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

Spring 2007EE130 Lecture 2, Slide 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 (#/cm 3 ) 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

Spring 2007EE130 Lecture 2, Slide 3 Bond Model of Electrons and Holes When an electron breaks loose and becomes a conduction electron, a hole is also created. 2-D representation:

Spring 2007EE130 Lecture 2, Slide 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:

Spring 2007EE130 Lecture 2, Slide 5 The Hole as a Positive Mobile Charge

Spring 2007EE130 Lecture 2, Slide 6 n i  cm -3 at room temperature Pure Si conduction

Spring 2007EE130 Lecture 2, Slide 7 Definition of Terms n = number of electrons/cm 3 p = number of holes/cm 3 n i = intrinsic carrier concentration In a pure semiconductor, n = p = n i

Spring 2007EE130 Lecture 2, Slide 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

Spring 2007EE130 Lecture 2, Slide 9 Energy Band Diagram EcEc EvEv Simplified version of energy band model, indicating bottom edge of the conduction band (E c ) top edge of the valence band (E v )  E c and E v are separated by the band gap energy E G electron energy distance

Spring 2007EE130 Lecture 2, Slide 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

Spring 2007EE130 Lecture 2, Slide 11 –Energy-band diagram: Shows only bottom edge of conduction band E c and top edge of valence band E v E c and E v are separated by the band-gap energy E G