Lecture #2

OUTLINE

• Electrons and holes

• Energy-band model

Read: Chapter 2 (Section 2.2)

EE130 Lecture 2, Slide 2Spring 2007

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. by applying an electric field

3. by changing the temperature

4. by irradiation

EE130 Lecture 2, Slide 3Spring 2007

Bond Model of Electrons and Holes

S i S i S i

S i S i S i

S i S i S i

Si Si Si

Si Si Si

Si Si Si

When an electron breaks loose and becomes a conduction electron, a hole is also created.

2-D representation:

EE130 Lecture 2, Slide 4Spring 2007

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 5Spring 2007

The Hole as a Positive Mobile Charge

EE130 Lecture 2, Slide 6Spring 2007

ni 1010 cm-3 at room temperature

Pure Si

conduction

EE130 Lecture 2, Slide 7Spring 2007

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 8Spring 2007

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 9Spring 2007

Energy Band Diagram

Ec

Ev

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

ele

ctro

n e

ne

rgy

distance

EE130 Lecture 2, Slide 10Spring 2007

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 11Spring 2007

– 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