(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 M2

610

M shell

K

L

dps

(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 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

(4.18)Hole and Electron Concentrations 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

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

np = ni2

(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 DGate

metal

silicon dioxidemonosilicon

(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

outin

(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