Photoconductor Readout Circuit

Abstract

Disclosed herein is a device including at least one photoconductor configured for exhibiting an electrical resistance dependent on an illumination of a light-sensitive region of the photoconductor; and at least one photoconductor readout circuit, where the photoconductor readout circuit includes at least one voltage divider circuit, where the voltage divider circuit includes at least one reference resistor Rref being arranged in series with the photoconductor, where the photoconductor readout circuit includes at least one amplifier device, where the photoconductor readout circuit includes at least one capacitor arranged between an input of the amplifier device and an output of the voltage divider circuit.

Claims

1. A device comprising at least one photoconductor configured for exhibiting an electrical resistance dependent on an illumination of a light-sensitive region of the photoconductor; and at least one photoconductor readout circuit, wherein the photoconductor readout circuit comprises at least one voltage divider circuit, wherein the voltage divider circuit comprises at least one reference resistor R.sub.ref being arranged in series with the photoconductor, wherein the photoconductor readout circuit comprises at least one amplifier device, wherein the photoconductor readout circuit comprises at least one capacitor arranged between an input of the amplifier device and an output of the voltage divider circuit.

2. The device according to claim 1, wherein the amplifier device is at least one charge amplifier or at least one transimpedance amplifier.

3. The device according to claim 1, wherein the amplifier device is configured for amplifying at least one output signal of voltage divider circuit.

4. The device according to claim 1, wherein the capacitor is configured for blocking a dark current of the photoconductor, wherein the capacitor is configured for filtering a dark DC current out of the at least one output signal of the voltage divider circuit.

5. The device according to claim 1, wherein the reference resistor is a dark photoconductor.

6. The device according to claim 1, wherein the photoconductor has a dark resistance R.sub.dark, wherein a ratio of the resistance of the reference resistor and the dark resistance R.sub.ref/R.sub.dark is 0.01≤R.sub.ref/R.sub.dark≤10.

7. The device according to claim 1, wherein the photoconductor readout circuit comprises at least one diode arranged between the capacitor and the amplifier device.

8. The device according to claim 1, wherein the device comprises at least one read-out integrated circuit.

9. The device according to claim 1, wherein the photoconductor readout circuit comprises at least one bias voltage source configured for applying at least one bias voltage to the photoconductor.

10. The device according to claim 9, wherein the bias voltage U.sub.bias is 0.001 V≥U.sub.bias≤5000 V.

11. The device according to claim 1, wherein the device comprises a plurality of photoconductors, wherein the photoconductors are arranged in an array.

12. The device according to claim 11, wherein the photoconductor readout circuit is configured for determining electrical resistance of each photoconductor of the plurality of photoconductors, wherein the device comprises at least one sample and hold circuit and at least one multiplexer.

13. The device according to claim 1, wherein the light-sensitive region comprises at least one photoconductive material selected from the group consisting of lead sulfide (PbS); lead selenide (PbSe); mercury cadmium telluride (HgCdTe); cadmium sulfide (CdS); cadmium selenide (CdSe); indium antimonide (InSb); indium arsenide (InAs); indium gallium arsenide (InGaAs); extrinsic semiconductors, and organic semiconductors.

14. A resistive transducer comprising at least one device according to claim 1, wherein the resistive transducer comprises at least one evaluation device configured for determining an output signal at at least one voltage output of the photoconductor readout circuit.

15. A method of using a device according to claim 1 referring to a device, for readout of one or more of at least one PbS sensor, at least one PbSe sensor, or at least one pixelated sensor array comprising a plurality of pixels, wherein each of the pixels comprises at least one PbS or PbSe sensor.

16. The device according to claim 9, wherein the bias voltage U.sub.bias is 1 V≥U.sub.bias≤500 V.

17. The device according to claim 9, wherein the bias voltage U.sub.bias is 2 V≥U.sub.bias≤50 V.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0063] Further optional details and features of the invention are evident from the description of preferred exemplary embodiments which follows in conjunction with the dependent claims. In this context, the particular features may be implemented alone or with features in combination.

[0064] The invention is not restricted to the exemplary embodiments. The exemplary embodiments are shown schematically in the figures. Identical reference numerals in the individual figures refer to identical elements or elements with identical function, or elements which correspond to one another with regard to their functions.

[0065] Specifically, in the figures:

[0066] FIG. 1 shows an exemplary embodiment of a device according to the present invention;

[0067] FIG. 2 shows a further exemplary embodiment of the device;

[0068] FIG. 3 shows an exemplary embodiment of a resistive transducer according to the present invention;

[0069] FIG. 4 shows a further exemplary embodiment of the resistive transducer;

[0070] FIGS. 5A and 5B show experimental results of amplified output voltages in response to a modulated electromagnetic signal of known intensity; and

[0071] FIG. 6 shows experimental results of output voltage response of an ROIC to a photoconductor array.

EXEMPLARY EMBODIMENTS

[0072] FIG. 1 illustrates, in a highly schematic fashion, an exemplary embodiment of a device 110. The device 110 comprises at least one photoconductor 112 configured for exhibiting an electrical resistance dependent on an illumination of a light-sensitive region of the photoconductor 112. The photoconductor 112 may be light sensitive element capable of exhibiting a specific electrical resistance R.sub.photo dependent on an illumination of the light-sensitive region the photoconductor 112. Specifically, the electrical resistance is dependent on the illumination of a material of the photoconductor 112. The photoconductor 112 may comprise a light-sensitive region comprising a photoconductive material. The light-sensitive region may comprise at least one photoconductive material selected from the group consisting of lead sulfide (PbS); lead selenide (PbSe); mercury cadmium telluride (HgCdTe); cadmium sulfide (CdS); cadmium selenide (CdSe); indium antimonide (InSb); indium arsenide (InAs); indium gallium arsenide (InGaAs); extrinsic semiconductors, e.g. doped Ge, Si, GaAs. However, other materials may be feasible. Further possible photoconductive materials are described in WO 2016/120392 A1, for example. For example, the photoconductor 112 may be a photoconductor commercially available under the brand name Hertzstueck™ from trinamiX GmbH, D-67056 Ludwigshafen am Rhein, Germany. A photoconductor 112 can, for example, be applied in light-sensitive detector circuits. The device 110 may comprise a plurality of photoconductors 112. The photoconductors may be arranged in an array. The photoconductors 112 of the array may be designed identical, in particular with respect to size and/or shape of their light-sensitive regions and/or photoconductive materials.

[0073] For example, the light-sensitive region may be illuminated by at least one illumination source 114. The illumination source 114 can for example be or comprise an ambient light source and/or may be or may comprise an artificial illumination source. By way of example, the illumination source 114 may comprise at least one infrared emitter and/or at least one emitter for visible light and/or at least one emitter for ultraviolet light. By way of example, the illumination source 114 may comprise at least one light emitting diode and/or at least one laser diode. The illumination source 114 can comprise in particular one or a plurality of the following illumination sources: a laser, in particular a laser diode, although in principle, alternatively or additionally, other types of lasers can also be used; a light emitting diode; an incandescent lamp; a neon light; a flame source; an organic light source, in particular an organic light emitting diode; a structured light source. Alternatively or additionally, other illumination sources can also be used. The illumination source 114 generally may be adapted to emit light in at least one of: the ultraviolet spectral range, the infrared spectral range. Most preferably, at least one illumination source is adapted to emit light in the NIR and IR range, preferably in the range of 800 nm and 5000 nm, most preferably in the range of 1000 nm and 4000 nm.

[0074] The illumination source 114 may comprise at least one non-continuous light source. Alternatively, the illumination source 114 may comprise at least one continuous light source. The light source may be an arbitrary light source having at least one radiating wavelength having an overlap to the sensitive wavelength of the photoconductor 112. For example, the light source may be configured for generating a Planckian radiation. For example, the light source may comprise at least one light emitting diode (LED) and/or at least one Laser source. For example, the light source may be configured for generating illumination by an exotherm reaction, like an oxidation of liquid or solid-material or Gas. For example, the light source may be configured for generating illumination out of fluorescent effects. The illumination source 114 may be configured for generating at least one modulated light beam. Alternatively, the light beam generated by the illumination source may be non-modulated and/or may be modulated by further optical means. The illumination source 114 may comprise at least one optical chopper device configured for modulating a light beam from the continuous light source. The optical chopper device may be configured for periodically interrupting the light beam from the continuous light source. For example, the optical chopper device may be or may comprise at least one variable frequency rotating disc chopper and/or at least one fixed frequency tuning fork chopper and/or at least one optical shutter. Due to the non-continuous illumination the output current may be a changing current signal, also denoted modulation current. The modulated current may be small comparted to dark current of the photoconductor 112.

[0075] For example, the light-sensitive region may be a two-dimensional or three-dimensional region which preferably, but not necessarily, is continuous and can form a continuous region. The photoconductor 112 can have one or else a plurality of such light-sensitive regions. In response to the illumination, the electrical resistance of the photoconductor 112 is adjusted and/or changed and/or varied. When the photoconductor 112 is illuminated the photoconductor 112 may exhibit a decrease in electrical resistance. The photoconductor 112 may lower its resistivity when illuminated. Specifically, the electrical resistance of the photoconductor 112 may decrease with increasing incident light intensity. The change between dark resistance and bright resistance is the quantity to be measured or to be read out, and may be denoted as output current of the photoconductor.

[0076] The device 110 comprises at least one photoconductor readout circuit 116. The photoconductor readout circuit 116 comprises at least one voltage divider circuit 118. The voltage divider circuit 118 comprises at least one reference resistor 120 being arranged in series with the photoconductor 112. The reference resistor 120 may be a resistor having a known electrical resistance R.sub.ref. The reference resistor 120 may be an arbitrary resistor adapted to allow determining voltage changes. The reference resistor 120 may be configured to allow determining and/or measuring the resistance R.sub.photo of the photoconductor 112.

[0077] The photoconductor 112 may have a dark resistance R.sub.dark. A ratio of the resistance of the reference resistor 120 and the dark resistance R.sub.ref/R.sub.dark may be 0.01≤R.sub.ref/R.sub.dark≤10. Preferably, the ratio R.sub.ref/R.sub.dark may be around 0.1. The dark resistance of the photoconductor may be 50Ω≤R.sub.dark≤500 MΩ. For example, the dark resistance of the array of photoconductors 112 may be 10 MΩ. The reference resistor 120 may be adjustable. The resistance value of the reference resistor 120 may be manually and/or automatically adjustable. In particular, the reference resistor 120 may be adjustable with respect to the voltage input signal and photoconductor characteristics, for example a noise index.

[0078] The reference resistor 120 may be a dark photoconductor 122. The photoconductor 112 and the dark photoconductor 122 may be designed identical or different from each other. Specifically, the dark photoconductor 122 may be a dark PbS-sensor. For example, the reference resistor 120 may comprise a photoconductor covered with at least one opaque mask, wherein the opaque mask prevents that light can pass to the light-sensitive region of the covered photoconductor. As outlined above, the device 110 may comprise a plurality of photoconductors 112 arranged in an array. For each illuminated pixel a dark pixel may be employed as reference resistor 120. To adjust, in particular to optimize, the dark resistance the size of the dark pixel may be adapted. For example, in a manufacturing process, all pixels may be coated with the same material, therefore, changing the pixel size may be the easiest way to adapt the pixel resistance.

[0079] The photoconductor readout circuit 116 may comprises at least one bias voltage source 124 configured for applying at least one bias voltage U.sub.bias to the photoconductor 112. In FIG. 1 a voltage at a common collector Vcc is exemplary shown. The photoconductor 112 may be electrically connected with the bias voltage source 124. The bias voltage may be the voltage applied across the photoconductor material. The bias voltage may be a direct current (DC) voltage. The bias voltage U.sub.bias is 0.001 V≥U.sub.bias≤5000 V, preferably 1 V≥U.sub.bias≤500 V, most preferably 2 V≥U.sub.bias≤50 V. The photoconductor 112 may be electrically connected with the reference resistor 120, as outlined above, arranged in series. The reference resistor 120 may be grounded. When the photoconductor 112 is illuminated the photoconductor 112 may exhibit a decrease in electrical resistance. The current having passed the photoconductor 112 may pass through the reference resistor 120 which may generate the output signal Va which depends on the electrical resistance R.sub.photo of the photoconductor 112. The use of the dark photoconductor 122 may allow eliminating the strong dependency of the output signal on the photoconductors 112 dark resistance. Moreover, the use of the dark photoconductor 122 may incorporate a temperature compensation concerning temperature dependence of the photoconductor 112. However, the voltage Va of the still is composed by more than 99% out of U.sub.bias/2 such that the signal of interest, introduced by the illumination of the light-sensitive region of the photoconductor 112 is less than 1%.

[0080] The photoconductor readout circuit 116 comprises the at least one capacitor 126, denoted as capacitor C.sub.b in the following. The capacitor 126 may have a capacity from 0.05 to 500 nF. Capacity of the capacitor C.sub.b may be, for example, 10 nF. The capacitor C.sub.b is arranged between the input of the amplifier device and an output of the voltage divider circuit. The capacitor 126 may be a filtering capacitor. The capacitor C.sub.b may be configured for blocking a dark current of the photoconductor 112. Specifically, the capacitor C.sub.b may be configured for filtering a dark DC current out of the output signal of the voltage divider circuit. The capacitor C.sub.b is configured for filtering for an alternating current (AC) signal component of the at least one output signal of the voltage divider circuit 118. The capacitor 126 may be configured for letting the AC signal component pass. The AC signal component may have no direct current (DC) component. The AC signal component may consist only of the signal of interest. In known photoconductor readout circuits, capacitors are used differently as proposed in the present invention. E.g. in a known voltage amplifier the capacitor is used as RC-high pass filter to remove the DC content and to amplify the AC content. Moreover, the filter requires further components such as CHP and RHP. For example, non-inverting amplifier circuit that is generally used in known circuits, includes at least one photoconductor, one reference resistor, one filter capacitor and one filter resistor. In contrast, the present invention uses the capacitor C.sub.b as high pass filter, but in addition the AC current is provided to at least one amplifier device. The charge amplifier and transimpedance amplifier according to the present invention have one less resistor in comparison. The photoconductor 112, reference resistor 120 and capacitor 126 form the filter such that less components are required. In addition, the generally used non-inverting amplifier has two resistors for the amplification in the non-inverting input. The charge amplifier and the transimpedance amplifier according to the present invention only has one component. Thus, the charge amplifier and the transimpedance amplifier according to the present invention have less components in comparison to prior art. Moreover, since often arrays of photoconductors are used with potentially hundreds of sensors, the reduction of components is very advantageous.

[0081] The photoconductor readout circuit 116 comprises at least one amplifier device 128. The amplifier device 128 may be configured for amplifying at least one output signal of voltage divider circuit 118, in particular the AC signal component having passed the capacitor 126. The amplifier device 128 may be at least one charge amplifier 130, as shown in FIG. 1, or at least one transimpedance amplifier 132. As shown in FIG. 2. The charge amplifier 130 may be an electronic device configured as integrator with high input impedances. The charge amplifier 130 may be configured to convert charge into voltage. The high input impedances may prevent leakage loss. The charge amplifier may comprise an operational amplifier 134. The charge amplifier 130 may comprise at least one capacitor C.sub.F in a feedback path. The capacitor C.sub.F in the feedback path may be configured for accumulating current over time. The transimpedance amplifier 132 may be an electronic device comprising the at least one operational amplifier 134 and a resistor R.sub.F in the feedback path. The transimpedance amplifier 132 may be configured for multiplying an input current with the resistance R.sub.F. The transimpedance amplifier 132 may be configured for increasing the input and for converting the input current into voltage.

[0082] Charge amplifiers 130 and transimpedance amplifiers 132 are well known circuits used to measure charge and current. A direct connection of the photoconductor 112 to the charge amplifier 130 or transimpedance amplifier 132 is not feasible due to large dark current of the photoconductor 112. The amplifiers dynamic range would be severely limited due to the small ratio of output current of the photoconductor to dark current. As a result, these circuits have not been explored for the use with photoconductors. The photoconductor readout circuit 116 according to the present invention allows that charge amplifiers 130 and transimpedance amplifiers 132 can be used with sensors, in particular photoconductors 112, that exhibit large dark currents. The capacitor 126, as outlined above, may be placed between the amplifiers input and the voltage divider output. The capacitor 126 may serve to block the dark current and shunt the small output current of the photoconductor 112 to the amplifier device 128.

[0083] Without wishing to be bound by theory, an output voltage v.sub.o of the charge amplifier 130 or transimpedance amplifier 132 may be determined as follows. With the capacitor 126, the charge amplifier 130 and the transimpedance amplifier 132 may be regarded as voltage devices and a Fourier steady state is assumed. A transfer function of the charge amplifier may be

[00003]$.Math.\frac{\Delta v_o}{v_a}.Math.=\mathrm{\tau \omega }\frac{C_b}{C_F},$

wherein v.sub.a is an input voltage of the charge amplifier 130, τ is the integration time of the charge amplifier 130 and ω is the modulation frequency of the illumination. Similarly, the relationship between the output voltage v.sub.o of the transimpedance amplifier 132 and the input voltage v.sub.a of the transimpedance amplifier 132 can be determined by

[00004]$.Math.\frac{\Delta v_o}{v_a}.Math.=\omega \frac{C_b}{R_F}.$

[0084] FIG. 3 shows an embodiment of a resistive transducer 136 comprising at least one device 110 according to the present invention, as described with respect to FIGS. 1 and 2. In addition, to the embodiments shown in FIGS. 1 and 2, the photoconductor readout circuit 116 may comprise at least one diode 138 arranged between the capacitor C.sub.B and the amplifier device 128. The diode 138 may be configured as a protective diode that protects amplification from voltage peaks. A cathode of the diode 138 may be connected to ground and an anode with the capacitor C.sub.B. The diode 138 may be configured for conducting current to ground and to protect current flow in the other direction. The use of a diode 138 may significantly accelerate the transient oscillation time at start up. The diode 138 may be or may comprise a TVS diode. The diode 138 may be configured for suppressing transient voltages. The diode 138 may be designed such that it does not functionally impact the circuit. The diode 138 may be used to protect the circuit from damage in the event of a transient voltage. The protection diode, such as a TVS ESD diode, is generally either a part of the read-out integrated circuit or a separate diode included for extra protection. Other mechanisms to protect the input are possible.

[0085] As outlined above, the device 110 may comprise a plurality of photoconductors 112 such as arranged in an array. The photoconductor readout circuit 116 may be configured for determining electrical resistance of each photoconductor 112 of the plurality of photoconductors 112. The device 110 may comprise at least one sample and hold circuit 139 and at least one multiplexer 140. The sample and hold circuit 139 may be configured for sampling voltage and for holding the value of the voltage at a constant level for a certain time period. The sample and hold circuit 139 may comprise at least one capacitor configured for storing electric charge. The sample and hold circuit 139 may comprise a switch 142 in parallel to the amplifier device 128 in order to discharge the circuit. If the switch 142 is closed, the capacitor may be charged over the amplifier device 128. When opening the switch 142, the capacitor may hold the voltage at a constant value which was present before opening of the switch. In principle it may be possible to perform the discharge via a resistor in parallel to the protective diode. However, the discharge via the sample and hold circuit may be much faster than through a resistor.

[0086] The device 110 may further comprise at least one analog-to-digital converter (ADC) 144 configured to converts an output signal of the photoconductor readout circuit 116 into a digital signal, specifically for further evaluation. In case of the device 110 comprises a plurality of photoconductors 112 and corresponding reference resistors 120, the device 110 may comprise for each pair of photoconductors 112 and corresponding reference resistors 120 at least one ADC 144. However, other arrangements are feasible.

[0087] The resistive transducer 136 furthermore comprises at least one evaluation device 146 adapted to determine a voltage output signal at at least one voltage output of the photoconductor readout circuit 116. The evaluation device 146 may be or may comprise one or more integrated circuits, such as one or more application-specific integrated circuits (ASICs), and/or one or more data processing devices, such as one or more computers, preferably one or more microcomputers and/or microcontrollers. Additional components may be comprised, such as one or more preprocessing devices and/or data acquisition devices, such as one or more devices for receiving and/or preprocessing of the voltage signal, such as one or more AD-converters and/or one or more filters. Further, the evaluation device 146 may comprise one or more data storage devices. Further, as outlined above, the evaluation device 146 may comprise one or more interfaces, such as one or more wireless interfaces and/or one or more wire-bound interfaces. The evaluation device 146 may particularly comprise at least one data processing device, in particular an electronic data processing device, which can be designed to determine at least one output voltage signal. The evaluation device 146 can also be designed to completely or partly control the at least one illumination source and/or to control the at least one voltage source and/or to adjust the at least one load resistor. The evaluation device 146 may further comprise one or more additional components, such as one or more electronic hardware components and/or one or more software components, such as one or more measurement units and/or one or more evaluation units and/or one or more controlling units.

[0088] As shown in FIG. 4, the device 110 may comprise at least one read-out integrated circuit (ROIC) 148. The reference resistor R.sub.ref and the capacitor C.sub.B may not form part of the ROIC 148 due to space constraints. In particular, large capacitors and resistors require a large area in the integrated circuit and may be prohibitively expensive. The ROIC 148 may comprise specific technology blocks like the ADC, multiplexer, sample and hold circuit. The ROIC 148 may comprise means for the dark signal cancellation, in particular filtering, and an amplification stage as well as means to access the output signal of each input. The photoconductor readout circuit 116 may be designed as at least one integrated circuit. The integrated circuit may comprise the amplifier device 128 and/or the sample and hold circuit 139 and/or the multiplexer 140. The integrated circuit may furthermore comprise the diode 138. The integrated circuit may furthermore comprise the at least one ADC. For example, the integrated circuit may be embodied as a microchip.

[0089] FIGS. 5A and 5B show experimental results of amplified output voltages, denoted ADC output in V, in response to a modulated electromagnetic signal of known intensity as a function of sample N, wherein N is a sample number from the ADC. A set number of samples N was read from the ADC at fixed time intervals between each sample. For the experimental setup, a light source with a power density of P.sub.d=10.1 μW/cm.sup.2 was used. The photoconductor readout circuit 116 comprising the charge amplifier 130 has been tested with several commercially available integrated circuits (ICs) under various conditions. Various capacitors C.sub.B, reference resistors R.sub.ref, integration times T and feedback capacitors C.sub.F have been tested to find the optimal signal to noise ratio and dynamic signal response of the system. FIG. 5A shows experimental results for C.sub.B=3.3 nF, R.sub.ref=1.2 MΩ and C.sub.F=3.0 pF. FIG. 5B shows experimental results for C.sub.B=3.3 nF, R.sub.ref=2.4 MΩ and C.sub.F=3.0 pF. For both Figures, the modulated electromagnetic signal has a frequency of 60 Hz, the integration time r was 640 μs.

[0090] FIG. 6 shows experimental results of output voltage response of an ROIC, denoted ADC output in V, to an photoconductor array according to the present invention based on varying C.sub.B and R.sub.ref values. The ADC output as a function of R.sub.dark/R.sub.ref was plotted. In particular, a PbS-Array of 380 μm×38 μm was used, having a dark resistance of 15 MΩ. For the experimental setup, a light source with a power density of P.sub.d=20 ρW/cm.sup.2 was used. For both FIG. 6, the modulated electromagnetic signal has a frequency of 16 Hz, the integration time T was 1000 μs. The bias voltage was 10 V and C.sub.F was 25.0 pF. Curve 150 shows the ADC output for C.sub.B=(10.0 R.sub.darkR.sub.ref)/(R.sub.dark+R.sub.ref). Curve 152 shows the ADC output for C.sub.B=(1.0 R.sub.darkR.sub.ref)/(R.sub.dark+R.sub.ref). Curve 154 shows the ADC output for C.sub.B=(0.1 R.sub.darkR.sub.ref)/(R.sub.dark+R.sub.ref).

LIST OF REFERENCE NUMBERS

[0091] 110 device [0092] 112 photoconductor [0093] 114 illumination source [0094] 116 photoconductor readout circuit [0095] 118 voltage divider circuit [0096] 120 reference resistor [0097] 122 dark reference resistor [0098] 124 bias voltage source [0099] 126 capacitor [0100] 128 amplifier device [0101] 130 charge amplifier [0102] 132 transimpedance amplifier [0103] 134 operational amplifier [0104] 136 resistive transducer [0105] 138 diode [0106] 139 sample and hold circuit [0107] 140 multiplexer [0108] 142 switch [0109] 144 analog-to-digital converter [0110] 146 evaluation device [0111] 148 read-out integrated circuit [0112] 150 curve [0113] 152 curve [0114] 154 curve