The performance of Pb0.8Sn0.2Te photodiodes and photoconductive detectors based on Pb1-x-ySnxGeyTe:In epitaxial films has been investigated in a wide temperature interval and at various background fluxes. It was found that dark current of photodiodes in the temperature range 30 K < T < 100 K was due to the generation-recombination in depletion region and at T < 30 K tunneling through defects in the depletion region dominated and incremental resistance R0tun at T < 30 K was exponential function of Rog-r at T = 77 K. Detectivity of more than 5 x 1012 cmHz1/2W-1 limited by preamplifier noise at a background of 1 x 1012 cm-2s-1 and T = 30 K was experimentally achieved. Pb1-x-ySnxGeyTe:In epitaxial film photoconductive detectors had sensitivity in the spectral range λ < 15μm and it exponentially depended on temperature and varied from 105 A/W at T = 10 K to 102 A/W at T = 30 K. Noise of the photoconductive detectors was independent of background flux when it varied from 1012 cm-2s-1 to 1018 cm-2s-1. Multielement photoconductive detectors based on these films were fabricated and D* = 1.7 x 1013 cmHz1/2W-1 at T ≤ 25 K was achieved.
Diamond has the highest radiation-damage level among radiation-detector semiconductor materials. Besides, low carbon
nucleus charge, Z=6, provides tissue equivalence of diamond detectors. This made it possible to create unique diamondbased
ionizing-radiation detectors possessing properties unachievable, for present time, for other materials. These
detectors have found the applications in a number of areas including thermonuclear plasma diagnostics at world leading
tokamaks and medical dosimetry.
Perfection and new developments of diamond-based detectors, including those on natural and synthetic crystals as well
as on CVD films, encounter a number of obstacles, the main of which is related to empirical approach to the
development in consequence of the lack of detailed understanding in physical mechanisms of such detector operation.
This paper is further development, theoretical and experimental, of our earlier proposed model of the operation of a
radiation detector based on high-resistivity semiconductor which first made it possible to explain main experimentally
observed peculiarities of characteristics of natural diamond detectors exposed to various kinds of radiation. The model is
based on the charge carrier recombination process that ensures the variation of carrier lifetimes depending on the space
charge value, in the whole detector volume. All calculations are conducted for two-level model of recombination which
fits this requirement. It is shown that a weak generation of free charge carriers from impurity levels in addition to the
main band-to-band generation can significantly increase the operation voltages of the detector. A decrease of detector
temperature is shown to widen the circle of the materials on which the detectors operating in accordance with the model
can be developed. Results of modeling of operation of the detector on detector samples based on silicon, as the most
perfect technologically developed material, are presented. The experimental studies were performed at a temperature of
14 K to ensure sufficiently low concentration of trapping impurity centers in this material. A technique is proposed for
evaluation of the concentration of trapping levels in high-resistivity semiconductors.
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.
Diamond has the highest radiation-damage level among radiation-detector semiconductor materials. Besides, low carbon nucleus charge, Z=6, provides tissue equivalence of diamond detectors. This made it possible to create unique natural diamond ionizing-radiation detectors possessing properties unachievable, for present time, for other materials. Perfection and new developments of diamond-based detectors encounter a number of obstacles, the main of which is related to empirical approach to the development in consequence of the lack of detailed understanding in physical mechanisms of such detector operation. This paper is an attempt to approach such understanding with the use of computer modeling of diamond detector operation and the comparison of this model with experimental results obtained for natural- and synthetic-diamond detectors. The model developed first makes it possible to explain main experimentally observed peculiarities of characteristics of natural diamond detectors exposed to hard X- and gamma rays, neutrons, alpha particles, and UV radiation. It is shown that for this purpose the model must be based on the charge carrier recombination process that ensures the variation of carrier lifetimes depending on the space charge value, in the whole detector volume. All calculations are conducted for two-level model of recombination which fits this requirement and, in particular, ensures experimentally observed lifetimes of charge carriers on the order of 10-8 s with the concentration of impurities of 1017 cm-3 real for natural diamond.
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.
KEYWORDS: Sensors, Diodes, Photoresistors, Photodiodes, Polonium, Signal processing, Electric field sensors, Infrared detectors, Diffusion, Temperature metrology
Experimental and theoretical studies of low-background behavior of extrinsic photoconductor detectors (EPDs) with a steady-state photoconductive gain G>1 and extrinsic double-injection photodiodes (EDIPs) were carried out. In both the cases, frequency response had two plateaus. It was established that four regions with various values of theoretical background limit of detectivity can be distinguished in the frequency range of EPDs. Temperature dependence of the 3-db cutoff frequency of low-frequency (LF) plateau for Si:Ga EPDs was found not to be described by existing models. This characteristic frequency sharply decreased with decreasing temperature (T<20K, background- limited current) and then became weakly dependent on temperature below 12 K. A model explaining this phenomenon through peculiarities of contact injection has been suggested. The responsivity of Ge:Hg EDIPs reached 2000 A/W, which corresponded to a gain of about 1000, and was about two orders of magnitude higher than that for EPDs of the same material. The ultimate detectivity in this case, nevertheless, was close to that of EPDs and the gain- bandwidth product was much greater. The decay segment between two plateaus on frequency response curve of EDIPs is more extended than that of EPDs, which is conditioned by great gain value for EDIPs. The high-frequency plateau of the curve is due to the displacement current, similar to EPD case, and the detectivity limit here is gain-dependent and lower than that for LF part of the curve, as well.
Diamond has the highest radiation-damage level among radiation-detector semiconductor materials. Besides, low carbon nucleus charge, Z equals 6, provides tissue equivalence of diamond detectors. However, essential restrictions are imposed on production of natural-diamond detectors by extremely low final yield of selection procedure and corresponding expensiveness of high-quality type IIa natural diamonds. The solution of this problem could be found through the development of single-crystal synthetic-diamond detectors. Basic radiation-response properties of high-pressure high- temperature (HPHT) single-crystal synthetic-diamond (SD) detectors and natural-diamond (ND) detectors made of extremely low nitrogen content (type IIa) material were comparatively studied under hard X-ray, UV, and alpha-particle irradiation. Four orders of magnitude higher steady-state responsivity to radiation has been observed for SD detectors. The gain evaluated under UV irradiation exceeded 6000 (the corresponding value of responsivity was above 1000 A/W). The study of alpha-particle-induced electromotive force (EMF) polarity has revealed the opposite type of surface bending of energy bands in synthetic and natural diamonds. The difference in detector performance could be explained in terms of presented model of charge carrier injection and transport in diamond.
KEYWORDS: Sensors, Photoresistors, Semiconductors, Field effect transistors, Electric field sensors, Neodymium, Transistors, Quantum efficiency, Information operations, Infrared sensors
An extrinsic semiconductor IR field-effect transistor of a new type has been developed and studied. It operates in a spectral range corresponding to its bulk material extrinsic photoresponsivity and possesses a quantum efficiency on the order of that of the same material photoconductor detectors. Its principle of operation is based on the variation of the near- contact electric field on a change in the concentration of free charge carriers in the bulk. A low-frequency current responsivity of 106-107 A/W for experimental SI:Ga samples has been attained under a background intensity of 1011 cm-2s-1. The low-frequency responsivity changes in inverse proportion to the level of background and can achieve a very high value with a decrease in it. The 3-dB cutoff frequency of photoresponse is close to the cutoff frequency of the low- frequency photoresponse plateau for the similar photoconductor detectors. A physical model has been developed which makes it possible to carry out approximate designing of such devices. Experimental samples of 16-element phototransistor linear arrays have been developed.
Operation of the low-background IR extrinsic photoconductor detectors and detector arrays depends essentially on the process of spreading of the time-dependent photocurrent. The spreading occurs due to the low non-steady-state screening of electric charges in the detectors. It induces the next non-steady-state affects: the increase in the photoresponse time, the essential difference between the photocurrents in the circuits of the source contact injecting free carriers into the detector and of the contact which serves as a drain for the carriers, and the high cross talk in the detector array. The results of the experimental study of these effects and some other ones in 36- and 48-element linear arrays (Si:Ga) and their theoretical description are presented. The theory is in a satisfactory agreement with the experimental data. Some consequences of these results, which are important for the space-based-astronomy applications of extrinsic detectors, are discussed.
The operation of low-background lR extrinsic photoconductor detectors and detector arrays is essentially dependent on the process of spreading time-dependent photocurrent, which is due to low non-steady-state screening of the electric charges in the detectors. The spreading leads to an increase in the photoresponse time, an essential difference between the non-steady-state photocurrents in the circuits of the source contacts injecting free-charge carriers into the detectors and those in the circuits of the contacts that serve as drains for the carriers, and a high non-steady-state cross talk in the detector arrays. The results of the experimental study of all these effects and some others in 36- and 48-element linear arrays (Si:Ga) as well as their theoretical description are presented. The theory developed is in satisfactory agreement with the experimental data. Some consequences of the results obtained, which are important in space-based astronomy applications of such detectors, are discussed.
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