1.1 Silicon Crystal Structure - University of California, …ee130/sp07/… · PPT file · Web view · 2007-01-19Crystal structure Crystallographic ... semiconductors have become

Semiconductor FundamentalsOUTLINE

• General material properties• Crystal structure• Crystallographic notation

Read: Chapter 1

EE130 Lecture 1, Slide 2Spring 2007

What is a Semiconductor?• Low resistivity => “conductor”• High resistivity => “insulator”• Intermediate resistivity => “semiconductor”

– conductivity lies between that of conductors and insulators

– generally crystalline in structure for IC devices• In recent years, however, non-crystalline semiconductors

have become commercially very important

polycrystalline amorphous crystalline

EE130 Lecture 1, Slide 3Spring 2007

Semiconductor Materials

Elemental:

Compound:

Alloy:

EE130 Lecture 1, Slide 4Spring 2007

From Hydrogen to Silicon

11s 2s 2p 3s 3p 3d

1 H 1 1s1

2 He 2 1s2

3 Li 2 1 1s2 2s1

4 Be 2 2 1s2 2s2

5 B 2 2 1 1s2 2s2 2p1

6 C 2 2 2 1s2 2s2 2p2

7 N 2 2 3 1s2 2s2 2p3

8 O 2 2 4 1s2 2s2 2p4

9 F 2 2 5 1s2 2s2 2p5

10 Ne 2 2 6 1s2 2s2 2p6

11 Na 2 2 6 1 1s2 2s2 2p6 3s1

12 Mg 2 2 6 2 1s2 2s2 2p6 3s2

13 Al 2 2 6 2 1 1s2 2s2 2p6 3s2 3p1

14 Si 2 2 6 2 2 1s2 2s2 2p6 3s2 3p2

15 P 2 2 6 2 3 1s2 2s2 2p6 3s2 3p3

16 S 2 2 6 2 4 1s2 2s2 2p6 3s2 3p4

17 Cl 2 2 6 2 5 1s2 2s2 2p6 3s2 3p5

18 Ar 2 2 6 2 6 1s2 2s2 2p6 3s2 3p6

Z Name Notation2 3

# of Electrons

EE130 Lecture 1, Slide 5Spring 2007

The Silicon Atom

• 14 electrons occupying the 1st 3 energy levels:– 1s, 2s, 2p orbitals filled by 10 electrons– 3s, 3p orbitals filled by 4 electrons

To minimize the overall energy, the 3s and 3p orbitals hybridize to form 4 tetrahedral 3sp orbitals

Each has one electron and is capable of forming a bond with a neighboring atom

EE130 Lecture 1, Slide 6Spring 2007

“diamond cubic” lattice

The Si Crystal

• Each Si atom has 4 nearest neighbors

• lattice constant = 5.431Å

EE130 Lecture 1, Slide 7Spring 2007

How Many Silicon Atoms per cm-3?• Number of atoms in a unit cell:

• 4 atoms completely inside cell• Each of the 8 atoms on corners are shared among cells count as 1 atom inside cell• Each of the 6 atoms on the faces are shared among 2 cells count as 3 atoms inside cell Total number inside the cell = 4 + 1 + 3 = 8

• Cell volume:(.543 nm)3 = 1.6 x 10-22 cm3

• Density of silicon atoms = (8 atoms) / (cell volume) = 5 x 1022 atoms/cm3

EE130 Lecture 1, Slide 8Spring 2007

Compound Semiconductors

G a

A s

• “zincblende” structure• III-V compound semiconductors: GaAs, GaP, GaN, etc.

important for optoelectronics and high-speed ICs

EE130 Lecture 1, Slide 9Spring 2007

Crystallographic Notation

Notation Interpretation( h k l ) crystal plane{ h k l } equivalent planes[ h k l ] crystal direction< h k l > equivalent directions

h: inverse x-intercept of planek: inverse y-intercept of planel: inverse z-intercept of plane

(Intercept values are in multiples of the lattice constant;h, k and l are reduced to 3 integers having the same ratio.)

Miller Indices:

EE130 Lecture 1, Slide 10Spring 2007

Crystallographic Planes and Si Wafers

Silicon wafers are usually cut along a {100} plane with a flat or notch to orient the wafer during IC fabrication:

EE130 Lecture 1, Slide 11Spring 2007

Unit cell:

View in <100> direction

Crystallographic Planes in Si

View in <110> direction

View in <111> direction

EE130 Lecture 1, Slide 12Spring 2007

Summary

• Crystalline Si:– 4 valence electrons per atom– diamond lattice

• each atom has 4 nearest neighbors– 5 x 1022 atoms/cm3

• Crystallographic notation– Miller indices are used to designate planes

and directions within a crystalline lattice