Introduction and Semiconductor Technology

woundcallousΗμιαγωγοί

1 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

80 εμφανίσεις

(4.
1
)

Semiconductor Devices


Atoms and electricity


Semiconductor structure


Conduction in semiconductors


Doping


epitaxy


diffusion


ion implantation


Transistors


MOS


CMOS


Implementing logic functions

(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


electron flow direction

V = IR

+

-

(4.
3
)


Electron Bands


Electrons circle nucleus in
defined
shells


K

2 electrons


L


8 electrons


M

18 electrons


N

32 electrons


Within each shell,
electrons are further
grouped into
subshells


s

2 electrons


p

6 electrons


d


10 electrons


f

14 electrons


electrons are assigned to
shells and subshells from
inside out


Si has 14 electrons: 2 K, 8
L, 4 M

2

6

10

M shell

K

L

d

p

s

(4.
4
)

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.
5
)

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.
6
)

Conduction in Crystal Lattices


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
)

(4.
7
)

Energy Bands in Semiconductors


The
space
between
the bands
is the
energy
gap
, or
forbidden
band

(4.
8
)

Insulators, Semiconductors, and Metals


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

(4.
9
)

Insulators, Semiconductors, and Metals
(continued)


Conduction
Band

Valence
Band

Conductor

Semiconductor

Insulator

(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

electron

pair creation

(4.
11
)

Improving Conduction by Doping


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)

(4.
12
)

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.
13
)

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.
14
)

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.
15
)

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


top

side

(4.
16
)

Diffusion of Dopants (continued)

Concentration of dopant in
surrounding atmosphere kept
constant per unit volume

Dopant deposited on
surface
-

constant
amount per unit area

(4.
17
)

Ion Implantation of Dopants


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 cm
2

»
typically 10
12

per cm
2

-

10
16

per cm
2


Flux is over entire surface of silicon


use masks to cover areas where implantation is
not wanted


Heat afterward to work into crystal lattice

(4.
18
)

Hole and Electron Concentrations


To produce reasonable levels of conduction
doesn’t require much doping


silicon has about 5 x 10
22

atoms/cm
3


typical dopant levels are about 10
15

atoms/cm
3


In undoped (intrinsic) silicon, the number of
holes and number of free electrons is equal,
and their product equals a constant


actually, n
i

increases with increasing
temperature


This equation holds true for doped silicon as
well, so increasing the number of free
electrons decreases the number of holes

np = n
i
2

(4.
19
)

Metal
-
Oxide
-
Semiconductor Transistors


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


p

n+

n+

S

D

Gate

metal

silicon dioxide

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


p

n+

n+

S

D

+

+

-

(4.
21
)

Complementary MOS Transistors


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

N

out

in

(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


p

n+

n+

S

D

p+

p+

S

D

n

(4.
23
)

Logic Functions Using CMOS

A

B

p

p

n

n

out

input 0

input 1

two input NAND
-

if
both inputs 1, both
p
-
channel are off,
both n
-
channel are
on, out is negative;
otherwise at least
one p
-
channel is
on and one n
-
channel off, and
out is positive