• In photon detectors, the light interacts with the detector material to produce free charge carriers photon-by-photon. The resulting miniscule electrical currents are amplified to yield a usable electronic signal. The detector material is typically some form of semiconductor, in which energies around an electron volt suffice to free charge carriers; this energy threshold can be adjusted from ~ 0.01 eV to 10eV by appropriate choice of the detector material.

• In thermal detectors, the light is absorbed in the detector material to produce a minute increase in its temperature. Exquisitely sensitive electronic thermometers react to this change to produce the electronic signal. Thermal detectors are in principle sensitive to photons of any energy, so long as they can be absorbed in a way that the resulting heat can be sensed by their thermometers. Thermal detectors are important for the X-ray and the far-infrared through mm-wave regimes.

• In coherent detectors, the electrical field of the photon interacts with a locally generated signal that downconverts its frequency to a range that is compatible with further electronic processing and amplification. Downconversion refers to a multi-step process in which the incoming photon electrical field is mixed with a local electrical field of slightly different frequency. The amplitude of the mixed signal increases and decreases at the difference frequency, termed the intermediate frequency (IF). This signal encodes the frequency of the input photon and its phase.

Three Ways to Detect Light

Following: Lord Rosse image of M33 vs. Hubble image demonstrate how critical detector technology is.

A modern electronic detector (in this case, the 2k X 2K photodiode arrays used in NIRCam for JWST)

Photon detector and semiconductor terminology:

Photon absorption and creation of free charge carriers in semiconductors is the basic process behind photo-detection. The electrical behavior of a semiconductor is usually indicated with an energy level (or band gap) diagram: the low energy levels indicate all crystal bonds in place, higher levels for free electrons, and the energy to break the bonds as the energy band gap. Compare the diagram of crystal structure (above) with the band gap diagrams (below). To free an electron in intrinsic material (1) requires a certain energy indicated by the band gap. It takes less energy to free charge carriers from impurities (2) and (3).

Terminology to describe detector behavior:

• quantum efficiency

• indirect vs. direct absorption

• linearity

• dynamic range

• resolution

• time response

• spectral response

• responsivity

• noise

• shot noise

• Johnson/kTC noise

• excess noise (e.g., electronic)

The net absorption is characterized by the absorption coefficient, a. Note the difference between direct and indirect absorption (e.g., silicon vs. GaAs). Quantum mechanical selection rules do not permit transfers at the band gap energy for indirect absorbers.

)3(,1)(

1

0

1)(

00da

eS

daeSS

ab

The quantum efficiency is:

A decent detector will have close to linear response over some range of signal, and will completely saturate at some high level. In between, it is possible to recover information – the range of signal where useful information can be obtained is the dynamic range.

We characterize the resolution in a number of ways, including the MTF. Although the time response can have complex behavior, we will deal mostly with simple resistance-capacitance (RC) behavior:

)9(0,/

0

tRCt

e

RC

v

outv

The spectral response drops abruptly to zero at the band gap or excitation energy. The responsivity is the amps out per watt of signal in. It rises linearly to the cutoff wavelength for an ideal detector (assuming one charge carrier per absorbed photon).

Johnson and kTC Noise

kTVC N2

1

2

1 2 R

kTdfI J

42

• The circuit below has both potential energy (charge on the capacitor) and kinetic energy

(Brownian motion of electrons in the resistor)

• From thermodynamics, we have kT/2 of energy with each degree of freedom, leading to:

• To control this type of noise, we need to operate

the detector cold and build it to have a very large

resistance.

• Must also operate cold to control thermally

generated currents

Here is what the very simplest

detector might look like

C R

In a simple photoconductor, the photon is absorbed between the

contacts. The performance is limited because this detector must have

an extremely long RC time constant (since R must be very large to

minimize thermal noise), and the geometry cannot be adjusted to

separate the R from the C.

Detector type #1, Si:X IBC

• Idea is to create a large resistance for all but the photo-electrons

•Physical structure to left, band diagram to right; structure is a thin intrinsic

layer, then to right of it a heavily doped absorbing layer, then to right, a contact

• An absorbed photon elevates an electron to the conduction band, from which

it can migrate to the contact unimpeded. Thermal charges in the impurity band

are blocked at the impurity layer, so dark current is low.

• Detector type of choice for 5 – 35mm

Use of these detectors in

an array requires some architecture

changes, to allow attaching the

readout (to the left in these drawings).

Also, very high purity must be

achieved in the silicon to allow for

complete depletion of the IR-active

layer, or the quantum efficiency will

suffer (or the bias will have to be set

too high, increasing the noise –

w is the width of the depleted region,

tB the blocking layer thickness, NA the

minority impurity concentration, and

Vb the bias).

Here is a state-of-the-art Si:As IBC

array (made by Raytheon for MIRI on

JWST).

It is 1024 X 1024 pixels and at ~

6.7K delivers dark current < 0.1 e/s,

read noise of ~ 15 e rms, and

quantum efficiency > 60% from 8 to

26mm (~ 5% of maximum still at

28.3mm and > 30% at 5mm). The

pixels are 25mm on a side. Impurities

are at the 1012 cm-3 level. That is,

within a pixel there are 1015 silicon

atoms and 2 X 104 impurity atoms (2

X 10-9 %)

A special process is used for the

readout circuit so it works well at

such low temperatures.

Similar devices but poorer readout

performance were made by DRS

Technologies for WISE.

Detector Type #2, Photodiodes

• A depletion region is created by doping to form a junction between n-type

and p-type material

An electron has a probability of 0.5 of lying at the Fermi energy level. Dopants shift the

Fermi level; n-type up and p-type down. When different doping is applied to adjacent

semiconductor volumes, electrons flow until the Fermi level is level, bending the bands.

When all the free charge carriers

have been captured or driven out

of a volume of semiconductor, it is

said to be depleted. When

backbiased (so external voltage

tries to drive electrons up the

contact potential), the resistance

is large.

When a photon is absorbed,

the freed charge carriers

diffuse through the material

until one of them

encounters the junction,

which it is driven across by

the internal field to create

the photocurrent. The

diffusion coefficient and

length are:

)17(,q

TkD

m

)18(.DL

where m is the mobility

(characterizes ability of charge

carrier to migrate) and is the

recombination time (goes as

T1/2).

L goes as T3/4

Diodes attached tostrong substrate (readout)and then thinned:SBRC InSb arrays, usedfor SIRTF and largeformat ‘Aladdin’ device

Diodes on “back” of transparent substrate:Rockwell HgCdTearrays, used forNICMOS and for largeformat ‘Hawaii’ devices

• For operation with low dark current, low temperatures are needed

• Absorbing layer may need to be thinned to 10 - 20mm to collect charge

carriers at these temperatures

• Two approaches are shown below

• The yields in doing this can be low, partly explaining the high prices of these

arrays.

Material Cutoff wavelength mm)

Si 1.1 (indirect)

Ge 1.8 (indirect)

InAs 3.4 (direct)

InSb 6.8 (direct)

HgCdTe ~1.2 - ~ 15 (direct)

GaInAs 1.65 (direct)

AlGaAsSb 0.75 – 1.7 (direct)

• Here are some photodiode materials and their cutoff wavelengths.

HgCdTe has a variable bandgap set by the relative amounts of Hg and Te in

the crystal. AlGaAsSb behaves similarly.

• Indirect absorbers will have poor QE just short of the cutoff

An example: Teledyne HgCdTe

for all the JWST instruments

except MIRI. Here is the

architecture of a pixel. Photons

come in from the right and the

contact to the readout is to the

left. The cap layer has the

Hg/Te ratio adjusted to

increase the bandgap, so free

charge carriers are repelled

without actually having a

discontinuity in the crystal.

These arrays come with either

2.5 or 5mm cutoff. A competing

technology is diodes in InSb,

with a cutoff just beyond 5mm.

An example: the HgCdTe

arrays for NIRCam and

other JWST instruments.

They are 2048 X 2048

pixels in size, with 18mm

pixels. Dark currents at ~

37K are 0.004 e/s for the

short cutoff and 0.01 e/s for

the long. The QE is > 90%

from 1mm to close to the

cutoff and > 70% between

0.5 and 1mm. The read

noise is ~ 6-7e rms.

Similar devices but

somewhat lower

performance have been

made by Raytheon for

VISTA.

Avalanche Photodiode (APD)

The APD allows gain and

even single photon counting

with a solid-state device.

Avalanche multiplication

occurs when the charge

carriers are accelerated to

high enough energy to free

additional charge carriers and

so one can produce an

avalanche of many. This is a

stochastic process and hence

brings extra noise, described

by the gain dispersion:

or by the excess noise factor,

where G is the gain and G is the standard deviation of the gain.

• To minimize G want the gain to occur in a very thin region of the detector

• Use bias to deplete absorption region

• Large field that produces avalanche is confined to the junction

• For many materials, avalanching involves both holes and electrons

• Silicon avalanche photodiodes have F 2

• For HgCdTe, the holes can hardly move. Technically we say the hole

mobility is much less than the electron mobility

• Therefore, can have gain in HgCdTe avalanche photodiodes of up to 30

with F < 1.1 – this makes possible detectors with impressive capabilities for

very rapid readout with low noise

• Another approach is to increase the gain to yield a pulse for a single absorbed

photon

• There is a dead time of hundreds of nsec to quench the avalanche

• Thus, this only works for very low photon rates

• Good when very accurate timing is needed on low signals

Avalanche Photodiode Design

The Array Revolution in Infrared Astronomy

1968, 5-m telescope,

3 nights, single

detector

~ 2000, 1.3-m

telescope, 8

seconds, 256 X

256

~ 2006, 6.5-m

telescope, 1 hour,

1024 X 1024

(4 minutes with the

2x2 2048 X 2048

NIRCam mosaic)

Detector Type #3: Image Intensifier

• Vacuum photodiode - similar in physics to semiconductor photodiode

• Lower quantum efficiency because photoelectron has to escape from the

photocathode in to the vacuum space (photoelectric effect)

• Operates well at room temperature because the vacuum has high resistance (the

limiting issue for its electrical performance is the thermal release of charge carriers

from the photocathode)

Band gap diagram for a photocathode

The work function, , is the energy required for the electron to escape

Some old-time image intensifiers

All suffer from signal-induced backgrounds. Their outputs are an amplified

version of the inputs and any leakage back to the input contaminates the signal.

Modern use is in the ultraviolet and with a microchannel array as the amplifier,

providing an electronic output with no feedback of light.

The microchannel is coated with a material that releases a number of electrons

when hit by an energetic electron. A high voltage is maintained from left to right

in this diagram to accelerate the electrons and create more at each impact.

However, this process is noisy because of the small number of electrons freed

on the first impact.

The GALEX image intensifiers use a microchannel plate for amplification and its

output goes to a grid of electrodes that encode the position where it hit the

photocathode with delay lines. That is, by measuring the relative output times of the

left and right ends of the x delay line, one determines the position.

Photomultipliers are also based on the vacuum photodiode with an

amplifier that works by multiple stages of avalanching. The first stage can

be manufactured to release many electrons (perhaps 20) and reduce the

contribution to the noise.

Infrared Detector Arrays

• Best performance with silicon integrated circuit readout • Cannot manufacture high quality electronics in other semiconductors

• CCD-type readout has charge transfer problems at cold temperatures

• Direct hybrid construction • Fields of indium bumps evaporated on detector array, readout amplifiers

• Aligned and squeezed together - very carefully

The readout: a source follower

simple integrating amplifier

• As charge accumulates on the gate

capacitance of the MOSFET, it

modulates the current in the channel

• To keep from saturating, the charge

is reset from time to time

How should we sample the output?

kTC or Reset Noise

kTVC N2

1

2

1 2 R

kTdfI J

42

• The circuit below has both potential energy (charge on the capacitor) and kinetic energy

(Brownian motion of electrons in the resistor)

• From thermodynamics, we have kT/2 of energy with each degree of freedom, leading to:

)24(.2

2

q

SCkT

NQ From the left expression we get:

For C = 10-13 F and T = 40K, the noise is about 45 electrons rms. A

NIRCam array has a read noise of 6 - 7 electrons. How is this done?

• To avoid reset or kTC noise, we have to use readout strategy (b) or ( c)

• Then, if R = 1019 (say), RC 106 sec

• ( c) lets us take out some forms of slow drift by sampling with the reset

switch closed. However it adds root2 more amplifier noise

• Modern arrays can support strategy (b) even for integrations of 1000 -

2000 seconds

• In general, the reset noise is

• The kTC noise is then (45e)*(1 - exp(-t/RC) ) = 0.1e for 2000 seconds

RCteq

kTCnoiseread /

21

with additional components from amplifier noise and other sources.

Vout

time

More readout options: “Fowler sampling” to the left and “multi-accum”

to the right. Both address averaging out the high frequency electronic

noise (e.g., from the output amplifier). Fowler sampling has an

advantage in principle for lower net read noise, multi-accum is more

robust against upsets, e.g., cosmic ray hits.

Now consider the operation of array of readout amplifiers:

This approach has

random access and

reads nondestructively.

That is, we can read

out any pixel we want,

and we can read it and

then leave it exactly as

it was with the same

signal. This allows

interesting read out

patterns, like Fowler

sampling (multiple

samples to drive down

amplifier noise) or

sampling up the

integration ramp.

Charge Coupled Devices

(CCDs)

CCDs are actually more

complex than infrared arrays,

so we have saved them for last.

The top shows the physical

arrangement of a pixel, and (b)

and ( c) the situation before and

after exposure to light. The

electrodes deplete the silicon

near them and create potential

wells where free charge carriers

collect.

To keep the photoelectrons

from getting trapped at the back

surface, special coatings are

applied that bend the bands to

repel them.

Charge Coupled Devices

(CCDs)

Note the resemblance to the

simple photoconductor.

Reading out a CCD

When it is time to read out the signal, it is transferred along the array by

manipulating the voltages on the electrodes. This illustration is for a 3-phase

CCD, but similar strategies work for 4-phase and, with a bit of a trick on the

electrode design, on 2-phase. The isolation of the charge packets provides the

high resistance necessary to minimize thermal noise.

The charges are brought to the output amplifier in (a) in-line transfer, (b )

interline transfer, or (c ) frame transfer architectures. Astronomers

generally prefer (a), in which case the chip continues to collect signal as it

is read out. However, when better time definition is needed, interline

transfer is used (for example, for X-ray detection).

)25(.2/1)0

2( NnCTE

N

The charge transfer efficiency must be extremely high. If the proportion of

charge lost in n transfers is , then the noise in transferring N0 charges is

Consider a high performance 2048 X 2048 CCD with 2 electrons read noise.

If it is 3-phase, it takes about 12000 transfers to transfer the most distant

charge package out. Consider a signal of 100 electrons. Then, if the CTE =

0.999999, the CTE noise is 1.5 electrons! Incredibly high charge transfer

efficiency is the key to the performance we need.

However, as we have drawn the CCD, the photo-electrons collect at the

interface between the electrode and the silicon crystal, where there are lots

of open crystal bonds that trap electrons and degrade the CTE. The noise

achievable with such “surface channel” CCDs is many hundreds of

electrons.

Another source of traps is damage by cosmic rays. The CCDs in the ACS

(Advanced Camera for Surveys) on HST are getting seriously damaged in

this way and their charge transfer efficiency is dropping.

The trapping problem is fixed

with a buried channel. A thin

layer of silicon (about a

micron) is doped oppositely

to the rest of the detector

wafer and the resulting

potential minimum causes the

electrons to collect in the bulk

crystal, not at the electrodes.

This does not work at low

temperatures (< 70K), and if

the wells are over-filled then

the CCD has some electrons

that revert to surface channel,

i.e., serious latent images

and bad CTE.

If the CCD is clocked out too fast, the electrons do not have time to

migrate completely to the next gate and the CTE is reduced, also causing

excess noise. One charge transfer mechanism is the thermal motion of

the electrons, which results in diffusion. The diffusion length is

)18(.DL

where D is the diffusion coefficient and is the recombination time.

)17(,q

TkD

m

and m is the mobility, a standard semiconductor parameter that is readily

available. We can re-interpret this as L is the distance between gates and

is the transfer time, and then calculate how long it takes to get all the

charge from one gate to another. A typical transfer time is 0.05ms, and we

might want to wait something like 8 transfer times to be sure all the

electrons got across, so it takes about 0.4ms per full transfer, or 1.2 ms per

pixel for a 3-phase device. If we have a 2k X 2k CCD, then it will require

about 5 seconds to read it out.

Other mechanisms assist the transfer, but this estimate is more or less

correct.

Another issue with over-

filled wells is that charge

can leak from one well to

its neighbor along the

charge transfer direction,

leading to “blooming.”

Some CCDs have extra

thick channel stops to

resist blooming, but this

reduces the quantum

efficiency and is not used

for astronomical detectors.

The buried channel CCD approach is very similar to the one using a

composition change in the HdCdTe cap layer in a Teledyne array to keep

the photo-electrons from getting trapped away from the junction.

Reading the signal out

Finally, the signal gets to the output amplifier. By using a floating gate to put

the matching charge (by capacitive coupling) on the MOSFET gate, we can

read it out while avoiding kTC noise. Since we retain the charge after

readout, we can take it to another amplifier and read it again to reduce the

net noise.

Some other aspects of CCD performance:

Pixel binning - does

not degrade the noise,

so is a painless way to

reduce the number of

effective pixels in the

array. This can reduce

data rates and also

effective read noise.

The CCD charge transfer process lends itself naturally to clocking charge

in one direction at a set rate. This capability can be useful in applications

where images drift across the detector array at a constant (relatively slow)

rate – the charge generated by a source can be moved across the CCD to

match the motion of the source. As a result, the CCD can integrate

efficiently on the moving scene of sources without physically moving

anything to track their motion.

Time-Delay-Integration (TDI)

Orthogonal charge transfer:

It is possible to arrange 4-

phase electrodes to allow

transfer of charge in either

direction and backwards

and forwards. As shown

here, transfer downward

goes 3-1-2-3, to the right

goes 4-2-1-4, and so forth.

Deep Depletion

Because silicon is an indirect absorber, CCDs tend to have poor QE near 1 micron.

The curves, in order of increasing absorption, are for 5, 10, 15, 30, and 50mm.

Just making the CCD thick reduces the

free electron collection efficiency

(particularly in the blue where the

electrons are freed in the first 10 – 20

microns). A deep depletion CCD solves

this problem by putting a transparent

contact on the back surface and placing a

bias voltage on it so drive the photo-

electrons into the wells.

CMOS detectors

are like taking

just the readout

for an infrared

array and placing

photodiodes on

the inputs of the

amplifiers.

CMOS (complementary metal oxide semiconductor) detectors

Because they can be made in

standard integrated circuit

foundries, they are relatively

cheap. They can also be made

with a lot of the support electronics

on the same silicon chip. To the

right is Sony’s diagram of how one

works.

CMOS detectors can be really big!

To the left, an X-ray detector; to the right a garden variety 18 Mpix camera detector.

However……..

For very low light levels (e.g., use in astronomy):

• CMOS detectors have poor fill factors (amplifiers compete for real estate on

the chip with the detectors)

• Questions about cryogenic performance: the fill factor issue can be fixed by

putting a grid of microlenses over the array (Canon does this for some of its

cameras), but it is not clear that such a device can be cooled. Thanks to Sony,

camera arrays are now available that are back illuminated:

An example: Fairchild CIS2521F

• 2560 X 2160 pixels

• 6.5 mm X 6.5 mm pixel pitch

• Readout speed maximum: 100

frames per second

• Read noise < 1.5 electrons rms

• Peak quantum efficiency > 52%