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1/28/2008
EE105 Fall 2007 1
Lecture 2
OUTLINE
• Basic Semiconductor Physics (cont’d)• Basic Semiconductor Physics (cont d)– Carrier drift and diffusion
• PN Junction Diodes– Electrostatics– Capacitance
EE105 Spring 2008 Lecture 2, Slide 1 Prof. Wu, UC Berkeley
Reading: Chapter 2.1‐2.2
Lecture 1, Slide 1
Dopant Compensation
• An N‐type semiconductor can be converted into P‐type material by counter‐doping it with acceptors
h hsuch that NA > ND.
• A compensated semiconductor material has both acceptors and donors.
P‐type material(NA > ND)
N‐type material(ND > NA)
EE105 Spring 2008 Lecture 2, Slide 2 Prof. Wu, UC Berkeley
A D
DA
i
DA
NNnn
NNp
−≈
−≈2
AD
i
AD
NNnp
NNn
−≈
−≈2
D A
1/28/2008
EE105 Fall 2007 2
Types of Charge in a Semiconductor
• Negative charges:– Conduction electrons (density = n)
– Ionized acceptor atoms (density = NA)
• Positive charges:– Holes (density = p)
– Ionized donor atoms (density = ND)
EE105 Spring 2008 Lecture 2, Slide 3 Prof. Wu, UC Berkeley
• The net charge density (C/cm3) in a semiconductor is
( ) AD NNnpq −+−=ρ
Carrier Drift
• The process in which charged particles move because of an electric field is called drift.
• Charged particles within a semiconductor move with an average velocity proportional to the electric field.– The proportionality constant is the carrier mobility.
→→
→→
= Ev ph μHole velocity
EE105 Spring 2008 Lecture 2, Slide 4 Prof. Wu, UC Berkeley
→→
−= Ev ne μ
Notation:μp ≡ hole mobility (cm2/V∙s)μn ≡ electron mobility (cm2/V∙s)
Electron velocity
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EE105 Fall 2007 3
Velocity Saturation• In reality, carrier velocities saturate at an upper limit, called the saturation velocity (vsat).
E
bv
bE
sat
0
0
0
1
μ
μ
μμ
=
+=
EE105 Spring 2008 Lecture 2, Slide 5 Prof. Wu, UC Berkeley
E
vEv
sat
0
0
1 μμ
+=
410 V/cmsatE ≈
Drift Current• Drift current is proportional to the carrier velocity and carrier concentration:
vh t A = volume from which all holes cross plane in time t
p v t A = # of holes crossing plane in time t
EE105 Spring 2008 Lecture 2, Slide 6 Prof. Wu, UC Berkeley
p vh t A = # of holes crossing plane in time t
q p vh t A = charge crossing plane in time t
q p vh A = charge crossing plane per unit time = hole current
Hole current per unit area (i.e. current density) Jp,drift = q p vh
1/28/2008
EE105 Fall 2007 4
Conductivity and Resistivity
• In a semiconductor, both electrons and holes conduct current:
EJEJ )(
• Conductivity
EEnpqJ
EqnEqpJJJ
EqnJEqpJ
npdrifttot
npdriftndriftpdrifttot
ndriftnpdriftp
σμμ
μμ
μμ
≡+=
+=+=
−−==
)(
)(
,
,,,
,,
[unit: mho/cm = S/cm]qp qnσ μ μ≡ +
EE105 Spring 2008 Lecture 2, Slide 7 Prof. Wu, UC Berkeley
• Resistivity
• Typical resistivity range for Si: 10‐3 ~ 103 Ω‐cm
[unit: mho/cm S/cm]p nqp qnσ μ μ≡ +
1 [Unit: -cm]ρσ
≡ Ω
Resistivity Example• Estimate the resistivity of a Si sample doped with phosphorus
to a concentration of 1015 cm‐3 and boron to a concentration of 1017 cm‐3. The electron mobility and hole mobility are 800
2/ 2/cm2/Vs and 300 cm2/Vs, respectively.
EE105 Spring 2008 Lecture 2, Slide 8 Prof. Wu, UC Berkeley
1/28/2008
EE105 Fall 2007 5
Electrical ResistanceV
+ _I
LV
L
tW
homogeneously doped sample
EE105 Spring 2008 Lecture 2, Slide 9 Prof. Wu, UC Berkeley
where ρ is the resistivity
ResistanceWtL
IVR ρ=≡ (Unit: ohms)
Carrier Diffusion• Due to thermally induced random motion, mobile particles tend to move from a region of high concentration to a region of low concentrationconcentration to a region of low concentration. – Analogy: ink droplet in water
• Current flow due to mobile charge diffusion is proportional to the carrier concentration gradient.– The proportionality constant is the diffusion constant.
dp
EE105 Spring 2008 Lecture 2, Slide 10 Prof. Wu, UC Berkeley
dxdpqDJ pp −=
Notation:Dp ≡ hole diffusion constant (cm2/s)Dn ≡ electron diffusion constant (cm2/s)
1/28/2008
EE105 Fall 2007 6
Diffusion Examples
• Non-linear concentration profile varying diffusion current
• Linear concentration profileconstant diffusion current
dp dp
dLxNp −
= exp⎟⎠⎞
⎜⎝⎛ −=
LxNp 1
EE105 Spring 2008 Lecture 2, Slide 11 Prof. Wu, UC Berkeley
LNqD
dxdpqDJ
p
pdiffp
=
−=
,
dd
p
pdiffp
Lx
LNqD
dxdpqDJ
−=
−=
exp
,
Diffusion Current
• Diffusion current within a semiconductor consists of hole and electron components:
• The total current flowing in a semiconductor is the
)(
,
,,
dxdpD
dxdnDqJ
dxdnqDJ
dxdpqDJ
pndifftot
ndiffnpdiffp
−=
=−=
EE105 Spring 2008 Lecture 2, Slide 12 Prof. Wu, UC Berkeley
• The total current flowing in a semiconductor is the sum of drift current and diffusion current:
diffndiffpdriftndriftptot JJJJJ ,,,, +++=
1/28/2008
EE105 Fall 2007 7
The Einstein Relation
• The characteristic constants for drift and diffusion are related:
• Note that at room temperature
q
kTD=
μ
mV26≅kT
EE105 Spring 2008 Lecture 2, Slide 13 Prof. Wu, UC Berkeley
• Note that at room temperature (300K)– This is often referred to as the “thermal voltage”.
mV26≅q
The PN Junction Diode
• When a P‐type semiconductor region and an N‐type semiconductor region are in contact, a PN junction d d f ddiode is formed. VD
ID
+–
EE105 Spring 2008 Lecture 2, Slide 14 Prof. Wu, UC Berkeley
1/28/2008
EE105 Fall 2007 8
Diode Operating Regions
• In order to understand the operation of a diode, it is necessary to study its behavior in three operation
l b b d f d bregions: equilibrium, reverse bias, and forward bias.
VD = 0 VD > 0VD < 0
EE105 Spring 2008 Lecture 2, Slide 15 Prof. Wu, UC Berkeley
Carrier Diffusion across the Junction• Because of the difference in hole and electron concentrations on each side of the junction, carriers diffuse across the junction:diffuse across the junction:
EE105 Spring 2008 Lecture 2, Slide 16 Prof. Wu, UC Berkeley
Notation:nn ≡ electron concentration on N‐type side (cm‐3)pn ≡ hole concentration on N‐type side (cm‐3)pp ≡ hole concentration on P‐type side (cm‐3)np ≡ electron concentration on P‐type side (cm‐3)
1/28/2008
EE105 Fall 2007 9
Depletion Region• As conduction electrons and holes diffuse across the junction, they leave behind ionized dopants. Thus, a region that is depleted of mobile carriers is formedregion that is depleted of mobile carriers is formed.– The charge density in the depletion region is not zero.
– The carriers which diffuse across the junction recombine with majority carriers, i.e. they are annihilated.
EE105 Spring 2008 Lecture 2, Slide 17 Prof. Wu, UC Berkeley
width=Wdep
quasi-neutral region
quasi-neutral region
The Depletion ApproximationIn the depletion region on the N side:
qNddE D==
ρGauss’s Law
( )bxqNE
dx
si
D
sisi
+=ε
εε
ρ(x)qND
In the depletion region on the P side:qN
dxdE A−
==εε
ρ a
1210 F/cmsiε =
EE105 Spring 2008 Lecture 2, Slide 18 Prof. Wu, UC Berkeley
x-qNA ( )xaqNE
dx
si
A
sisi
−=ε
εε
DA bNaN =
-b
1/28/2008
EE105 Fall 2007 10
Potential Distribution
• In the depletion region, the electric potential is quadratic since the electric field is linear
• The potential difference between the N and the P side is called built‐in potential, V0
V(x)V0
dVEdx
= −
EE105 Spring 2008 Lecture 2, Slide 19 Prof. Wu, UC Berkeley
xa-b0
dxV E dx= − ⋅∫
PN Junction in Equilibrium
• In equilibrium, the drift and diffusion components of current are balanced; therefore the net current fl hflowing across the junction is zero.
diffndriftn
diffpdriftp
JJ
JJ
,,
,,
−=
−=
EE105 Spring 2008 Lecture 2, Slide 20 Prof. Wu, UC Berkeley
0,,,, =+++= diffndiffpdriftndriftptot JJJJJ
1/28/2008
EE105 Fall 2007 11
Built‐in Potential, V0
• Because of the electric field in the depletion region, there exists a potential drop across the junction:
( ) ( ) ln ln
p
n
p p p p
pa
p pb p
p p A
dp dV dpqp E qD p Ddx dx dx
dpdV Dp
D p NkTV b V a
μ μ
μ−
⎛ ⎞= ⇒ − =⎜ ⎟⎝ ⎠
⇒ − =
⇒ − − = =
∫ ∫
b− a
EE105 Spring 2008 Lecture 2, Slide 21 Prof. Wu, UC Berkeley
( )2 ( ) ( ) ln ln
/p n i D
V b V ap q n Nμ
⇒ = =
ln 20i
DA
nNN
qkTV = (Unit: Volts)
Built‐In Potential Example
• Estimate the built‐in potential for PN junction below.
N P
ND = 1018 cm-3 NA = 1015 cm-3
( ) ( ) ( )18 15
130 2 20
10 10ln 26mV ln 26mV ln 1010
D A
i
N NkTVq n
⎛ ⎞ ⎛ ⎞= = =⎜ ⎟ ⎜ ⎟
⎝ ⎠⎝ ⎠
EE105 Spring 2008 Lecture 2, Slide 22 Prof. Wu, UC Berkeley
0
Note: ln(10) 26mV 2.3 60mV
60mV 13 780mV
kTq
V
≅ × ≅
= × =
1/28/2008
EE105 Fall 2007 12
PN Junction under Reverse Bias • A reverse bias increases the potential drop across the junction. As a result, the magnitude of the electric field increases and the width of the depletion region widensincreases and the width of the depletion region widens.
( ) 112 0 Rsi
dep VVNNq
W +⎟⎟⎠
⎞⎜⎜⎝
⎛+=
ε
EE105 Spring 2008 Lecture 2, Slide 23 Prof. Wu, UC Berkeley
DA NNq ⎠⎝
Diode Current under Reverse Bias• In equilibrium, the built‐in potential effectively prevents carriers from diffusing across the junction.
• Under reverse bias, the potential drop across the junction increases; therefore, negligible diffusion current flows. A very small drift current flows, limited by the rate at which minority carriers diffuse from the quasi‐neutral regions into the depletion region.
EE105 Spring 2008 Lecture 2, Slide 24 Prof. Wu, UC Berkeley
1/28/2008
EE105 Fall 2007 13
PN Junction Capacitance• A reverse‐biased PN junction can be viewed as a capacitor. The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR.junction capacitance (Cj) varies with VR.
2
[F/cm ]
sij
dep
CWε
=
EE105 Spring 2008 Lecture 2, Slide 25 Prof. Wu, UC Berkeley
[F/cm ]
Voltage‐Dependent Capacitance
0CC j=
00
0
12
1
VNNNNqC
VV
C
DA
DAsij
Rj
+=
+
εVD
EE105 Spring 2008 Lecture 2, Slide 26 Prof. Wu, UC Berkeley
εsi ≅ 10‐12 F/cm is the permittivity of silicon
1/28/2008
EE105 Fall 2007 14
Reverse‐Biased Diode Application
• A very important application of a reverse‐biased PN junction is in a voltage controlled oscillator (VCO), h h k hwhich uses an LC tank. By changing VR, we can
change C, which changes the oscillation frequency.
LCfres
121
=
EE105 Spring 2008 Lecture 2, Slide 27 Prof. Wu, UC Berkeley
LCfres 2π
Summary• Current flowing in a semiconductor is comprised of drift and diffusion components:
A i d l t d f bil h i t t th j ti
dxdpqD
dxdnqDEqnEqpJ pnnptot −++= μμ
• A region depleted of mobile charge exists at the junction between P‐type and N‐type materials.– A built‐in potential drop (V0) across this region is established by the charge density profile; it opposes diffusion of carriers across the junction. A reverse bias voltage serves to enhance the potential drop across the depletion region, resulting in
EE105 Spring 2008 Lecture 2, Slide 28 Prof. Wu, UC Berkeley
very little (drift) current flowing across the junction.
– The width of the depletion region (Wdep) is a function of the bias voltage (VD).
ln 20i
DA
nNN
qkTV =( ) 112 0 D
DA
sidep VV
NNqW −⎟⎟
⎠
⎞⎜⎜⎝
⎛+=
ε