Transcript Introduction and Semiconductor Technology

Semiconductor Devices




Atoms and electricity
Semiconductor structure
Conduction in semiconductors
Doping
– epitaxy
– diffusion
– ion implantation
 Transistors
– MOS
– CMOS
 Implementing logic functions
(4.1)
(4.2)
Electricity
 Electricity is the flow of electrons
 Good conductors (copper) have easily
released electrons that drift within the metal
 Under influence of electric field, electrons
flow in a current
– magnitude of current depends on magnitude
of voltage applied to circuit, and the
resistance in the path of the circuit
 Current flow governed by Ohm’s Law V = IR
+
-
electron flow direction
(4.3)
Electron Bands
 Electrons circle nucleus in
defined shells
–K
–L
–M
–N
2 electrons
8 electrons
18 electrons
32 electrons
–s
–p
–d
–f
2 electrons
6 electrons
10 electrons
14 electrons
L
K
 Within each shell,
electrons are further
grouped into subshells
 electrons are assigned to
shells and subshells from
inside out
– Si has 14 electrons: 2 K, 8
L, 4 M
M shell
d
10
p
6
s
2
Semiconductor Crystalline Structure
 Semiconductors have a
regular crystalline
structure
– for monocrystal, extends
through entire structure
– for polycrystal, structure
is interrupted at irregular
boundaries
 Monocrystal has uniform
3-dimensional structure
 Atoms occupy fixed
positions relative to one
another, but are in
constant vibration about
equilibrium
(4.4)
Semiconductor Crystalline Structure
 Silicon atoms have 4
electrons in outer shell
– inner electrons are
very closely bound to
atom
 These electrons are
shared with neighbor
atoms on both sides to
“fill” the shell
– resulting structure is
very stable
– electrons are fairly
tightly bound
» no “loose” electrons
– at room temperature,
if battery applied,
very little electric
current flows
(4.5)
Conduction in Crystal Lattices
(4.6)
 Semiconductors (Si and Ge) have 4 electrons
in their outer shell
– 2 in the s subshell
– 2 in the p subshell
 As the distance between atoms decreases
the discrete subshells spread out into bands
 As the distance decreases further, the bands
overlap and then separate
– the subshell model doesn’t hold anymore, and
the electrons can be thought of as being part
of the crystal, not part of the atom
– 4 possible electrons in the lower band
(valence band)
– 4 possible electrons in the upper band
(conduction band)
Energy Bands in Semiconductors
 The
space
between
the bands
is the
energy
gap, or
forbidden
band
(4.7)
Insulators, Semiconductors, and Metals
(4.8)
 This separation of the valence and
conduction bands determines the electrical
properties of the material
 Insulators have a large energy gap
– electrons can’t jump from valence to
conduction bands
– no current flows
 Conductors (metals) have a very small (or
nonexistent) energy gap
– electrons easily jump to conduction bands
due to thermal excitation
– current flows easily
 Semiconductors have a moderate energy
gap
– only a few electrons can jump to the
conduction band
» leaving “holes”
– only a little current can flow
Insulators, Semiconductors, and Metals
(continued)
Conduction
Band
Valence
Band
Conductor
Semiconductor
Insulator
(4.9)
(4.10)
Hole - Electron Pairs
 Sometimes thermal energy is enough to cause
an electron to jump from the valence band to
the conduction band
– produces a hole - electron pair
 Electrons also “fall” back out of the conduction
band into the valence band, combining with a
hole
pair elimination
hole
pair creation
electron
Improving Conduction by Doping
(4.11)
 To make semiconductors better conductors,
add impurities (dopants) to contribute extra
electrons or extra holes
– elements with 5 outer electrons contribute an
extra electron to the lattice (donor dopant)
– elements with 3 outer electrons accept an
electron from the silicon (acceptor dopant)
Improving Conduction by Doping (cont.)
 Phosphorus and arsenic
are donor dopants
– if phosphorus is
introduced into the silicon
lattice, there is an extra
electron “free” to move
around and contribute to
electric current
» very loosely bound to
atom and can easily
jump to conduction
band
– produces n type silicon
» sometimes use + symbol
to indicate heavier
doping, so n+ silicon
– phosphorus becomes
positive ion after giving
up electron
(4.12)
Improving Conduction by Doping (cont.)
 Boron has 3
electrons in its outer
shell, so it
contributes a hole if
it displaces a silicon
atom
– boron is an
acceptor dopant
– yields p type silicon
– boron becomes
negative ion after
accepting an
electron
(4.13)
Epitaxial Growth of Silicon
 Epitaxy grows silicon on
top of existing silicon
– uses chemical vapor
deposition
– new silicon has same
crystal structure as
original
 Silicon is placed in
chamber at high
temperature
– 1200 o C (2150 o F)
 Appropriate gases are fed
into the chamber
– other gases add
impurities to the mix
 Can grow n type, then
switch to p type very
quickly
(4.14)
Diffusion of Dopants
 It is also possible to
introduce dopants into
silicon by heating them so
they diffuse into the silicon
– no new silicon is added
– high heat causes
diffusion
 Can be done with constant
concentration in
atmosphere
– close to straight line
concentration gradient
 Or with constant number of
atoms per unit area
– predeposition
– bell-shaped gradient
 Diffusion causes spreading
of doped areas
(4.15)
top
side
Diffusion of Dopants (continued)
Concentration of dopant in
surrounding atmosphere kept
constant per unit volume
(4.16)
Dopant deposited on
surface - constant
amount per unit area
Ion Implantation of Dopants
(4.17)
 One way to reduce the spreading found with
diffusion is to use ion implantation
– also gives better uniformity of dopant
– yields faster devices
– lower temperature process
 Ions are accelerated from 5 Kev to 10 Mev and
directed at silicon
– higher energy gives greater depth penetration
– total dose is measured by flux
» number of ions per cm2
» typically 1012 per cm2 - 1016 per cm2
 Flux is over entire surface of silicon
– use masks to cover areas where implantation is
not wanted
 Heat afterward to work into crystal lattice
Hole and Electron Concentrations
(4.18)
 To produce reasonable levels of conduction
doesn’t require much doping
– silicon has about 5 x 1022 atoms/cm3
– typical dopant levels are about 1015
atoms/cm3
 In undoped (intrinsic) silicon, the number of
holes and number of free electrons is equal,
and their product equals a constant
– actually, ni increases with increasing
temperature
np = ni2
 This equation holds true for doped silicon as
well, so increasing the number of free
electrons decreases the number of holes
Metal-Oxide-Semiconductor Transistors
(4.19)
 Most modern digital devices use MOS transistors,
which have two advantages over other types
– greater density
– simpler geometry, hence easier to make
 MOS transistors switch on/off more slowly
 MOS transistors consist of source and drain
diffusions, with a gate that controls whether the
transistor is on
S
Gate
n+
D
n+
metal
silicon dioxide
p
monosilicon
(4.20)
MOS Transistors (continued)
 Making gate positive (for n channel device)
causes current to flow from source to drain
– attracts electrons to gate area, creates
conductive path
 For given gate voltage, increasing voltage
difference between source and drain increases
current from source to drain
+
S
n+
D
n+
p
+
-
Complementary MOS Transistors
(4.21)
 A variant of MOS transistor uses both n-channel and
p-channel devices to make the fundamental
building block (an inverter, or not gate)
– lower power consumption
– symmetry of design
 If in = +, n-channel device is on, p-channel is off, out
is connected to  If in = -, n-channel is off, p-channel is on, out is
connected to +
 No current flows through battery in either case!!
P
out
in
N
(4.22)
CMOS (continued)
 CMOS geometry (and manufacturing
process) is more complicated
 Lower power consumption offsets that
 Bi-CMOS combines CMOS and bipolar
(another transistor type) on one chip
– CMOS for logic circuits
– Bi-polar to drive larger electrical circuits off the
chip
S
D
S
n+
n+
p+
p
D
n
p+
(4.23)
Logic Functions Using CMOS
p
A
p
B
input 0
out
two input NAND - if
n
both inputs 1, both
p-channel are off,
both n-channel are
n
on, out is negative;
otherwise at least
one p-channel is
on and one ninput 1 channel off, and
out is positive