1. Review of Semiconductor Physics, PN Junction Diodes and Resistors
Semiconductor fundamentals
Doping
Pn junction
The Diode Equation
Zener diode
LED
Resistors

2. What Is a Semiconductor?
                                                                                                                                                                                                          
Many materials, such as most metals, allow electrical current to flow through them
These are known as conductors
Materials that do not allow electrical current to flow through them are called insulators
Pure silicon, the base material of most transistors, is considered a semiconductor because its conductivity can be modulated by the introduction of impurities

3. Semiconductors
A material whose properties are such that it is not quite a conductor, not quite an insulator
Some common semiconductors
elemental
Si - Silicon (most common)
Ge - Germanium
compound
GaAs - Gallium arsenide
GaP - Gallium phosphide
AlAs - Aluminum arsenide
AlP - Aluminum phosphide
InP - Indium Phosphide

4. Crystalline Solids
In a crystalline solid, the periodic arrangement of atoms is repeated over the entire crystal
Silicon crystal has a diamond lattice

5. Crystalline Nature of Silicon
Silicon as utilized in integrated circuits is crystalline in nature
As with all crystalline material, silicon consists of a repeating basic unit structure called a unit cell
For silicon, the unit cell consists of an atom surrounded by four equidistant nearest neighbors which lie at the corners of the tetrahedron

6. What’s so special about Silicon?
Cheap and abundant
Amazing mechanical, chemical and electronic properties
The material is very well-known to mankind
SiO2: sand, glass
Si is column IV of the periodic table
Similar to the carbon (C) and the germanium (Ge)
Has 3s² and 3p² valence electrons

7. Nature of Intrinsic Silicon
Silicon that is free of doping impurities is called intrinsic
Silicon has a valence of 4 and forms covalent bonds with four other neighboring silicon atoms

8. 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

9. 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

10. 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)

11. Energy Bands in Semiconductors
The space between the bands is the energy gap, or forbidden band

12. 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

13. Insulators, Semiconductors, and Metals (continued)
Conduction Band
Valence Band
Conductor
Semiconductor
Insulator

14. 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
pair creation
hole
electron

15. 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)

16. 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

17. 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

18. 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

19. Diffusion of Dopants no new silicon is added
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

20. Diffusion of Dopants (continued)
Concentration of dopant in surrounding atmosphere kept constant per unit volume
Dopant deposited on surface - constant amount per unit area

21. 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 cm 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

22. 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

23. INTRINSIC (PURE) SILICON
At 0 Kelvin Silicon density is 5*10²³ particles/cm³
Silicon has 4 valence electrons, it covalently bonds with four adjacent atoms in the crystal lattice
Higher temperatures create free charge carriers.
A “hole” is created in the absence of an electron.
At 23C there are 10¹º particles/cm³ of free carriers

24. There are two types of doping
N-type and P-type.
The N in N-type stands for negative.
A column V ion is inserted.
The extra valence electron is free to move about the lattice
The P in P-type stands for positive.
A column III ion is inserted.
Electrons from the surrounding Silicon move to fill the “hole.”

25. Energy-band Diagram
A very important concept in the study of semiconductors is the energy-band diagram
It is used to represent the range of energy a valence electron can have
For semiconductors the electrons can have any one value of a continuous range of energy levels while they occupy the valence shell of the atom
That band of energy levels is called the valence band
Within the same valence shell, but at a slightly higher energy level, is yet another band of continuously variable, allowed energy levels
This is the conduction band

26. Band Gap
Between the valence and the conduction band is a range of energy levels where there are no allowed states for an electron
This is the band gap
In silicon at room temperature [in electron volts]:
Electron volt is an atomic measurement unit, 1 eV energy is necessary to decrease of the potential of the electron with 1 V.

27. Impurities
Silicon crystal in pure form is good insulator - all electrons are bonded to silicon atom
Replacement of Si atoms can alter electrical properties of semiconductor
Group number - indicates number of electrons in valence level (Si - Group IV)

28. Impurities Replace Si atom in crystal with Group V atom
substitution of 5 electrons for 4 electrons in outer shell
extra electron not needed for crystal bonding structure
can move to other areas of semiconductor
current flows more easily - resistivity decreases
many extra electrons --> “donor” or n-type material
Replace Si atom with Group III atom
substitution of 3 electrons for 4 electrons
extra electron now needed for crystal bonding structure
“hole” created (missing electron)
hole can move to other areas of semiconductor if electrons continually fill holes
again, current flows more easily - resistivity decreases
electrons needed --> “acceptor” or p-type material

29. COUNTER DOPING Insert more than one type of Ion
The extra electron and the extra hole cancel out

30. A LITTLE MATH pi-number of holes in intrinsic silicon= 10¹º/cm³
n= number of free electrons
p=number of holes
ni=number of electrons in intrinsic silicon=10¹º/cm³
pi-number of holes in intrinsic silicon= 10¹º/cm³
Mobile negative charge = -1.6*10-19 Coulombs
Mobile positive charge = 1.6*10-19 Coulombs
At thermal equilibrium (no applied voltage) n*p=(ni) (room temperature approximation)
The substrate is called n-type when it has more than 10¹º free electrons (similar for p-type)

31. P-N Junction Also known as a diode
One of the basics of semiconductor technology -
Created by placing n-type and p-type material in close contact
Diffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type

32. P-N Junction
Region of charges left behind (dopants fixed in crystal lattice)
Group III in p-type (one less proton than Si- negative charge)
Group IV in n-type (one more proton than Si - positive charge)
Region is totally depleted of mobile charges - “depletion region”
Electric field forms due to fixed charges in the depletion region
Depletion region has high resistance due to lack of mobile charges

33. THE P-N JUNCTION

34. The Junction 
The “potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region

35. Biasing the P-N Diode THINK OF THE DIODE AS A SWITCH Forward Bias
Applies - voltage to the n region and + voltage to the p region
CURRENT!
Reverse Bias
Applies + voltage to n region and – voltage to p region
NO CURRENT

36. P-N Junction – Reverse Bias
positive voltage placed on n-type material
electrons in n-type move closer to positive terminal, holes in p-type move closer to negative terminal
width of depletion region increases
allowed current is essentially zero (small “drift” current)

37. P-N Junction – Forward Bias
positive voltage placed on p-type material
holes in p-type move away from positive terminal, electrons in n-type move further from negative terminal
depletion region becomes smaller - resistance of device decreases
voltage increased until critical voltage is reached, depletion region disappears, current can flow freely

38. P-N Junction - V-I characteristics
Voltage-Current relationship for a p-n junction (diode)

39. Current-Voltage Characteristics
THE IDEAL DIODE
Positive voltage yields finite current
Negative voltage yields zero current
REAL DIODE

40. The Ideal Diode Equation

41. Semiconductor diode - opened region
The p-side is the cathode, the n-side is the anode
The dropped voltage, VD is measured from the cathode to the anode
Opened: VD  VF:
VD = VF
ID = circuit limited, in our model the VD cannot exceed VF

42. Semiconductor diode - cut-off region
Cut-off: 0 < VD < VF:
ID  0 mA

43. Semiconductor diode - closed region
Closed: VF < VD  0:
VD is determined by the circuit, ID = 0 mA
Typical values of VF: 0.5 ¸ 0.7 V

44. Zener Effect Zener break down: VD <= VZ:
VD = VZ, ID is determined by the circuit.
In case of standard diode the typical values of the break down voltage VZ of the Zener effect V
Zener diode
Utilization of the Zener effect
Typical break down values of VZ : V

45. LED
Light emitting diode, made from GaAs
VF=1.6 V
IF >= 6 mA

46. Resistor in an Integrated Circuit