Lecture #2
OUTLINE
• Electrons and holes
• Energy-band model
Read: Chapter 2 (Section 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:
1.
by adding special impurity atoms ( dopants )
2.
3.
4.
by applying an electric field
by changing the temperature
by irradiation
Spring 2007
EE130 Lecture 2, Slide 2
Bond Model of Electrons and Holes
2-D representation:
When an electron breaks
loose and becomes a
conduction electron, a
hole is also created.
Spring 2007
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
EE130 Lecture 2, Slide 3
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:
Spring 2007
EE130 Lecture 2, Slide 4
The Hole as a Positive Mobile Charge
Spring 2007
EE130 Lecture 2, Slide 5
Pure Si
conduction
ni  1010 cm-3 at room temperature
Spring 2007
EE130 Lecture 2, Slide 6
Definition of Terms
n = number of electrons/cm3
p = number of holes/cm3
ni = intrinsic carrier concentration
In a pure semiconductor,
n = p = ni
Spring 2007
EE130 Lecture 2, Slide 7
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
Spring 2007
EE130 Lecture 2, Slide 8
Energy Band Diagram
electron energy
Ec
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
Spring 2007
EE130 Lecture 2, Slide 9
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
Spring 2007
EE130 Lecture 2, Slide 10
– 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
Spring 2007
EE130 Lecture 2, Slide 11