Read: Chapter 2 (Section 2.2) 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: by adding special impurity atoms ( dopants ) by applying an electric field by changing the temperature by irradiation EE130 Lecture 2, Slide 2

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

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

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

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

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

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

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

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

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