Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-12 GTBL042-Callister-v2 August 13, 2007 18:22

478 • Chapter 12 / Electrical Properties

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

P

(5+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

(a)

 Field

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Free electron

P

(5+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

(b)

 Field

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

P

(5+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

Si

(4+)

(c)

Figure 12.12 Extrinsicn-type semiconduction

model (electron bonding). (a) An impurity

atom such as phosphorus, having five valence

electrons, may substitute for a silicon atom.

This results in an extra bonding electron, which

is bound to the impurity atom and orbits it.

(b) Excitation to form a free electron. (c)The

motion of this free electron in response to an

electric field.

valence–conduction band transitions occur, as in Figure 12.6b, but to a negligible

degree. Thus, the number of electrons in the conduction band far exceeds the num-

ber of holes in the valence band (ornp), and the first term on the right-hand side

of Equation 12.13 overwhelms the second; that is,

σ∼=n|e|μe (12.16)

For ann-type

extrinsic

semiconductor,

dependence of

conductivity on

concentration and

mobility of electrons