1. Read: Chapter 2 (Section 2.2)
Lecture #2
OUTLINE
Electrons and holes
Energy-band model
Read: Chapter 2 (Section 2.2)

2. Electronic Properties of Si
 Silicon is a semiconductor material.
Pure Si has a relatively high electrical resistivity at room temperature.
 There are 2 types of mobile charge-carriers in Si:
Conduction electrons are negatively charged;
Holes are positively charged.
 The concentration (#/cm3) of conduction electrons & holes in a semiconductor can be modulated in several ways:
by adding special impurity atoms ( dopants )
by applying an electric field
by changing the temperature
by irradiation
EE130 Lecture 2, Slide 2

3. Bond Model of Electrons and Holes
2-D representation: Si
When an electron breaks loose and becomes a
conduction electron, a hole is also created.
EE130 Lecture 2, Slide 3

4. What is a Hole?
Mobile positive charge associated with a half-filled covalent bond
Treat as positively charged mobile particle in the semiconductor
Fluid analogy:
EE130 Lecture 2, Slide 4

5. The Hole as a Positive Mobile Charge
EE130 Lecture 2, Slide 5

6. ni  1010 cm-3 at room temperature
Pure Si
conduction
ni  1010 cm-3 at room temperature
EE130 Lecture 2, Slide 6

7. n = number of electrons/cm3 p = number of holes/cm3
Definition of Terms
n = number of electrons/cm3
p = number of holes/cm3
ni = intrinsic carrier concentration
In a pure semiconductor,
n = p = ni
EE130 Lecture 2, Slide 7

8. Si: From Atom to Crystal
Energy states in Si atom  energy bands in Si crystal
The highest nearly-filled band is the valence band
The lowest nearly-empty band is the conduction band
EE130 Lecture 2, Slide 8

9. Energy Band Diagram Ec Ev
electron energy Ev
distance
Simplified version of energy band model, indicating
bottom edge of the conduction band (Ec)
top edge of the valence band (Ev)
Ec and Ev are separated by the band gap energy EG
EE130 Lecture 2, Slide 9

10. Summary
In a pure Si crystal, conduction electrons and holes are formed in pairs.
Holes can be considered as positively charged mobile particles which exist inside a semiconductor.
Both holes and electrons can conduct current.
Splitting of allowed atomic energy levels occurs in a crystal
Separation between energy levels is small, so we can consider them as bands of continuous energy levels
Highest nearly-filled band is the valence band
Lowest nearly-empty band is the conduction band
EE130 Lecture 2, Slide 10

11. Energy-band diagram:
Shows only bottom edge of conduction band Ec and top edge of valence band Ev
Ec and Ev are separated by the band-gap energy EG
EE130 Lecture 2, Slide 11