Near-Infrared Detector Arrays M. Robberto (with several slides grabbed from J. Beletic, K. Hodapp et al.)

Intrinsic materials material  c (  m) E g (eV) AgCl CdS GaP CdSe CdTe GaAs Si Ge PbS InAs PbTe PbSe InSb Pb 1-x Sn x Te<12.4>0.10 Hg 1-x Cd x Te<12.4>0.10 The bandgap depends on the temperature e.g. for InSb: E g = 0.24 eV and β = -2x10 -4 eV

NASA CDR Extrinsic materials P-type ◄ ► N-type material  c (  m) E g (eV) Ge:Au Ge:Hg Ge:Cd Ge:Cu Ge:Zn Ge:Be Ge:B Ge:Ga Ge:Li Si:In Si:Mg Si:Ga Si:Bi Si:Al Si:As

Homework 1 Small fractional changes in x lead to large fractional changes in the gap energy. How well we need to control x at room Temperature to have a 2% uncertainty in response at cutoff for – HgCdTe 1.72micron cutoff at 145K [WFC3] – HgCdTe 2.5micron cutoff at 77K [ground based] – HgCdTe 5micron cutoff at 35K [JWST] – HgCdTe 10micron cutoff at 35K [NEOCAM] Among these, that is the most demanding material to grow?

Cross-section of HgCdTe detector p-on-n P-on-N design

PN junction Semiconductor EF = Fermi Level => ½ occupancy at high T

PN junction N-typeP-type Doped semiconductors Impurities (doping) move the E F closer to the valence (P- type) or conduction (N=type) bands.

PN junction N-typeP-type P-N Junction When the two materials are brought into electrical contact, the electrons and hole diffuse. Recombination occurs until the Fermi levels are in equilibrium. Depletion or Space Charge region E

Depletion Region Not Neutral: there is an electric field from the N-type (+ charged) to the P-type (- charged) Free (depleted) of mobile carriers: extremely low conductivity, or high resistivity. An insulator between two charge distributions is a capacitance. The development of the electric field eventually stops the diffusion: “ built-in voltage ” or “ contact potential ” The electric field facilitates the flow of charges in one direction and prevents in the other: diode E

PN junction Reverse biased P-N Junction Reverse bias: apply voltage with the same polarity of the contact potential + Voltage to the N-type - Voltage to the P-type makes depletion region wider and increases the resistance of the junction. (but do not exagerate! => breakdown) Forward bias: smaller depletion region, eventually no E: high conductivity N-typeP-type E+E b

PN junction illuminated Reverse biased P-N Junction Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is kept constant, or recombines with a hole and discharges the junction If the bias is “ floating ”, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the reverse direction with respect to the original one that set the junction. (Same is true for photogenerated holes in N-type material). N-typeP-type

PN junction illuminated Reverse biased P-N Junction Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is kept constant, or recombines with a hole and discharges the junction If the bias is “ floating ”, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the reverse direction with respect to the original one that set the junction. (Same is true for photogenerated holes in N-type material). N-typeP-type

PN junction illuminated Reverse biased P-N Junction Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a hole-electron pair. They will eventually recombine. However, if the electron (minority carrier in the P-type material), reaches the junction before recombination, it will be swept on the other side. There it becomes a majority carrier. It will be sensed out if the bias is kept constant, or recombines with a hole and discharges the junction If the bias is “ floating ”, the other original hole, a majority carrier in the sea of holes, will drift until recombination, calling an electron from ground. A current is generated in the reverse direction with respect to the original one that set the junction. (Same is true for photogenerated holes in N-type material). N-typeP-type

Back to zero bias and beyond N-typeP-type P-N Junction Eventually the junction is discharged but photons are still absorbed. The diffusion current pushes back to maintain the built-in bias. Dark and photocurrent therefore work in different directions. An equilibrium is reached: saturation.

Reset

Photon detection Do you see the cross-talk/MTF problem?

End of integration

Reading out the generated charges “Hybrid CMOS sensors” Indium bumps are aligned, squeezed and distorted to establish electric contact between detector layer and multiplexer: COLD-WELDING The addressing and readout electronics is built on Silicon. More standard technology (still >10 7 transistors).

HAWAII-2: Photolithographically Abut 4 CMOS Reticles to Produce Each ROIC Twelve ROICs per 8” Wafer Readout Provides Low Read Noise for Visible and MWIR

RSC Approach H A W A I I - 2 R G HgCdTe detector –substrate removed to achieve 0.6 µm sensitivity H gCdTe A stronomy W ide A rea I nfrared I mager with 2 k 2 Resolution, R eference pixels and G uide Mode Specifically designed multiplexer –highly flexible reset and readout options –optimized for low power and low glow operation –three-side close buttable Two-chip imaging system: MUX + ASIC –convenient operation with small number of clocks/signals –lower power, less noise

HAWAII-2RG

Block Diagram All pads located on one side (top) Approx. 110 doubled I/O pads (probing and bonding) Three-side close buttable 18 µm pixels Total dimensions: 39 x 40.5 mm²

Output Options Slow scan direction selectable Single output for all 2048 x 2048 pixels (guide mode always uses single output) Fast scan direction selectable Single Output Mode default scan directions Fast scan direction individually selectable for each subblock Separate output for each subblock of 512 x 2048 pixels Slow scan direction selectable 4 Output Mode default scan directions

Output Options (2) Slow scan direction selectable 32 Output Mode Separate output for each subblock of 64 x 2048 pixels Four different patterns for fast scan direction selectable default scan directions

Interleaved readout of full field and guide window Guide window Full field FPA Switching between full field and guide window is possible at any time  any desired interleaved readout pattern can be realized Three examples for interleaved readout: 1. Read guide window after reading part of the full field row 2. Read guide window after reading one full field row 3. Read guide window after reading two or more full field rows

Reset Schemes

MIRI Detectors: Si:As IBC Extrinsic (vs. HgCdTe, intrinsic) Blocked Impurity Band (BIB) extrinsic (vs. “Bulk”)

READOUT INTEGRATED CIRCUIT (ROIC) 1024 × 1024 / 25 μm pixels 7 K Operation Source-Follower-per-Detector (SFD) PMOS input circuit Low Noise: 10 – 12 e- rms with Fowler-8 Low Read Glow Low Power: < 0.5 mW 4 outputs with interleaved columns Reference pixels on all outputs mimic "dark" detectors Reference output averages noise from 8 "dark" reference pixels 2.75 second read time at 10 μsec per sample (100 kHz pixel data rate)

Time Diode Bias Voltage 0.5 V 0 V Reset Readout Reset kTC Noise Reset-Read Sampling

Reset Noise in Capacitors Energy stored in a capacitor: Noise floor energy: E_n = ½kT Noise Charge: E=E n Problem: Calculate the Reset noise for JWST detectors, assuming: C= 50 fF, T=37 K

Time Diode Bias Voltage 0.5 V 0 V Reset Open ShutterClose Shutter Readout Reset Readout kTC noise CDS Signal Double Correlated Sampling

Time Diode Bias Voltage 0.5 V 0 V Reset Readout Reset Readout kTC noise MCS Signal Fowler (multi) Sampling

Time Diode Bias Voltage 0.5 V 0 V Reset Up-the-ramp Readout kTC noise MCS Signal Up-the-Ramp Sampling