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Detector zoology
AlN
6.0
III-Nitrides
(c ~ 1.6 a0)
Zincblend
GaN
ZnS
GaN
3.0
2.0
0.4
InN
Theory
InN
GaP
ZnSe
CdS ZnTe
AlP
AlAs
CdSe
AlSb
InP
CdTe
Ge
Al2O3
3C-SiC
ZnO
1.0
6H-SiC
GaAs
Si
GaSb
InAs
InSb
3.0
3.5
4.0
4.5
Lattice Constant (Å)
5.0
5.5
6.0
0.5
0.6
0.7
1.0
2.0
5.0
6.5
Wavelength (㎛)
0.3
4.0
Al2O3
Bandgap (eV)
E(eV)=1.24/ λ(㎛)
AlN
Theory
5.0
0.0
2.5
0.2
Direct gap
Indirect gap
Photon detection devices
Photons to thermal energy
(phototube)
Metal-Semicon. photoconductor
(Schottky-barrier photodiode)
The External Photoeffect: Photoelectron Emission
 Photogenerated electrons escape from the material as free electrons.  photoelectrons
metal
< Phototube >
semiconductor
< Photomultiplier tube (PM tube) >
The Internal Photoeffect: Photoconductivity
 Excited carriers remain within the material, serve to increase electrical conductivity.
Generation: Absorbed photons generate free carriers (electrons and holes).
Transport: An applied electric field induces these carriers to move,
which results in a circuit current.
Amplification: large electric fields enhance the responsivity of the detector.
Here we will discuss three types of semiconductor photodetectors
Photoconductors
Photodiodes (PD)
Avalanche photodiodes (APD)
Quantum efficiency
Responsivity
Response time.
Photon noise
Photoelectron noise
Gain noise
Quantum efficiency of photodetectors
Internal Quantum Efficiency
int 
Number of Collected electrons
 1 e  d 
Number of Photons *Entering* detector
External Quantum Efficiency
ext 
i /q
Number of Collected electrons
 1 RF   1 e  d   ph
Number of Photons *Incident* on detector
Po / h
Fresnel loss
Fraction absorbed
in detection region
Surface recombination effect
Responsivity and Response time
Responsivity
R
i
Photo Current (Amps)
q
 ph  ext
Incident Optical Power (Watts) Po
h
 Photocurrent : i ph  RPo
Transit time
Holes and electrons move at different speed
inside the material so that the transit time
spreads so that, even if delayed, the response
is not a delta in time.
Then there is the intrinsic time of the junction
related to the intrinsic capacitance (the junction
is always equivalent to an RC circuit…)
Photoconductors
Photoconductors
Photodiodes
Photodiodes
n
P
+
- ip
Two operation modes of PN photodiodes
Short-circuit (photoconductive)
operation of PDs
Open-circuit (photovotaic)
operation of PDs
Open-circuit (photovotaic) operation of PDs
Photovoltage Vp
across the device that increases
with increasing photon flux.
This mode of operation is used,
for example, in solar cells
Short-circuit operation of PDs
Reverse-biased PDs
p-i-n Photodiodes (PIN PDs)
Heterojunction Photodiodes
Schottky-barrier Photodiodes
(Metal-semiconductor PDs)
A thin semitransparent metallic
film is used in place of the p-type (or n-type)
layer in the p-n junction photodiode.
•Simple to fabricate
•Quantum efficiency:
Medium
Problem: Shadowing of absorption region
by contacts
•Capacitance: Low
•Bandwidth: High
Can be increased by thinning absorption layer and
backing with a non absorbing material. Electrodes
must be moved closer to reduce transit time.
To increase speed,
decrease electrode spacing
and absorption depth
Absorption
layer
Non absorbing substrate
Avalanche Photodiodes (APD)
APD with only one type of carrier (e or h) is desirable.
•
•
•
High resistivity p-doped layer increases electric field across absorbing region
High-energy electron-hole pairs ionize other sites to multiply the current
Leads to greater sensitivity
light absorption
intrinsic region
(very lightly doped p region)
High resistivity p region
larger charge density
APD with only one type of carrier (e or h) is desirable.
: ionization coefficients of e and h
Ionization ratio :
e  h  e  …..
The ideal case of single-carrier multiplication is achieved when
APD gain