasnizah Aris, 2008
WAFER FABRICATION
Resistor
• The simplest electronic device.
• In the IC fabrication, patterned
doped silicon normally used to
make resistorsTaklimat
with resistance
UniMAP
determined by the length, line
width, junction depth and
dopant Universiti
concentration.Malaysia Perlis
• Poly silicon also used a resistor.
Hasnizah Aris, 2008
WAFER FABRICATION
Example 1
• Many people use polysilicon to form gates
and local interconnect. Resistivity of
Taklimat
UniMAP
polysilicon is
determined
by dopant
concentration, about 1022cm-3, and ρ= 200
μΩ.cm. Assume polysilicon gate and local
Universiti
Malaysia
Perlis
interconnect
line width,
height, and
length
are 1μm, 1μm and 100μm respectively.
Calculate the resistance.
Hasnizah Aris, 2008
WAFER FABRICATION
Solution 1
•R = ρl / wh
= 200 μΩ.cm x (100 x 10-4) cm /
-4cm)
[(1x10-4cm)
x 1X10UniMAP
Taklimat
= 2 x 108μΩ
= 200
Ω
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Capacitor
• One of the most important IC
components
• When two conducting materials are
UniMAP
separatedTaklimat
by a dielectric,
a capacitor
is formed.
• Charge storage device
Universiti
Malaysia
• Memory
devices esp.
DRAM Perlis
• Challenge: Reduce capacitor size
while keeping the capacitance
Hasnizah Aris, 2008
WAFER FABRICATION
Taklimat UniMAP
• ε0-Absolute permittivity of vacuum(8.85 x
10-12F/m
Universiti
Malaysia
Perlis
• κ-Dielectric constant
Hasnizah Aris, 2008
WAFER FABRICATION
Example 2
• Calculate the capacitance for a capacitor
shown with h = l = 10 μm. Assume the
dielectric between
the 2UniMAP
conducting plates
Taklimat
is silicon dioxide, with k=3.9 and d=1000
Å.
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Solution 2
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Diode
• p-n junction
• Allow electric conduction only in one way
(positively biased)
• When p-type and n-type semiconductors join
together, they form a p-n junction diode.
• Holes in p-type region will diffuse to the n-type
region, and electrons in n-type region will diffuse to
the p-type region (at thermal equilibrium, without
applied bias).
• The area dominated by minority carriers is called
the transition region.
• The voltage across the transition region given by;
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008
WAFER FABRICATION
Taklimat UniMAP
Universiti Malaysia Perlis
Hasnizah Aris, 2008