Basic Physics of Semiconductors, Lecture notes of Analog Electronics

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ANALOGUE ELECTRONICS

AENG001- 4 - 2

CHAPTER 2

Basic Physics of Semiconductors

Chapter 2 Basic Physics of Semiconductors

 2.1 Semiconductor materials and their properties

 2.2 PN-junction diodes

 2.3 Reverse Breakdown

Charge Carriers in Semiconductor

 To understand PN junction’s IV characteristics, it is

important to understand charge carriers’ behavior in solids,

how to modify carrier densities, and different mechanisms

of charge flow.

Periodic Table

 This abridged table contains elements with three to five

valence electrons, with Si being the most important.

Electron-Hole Pair Interaction

 With free electrons breaking off covalent bonds, holes are

generated.

 Holes can be filled by absorbing other free electrons, so

effectively there is a flow of charge carriers.

Free Electron Density at a Given Temperature

 Eg , or bandgap energy determines how much effort is

needed to break off an electron from its covalent bond.

 There exists an exponential relationship between the free-

electron density and bandgap energy.

0 15 3

0 10 3

15 3 / 2 3

( 600 ) 1. 54 10 /

( 300 ) 1. 08 10 /

/ 2

5. 2 10 exp

n T K electrons cm

n T K electrons cm

electrons cm kT

E n T

i

i

g i

  

  

  

Doping (P type)

 If Si is doped with B (boron), then it has more holes, or

becomes type P.

Summary of Charge Carriers

First Charge Transportation Mechanism: Drift

 The process in which charge particles move because of an

electric field is called drift.

 Charge particles will move at a velocity that is proportional

to the electric field.

 

 

v E

v E

e n

h p

Current Flow: General Case

 Electric current is calculated as the amount of charge in v

meters that passes thru a cross-section if the charge travel

with a velocity of v m/s.

I  v  W  h  n  q

Velocity Saturation

 A topic treated in more advanced courses is velocity

saturation.

 In reality, velocity does not increase linearly with electric

field. It will eventually saturate to a critical value.

E

v

E

v

b

v

bE

sat

sat

0

0

0

0

Second Charge Transportation Mechanism:

Diffusion

 Charge particles move from a region of high concentration

to a region of low concentration. It is analogous to an every day example of an ink droplet in water.

Example: Linear vs. Nonlinear Charge Density

Profile

 Linear charge density profile means constant diffusion

current, whereas nonlinear charge density profile means

varying diffusion current.

L

N

qD dx

dn J (^) n  qDn   n 

d d

n n L

x

L

qD N

dx

dn J qD

  exp

Einstein's Relation

 While the underlying physics behind drift and diffusion

currents are totally different, Einstein’s relation provides a

mysterious link between the two.

q

D kT 