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EE471 Course Notes: Lecture 1
These notes summarize the information presented in Lecture 1.
Contents
1. CARRIER DENSITIES IN SEMICONDUCTORS.
.................................................................................................
1
1.1 INTRINSIC SILICON.
.....................................................................................................................................................
1
1.2 DOPED (EXTRINSIC) SEMICONDUCTOR  NTYPE AND PTYPE SEMICONDUCTORS.
...............................................
2
1.3 SUMMARY OF EQUATIONS.
.........................................................................................................................................
5
APPENDIX A: PHYSICAL CONSTANTS.
...................................................................................................................
6
APPENDIX B: PROPERTIES OF SILICON AND GAAS AT 300K AND SOME OTHERS .
.............................
7
1. Carrier Densities in Semiconductors
1.1 Intrinsic Silicon
"Intrinsic silicon" is a pure silicon crystal (no dopants). The discussion of intrinsic silicon
developed the following (values of physical and semiconductor constants are given in the
appendix.
•
The general equation for the density of electrons (of holes) in terms of the effective density
of states in the conduction band (valence band), the Fermi energy, and the thermal energy
factor
kT.
The equations for the carrier densities in general
(i.e., also for doped silicon) and
values of parameters (for silicon at 300K) are
n
=
N
C
"
exp
#
E
C
#
E
f
kT
$
%
’
(
)
p
=
N
V
"
exp
#
E
f
#
E
V
kT
$
%
’
(
)
kT
=
0.0257 eV at 300
°
K
N
C
=
2.88
"
10
19
cm
3
N
C
=
2.66
"
10
19
cm
3
•
The intrinsic carrier density
n
i
(electrons and holes have same density in intrinsic silicon
since they are created in pairs) in terms of the densities of state and the energy band gap
E
g
=
E
C
"
E
V
, where
E
C
and
E
V
are the conduction and valence band energy levels,
respectively. The equation for and value (for silicon and at 300K) of the intrinsic carrier
density are
n
i
=
N
C
N
V
"
exp
#
E
g
2
kT
$
%
’
(
)
n
i
300
°
K
( )
=
9.65
"
10
9
cm
3
•
The "intrinsic Fermi energy"
E
i
"
E
V
+
E
g
/2
(middle of band gap) as the Fermi level for
intrinsic semiconductor. The "approximately equal" sign is used because we have ignored a
small term (of order
kT
). We will simply draw the intrinsic Fermi energy in the middle of the
band gap.
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The general equation for the density of electrons and of holes in terms of the Fermi level for
the given sample, the thermal energy factor, and the intrinsic silicon parameters
(
n
i
and
E
i
). The equations are generated from the equations above for
n
and
p
and are
fully equivalent to those equations.
n
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This note was uploaded on 10/27/2009 for the course ECE 5 taught by Professor Chavez during the Spring '09 term at Stevens.
 Spring '09
 chavez

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