Optical detection circuit and optical detection device

Abstract

To detect highly accurately a signal component caused by input light to a photodiode even when a dark current generating in the photodiode increases.SOLUTION: An optical detection circuit 3 comprises: a photodiode 4 that has an optical absorption layer composed of InAsSb; a first wiring 91 that applies a reverse bias voltage Vb to the photodiode 4; a TIA 5 that has a first input terminal 51a to which a reference voltage Va is input, and a second input terminal 51b that is connected to the photodiode 4; an error amplifier 6 that has a first input terminal 6a that is connected to the first input terminal 51a or second input terminal 51b of the TIA 5, and a second input terminal 6b that is connected to an output terminal 51c of the TIA 5, in which a cycle corresponding to a maximum frequency in a frequency band is larger than a pulse width of cyclical pulse light to be input to the photodiode 4,; and a current extraction circuit 7 that extracts a current from the photodiode 4 in accordance with an output voltage from the error amplifier 6.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a photodetection circuit and a photodetection device. [Background technology]

[0002] Patent Document 1 discloses a method for suppressing dark current occurring in a photodiode containing In and Sb by applying zero bias between the electrodes of the photodiode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2008-103742 A Summary of the Invention [Problem to be solved by the invention]

[0004] Conventionally, charge amplifier and transimpedance amplifier readout circuits are known as readout circuits that convert the current output from a photodiode into a voltage signal. These methods are mainly used for photodiodes that are sensitive to wavelengths less than 3.0 μm (e.g., those with an InGaAs light absorption layer). Photodiodes that are sensitive to wavelengths less than 3.0 μm have a parallel resistance of several MΩ or more, which allows the amplifier gain to be large. Even if the parallel resistance is several MΩ or more, the effect of dark current cannot be ignored when a reverse bias is applied, so a circuit configuration that operates at zero bias is desirable.

[0005] On the other hand, photodiodes having sensitivity to wavelengths of 3.0 μm or more include, for example, those having an InAsSb light absorption layer. The parallel resistance of such photodiodes having sensitivity to relatively long wavelengths is about several tens of Ω to several kΩ, and it is difficult to increase the gain of the amplifier. Therefore, it is desired to increase the sensitivity of the photodiode itself. For example, the sensitivity can be increased by applying a reverse bias to a photodiode having an InAsSb light absorption layer. However, when a reverse bias is applied to the photodiode, the dark current generated in the photodiode increases. When the dark current increases, the proportion of the stationary component caused by the dark current in the output voltage from the readout circuit increases, making it difficult to accurately detect the signal component caused by the input light to the photodiode.

[0006] An object of the present disclosure is to provide a photodetection circuit and a photodetection device that are capable of accurately detecting a signal component resulting from light input to a photodiode even if dark current generated in the photodiode increases. [Means for solving the problem]

[0007] [1] A photodetection circuit according to the present disclosure includes a photodiode having a light absorption layer made of InAsSb, a first wiring connected to the cathode of the photodiode and applying a reverse bias voltage to the photodiode, a transimpedance amplifier having a first input terminal to which a reference voltage is input and a second input terminal connected to the anode of the photodiode, an error amplifier having a first input terminal connected to the first input terminal or the second input terminal of the transimpedance amplifier and a second input terminal connected to the output terminal of the transimpedance amplifier, and having a period corresponding to a maximum frequency in a frequency band that is greater than the pulse width of periodic pulsed light input to the photodiode, and a current extraction circuit connected between the anode of the photodiode and a second wiring having a lower potential than the first wiring and extracting a current from the photodiode in accordance with the output voltage from the error amplifier.

[0008] [2] Another photodetection circuit according to the present disclosure includes a photodiode having light sensitivity to wavelengths of 3.0 μm or more, a first wiring connected to the cathode of the photodiode and applying a reverse bias voltage to the photodiode, a transimpedance amplifier having a first input terminal to which a reference voltage is input and a second input terminal connected to the anode of the photodiode, an error amplifier having a first input terminal connected to the first input terminal or the second input terminal of the transimpedance amplifier and a second input terminal connected to the output terminal of the transimpedance amplifier, and having a period corresponding to a maximum frequency in a frequency band that is greater than the pulse width of periodic pulsed light input to the photodiode, and a current extraction circuit connected between the anode of the photodiode and a second wiring at a lower potential than the first wiring and extracting a current from the photodiode in accordance with the output voltage from the error amplifier.

[0009] When a periodic pulse light is incident on the photodiode of the photodetection circuit of [1] and [2] above, the periodic pulse signal generated in the photodiode is input to the transimpedance amplifier and converted into a periodic pulse signal component of the output voltage from the transimpedance amplifier. Since the pulse width of this pulse signal component (i.e., the pulse width of the pulse light input to the photodiode) is smaller than the period corresponding to the maximum frequency in the frequency band of the error amplifier, the amplification of this pulse signal component by the error amplifier is suppressed. On the other hand, since the photodiode has a light absorption layer made of InAsSb or has light sensitivity to wavelengths of 3.0 μm or more, a large dark current is generated in the photodiode by the reverse bias voltage applied from the first wiring. This dark current is also input to the transimpedance amplifier and converted into a steady-state component of the output voltage from the transimpedance amplifier. This steady-state component is amplified by the error amplifier and input to the current sink circuit. The current sink circuit sinks a current corresponding to the magnitude of the steady-state component, i.e., the dark current, from the anode of the photodiode. As a result of such feedback operation, the dark current flowing from the photodiode to the transimpedance amplifier is reduced. Therefore, according to the photodetection circuits of [1] and [2] above, even if the dark current generated in the photodiode increases, it is possible to accurately detect the pulse signal component caused by the light input to the photodiode.

[0010] [3] In the photodetection circuit of [2] above, the photodiode may have a light receiving sensitivity to wavelengths of 5.0 μm or more. In this case, the dark current caused by application of a reverse bias voltage increases further. Therefore, the configuration of the photodetection circuit of [2] above becomes even more effective.

[0011] [4] In any of the photodetection circuits [1] to [3] above, the current sink circuit may have a first transistor including a control terminal connected to the output terminal of the error amplifier, a first current terminal connected to the anode of the photodiode, and a second current terminal connected to the second wiring. In this case, the current sink circuit can be easily configured.

[0012] [5] In the photodetection circuit of [4] above, the current sink circuit may further include a second transistor connected in series with the first transistor and having a diode-connected current terminal and control terminal. In comparison with a photodiode (e.g., having an InGaAs light absorption layer) having sensitivity to wavelengths less than 3.0 μm, in any of the photodetection circuits of [1] to [4] above, the dark current of the photodiode when a reverse bias voltage is applied is significantly larger. According to the knowledge of the present inventor, the dark current of a photodiode having an InAsSb light absorption layer is 1000 times or more larger than the dark current of a photodiode having an InGaAs light absorption layer. If such a large current is to be extracted by only a single first transistor, the size of the first transistor will be large. By connecting a second transistor having a diode-connected current terminal and control terminal in series with the first transistor, the size of the entire transistor circuit can be made smaller than the case of only a single first transistor.

[0013] [6] Any of the photodetection circuits [1] to [5] above may further include a smoothing capacitor connected to a node between the output terminal of the error amplifier and the current sink circuit. In this case, periodic signal components contained in the output voltage from the error amplifier can be further reduced, making it possible to accurately sink only the dark current.

[0014] [7] Any of the photodetection circuits of [1] to [6] above may further include a switch. One end of the switch is connected to the node, and the other end of the switch is connected to the output terminal of the error amplifier. The switch switches the connection state between the node and the output terminal of the error amplifier. When the switch is in the connected state, the photodetection circuit can achieve the above-mentioned effect. Even after the switch is switched from the connected state to the disconnected state, the smoothing capacitor maintains the voltage input to the current sink circuit, or the gate voltage is maintained by the gate capacitance of the transistor constituting the current sink circuit, so that the current sink circuit continues to sink the dark current, which is a stationary component, from the photodiode. Furthermore, since the switch separates the error amplifier from the current sink circuit, even if a signal component having a pulse width larger than a period corresponding to the maximum frequency in the frequency band of the error amplifier is output from the photodiode, the current sink circuit does not sink the signal current from the photodiode, and the signal current is amplified by the transimpedance amplifier. In this way, according to the photodetection circuit of [7] above, by switching the switch as necessary, it is possible to amplify signal components with large pulse widths in addition to signal components with small pulse widths.

[0015] [8] A photodetection device according to the present disclosure includes a light source that outputs periodic pulsed light, and any one of the photodetection circuits [1] to [7] above. The pulsed light from the light source is input to a photodiode. The period corresponding to the maximum frequency in the frequency band of the error amplifier is greater than the pulse width of the pulsed light. According to this photodetection device, by including any one of the photodetection circuits [1] to [7] above, it is possible to accurately detect a signal component resulting from the light input to the photodiode even if the dark current generated in the photodiode increases. Effect of the Invention

[0016] According to the present disclosure, it is possible to provide a photodetection circuit and a photodetection device that can accurately detect a signal component resulting from light input to a photodiode even if the dark current generated in the photodiode increases. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram illustrating a schematic configuration of a light detection device according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a circuit diagram showing a configuration of the photodetection circuit. [Diagram 3] FIG. 3 is a diagram illustrating an example of a laminated structure of a photodiode. [Figure 4] Fig. 4(a) is a graph showing the relationship between the magnitude of the reverse bias voltage and the parallel resistance of a photodiode having the structure shown in Fig. 3. Fig. 4(b) is a graph showing the relationship between the magnitude of the reverse bias voltage and the light receiving sensitivity of a photodiode having the structure shown in Fig. 3. [Diagram 5] FIG. 5 is a circuit diagram of a photodetection circuit including a specific configuration example of a current sink circuit. [Figure 6] FIG. 6 is a circuit diagram showing a conventional photodetection circuit using a charge amplifier system. [Figure 7] FIG. 7 is a circuit diagram showing a photodetection circuit of the TIA type. [Figure 8] FIG. 8 is a graph showing a simulation result regarding an output voltage waveform from an amplifier when a reverse bias voltage is applied to a photodiode in a TIA type photodetection circuit. [Figure 9] FIG. 9 is a graph showing the results of measuring the output voltage from the TIA when the photodetector circuit of this embodiment is designed and fabricated as a prototype. [Figure 10] FIG. 10 is a circuit diagram showing a configuration of a photodetection circuit according to one modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, embodiments of a photodetection circuit and a photodetection device according to the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals and duplicated descriptions will be omitted.

[0019] FIG. 1 is a diagram illustrating a schematic configuration of a photodetector 1 according to an embodiment of the present disclosure. As illustrated in FIG. 1, the photodetector 1 of this embodiment includes a light source 2 that outputs periodic pulsed light L, and a photodetection circuit 3 that detects the pulsed light L. The pulsed light L output from the light source 2 is mid-infrared light. The wavelength of the pulsed light L is, for example, 3.0 μm or more, 5.0 μm or more, or 7.0 μm or more. The wavelength of the pulsed light L is, for example, 10 μm or less. The pulse width (full width at half maximum) of the pulsed light L is, for example, 50 nanoseconds or more and 1 microsecond or less, and is 100 nanoseconds in one embodiment. The repetition frequency of the pulsed light L is, for example, 10 kHz or more and 1 MHz or less, and is 500 kHz in one embodiment. The photodetector 1 may have a semiconductor laser element that outputs the pulsed light L.

[0020] Fig. 2 is a circuit diagram showing a configuration of the photodetection circuit 3. As shown in Fig. 2, the photodetection circuit 3 includes a photodiode 4, a transimpedance amplifier (TIA) 5, an error amplifier 6, a current sink circuit 7, a smoothing capacitor 8, a first wiring 91, and a second wiring 92.

[0021] The photodiode 4 receives the pulsed light L from the light source 2. The photodiode 4 has a light sensitivity to the wavelength of the pulsed light L, which is mid-infrared light output from the light source 2. That is, the photodiode 4 has a light sensitivity to wavelengths of, for example, 3.0 μm or more, 5.0 μm or more, or 7.0 μm or more. The photodiode 4 also has a light sensitivity to wavelengths of, for example, 10 μm or less. Therefore, the photodiode 4 contains In, As, and Sb in the light absorption layer. The photodiode 4 may have a light sensitivity to wavelengths of 5 μm or more. The photodiode 4 receives the pulsed light L and outputs a signal current J1 having a magnitude corresponding to the light intensity of the pulsed light L.

[0022] FIG. 3 is a diagram showing a schematic example of a laminated structure of the photodiode 4. In the example shown in FIG. 3, the photodiode 4 includes a semiconductor substrate 41, a buffer layer 42, an n-type semiconductor layer 43, a light absorbing layer 44, and a p-type semiconductor layer 45. The semiconductor substrate 41 is, for example, a GaAs substrate. The buffer layer 42 is provided on a major surface 41a of the semiconductor substrate 41. The buffer layer 42 is made of, for example, n-type InSb. The n-type semiconductor layer 43 is provided on the buffer layer 42. The n-type semiconductor layer 43 is made of, for example, n-type InAsSb. The light absorbing layer 44 is provided on the n-type semiconductor layer 43. The light absorbing layer 44 is made of, for example, undoped InAsSb. The p-type semiconductor layer 45 is provided on the light absorbing layer 44. The p-type semiconductor layer 45 is made of, for example, p-type InAsSb. The band gap of the light absorbing layer 44 is smaller than the band gap of the n-type semiconductor layer 43 or the p-type semiconductor layer 45 .

[0023] The n-type semiconductor layer 43, the light absorption layer 44, and the p-type semiconductor layer 45 configure a mesa structure 46. A trench 47 for insulation is formed around the mesa structure 46 in a plan view. The depth of the trench 47 reaches from the n-type semiconductor layer 43 to the semiconductor substrate 41. A cathode electrode (not shown) is provided on the exposed surface of the n-type semiconductor layer 43, and the cathode electrode is in ohmic contact with the n-type semiconductor layer 43. An anode electrode (not shown) is provided on the exposed surface of the p-type semiconductor layer 45, and the anode electrode is in ohmic contact with the p-type semiconductor layer 45.

[0024] Referring again to FIG. 2, the cathode of the photodiode 4 is connected to the first wiring 91. The first wiring 91 applies a reverse bias voltage Vb to the photodiode 4. The photodiode 4 further outputs a dark current J2 caused by the reverse bias voltage Vb. FIG. 4(a) is a graph showing the relationship between the magnitude of the reverse bias voltage Vb and the parallel resistance of the photodiode 4 having the structure shown in FIG. 3. FIG. 4(b) is a graph showing the relationship between the magnitude of the reverse bias voltage Vb and the light receiving sensitivity (InAsSb relative sensitivity) of the photodiode 4 having the structure shown in FIG. 3. In FIG. 4(a), plot P1 shows the parallel resistance at -10°C, plot P2 shows the parallel resistance at 0°C, and plot P3 shows the parallel resistance at 23.6°C. As shown in these figures, at any temperature, the parallel resistance increases as the absolute value of the reverse bias voltage increases from 0V, and the parallel resistance is maximum when a reverse bias voltage of approximately 0.1V is applied. When the temperature of the photodiode 4 is 23.6° C., the parallel resistance for a reverse bias voltage of 0.1 V is approximately 150 Ω. Therefore, the dark current J2 is J2=0.1 V / 150 Ω=0.67 mA.

[0025] The TIA 5 is connected to the anode of the photodiode 4 and converts the current output from the photodiode 4 into a voltage. The TIA 5 has an amplifier 51 and a feedback resistor 52. The amplifier 51 has a first input terminal 51a, a second input terminal 51b, and an output terminal 51c. In one example, the first input terminal 51a is a non-inverting input terminal, and the second input terminal 51b is an inverting input terminal. A reference voltage Va is input to the first input terminal 51a. The second input terminal 51b is connected to the anode of the photodiode 4. The feedback resistor 52 is connected between the second input terminal 51b and the output terminal 51c. The TIA 5 converts the current input to the second input terminal 51b into a voltage with a gain determined by the feedback resistor 52.

[0026] The error amplifier 6 has a first input terminal 6a which is a non-inverting input terminal, a second input terminal 6b which is an inverting input terminal, and an output terminal 6c. The error amplifier 6 amplifies a difference voltage between a voltage input to the first input terminal 6a and a voltage input to the second input terminal 6b, and outputs the amplified difference voltage from the output terminal 6c. The first input terminal 6a is connected to the first input terminal 51a or the second input terminal 51b of the amplifier 51. The second input terminal 6b is connected to the output terminal 51c of the amplifier 51. Note that FIG. 2 shows an example in which the first input terminal 6a is connected to the second input terminal 51b of the amplifier 51. The period corresponding to the maximum frequency in the frequency band of the error amplifier 6 is greater than the pulse width of the pulse light L input to the photodiode 4. When the pulse width of the pulse light L is 100 nanoseconds, the frequency band of the error amplifier 6 is, for example, 17 kHz or less.

[0027] The current sink circuit 7 is connected between the anode of the photodiode 4 and the second wiring 92. The second wiring 92 is a wiring with a lower potential than the first wiring 91, for example, a reference potential line (GND line) of the light detection circuit 3. The current sink circuit 7 is further connected to the output terminal 6c of the error amplifier 6, and sinks a current from the photodiode 4 according to the output voltage from the error amplifier 6, and passes the current to the second wiring 92. The magnitude of the current sinked by the current sink circuit 7 is, for example, about 1 mA when the photodiode 4 has an InAsSb light absorption layer and the reverse bias voltage Vb is 0.1 V.

[0028] Fig. 5 is a circuit diagram of the photodetection circuit 3 including a specific configuration example of the current sink circuit 7. The current sink circuit 7 shown in Fig. 5 has a first transistor 71 and a second transistor 72. The first transistor 71 includes a control terminal 71a connected to the output terminal 6c of the error amplifier 6, a first current terminal 71b connected to the anode of the photodiode 4, and a second current terminal 71c connected to the second wiring 92 via the second transistor 72.

[0029] The second transistor 72 is connected in series with the first transistor 71 between the first transistor 71 and the second wiring 92. The second transistor 72 includes a control terminal 72a, a first current terminal 72b connected to the second current terminal 71c of the first transistor 71, and a second current terminal 72c connected to the second wiring 92. The control terminal 72a is diode-connected to the first current terminal 72b. The first transistor 71 and the second transistor 72 are, for example, MOSFETs.

[0030] The smoothing capacitor 8 is connected between a node N1 between the output terminal 6c of the error amplifier 6 and the current sink circuit 7, and the second wiring 92. That is, one electrode of the smoothing capacitor 8 is connected to the node N1, and the other electrode is connected to the second wiring 92. The smoothing capacitor 8 smoothes the output voltage from the error amplifier 6.

[0031] The effects obtained by the photodetector 1 and photodetection circuit 3 of this embodiment described above will be described together with the problems of the conventional photodetection circuit. As described above, the parallel resistance of the photodiode 4 sensitive to wavelengths of 3.0 μm or more is about several tens of Ω to several kΩ, making it difficult to increase the gain of the amplifier. Therefore, it is desirable to increase the sensitivity of the photodiode 4 itself. For example, the sensitivity can be increased by applying a reverse bias voltage Vb to the photodiode 4.

[0032] 6 is a circuit diagram showing a conventional charge amplifier type photodetection circuit 100A. In the photodetection circuit 100A, a bias voltage Vb1 is applied to the cathode of a photodiode 104, and the bias voltage Vb1 is input to a non-inverting input terminal 105a of an amplifier 105. A signal current J3 output from the anode of the photodiode 104 is input to an inverting input terminal 105b of the amplifier 105. A capacitor 106 is connected between the inverting input terminal 105b and an output terminal 105c of the amplifier 105. Furthermore, a switch 107 for resetting the charge accumulated in the capacitor 106 is connected in parallel with the capacitor 106.

[0033] In this way, the charge amplifier method employs a circuit configuration (auto-zero method) for minimizing the reverse bias voltage applied to the photodiode 104. Therefore, applying a reverse bias voltage to the photodiode 104 deviates from the purpose of the charge amplifier method. In addition, since the gain of the amplifier 105 is large in the charge amplifier method, a signal current from a photodiode having a small parallel resistance, for example, a photodiode having sensitivity to wavelengths longer than the near-infrared region, does not easily flow to the amplifier 105. Therefore, if the photodetection circuit 100A of the charge amplifier method is used with a photodiode having a small parallel resistance, the sensitivity of the entire photodetection circuit 100A decreases.

[0034] FIG. 7 is a circuit diagram showing a photodetection circuit 100B of the TIA system. In the photodetection circuit 100B, a feedback resistor 108 is provided instead of the capacitor 106 and the switch 107 of the photodetection circuit 100A of the charge amplifier system. According to the TIA system, the gain of the amplifier 105 is smaller than that of the charge amplifier system. In addition, the magnitude of the gain of the amplifier 105 can be set according to the magnitude of the parallel resistance of the photodiode 104. This makes it possible to read out the signal current from the photodiode with high sensitivity. However, when the parallel resistance of the photodiode 104 is small, if a reverse bias voltage is applied to the photodiode 104 in the photodetection circuit 100B, a large stationary component (offset component) is included in the output voltage from the amplifier 105 due to the dark current generated in the photodiode 104. Therefore, when a reverse bias voltage is applied to the photodiode 104, it is difficult to detect only the signal current in the conventional TIA system.

[0035] FIG. 8 is a graph showing a simulation result of an output voltage waveform from the amplifier 105 when a reverse bias voltage is applied to the photodiode 104 in the TIA-type photodetection circuit 100B. Here, InAsSb is assumed as the light absorption layer of the photodiode 104, the parallel resistance of the photodiode 104 is set to 100Ω, and the resistance value of the feedback resistor 108 is set to 1 kΩ. FIG. 8 shows a graph G11 when the reverse bias voltage is set to 0.00V, a graph G12 when the reverse bias voltage is set to 0.01V, a graph G13 when the reverse bias voltage is set to 0.05V, and a graph G14 when the reverse bias voltage is set to 0.1V. In these graphs G11 to G14, the pulse waveform PL is a pulse waveform assuming the incidence of pulsed light L. Referring to FIG. 8, it can be seen that the dark current of the photodiode 104 increases as the absolute value of the reverse bias voltage increases, and the stationary component of the output voltage from the amplifier 105 moves away from the reference voltage (2.5V). At a reverse bias of 0.1 V (graph G14), the steady-state component of the output voltage alone is 1.0 V, approaching the saturation voltage VT of the TIA. In this state, the top of the pulse waveform PL exceeds the saturation voltage VT of the TIA, and an appropriate pulse waveform PL cannot be obtained.

[0036] In response to the above problem, in the photodetection circuit 3 of this embodiment, when the periodic pulse light L is incident on the photodiode 4, the periodic signal current J1 generated in the photodiode 4 is input to the TIA 5 and converted into a periodic signal component of the output voltage from the TIA 5. Since the frequency of this signal component, i.e., the repetition frequency of the pulse light L input to the photodiode 4, is greater than the frequency band of the error amplifier 6, the signal component is prevented from being amplified by the error amplifier 6. On the other hand, since the photodiode 4 has a light absorption layer 44 made of InAsSb or has light sensitivity to wavelengths of 3 μm or more, a large dark current J2 is generated in the photodiode 4 by the reverse bias voltage Vb applied from the first wiring 91. The dark current J2 is also input to the TIA 5 and converted into a stationary component of the output voltage from the TIA 5. Since the frequency of this stationary component is included in the frequency band of the error amplifier 6, the stationary component is amplified by the error amplifier 6 and input to the current sink circuit 7. The current sink circuit 7 sinks a current according to the magnitude of the stationary component, i.e., the dark current J2, from the anode of the photodiode 4. As a result of such feedback operation, the dark current J2 flowing from the photodiode 4 to the TIA 5 is reduced. Therefore, according to the photodetection circuit 3 of this embodiment, even if the dark current J2 generated in the photodiode 4 increases, it is possible to accurately detect the signal component caused by the pulsed light L input to the photodiode 4.

[0037] FIG. 9 is a graph showing the results of measuring the output voltage from the TIA 5 when the photodetection circuit 3 of this embodiment is designed and fabricated. In this prototype, a photodiode 4 having an InAsSb light absorption layer is used, and the resistance value of the feedback resistor 52 is set to 1 kΩ. FIG. 9 shows a graph G21 when the reverse bias voltage Vb is 0.01 V, a graph G22 when the reverse bias voltage Vb is 0.10 V, a graph G23 when the reverse bias voltage Vb is 0.15 V, a graph G24 when the reverse bias voltage Vb is 0.20 V, and a graph G25 when the reverse bias voltage Vb is 0.25 V. In the figure, a graph G26 shows the trigger signal waveform, and a double-headed arrow W shows the irradiation period (pulse width 100 nanoseconds) of the pulsed light L. Referring to FIG. 9, it can be seen that the steady component of the output voltage (component when the pulsed light L is not incident) is constant at approximately 2.5 V, regardless of the magnitude of the reverse bias voltage Vb, due to the action of the current extraction circuit 7. Furthermore, referring to FIG. 9, it can be seen that the sensitivity of the photodetection circuit 3 improves as the reverse bias voltage Vb increases.

[0038] The suitable resistance value of the feedback resistor 52 depends on the magnitude of the parallel resistance of the photodiode 4. The photodiode 4 of this embodiment has a light receiving sensitivity at wavelengths of 3 μm or more (or has an InAsSb light absorbing layer). The parallel resistance value of the photodiode 4 also depends on the PN junction area (pixel size), but the parallel resistance value of the photodiode 4 having such a light receiving sensitivity is at most about 300Ω (see FIG. 4). When confirmed by simulation, if the feedback resistor 52 is 1 kΩ or less, all of the signal current from the photodiode 4 flows to the feedback resistor 52, but if the feedback resistor 52 is 2 kΩ or more, the signal current flowing to the feedback resistor 52 decreases. Therefore, if the photodiode 4 has a light receiving sensitivity at wavelengths of 3 μm or more (or has an InAsSb light absorbing layer), the feedback resistor 52 should be less than 2 kΩ (for example, 1 kΩ).

[0039] As described above, the photodiode 4 may have light sensitivity to wavelengths of 5 μm or more. In this case, the dark current J2 caused by application of the reverse bias voltage Vb increases further. Therefore, the configuration of the photodetection circuit 3 of this embodiment becomes even more effective.

[0040] As in the present embodiment, the current sink circuit 7 may have a first transistor 71 including a control terminal 71a connected to the output terminal 6c of the error amplifier 6, a first current terminal 71b connected to the anode of the photodiode 4, and a second current terminal 71c connected to the second wiring 92. In this case, the current sink circuit 7 can be easily configured.

[0041] As in this embodiment, the current sink circuit 7 may further include a second transistor 72 connected in series with the first transistor 71, with the first current terminal 72b and the control terminal 72a being diode-connected. Compared with a photodiode having sensitivity to wavelengths less than 3.0 μm (e.g., one having an InGaAs light absorption layer), the photodiode 4 of this embodiment having sensitivity to wavelengths of 3.0 μm or more (or having an InAsSb light absorption layer) generates a significantly large dark current J2 when a reverse bias voltage Vb is applied. According to the knowledge of the inventor, the dark current J2 of the photodiode 4 having an InAsSb light absorption layer is 1000 times or more the dark current of a photodiode having an InGaAs light absorption layer. If such a large current is to be extracted by only a single first transistor 71, the size of the first transistor 71 will be large. By connecting the second transistor 72, whose first current terminal 72b and control terminal 72a are diode-connected, in series with the first transistor 71, the overall dimensions of the transistor circuit can be made smaller than in the case of using only a single first transistor 71.

[0042] As in this embodiment, the photodetection circuit 3 may further include a smoothing capacitor 8 connected to a node N1 between the output terminal 6c of the error amplifier 6 and the current sink circuit 7. In this case, the periodic signal component contained in the output voltage from the error amplifier 6 can be further reduced, and only the dark current J2 can be accurately extracted.

[0043] [Variations] FIG. 10 is a circuit diagram showing a configuration of a photodetection circuit 3A according to a modification of the above embodiment. The photodetection circuit 3A of this modification further includes a switch 10 in addition to the configuration of the photodetection circuit 3 of the above embodiment. One end of the switch 10 is connected to a node N1. The other end of the switch 10 is connected to an output terminal 6c of the error amplifier 6. The switch 10 switches a connection state between the node N1 and the output terminal 6c of the error amplifier 6. That is, when the switch 10 is in a connected state, the output terminal 6c of the error amplifier 6 and the node N1 are electrically connected. When the switch 10 is in a non-connected state, the output terminal 6c of the error amplifier 6 and the node N1 are electrically separated. The switch 10 may be a mechanical switch such as an electromagnetic relay, or a semiconductor switch such as a transistor.

[0044] When the switch 10 is in a connected state, the photodetection circuit 3A performs the same operation as the photodetection circuit 3 of the above embodiment, and can achieve the same effect as the photodetection circuit 3. At this time, the smoothing capacitor 8 is charged by the output voltage from the error amplifier 6, so that the voltage across the smoothing capacitor 8 is equal to the output voltage from the error amplifier 6. After that, the switch 10 is switched from a connected state to a disconnected state. After that, the voltage across the smoothing capacitor 8, which is equal to the output voltage from the error amplifier 6 at the time of switching the switch 10, continues to be input to the current sink circuit 7. Therefore, the current sink circuit 7 continues to sink the dark current J2, which is a stationary component, from the photodiode 4. Note that the smoothing capacitor 8 is not an essential element, and similar operation is possible even if another element that stores charge is provided. For example, the gate voltage is maintained by the gate capacitance of the transistor 71 (see FIG. 5) constituting the current sink circuit 7 storing charge, so that the current sink circuit 7 continues to sink the dark current J2, which is a stationary component, from the photodiode 4, even when the switch 10 is in a disconnected state. Furthermore, because switch 10 isolates error amplifier 6 from current sink circuit 7, even if signal current J1 having a pulse width greater than a period corresponding to the maximum frequency in the frequency band of error amplifier 6 is output from photodiode 4, signal current J1 is amplified by TIA 5 without being acted upon by current sink circuit 7. In this way, according to photodetection circuit 3A of this modification, signal current J1 with a large pulse width can be amplified in addition to signal current J1 with a small pulse width by switching switch 10 as necessary.

[0045] The photodetection circuit and photodetection device according to the present disclosure are not limited to the above-mentioned embodiment, and various other modifications are possible. For example, the photodiode having an InAsSb light absorption layer is exemplified in the above-mentioned embodiment, but the light absorption layer of the photodiode having light sensitivity at wavelengths of 3.0 μm or more may be made of InAs or InSb. In addition, if the signal component of the pulsed light L is sufficiently reduced in the error amplifier 6, the smoothing capacitor 8 can be omitted.

[0046] In the above embodiment, the current sink circuit has been exemplified as having the first transistor 71 and the second transistor 72, but the current sink circuit may have only the single first transistor 71. In addition, the current sink circuit can have various configurations, not limited to the configuration shown in Fig. 5, as long as it has the function of sinking a current from the photodiode in response to the output voltage from the error amplifier. [Explanation of symbols]

[0047] 1...photodetector, 2...light source, 3,3A...photodetector circuit, 4...photodiode, 5...transimpedance amplifier (TIA), 6...error amplifier, 6a...first input terminal, 6b...second input terminal, 6c...output terminal, 7...current sink circuit, 8...smoothing capacitor, 10...switch, 41...semiconductor substrate, 42...buffer layer, 43...n-type semiconductor layer, 44...light absorption layer, 45...p-type semiconductor layer, 46...mesa structure, 47...trench, 51...amplifier, 51a...first input terminal, 51b...second input terminal, 51c...output terminal, 52...feedback resistor, 71...first transistor, 71a...control terminal, 71b ...first current terminal, 71c...second current terminal, 72...second transistor, 72a...control terminal, 72b...first current terminal, 72c...second current terminal, 91...first wiring, 92...second wiring, 100A, 100B...light detection circuit, 104...photodiode, 105...amplifier, 105a...non-inverting input terminal, 105b...inverting input terminal, 105c...output terminal, 106...capacitor, 107...switch, 108...feedback resistor, J1, J3...signal current, J2...dark current, L...pulsed light, N1...node, PL...pulse waveform, Va...reference voltage, Vb...reverse bias voltage, Vb1...bias voltage, VT...saturation voltage.

Claims

1. a photodiode having a light absorption layer which is an InAsSb layer, an InAs layer, or an InSb layer, and having a cathode and an anode; a first wiring connected to the cathode of the photodiode and applying a reverse bias voltage to the photodiode; a second wiring having a lower potential than the first wiring; a transimpedance amplifier having a first input terminal to which a reference voltage is input, a second input terminal connected to the anode of the photodiode, and an output terminal; an error amplifier having a first input terminal connected to the first input terminal or the second input terminal of the transimpedance amplifier, a second input terminal connected to the output terminal of the transimpedance amplifier, and an output terminal, the period corresponding to the maximum frequency in a frequency band being greater than the pulse width of periodic pulsed light input to the light absorption layer of the photodiode; a current sink circuit connected between the anode of the photodiode and the second wiring, and sinking a current from the photodiode in accordance with an output voltage from the output terminal of the error amplifier; A photodetection circuit comprising:

2. A photodiode having a light absorption layer, a cathode, and an anode, and having light sensitivity at wavelengths of 3.0 μm or more; a first wiring connected to the cathode of the photodiode and applying a reverse bias voltage to the photodiode; a second wiring having a lower potential than the first wiring; a transimpedance amplifier having a first input terminal to which a reference voltage is input, a second input terminal connected to the anode of the photodiode, and an output terminal; an error amplifier having a first input terminal connected to the first input terminal or the second input terminal of the transimpedance amplifier, a second input terminal connected to the output terminal of the transimpedance amplifier, and an output terminal, the period corresponding to the maximum frequency in a frequency band being greater than the pulse width of periodic pulsed light input to the light absorption layer of the photodiode; a current sink circuit connected between the anode of the photodiode and the second wiring, and sinking a current from the photodiode in accordance with an output voltage from the output terminal of the error amplifier; A photodetection circuit comprising:

3. 3. The photodetector circuit according to claim 2, wherein said photodiode has sensitivity to light having a wavelength of 5.0 [mu]m or greater.

4. A photodetection circuit as described in claim 2 or 3, wherein the light absorption layer contains In, As and Sb.

5. 4. The photodetection circuit according to claim 1, wherein the current sink circuit has a first transistor including a control terminal connected to the output terminal of the error amplifier, a first current terminal connected to the anode of the photodiode, and a second current terminal connected to the second wiring.

6. 6. The photodetector circuit according to claim 5, wherein the current sink circuit further comprises a second transistor connected in series with the first transistor, the second transistor having a current terminal and a control terminal, the current terminal and the control terminal being diode-connected.

7. 4. The photodetection circuit according to claim 1, further comprising a smoothing capacitor connected to a node between the output terminal of the error amplifier and the current sink circuit.

8. An optical detection circuit as described in claim 7, having one end and another end, the one end connected to the node and the other end connected to the output terminal of the error amplifier, and further comprising a switch that switches the connection state between the node and the output terminal of the error amplifier.

9. A photodetection circuit as described in any one of claims 1 to 3, having one end and another end, the one end connected to the current sink circuit and the other end connected to the output terminal of the error amplifier, and further comprising a switch that switches the connection state between the current sink circuit and the output terminal of the error amplifier.

10. A photodetection circuit described in any one of claims 1 to 3, wherein the second wiring is a reference potential line of the photodetection circuit.

11. A photodetector circuit according to any one of claims 1 to 3; a light source that outputs the periodic pulsed light; Equipped with the periodic pulsed light from the light source is input to the light absorption layer of the photodiode; A photodetector, wherein a period corresponding to a maximum frequency in a frequency band of the error amplifier is greater than a pulse width of the periodic pulsed light.

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