7/31/2019 Electronics Lect2
1/36
7/31/2019 Electronics Lect2
2/36
7/31/2019 Electronics Lect2
3/36
7/31/2019 Electronics Lect2
4/36
7/31/2019 Electronics Lect2
5/36
Conductors
Conductors have loosely bound electrons in their outer shellThese electrons require a small amount of energy to free themfor conduction
Lets apply a potential difference across the conductor above
The force on each electron is enough to free it from its orbitand it can jump from atom to atom the conductor conducts
Conductors are said to have a low resistivity / resistance
7/31/2019 Electronics Lect2
6/36
Conductors
Good conductors have low resistance soelectrons flow through them with ease.
Best element conductors include: Copper, silver, gold, aluminum, & nickel
Alloys are also good conductors: Brass & steel
Good conductors can also be liquid: Salt water
7/31/2019 Electronics Lect2
7/36
Conductor Atomic Structure The atomic structure of
good conductors usuallyincludes only one electronin their outer shell.
It is called a valence electron. It is easily striped from the
atom, producing currentflow.
CopperAtom
7/31/2019 Electronics Lect2
8/36
7/31/2019 Electronics Lect2
9/36
The Silicon, Si, AtomSilicon has a valency of 4 i.e. 4electrons in its outer shell
Each silicon atom shares its 4outer electrons with 4neighbouring atoms
These shared electrons bonds are shown ashorizontal and vertical linesbetween the atoms
This picture shows theshared electrons
7/31/2019 Electronics Lect2
10/36
Silicon the crystal latticeIf we extend thisarrangement throughout apiece of silicon
We have the crystal lattice of silicon
This is how silicon lookswhen it is cold
It has no free electrons it cannot conduct electricity therefore it behaves like aninsulator
7/31/2019 Electronics Lect2
11/36
Electron Movement in SiliconHowever, if we apply a littleheat to the silicon.
An electron may gainenough energy to breakfree of its bond
It is then available forconduction and is free to
travel throughout thematerial
7/31/2019 Electronics Lect2
12/36
Hole Movement in SiliconLets take a closer look atwhat the electron has leftbehind
There is a gap in the bond what we call a hole
7/31/2019 Electronics Lect2
13/36
Hole Movement in SiliconThis hole can also move
An electron in a nearbybond may jump into thishole
Effectively causing the holeto move
Like this
7/31/2019 Electronics Lect2
14/36
Semiconductor Valence Orbit
The main characteristicof a semiconductorelement is that it hasfour electrons in itsouter or valence orbit.
7/31/2019 Electronics Lect2
15/36
Crystal Lattice Structure The unique capability of
semiconductor atoms istheir ability to linktogether to form aphysical structure called
a crystal lattice. The atoms link together
with one another sharingtheir outer electrons.
These links are calledcovalent bonds. 2D Crystal Lattice
Structure
7/31/2019 Electronics Lect2
16/36
Semiconductors
An intrinsic semiconductor, also called anundoped semiconductor or i-typesemiconductor, is a pure semiconductorwithout any significant dopant species present.
An extrinsic semiconductor is asemiconductor that has been doped .
7/31/2019 Electronics Lect2
17/36
Doping Relying on heat or light for conduction does not
make for reliable electronics To make the semiconductor conduct electricity,
other atoms called impurities must be added. Impurities are different elements. This process is called doping .
7/31/2019 Electronics Lect2
18/36
Semiconductors can be Conductors
An impurity, or element
like arsenic, has 5 valenceelectrons. Adding arsenic (doping)
will allow four of thearsenic valence electronsto bond with theneighboring silicon atoms.
The one electron left overfor each arsenic atombecomes available toconduct current flow.
7/31/2019 Electronics Lect2
19/36
The Phosphorus AtomPhosphorus is number 15 in the periodic table
It has 15 protons and 15electrons 5 of theseelectrons are in its outershell
7/31/2019 Electronics Lect2
20/36
Doping Making n-type Silicon
Suppose we remove asilicon atom from thecrystal lattice
and replace it with a
phosphorus atom
We now have an electron that is not bonded it is thus free for conduction
7/31/2019 Electronics Lect2
21/36
Doping Making n-type SiliconLets remove another silicon atom
and replace it with aphosphorus atom
As more electrons areavailable for conduction wehave increased theconductivity of the material
If we now apply a potential difference across thesilicon
Phosphorus is called thedopant
7/31/2019 Electronics Lect2
22/36
Extrinsic Conduction n-type Silicon
A current willflow
Note:The negativeelectrons move
towards thepositiveterminal
7/31/2019 Electronics Lect2
23/36
From now onn-type will beshown likethis.
N-type Silicon
This type of silicon is called n-type
This is because the majority charge carriers arenegative electronsA small number of minority charge carriers holes will exist due to electrons-hole pairs being created inthe silicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons
7/31/2019 Electronics Lect2
24/36
The Boron AtomBoron is number 5 in theperiodic table
It has 5 protons and 5electrons 3 of theseelectrons are in its outershell
7/31/2019 Electronics Lect2
25/36
Doping Making p-type SiliconAs before, we remove asilicon atom from thecrystal lattice
This time we replace it witha boron atom
Notice we have a hole in a bond this hole is thus free for conduction
7/31/2019 Electronics Lect2
26/36
7/31/2019 Electronics Lect2
27/36
P-type Silicon
This type of silicon is called p-type
This is because the majority charge carriers are positive holesA small number of minority charge carriers electrons will exist due to electrons-hole pairs being created in thesilicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons
From now onp-type will beshown likethis.
7/31/2019 Electronics Lect2
28/36
The p-n Junction
Suppose we join a piece of p-type silicon to a pieceof n-type silicon
We get what is called a p-n junction
Remember both pieces are electrically neutral
7/31/2019 Electronics Lect2
29/36
The p-n JunctionWhen initially joined electronsfrom the n-type migrate intothe p-type less electrondensity there
When an electron fills ahole both the electronand hole disappear as thegap in the bond is filled
This leaves a region with no free charge carriers the depletion layer this layer acts asan insulator
7/31/2019 Electronics Lect2
30/36
The p-n JunctionAs the p-type has gained electrons it is left with anoverall negative charge
As the n-type has lost electrons it is left with anoverall positive charge
Therefore there is a voltage across the junction the junction voltage for silicon this isapproximately 0.6 V
0.6 V
7/31/2019 Electronics Lect2
31/36
The Reverse Biased P-N JunctionTake a p-n junction
Apply a voltage acrossit with the
p-type negative
n-type positive
Close the switch
The voltage sets up anelectric field throughout the
junction The junction is said to be reverse biased
7/31/2019 Electronics Lect2
32/36
The Reverse Biased P-N JunctionNegative electrons in then-type feel an attractiveforce which pulls themaway from the depletionlayer
Positive holes in the p-type also experience anattractive force which
pulls them away from thedepletion layer
Thus, the depletion layer ( INSULATOR) is widened and nocurrent flows through thep-n junction
7/31/2019 Electronics Lect2
33/36
The Forward Biased P-N JunctionTake a p-n junction
Apply a voltage acrossit with the
p-type postitive
n-type negative
Close the switch
The voltage sets up anelectric field throughout the
junction The junction is said to be forward
biased
7/31/2019 Electronics Lect2
34/36
The Forward Biased P-N JunctionNegative electrons in then-type feel a repulsiveforce which pushes theminto the depletion layer
Positive holes in the p-type also experience arepulsive force whichpushes them into thedepletion layer
Therefore, the depletion layer is eliminated and a currentflows through the p-n junction
7/31/2019 Electronics Lect2
35/36
The Forward Biased P-N JunctionAt the junction electronsfill holes
They are replenished bythe external cell and
current flows
Both disappear as theyare no longer free for
conduction
This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V
7/31/2019 Electronics Lect2
36/36
The Forward Biased P-N JunctionIf we apply a higher voltage
The electrons feel agreater force and move
faster
The current will begreater and will looklike
The p-n junction is called a DIODEand isrepresented by the symbol
The arrow shows thedirection in which itconducts current
this.