TripleSpec Update
TripleSpec Update
TripleSpec Update
TripleSpec Update
TripleSpec Update
TripleSpec Update
TripleSpec Update
Detector Characterization
The performance of detectors can be characterized
in terms of:
Responsivity -- the number of amps (electrons) out for a
Watt of incident power
For G=1 and =1
R∝
1
I nj ∝
A
1
R
Noise Equivalent Power (NEP) -- The amount of incident
power needed to produce a signal equal to the RMS noise.
Detectivity (D) - The inverse of NEP (for those who don't
golf).
Normalized Detectivity (D*) -- D divided by sqrt(detector
area)
this provides a more geometry independent
assessment of detector performance since, for Johnson
noise limited performance,
R∝
1
A
I nj ∝
1
R
Extrinsic Semiconductors
Introducing an impurity into an intrinsic
semiconductor can introduce energy
states with very small bandgaps
relative to the intrinsic conduction
(donors) or valence (acceptors) bands.
At room temperatures doping creates a
well-controlled population of conducting
electrons/holes – semiconductor devices.
At cryogenic temperatures (kT <<<
bandgap energy) dopants create bound
states/conduction levels with small energy
difference – long wavelength detectors.
Extrinsic Semiconductor Detectors
Doping levels must be low (1016 cm−3 ) in order to avoid dark
conduction due to ''hopping''.
Requires centimeter thick detector layers to obtain necessary
absorption for good quantum efficiency.
Stressed Photoconductors
Even extrinsic semiconductors have large bandgaps relative to
the energy of a 200um photon (0.005 eV)
Mechanical stress can weaken the bond of an acceptor (hole) in
a p-type extrinsic material.
The Spitzer 160um array is constructed with discrete stressed
Ge:Ga detectors.
http://fifils.mpe-garching.mpg.de/pics/det/spect-resp.gif
http://mips.as.arizona.edu/MIPS/
http://fifils.mpe-garching.mpg.de/detector.html
Stressed Photoconductors
Even extrinsic semiconductors have large bandgaps relative to
the energy of a 200um photon (0.005 eV)
Mechanical stress can weaken the bond of an acceptor (hole) in
a p-type extrinsic material.
The Spitzer 160um array is constructed with discrete stressed
Ge:Ga detectors.
http://mips.as.arizona.edu/MIPS/
Stressed Photoconductors
Even extrinsic semiconductors have large bandgaps relative to
the energy of a 200um photon (0.005 eV)
Mechanical stress can weaken the bond of an acceptor (hole) in
a p-type extrinsic material.
The Spitzer 160um array is constructed with discrete stressed
Ge:Ga detectors.
http://mips.as.arizona.edu/MIPS/
Blocked Impurity Band (BIB) Extrinsic
Photoconductors
In order to avoid “hopping” (valence band
conduction) doping levels must be kept low in
biased extrinsic semiconductors.
At these low doping levels, physical thickness
must be large to obtain good quantum
efficiency.
Large arrays become impractical due to
detector size.
If a heavily doped layer, however, is
insulated from conducting leads hopping is
avoided.
Blocked Impurity Band detectors (BIB's)
provide the best of both worlds.
An undoped (''insulating'') layer thwarts
conduction through the device in the
heavily doped impurity band.
The applied bias...
Photons can still promote carriers to the
conduction band of the intrinsic material
BIBs in Space
Arsenic doped Silicon (Si:As) BIB
detectors provide high quantum
efficiency in array formats as large
as 1024x1024
Quantum efficiency can be close to
100%
Dark currents range from 10,000 e-/s
at 10K to 10 e-/s at 6K
Spitzer IRS uses 128x128 Si:As (520um) and Si:Sb (18-38um) BIB
arrays.
Spitzer MIPS 24um is a 128x128
Si:As BIB
WISE (2009 launch) will use two
1024x1024 Si:As BIB arrays for its 12
and 23um imaging channels.
DRS Technologies BIB Arrays
BIBs in Space
Arsenic doped Silicon (Si:As) BIB
detectors provide high quantum
efficiency in array formats as large
as 1024x1024
Quantum efficiency can be close to
100%
Dark currents range from 10,000 e-/s
at 10K to 10 e-/s at 6K
Spitzer IRS uses 128x128 Si:As (520um) and Si:Sb (18-38um) BIB
arrays.
Spitzer MIPS 24um is a 128x128
Si:As BIB
WISE (2009 launch) will use two
1024x1024 Si:As BIB arrays for its 12
and 23um imaging channels.
DRS Technologies BIB Arrays
Short Wavelength Infrared Arrays: InSb as
a (poor) Photoconductor
Indium Antimonide (InSb) is a particularly attractive intrinsic
semiconductor for near-infrared detection (bandgap -> 5.5um)
7
5
2 −1 −1
Given =10 ; =10 cm V s in this material.
G = 10
−2
V
l
2
InSb will break down at about 1V, so the thickness of such a
detector would have to be 1mm in order to achieve G=1.
Given the high carrier mobility such a thin detector would have
a small resistance and, thus, large Johnson noise.
At the same time, the detector would exhibit poor quantum
efficiency due to (not to mention a potentially poor quantum
efficiency due to a small absorption cross section.
These two competing goals – thick for absorption, thin for G=1,
seem to rule out this ideal material as a viable detector.
HgCdTe has similar properties.... so how to make effective short
wavelength detectors???
particularly
for arrays...
Diodes
If materials with different Fermi energies are brought into
contact, the Fermi levels must reach equilibrium in the two
materials through the migration of carriers.
This situation is achieved in practice by joining a donor doped
extrinsic semiconductor (n-type) with acceptor doped material (ptype)
–
–
Donor electrons in the n-type material find it energetically
favorable to migrate into the p-type region and fill holes.
This migration creates a “depletion region” at the p/n junction
which is free of both donors and acceptors.
Diodes
If materials with different Fermi energies are brought into
contact, the Fermi levels must reach equilibrium in the two
materials through the migration of carriers.
This situation is achieved in practice by joining a donor doped
extrinsic semiconductor (n-type) with acceptor doped material (ptype)
–
–
Donor electrons in the n-type material find it energetically
favorable to migrate into the p-type region and fill holes.
The displacement of charges creates a significant “contact
potential” from the electric field across the depletion region
Diode Behavior
The depletion region has a high resistance and prevents
conduction through the detector – good for minimizing Johnson
noise.
Placing a voltage across the diode can either enhance or
degrade/eliminate the potential barrier presented by the depletion
region.
A diode will not conduct when a reverse bias is applied which
adds to the contact potential barrier.
The diode will begin to conduct when a forward bias is applied in
excess of the junction potential barrier.
Non-ideal Diode Behavior
In reality, a small contaminant of donors exists in the p-type
material and vice versa.
These minority carriers create a small leakage current when the
diode is reverse biased.
The reverse current (Io) is proportional to the density of these
contaminants.
At high reverse bias tunneling can begin to add to the reverse
current. Ultimately avalanche breakdown forces the junction into
conduction at extreme reverse bias level.
Explaining Diode Conduction
Diode current vs. applied voltage bias, Vb, is given by
qV b
Io =
I = I 0 e kT − 1
–
qAD
L
N
where A is the junction area, D is the diffusion coefficient, L is the
diffusion length and N is the carrier density.
How does this come about?
The pn junction is a barrier of width, w, permeated by an electric field which
yields a contact potential difference, Vo.
One can solve the diffusion equation for carrier density through the junction
under this opposing potential, yielding
for the carrier density on
qV
np
eithe side of the junction.
= e kT
nn
o
Under a bias, a carrier that makes it across the junction contributes to the
current.
since np is uninfluenced by Vb.
q V −V 
qV
np
= e kT
= n n e kT
nn
o
b
b
The carriers which cross the junction
must then make it through the (hostile)
oppositely doped region – thus the constant dependent on diffusion.
Photodiode Detection
The migration/diffusion of carriers across the
junction creates
a region depleted of carriers
an electric field across the region due to the
excess of opposite type carriers on either side.
An electron/hole pair created by a photon near
this depletion region will be separated and swept
out by the electric field if the electron can diffuse
to the region before recombining.
prevents rapid recombination
creates a ''photocurrent'' which will flow
through a wire connecting the ''p'' and ''n''
type materials
the “assured” detection of the electron makes
the photoconductive gain = 1 in this
configuration
high resistance and high gain make for an
attractive detector
Detection of Photocurrent