Download Wykład VI - Politechnika Wrocławska

Lecture 12
Semiconducting junctions
The PN-Junction
One of the simplest bipolar devices, important
for the understanding of more complex devices
(bipolar = both electrons and holes contribute
to device characteristics).
Semiconductor devices:
Inhomogeneous semiconductors
All solid-state electronic and opto-electronic devices are
based on doped semiconductors.
In many devices the doping and hence the carrier
concentrations are non-homogeneous.
In the following section we will consider the p-n junction
which is an important part of many semiconductor
devices and which illustrated a number of key effects
Diode
Nonlinear I-V characteristics
Forward bias
n
+
I
+
+
+
+
+
+
+
+
+ + + + +
-- - -
p
A
Reverse bias
V
-
--
-
The p-n semiconductor junction:
p-type / n-type semiconductor interface
We will consider the p-n
interface to be abrupt. This is a
good approximation.
n-type ND donor atoms per m3
p-type NA acceptor atoms per m3
Consider temperatures ~300K
Almost all donor and acceptor
atoms are ionised.
p-n interface at x=0.
impurity atoms m-3
ND
NA
p-type
n-type
x=0
xa
ND (x) = ND (x>0) = 0 (x<0)
NA (x) = NA (x<0)
(x>0) = 0 (x>0)
(x<0)
p-type semiconductor
Electron and
hole transfer
n-type semiconductor
Electrons
EC
m
EC
m
EV
EV
Holes
Consider bringing into contact p-type and n-type semiconductors.
n-type semiconductor: Chemical potential, m (Fermi level) below
bottom of conduction band
p-type semiconductor: Chemical potential, m above top of valence
band.
Electrons diffuse from n-type into p-type filling empty valence states.
EC
Electrons
Band
Bending
EC
m
EV
Holes
e0
EV
p-type semiconductor
n-type semiconductor
Electrons diffuse from n-type into p-type filling empty valence band states.
The p-type becomes negatively charged with respect to the n-type material.
Electron energy levels in the p-type rise with respect to the n-type material.
A large electric field is produced close to the interface.
Dynamic equilibrium results with the chemical potential (Fermi level)
constant throughout the device.
Note: Absence of electrons and hole close to interface -- depletion region
Junction
At equilibrium the Fermi level gradient equals zero!
dE F
0
dx
p-n junction
IV characteristics :
I
The principle working of a pn-junction
P-doped
N-doped
Positively charged
holes + negatively
charged immobile
acceptors
Negatively charged
electrons +
positively charged
immobile donors
N-doped
P-doped
holes
- +
electrons
No electrons or holes, only charged donors/acceptors
(DEPLETION LAYER)
The principle working of a pn-junction
No Voltage
IV characteristics :
- +
holes
electrons
Current
I
N-doped
P-doped
Forward bias
current
+
-+
holes
electrons
-
Voltage
“No” current
(Leakage current)
Reverse bias
“no” current
-
holes
-
+
electrons
+
Circuit symbol:
Large current
Ec
No bias
Ev
Forward
bias
Ec
Drift (thermally exc.)
Diffusion (E-field)
ee-
jdrift  jdiffusion
jdiffusion  jdrift
j
V
Reverse E
c
bias
V
jdiffusion  jdrift
0
0
V
I-V Characteristics
Hole current:
•
diffusion Ipd = C1Npexp (-eVbi/(kT))
•
drift Ipu = CNpn = Ipd = C1Npexp (-eVbi/(kT))
•
at forward bias IpF = C1 Np exp (-e(Vbi- V) /(kT))
•
Ip = IpF - Ipu = C1Np exp (-e(Vbi- V) /(kT)) – C1Np exp (-eVbi/(kT)) =
C1Npexp [-eVbi/(kT)][exp(eV/(kT)-1] =Ipd [exp(eV/(kT))-1]
Electron current:
In = Ind [exp(eV/(kT))-1
with Ind = C2Nn exp (-eVbi/(kT))
I = Io [exp(eV/(kT)-1]
Io = Ind + Ipd = (C1 Np + C2Nn) exp (-eVbi/(kT))
Rectifier
Ac transfers into dc
b)
a)
I
t
photodiode
Based on the photovoltaic effect
-solar cell
-photodetectors
Avalanche diode
• Powielanie lawinowe (Vprzebicia>6Eg/e)
- elektrony
p
uzyskują energię
- aby kreować pary elektron-dziura
przez zderzenie nieelastyczne
+
n
Zener diode
Wykład VI
photodiode
Light is absorbed if hf  Eg ; EHP are created; electric field separates
carriers
• Short-circuit (U = 0)
E
ID (A)
C
hf
E
EF
E
C
V
VD (V)
0
E
V
-
Isc
Isc = q Nph(Eg)
photodiode
• Open circuit
ID (A)
EC
EC
qVOC
Voc
EV
qVbi
EV
Id = Io [exp(eVoc /kT)-1]
This current balances photogenerated current, Isc
Isc – Id = 0
I sc
kT
kT I sc
Voc 
ln(  1) 
ln
q
Io
q
Io
VD (V)
Solar cell
Transfers solar energy into electric energy
P = I x U=I2 x R= U2/R
LED
Ge
Si
GaAs
Semiconductor laser
EFC  EFV  0