Post on 26-Mar-2015
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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
+ +
+
++
-
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--
A
Diode
Nonlinear I-V characteristics
V
I
Forward bias
Reverse bias
np
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+
-
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-
--
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+
-
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AA
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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.
impurity atoms m-3
NA
ND
x x = 0
p-type n-type
ND (x) = ND (x>0) = 0 (x<0)NA (x) = NA (x>0) = 0 (x<0)
impurity atoms m-3
NA
ND
x x = 0
p-type n-type
impurity atoms m-3
NA
ND
x x = 0
p-type n-type
ND (x) = ND (x>0) = 0 (x<0)NA (x) = NA (x<0) = 0 (x>0)
p-n interface at x=0.
Electron andhole transfer
Consider bringing into contact p-type and n-type semiconductors.
n-type semiconductor: Chemical potential, (Fermi level) below bottom of conduction band
p-type semiconductor: Chemical potential, above top of valence band.
Electrons diffuse from n-type into p-type filling empty valence states.
n-type semiconductorp-type semiconductor
EC
EV
EC
EV
EC
EV
EC
EV
Electrons
Holes
EC
EV
EC
EV
e0
p-type semiconductor n-type semiconductor
Electrons
Holes
EC
EV
EC
EV
e0
p-type semiconductor n-type semiconductor
EC
EV
EC
EV
e0
p-type semiconductor n-type semiconductor
Electrons
Holes
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
Band Bending
Junction
0dx
dEF
At equilibrium the Fermi level gradient equals zero!
p-n junction
I
IV characteristics :
The principle working of a pn-junction
P-doped
N-doped
Negatively charged
electrons + positively charged
immobile donors
Positively charged holes +
negatively charged immobile acceptors
+-
No electrons or holes, only charged donors/acceptors (DEPLETION LAYER)
electrons
holes
P-doped
N-doped
The principle working of a pn-junction
+- electrons
holes
No Voltage
P-doped N-doped
+ -+- electrons
holes
Forward bias
current
- ++- electrons
holes
Reverse bias
“no” current
“No” current(Leakage current)
Large current
Current
Voltage
Circuit symbol:
I
IV characteristics :
Ec
Ev
Ec
Ec
e-e-Drift (thermally exc.) Diffusion (E-field)
diffusiondrift jj
driftdiffusion jj
driftdiffusion jj
No bias
Forward bias
Reversebias
V
V
V0
0
j
I-V CharacteristicsHole 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
a) b)
I
t
Based on the photovoltaic effect-solar cell-photodetectors
photodiode
Avalanche diode
• Powielanie lawinowe (Vprzebicia>6Eg/e)
p
n
-elektrony
uzyskują energię
--
+
aby kreować pary elektron-dziuraprzez zderzenie nieelastyczne
Wykład VI
Zener diode
Light is absorbed if ; EHP are created; electric field separates carriers
• Short-circuit (U = 0)
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EC
EV
EC
EV
F
0
hf
Isc = q Nph(Eg)
ID (A)
VD (V)
Isc
photodiodeghf E
photodiode• Open circuit
EC
EV
EC
EV
qVbi
qVOC
ID (A)
VD (V)
Voc
Id = Io [exp(eVoc /kT)-1]
Isc – Id = 0
This current balances photogenerated current, Isc
ln( 1) lnsc scoc
o o
I IkT kTV
q I q I
Solar cell
Transfers solar energy into electric energy
P = I x U=I2 x R= U2/R
LED
Ge Si GaAs
Semiconductor laser
0FC FVE E