A numerical model of the current noise spectral density in elements of infrared focal plane array based on HgCdTe
photodiodes has been developed. Model is based on Langevine method and applied to the photodiode with p+-n-junction
and base of finite length d. Dominated dark current diffusion mechanism and random nature of thermal generationrecombination
and scattering processes determined the diffusion current fluctuations has been taken into account.
The model main peculiar properties are the stochastic boundary conditions on the interface between the depletion and
quasineutral regions. Current noise spectral density of the diode with thin base d < Lp, where Lp is the hole diffusion
length in n-region, has been calculated. In thin base diodes with blocking contact to substrate, in which recombination
velocity S = 0, a noise suppression effect is revealed. At noticeable reverse junction biases |qV| > 3kT the diffusion current
noise suppression is to be observed in whole frequency band ωtfl
<< 1, where tfl is the hole flight time through the
depletion region. In this case the diffusion current noise spectral density is less than in diodes with thick base (d >> Lp)
by a factor th(d/Lp). At slight biases |qV| < 3kT the diffusion current noise suppression occurs only in limited frequency
band ωτ < 1, where τ is the minority carriers lifetime. At high frequencies ωτ >> 1 diffusion current noise comes out
of fluctuations caused by scattering processes and is independent on the diode structure. Photocurrent noise spectral
density has been calculated too. Model developed is useful for the photodiode elements and arrays optimization.
MCT was independently synthesized in Soviet Union one year later than in UK. MCT investigations and technology
development virtually started from the material origin. Main milestones of this way from early days to the present are
reviewed. Deep researches and MCT based device development spring from the projects which Scientific Research Institute
of Applied Physics (now ORION Research and Production Association) charged with in 1969. Gradual material,
photoresistor and photodiode technology developments were carried out in 1970-1990 and resulted in n-type single crystals
MCT industrial production mastery, photoconductive detectors series up to 200 elements and high frequency heterodyne
detectors production. New generation devices - focal plane arrays and MCT epitaxial technology were developed
in 1980-2000. MCT FPA and heteroepitaxial technology enhancement since 2000 led to production of the family of long
linear and staring second generation arrays in various formats and package configurations. Third generation devices pointed
on advanced MCT heteroepitaxial technologies and new type photosensitive structures creation are under development.
Original investigations of some interesting phenomena in MCT and device structures such as injection heat transfer and
negative differential conductance in MCT diodes, metal-tunnel transparent insulator structures are also presented.
Infrared second generation photodetectors developments have been carried out in RD&P Center "Orion" for creation short
wavelength IR (SWIR, 1 to 3μm), medium wavelength IR (MWIR, 3 to 5 μm) and long wavelength IR (LWIR, 8 to 1 μm) on the
base of lead chalcogenides (PbS, PbSe), indium antimonide (InSb) and mercury cadmium telluride (CdHg1Te). Performance
and operational functionality of 2xl28 (PbS, PbSe), 256x256 (InSb), and 2x256, 4x288 256x256, 384x288, 768x576 (MCT) focal
plane arrays (FPA) are specified. Requirements to the FPA for main directions of thermovision systems applications are given.
Bias-dependent response of an extrinsic double-injection IR detector under irradiation from extrinsic and intrinsic
responsivity spectral ranges was obtained analytically and through numerical modeling. The model includes the
transient response and generation-recombination noise as well. It is shown that a great increase in current responsivity
(by orders of magnitude) without essential change in detectivity can take place in the range of extrinsic responsivity for
detectors on semiconductor materials with long-lifetime minority charge carriers if double-injection photodiodes are
made on them instead photoconductive detectors. Field dependence of the lifetimes and mobilities of charge carriers
essentially influences detector characteristics especially in the voltage range where the drift length of majority carriers
is greater than the distance between the contacts.
The model developed is in good agreement with experimental data obtained for n-Si:Cd, p-Ge:Au, and Ge:Hg diodes,
as well as for diamond detectors of radiations. A BLIP-detection responsivity of about 2000 A/W (for a wavelength of
10 micrometers) for Ge:Hg diodes has been reached in a frequency range of 500 Hz under a background of 6 x 1011
cm-2s-1 at a temperature of 20 K. Possibilities of optimization of detector performance are discussed.
Extrinsic double-injection photodiodes and other detectors of radiations with internal gain based on double injection are
reasonable to use in the systems liable to strong disturbance action, in particular to vibrations, because high responsivity
can ensure higher resistance to interference.
Double injection into extrinsic semiconductor infrared detectors can lead, in some cases, to considerable increase of their current responsivity without essential change of detectivity. These detectors are called extrinsic double-injection photodiodes. They are especially effective under low background conditions. The BLIP-detection responsivity of studied Ge:Hg double injection photodiodes has reached about 2000 A/W at a wavelength of 10 micrometers under a background of 6 x 1011cm-2s-1. This is more than 100 times the responsivity of the same material and same size photoconductive detector used under the same conditions. Under periodically pulsing bias, when a sufficiently high steady-state voltage is applied to such a diode on which relatively short voltage pulses of rectangular shape are imposed, the realization of charge accumulation in the detector bulk proves to be possible for the period of voltage variation and readout with amplification during the voltage pulse. A model of this effect was developed. It was shown, in particular, that the pulse readout current of the diode under some conditions was equal to its steady-state current multiplied by the ratio of the integration time to the readout time, i.e., great amplification takes place during the readout: the reading-out charge equals the charge generated in diode bulk for the pulse voltage variation period multiplied by the steady-state photocurrent gain of the diode.
The pulse responsivity for Ge:Hg diodes has reached about 105 A/W. It was determined as the ratio of the pulse readout current to the power of steady-state incident radiation flux.
This operation mode is especially convenient for detector arrays.
Indium antimonide MWIR Focal Plane Array (FPA) have been developed and investigated. FPA consists of two dimensional anys of InSb photodiodes bonded by indium bumps with CMOS-multiplexer and Split-Stirling cooler. Noise equivalent power NEP≈7•10-13 W/pixel and dynamic range 60÷70 dB at frame frequency (800÷1000) Hz.
The results of development researches and tests of the 4x288 focal plane array (FPA) for a spectral range 8-12 microns are given. The cooled photoreception module of the FPA represents hybrid assembly of a matrix photosensitive element based on liquid phase epitaxy (LPE) mercury-cadmium-telluride (MCT) photodiodes and cooled silicon readout integrated circuit (ROIC). Connection of the MCT and ROIC is carried out by means of In bumps. Cooled silicon readout circuits have been made by n-MOS technology with no TDI stages at focal plane. For 11.5 μm cutoff wavelength detectivity is higher then 1x1011 cmW-1Hz1/2 for 4x288 FPA with four TDI elements.
Potential barriers in Schottky diodes with a metal-tunnel transparent dielectric based on CdxHg1-xTe (CMT) with x~0.2 have been studied. We used In, Tn, Al and Cr as metal barriers. Superthin dielectric, Al2O3 and fluorine plasma films were deposited between the CMT surface and a metal.
MWIR 128x128 Focal Plane Array (FPA) performance has been investigated. FPA has been fabricated on the base of HgCdTe active layers grown on (111)B GaAs substrate by Metal Organic Chemical Vapor Deposition (MOCVD). Histograms and diagrams of photodiodes current, responsivity and detectivity have been plotted for FPA with cutoff value 5,1μm at T=200K.
Development and production of up-to-date IR focal plane arrays (FPA) in leading foreign countries are analyzed. Cadmium-mercury telluride (CMT) FPA are discussed for near, middle and long infra-red (IR) ranges including those ones operating in the time delay and integration (TDI) mode; FPA of indium antimonide, platinum silicide, lead selenide and telluride; quantum well infra-red photodetectos (QWIP) based on heterostructures of gallium arsenide/gallium-aluminum arsenide; uncooled FPA - microbolometric and pyroelectric; FPA of lead sulfide and indium-gallium arsenide; deep-cooled FPA of extrinsic silicon. Trends of further development of FPA for the thermal imaging systems of a new generation are discussed.
xResults of studying photodetectors (FD) and photodetecting assemblies (PDA) provided for receiving lased radiation in 0.2÷12 μm range are given in the paper. A possibility of using low-frequency CdxHg1-xTe photoresistors for recording CO2-laser pulses of up to 20 ns duration is discussed.
Results of the latest developments of technologies of photodetectors and photodetective assemblies including multi-element and matrix ones for IR-spectral range were discussed. Technologies for photodetectors based on CMT photodiodes for thermal imaging equipment operating on TDI mode (2×256, 4×288) or in a “staring” mode (384×288) as well as on the basis of photodiodes of indium antimonide received a large development effort. Technologies for manufacturing of high-speed photodetective assemblies for recording of pulse radiation in a wide interval of wavelengths from 0,3 to 11 μm for laser direction finding and metrology are actively developed. An analysis of up-to-date state of photoelectronics was carried out and main physics and technological problems of making photosensitive materials, photosensitive elements, cooling and image processing systems was observed.
The results are presented on the basis of characteristics of p-type CdHgTe (CMT) diodes with metal-tunnel transparent dielectric - semiconductor (MTTD photodiodes) in the 8-11 μm spectral range.
Results of studying photodetectors (FD) and photodetecting assemblies (PDA) provided for receiving lased radiation in the 0.2÷12 tm range are given in the paper. A possibility of using low-frequency CdXHgI-X Te photoresistors for recording CO2 — laser pulses of up to 20 ns duration is discussed. Photodiodes and photoresistors are compared when operating in an optical detector with heterodyning.
Development results of photodetective assemblies for 1.5 ÷ 2.5, 3 ÷ 5 and 8 ÷ 14 micrometer spectral ranges achieved last time in RD&P Center 'Orion' are discussed. In thermal imaging and heat location systems with high space and temperature resolution photodetectors with a minimum size of sensitive elements and a maximum signal/noise ratio at the output of readout microelectronics are used. Submatrix photodetectors (MxN) operating in the time delay and integration mode (TDI) as well as 'staring' arrays (N X N) were the most extensively developed. Comparative parameters and characteristics of photodetectors with intermediate and deep cooling based on PbS (2x1536), PbSe (1x256), InSb (384x288), CdHgTe (2x256, 4x128, 128x128, 384x288) with photosensitive element sizes from 100 to 30 micrometer are given. Thermoelectric cooling or microcryogenic Stirling coolers provide operating temperatures. Development results of high-speed photodiodes based on Ge, Si and InGaAsP for optical communication and range finding in 0.8 ÷ 1.7 μm spectral range as well as CdHgTe photodiodes with high speed of up to 1 GHz for laser detection and ranging at 10.6 μm wavelength are given.
We theoretically study physical processes in new promising hybrid IR FPAs based on HgCdTe p-n junctions and analyze them ultimate performance for 3 - 5 micrometers and 8 - 10 micrometers spectral ranges. Architecture of these FPAs are much simpler than that of existing FPAs: IR-sensitive HgCdTe p-n junctions are used as switches themselves, and capacitors used as strong elements occupy all the area under each p-n junction. These capacitors can be produced on the base of dielectrics with relatively high permittivity (of TiO2, type or integrated ferroelectrics). In contrast to CCD and CID, the proposed FPA does not use charge transfer between electrodes separated in space. We formulate requirements to the parameters of photosensitive elements and storage capacitors to reach the largest integration time and threshold characteristics close to the theoretical limits. It is shown that in principle the considered FPAs have unique parameters and 1/f noise of amplifiers can be suppressed in them. FPA for 3 - 5 micrometers spectral range with p-n junction of 20 X 20 micrometers 2 area can operate in BLIP mode at background temperature 300 K;its photosignal integration time equal to the persistence of human eye and format can reach 1024 X 1024 pixels. For 8 - 10 micrometers range these parameters are 300 microsecond(s) and 256 X 256 pixels, respectively, when TiO2 storage capacitors are used.
Experimental and theoretical results are presented of photosensitive semiconductor structures as well as the main developments of modern semiconductor photoresistors, photodiodes, including injection ones, based on polycrystalline and monocrystalline materials, multilayer structures and superlattices for the visual-far infrared spectral range. Performance of multielement photodetectors based on lead chalcogenides, germanium and silicon, AIIIBV compounds, and CMT structures is described.
Experimental and theoretical results of photosensitive semiconductor structures as well as the main developments of modern semiconductor photoresistors, photodiodes, including injection ones, based on polycrystalline and monocrystalline materials, multilayer structures and superlattices for visual far infrared spectral range are presented. Performance of multielement photodetectors based on lead chalcogenides, germanium and silicon, AIIIBV compounds, and CMT structures are described.
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