Title: Semiconductor Devices
1Semiconductor Devices
- Atoms and electricity
- Semiconductor structure
- Conduction in semiconductors
- Doping
- epitaxy
- diffusion
- ion implantation
- Transistors
- MOS
- CMOS
- Implementing logic functions
2Electricity
- 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 Ohms Law
V IR
electron flow direction
-
3Electron 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
L
K
M shell
10
d p s
6
2
4Semiconductor 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
5Semiconductor 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
6Conduction 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 doesnt 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)
7Energy Bands in Semiconductors
- The space between the bands is the energy gap, or
forbidden band
8Insulators, 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 cant 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
9Insulators, Semiconductors, and Metals (continued)
Conduction Band
Valence Band
Conductor
Semiconductor
Insulator
10Hole - 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
pair creation
hole
electron
11Improving 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)
12Improving 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
13Improving 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
14Epitaxial 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
15Diffusion 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
16Diffusion of Dopants (continued)
Concentration of dopant in surrounding atmosphere
kept constant per unit volume
Dopant deposited on surface - constant amount per
unit area
17Ion 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 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
18Hole and Electron Concentrations
- To produce reasonable levels of conduction
doesnt 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 - This equation holds true for doped silicon as
well, so increasing the number of free electrons
decreases the number of holes
np ni2
19Metal-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
Gate
S
D
metal
n
n
silicon dioxide
p
monosilicon
20MOS 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
21Complementary 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
out
in
N
22CMOS (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
D
n
n
p
p
n
p
23Logic Functions Using CMOS
p
A
p
B
out
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
n
n
input 0
input 1