1. Introduction to Semiconductors
COE 360
Principles of VLSI Design
Introduction to Semiconductors

3. Electronic Materials Electronic materials include:
Conductors: have low resistance which allows electrical current flow
Insulators: have high resistance which suppresses electrical current flow
Semiconductors: can allow or suppress electrical current flow

4. Insulators, Semiconductors, and Conductors
Conduction Band
Valence Band
attractiveness
Conductor
Semiconductor
Insulator

5. Energy Bands

6. Semiconductors Old Days – Germanium (Ge) Now – (Si)
Now – Gallium Arsenide (GaAs) used for high speed and optical devices.
New – Silicon Carbide (SiC) – High voltage Schottky diodes.

7. Electron Bands Si has 14 electrons: 2 K, 8 L, 4 M
Electrons circle nucleus in defined shells
K 2 electrons
L 8 electrons
M 18 electrons
N 32 electrons
electrons are assigned to shells and subshells from inside out
Si has 14 electrons: 2 K, 8 L, 4 M L K

8. Conductor Isolated copper Atom Electron Core Valence orbit
has only one
Electron and is
loosely bound
to core
29P
Isolated copper Atom

9. Semiconductor Isolated silicon atom Energy r1 r2 r3 Center of core
Electron r3 r1 r2 r2
14P r3 r1
Center of core
Valence orbit
has four electrons
Isolated silicon atom

10. Semiconductors
Silicon and Germanium are group 4 elements – they have 4 electrons in their valence shell.
Valence Electron Si

11. Silicon
When two silicon atoms are placed close to one another, the valence electrons are shared between the two atoms, forming a covalent bond.
Covalent bond Si Si

12. Silicon Si

13. Si Si Si Si Si

14. Covalent Bond Silicon crystal Electron An electron shared by two
14P
An electron
shared by two
neighboring atoms
to form a covalent
bond.
14P
14P
14P
14P
Silicon crystal

15. => At 0 ºK, silicon is an insulator.
At 0 ºK, no electrons will move because they are bound to their individual atoms.
=> At 0 ºK, silicon is an insulator.

16. Silicon
As temperature increases, the valence electrons gain thermal energy.
If a valence electron gains enough energy, it may break its covalent bond and moves away from its original position.
This electron is free to move within the crystal.

17. Si + -
If a negatively (-) charged electron breaks its bond and moves away from its original position, a positively charged “empty state” is left in its original position.

18. This electron can fill the empty state.Si Si + Si
This electron can fill the empty state. Si Si
Empty state originally here.

19. Si Si Si Si + Si Si Si Si Si
Empty state now here. Si

20. Si Si Si Si + Si Si Si Si Si Si

21. Si Si Si Si Si Si Si + Si Si Si

22. Semiconductors
As temperature increases, more bonds are broken creating more negative free electrons and more positively charged empty states.
To break a covalent bond, a valence electron must gain a minimum energy Eg, called the energy band gap.

23. Energy bands Electron (in conduction band) Conduction band Hole
(in valence band)
Higher band higher energy
Valence band
14P
2nd band
1st band

24. Thermal energy produces free electron and
hole pair
Electron
(in conduction band)
Hole
(in valence band)
14P
14P
14P
14P
14P

25. Recombination of free electron and hole
(in conduction band)
Hole
(in valence band)
14P
14P
14P
14P
14P

26. Hole/electron flow through a semiconductor+ -
Free Electron (in conduction band)
14P
14P
14P A C D F B E
14P
14P
14P
Hole
(in valence band)
The electron moves F-E-D-C-B-A
The hole moves A-B-C-D-E-F (pseudo movement)

27. Carrier Concentrations
The concentrations of holes and free electrons are important quantities in the behavior of semiconductors.
Carrier concentration is given as the number of particles per unit volume, or
Carrier concentration =

28. Intrinsic Semioconductor
An intrinsic semiconductor is a single crystal semiconductor with no other types of atoms in the crystal.
Pure silicon
Pure germanium
Pure gallium arsenide.

29. Intrinsic Semiconductors
n = the concentration of free electrons in an intrinsic semiconductor.
p = the concentration of holes in an intrinsic semiconductor.
Mass Action Law

30. Intrinsic and extrinsic semiconductor
Intrinsic = pure
Extrinsic = impure or doped

31. Doping Doping means mixing a pure semiconductor with
impurities to increase its electrical conductivity
Increasing the number of electrons by mixing pentavalent elements (having a valency of five. ) such as phosphorous, arsenic, antimony (adding donor impurities)
Increasing the number of holes by mixing trivalent elements such as aluminum, boron, gallium (adding acceptor impurities)
having a valency of five.

32. Has many free electrons in conduction band and few holes
N-type semiconductor
Has many free electrons in conduction band and few holes
in valence band
Free Electron
Phosphorous atom
14P
14P
15P
14P
14P

33. Has few free electrons in conduction band and many holes
P-type semiconductor
Has few free electrons in conduction band and many holes
in valence band
Hole
Aluminum atom
14P
14P
13P
14P
14P

34. Majority and minority carriers
Electrons are
Majority carriers in N-type semiconductor
Minority carriers in P-type semiconductor
Holes are
Majority carriers in P-type semiconductor
Minority carriers in N-type semiconductor

35. See you next time

36. N-type Semiconductor
When we add a pentavalent impurity to pure semiconductor we get n-type semiconductor. As
N-type Si
Pure si

37. P-type Semiconductor
When we add a Trivalent impurity to pure semiconductor we get p-type semiconductor. Ga
P-type Si
Pure si

38. P and N type Semiconductors
Acceptor ion
Donor ion N + - - - + + + - + - + + + - - - + - + + - -
Minority hole
Minority electron
Majority holes
Majority electrons

39. P and N type Semiconductors
Acceptor ion
Donor ion N + - - - + + + - + - + + + - - - + - + + - -
Minority hole
Minority electron
Majority holes
Majority electrons

40. Carrier Concentrations
For any semiconductor in thermal equilibrium nopo=ni2, where
no = the concentration of free electrons.
po = the concentration of holes.
ni = the intrinsic carrier concentration
ND: ionized donor concentration (cm-3)
NA: ionized acceptor concentration (cm-3)
Charge neutrality condition: ND + p = NA + n
At thermal equilibrium, np = ni2 (“Law of Mass Action”)

41. Extrinsic Carrier Concentrations
For an n-type semiconductor with donor impurities, the concentration of donor impurities is ND with units #/cm3.
The concentration of free electrons in the n-type semiconductor is approximately no  ND.

42. Extrinsic Carrier Concentrations
Since nopo=ni2 for any semiconductor in thermal equilibrium, and
For an n-type semiconductor, no  ND
Where po is the concentration of holes in the n-type semiconductor.

43. Extrinsic Carrier Concentrations
For a p-type semiconductor with acceptor impurities, the concentration of acceptor impurities is NA with units #/cm3.
The concentration of holes in the p-type semiconductor is approximately po  NA.

44. Extrinsic Carrier Concentrations
Since nopo=ni2 for any semiconductor in thermal equilibrium, and
For a p-type semiconductor, po  NA
Where no is the concentration of free electrons in the p-type semiconductor.

45. Terminology donor: impurity atom that increases n
acceptor: impurity atom that increases p
n-type material: contains more electrons than holes
p-type material: contains more holes than electrons
majority carrier: the most abundant carrier
minority carrier: the least abundant carrier
intrinsic semiconductor: n = p = ni
extrinsic semiconductor: doped semiconductor
EE130 Lecture 3, Slide 45

46. Example 1
Calculate the thermal equilibrium electron and hole concentrations.
Consider silicon at T = 300 K doped with phosphorous at a concentration of Nd = 1016 cm-3 and ni = 1.5 x 1010 cm-3.

47. For part (a) – it is p-type For part (b) – it is n-type
Example 2
Calculate the majority and minority carrier concentrations in silicon at T = 300K if ni = 1.5 x 1010 cm-3.
Na = 1017cm-3
Nd = 5 x 1015cm-3
For part (a) – it is p-type
For part (b) – it is n-type
a) majority = 1017cm-3 minority 2.25x 103 cm-3
b) Majority 5 x 1015cm-3, minority 4.5 x 104 cm-3