Light detection circuit and light detection device

Abstract

A light detection circuit according to the present invention is provided with: a photodiode having a light absorption layer that is an InAsSb layer; a first wiring that applies a reverse bias voltage to the photodiode; a TIA having a first input terminal to which a reference voltage is input, and a second input terminal connected to the photodiode; an error amplifier having a first input terminal connected to the first input terminal or the second input terminal of the TIA, and a second input terminal connected to the output terminal of the TIA, the error amplifier having a period corresponding to the maximum frequency in the frequency band greater than the pulse width of the periodic pulsed light input to the photodiode; and a current extraction circuit that extracts a current from the photodiode on the basis of the output voltage from the error amplifier.

Description

Technical Field

[0001] The present disclosure relates to a light detection circuit and a light detection device. This application claims priority based on Japanese Application No. 2022-192150 filed on November 30, 2022, and incorporates all the descriptions recorded in the above-mentioned Japanese application. Background Art

[0002] Patent Document 1 discloses a method of suppressing dark current generated in a photodiode by setting a zero bias voltage between the electrodes of a photodiode containing In and Sb.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-103742. Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] Currently, as a readout circuit that converts the current output from a photodiode into a voltage signal, a charge amplifier type and a transimpedance amplifier type readout circuits are known. These methods are mainly used for photodiodes (e.g., photodiodes having an InGaAs light absorption layer) that are sensitive to wavelengths less than 3.0 μm. Photodiodes that are sensitive to wavelengths less than 3.0 μm have a parallel resistance of several MΩ or more, so the gain of the amplifier can be increased. Even if the photodiode has a parallel resistance of several MΩ or more, the influence of dark current cannot be ignored when a reverse bias voltage is applied to the photodiode. Therefore, a circuit structure that operates with a zero bias voltage is desired.

[0008] On the other hand, as a photodiode that is sensitive to wavelengths of 3.0 μm or more, for example, there is a photodiode having an InAsSb light absorption layer. The parallel resistance of such a photodiode that is sensitive to relatively long wavelengths is on the order of several tens of Ω to several kΩ, so it is difficult to increase the gain of the amplifier. Therefore, it is desired to improve the sensitivity of the photodiode itself. For example, the sensitivity can be improved by applying a reverse bias voltage to a photodiode having an InAsSb light absorption layer. However, if a reverse bias voltage is applied to the photodiode, the dark current generated in the photodiode increases. If the dark current increases, the proportion of the steady component caused by the dark current in the output voltage from the readout circuit becomes higher. Therefore, it is difficult to detect the signal component caused by the input light to the photodiode with high precision.

[0009] An object of the present disclosure is to provide a photodetection circuit and a photodetection device that can accurately detect a signal component caused by incident light on a photodiode even when the dark current generated in the photodiode increases.

[0010] Technical means for solving the problem

[0011] [1] The photodetection circuit of the present disclosure includes: a photodiode having a light absorption layer that 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 to apply a reverse bias voltage to the photodiode; a second wiring having a potential lower than that of 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, and having a period corresponding to the maximum frequency in the frequency band greater than the pulse width of a pulsed light having a periodicity of the light absorption layer of the input photodiode; and a current extraction circuit connected between the anode of the photodiode and the second wiring to extract a current from the photodiode according to the output voltage from the output terminal of the error amplifier.

[0012] [2] Another photodetection circuit of the present disclosure includes: a photodiode having a light absorption layer, a cathode, and an anode, and having photosensitivity to a wavelength of 3.0 μm or more; a first wiring connected to the cathode of the photodiode to apply a reverse bias voltage to the photodiode; a second wiring having a potential lower than that of 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, and having a period corresponding to the maximum frequency in the frequency band greater than the pulse width of a pulsed light having a periodicity of the light absorption layer of the input photodiode; and a current extraction circuit connected between the anode of the photodiode and the second wiring to extract a current from the photodiode according to the output voltage from the output terminal of the error amplifier.

[0013] If periodic pulsed light is incident on the photodiode of the optical detection circuits of [1] and [2] above, the periodic pulse signal generated in the photodiode is input to the transimpedance amplifier and is converted into a periodic pulse signal component in the output voltage from the transimpedance amplifier. The pulse width of this pulse signal component (i.e., the pulse width of the pulsed light incident on the photodiode) is less than the period corresponding to the maximum frequency in the frequency band of the error amplifier. Therefore, amplification of this pulse signal component by the error amplifier is suppressed. On the other hand, the photodiode has a light absorption layer that is an InAsSb layer or has photosensitivity to wavelengths of 3.0 μm or more. Therefore, 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 is converted into a steady component in the output voltage from the transimpedance amplifier. This steady component is amplified by the error amplifier and is input to the current extraction circuit. The current extraction circuit extracts a current corresponding to the magnitude of the steady component, i.e., the dark current, from the anode of the photodiode. As a result of such a feedback operation, the dark current flowing from the photodiode to the transimpedance amplifier is reduced. Therefore, according to the optical detection circuits of [1] and [2] above, even if the dark current generated in the photodiode increases, the pulse signal component caused by the input light to the photodiode can be detected with high precision.

[0014] [3] In the optical detection circuit of [2] above, it is also possible that the photodiode has photosensitivity to wavelengths of 5.0 μm or more. In this case, the dark current generated due to the application of the reverse bias voltage further increases. Therefore, the structure of the optical detection circuit of [2] above is more effective.

[0015] [4] In the optical detection circuit of [2] or [3] above, it is also possible that the light absorption layer contains In, As, and Sb. For example, in such a case, the photodiode can have photosensitivity to wavelengths of 3.0 μm or more or 5.0 μm or more.

[0016] [5] In the optical detection circuit of any one of [1] to [4] above, it is also possible that the current extraction 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. In this case, the current extraction circuit can be simply configured.

[0017] [6]In the optical detection circuit of [5] above, it is also possible that the current extraction circuit further includes: a second transistor, which is connected in series with the first transistor and has a current terminal and a control terminal, and the current terminal and the control terminal are diode-connected. Compared with a photodiode that is sensitive to wavelengths less than 3.0 μm (for example, a photodiode having an InGaAs light absorption layer), in the optical detection circuit of [5] above, the dark current of the photodiode when a reverse bias voltage is applied is significantly larger. In the opinion of the inventors of the present invention, the dark current of a photodiode having an InAsSb light absorption layer is more than 1000 times that of a photodiode having an InGaAs light absorption layer. If such a large current is extracted only by a single first transistor, the size of the first transistor will increase. By connecting the second transistor with the current terminal and the control terminal diode-connected in series with the first transistor, compared with the case of only a single first transistor, the overall size of the transistor circuit can be reduced.

[0018] [7]The optical detection circuit according to any one of [1] to [6] above may further include: a smoothing capacitor connected to a node between the output terminal of the error amplifier and the current extraction circuit. In this case, the periodic signal component included in the output voltage from the error amplifier can be further reduced, and only the dark current can be extracted with high precision.

[0019] [8]The optical detection circuit according to any one of [1] to [7] above may further include: a switch having one end and the other end. One end of the switch is connected to the above 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 above node and the output terminal of the error amplifier. Alternatively, [9] one end of the switch is connected to the current extraction circuit, 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 current extraction circuit and the output terminal of the error amplifier. When the switch is in the connected state, the optical detection circuit can achieve the above effects. In addition, after the switch is switched from the connected state to the non-connected state, the smoothing capacitor also maintains the voltage input to the current extraction circuit, or maintains the gate voltage through the gate capacitance of the transistor constituting the current extraction circuit, etc., so that the current extraction circuit continuously extracts the dark current, which is the steady component from the photodiode. In addition, since the error amplifier and the current extraction circuit are separated by the switch, when a signal component having a pulse width larger than the period corresponding to the maximum frequency in the frequency band of the error amplifier is output from the photodiode, the current extraction circuit does not extract the signal current from the photodiode, and the signal current is amplified by the transimpedance amplifier. Thus, according to the optical detection circuit of [8] or [9] above, by switching the switch as needed, not only can signal components with a small pulse width be amplified, but also signal components with a large pulse width can be amplified.

[0020]

[10] In the optical detection circuit according to any one of [1] to [9] above, the second wiring may be a reference potential line of the optical detection circuit.

[0021]

[11] The optical detection device of the present disclosure includes: a light source that outputs periodic pulsed light, and the optical detection circuit according to any one of [1] to

[10] above. The periodic pulsed light from the light source is input to the light absorption layer of the photodiode. The period corresponding to the maximum frequency in the frequency band of the error amplifier is longer than the pulse width of the periodic pulsed light. According to this optical detection device, by including the optical detection circuit according to any one of [1] to

[10] above, even if the dark current generated in the photodiode increases, the signal component caused by the input light to the photodiode can be detected with high precision.

[0022] Advantages of the Invention

[0023] According to the present disclosure, an optical detection circuit and an optical detection device can be provided that can detect the signal component caused by the input light to the photodiode with high precision even if the dark current generated in the photodiode increases. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a diagram schematically showing the structure of an optical detection device according to an embodiment of the present disclosure.

[0025] Figure 2 is a circuit diagram showing the structure of an optical detection circuit.

[0026] Figure 3 is a diagram schematically showing an example of the stacked structure of a photodiode.

[0027] Figure 4 is a diagram showing the relationship between the magnitude of the reverse bias voltage and the characteristics of the photodiode. Figure 4 Part (a) of... 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... Figure 3 shown. Figure 4 Part (b) of... is a graph showing the relationship between the magnitude of the reverse bias voltage and the photosensitivity of a photodiode having the structure shown in... Figure 3 shown.

[0028] Figure 5 is a circuit diagram of an optical detection circuit including a specific structural example of a current extraction circuit.

[0029] Figure 6 is a circuit diagram showing an optical detection circuit of a charge amplifier type.

[0030] Figure 7 is a circuit diagram showing an optical detection circuit of a TIA type.

[0031] Figure 8 It is a diagram showing simulation results related to the output voltage waveform from an amplifier when a reverse bias voltage is applied to a photodiode in a photodetection circuit in the TIA mode.

[0032] Figure 9 It is a diagram showing the results of designing and prototyping a photodetection circuit of an embodiment and measuring the output voltage from the TIA.

[0033] Figure 10 It is a circuit diagram showing the structure of a photodetection circuit of a modification example. Detailed Embodiment

[0034] Hereinafter, embodiments of the photodetection circuit and the photodetection device of the present disclosure will be described in detail with reference to the drawings. In addition, in the description of the drawings, the same reference numerals are assigned to the same elements, and repeated descriptions are omitted.

[0035] Figure 1 It schematically shows the structure of a photodetection device 1 according to an embodiment of the present disclosure. As Figure 1 shown, the photodetection device 1 of the present embodiment includes a light source 2 that outputs a 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 light source 2 may have a semiconductor laser element that outputs the pulsed light L.

[0036] Figure 2 It is a circuit diagram showing the structure of the photodetection circuit 3. As Figure 2 shown, the photodetection circuit 3 includes a photodiode 4, a transimpedance amplifier (TIA) 5, an error amplifier 6, a current extraction circuit 7, a smoothing capacitor 8, a first wiring 91, and a second wiring 92.

[0037] The photodiode 4 inputs the pulsed light L from the light source 2. The photodiode 4 has photosensitivity to the wavelength of the mid-infrared light, i.e., the pulsed light L, output from the light source 2. That is, the photodiode 4 has photosensitivity to a wavelength of, for example, 3.0 μm or more, 5.0 μm or more, or 7.0 μm or more. Alternatively, the photodiode 4 has photosensitivity to a wavelength of, for example, 10 μm or less. Therefore, the photodiode 4 includes In, As, and Sb in the light absorption layer. 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.

[0038] Figure 3 is a diagram schematically showing an example of the stacked structure of the photodiode 4. In Figure 3 the example shown, the photodiode 4 includes a semiconductor substrate 41, a buffer layer 42, an n-type semiconductor layer 43, a light absorption 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 the main surface 41a of the semiconductor substrate 41. The buffer layer 42 is composed 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, for example, an n-type InAsSb layer. The light absorption layer 44 is provided on the n-type semiconductor layer 43. The light absorption layer 44 is, for example, an undoped InAsSb layer. The p-type semiconductor layer 45 is provided on the light absorption layer 44. The p-type semiconductor layer 45 is, for example, a p-type InAsSb layer. The bandgap of the light absorption layer 44 is smaller than the bandgaps of the n-type semiconductor layer 43 and the p-type semiconductor layer 45.

[0039] The n-type semiconductor layer 43, the light absorption layer 44, and the p-type semiconductor layer 45 constitute a mesa structure 46. A trench 47 for insulation is formed around the mesa structure 46 in a top view. The trench 47 has a depth reaching 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. The cathode electrode makes an 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. The anode electrode makes an ohmic contact with the p-type semiconductor layer 45.

[0040] Refer again to Figure 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 also outputs a dark current J2 generated due to the reverse bias voltage Vb. Figure 4 is a diagram showing the relationship between the magnitude of the reverse bias voltage and the characteristics of the photodiode. Figure 4 The (a) part of Figure 3 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 Figure 4 shown structure. Figure 3 The (b) part of Figure 4 is a graph showing the relationship between the magnitude of the reverse bias voltage Vb and the photosensitivity (InAsSb relative sensitivity) of the photodiode 4 having the Figure 4As shown, at any temperature, as the absolute value of the reverse bias voltage Vb increases from 0 V, the parallel resistance increases, and when a reverse bias voltage of approximately 0.1 V in absolute value is applied, the parallel resistance becomes maximum. When the temperature of the photodiode 4 is 23.6 °C, the parallel resistance with respect to the reverse bias voltage (absolute value) of 0.1 V is approximately 150 Ω. Therefore, the dark current J2 becomes J2 = 0.1 V / 150 Ω = 0.67 mA.

[0041] Refer again to Figure 2 . The TIA 5 is connected to the anode of the photodiode 4 and converts the current output from the photodiode 4 (the combined current of the signal current J1 and the dark current J2) 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.

[0042] The error amplifier 6 has a first input terminal 6a as a non-inverting input terminal, a second input terminal 6b as an inverting input terminal, and an output terminal 6c. The error amplifier 6 amplifies the difference voltage between the voltage input to the first input terminal 6a and the 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. In Figure 2 an example where the first input terminal 6a is connected to the second input terminal 51b of the amplifier 51 is shown. The period corresponding to the maximum frequency in the frequency band of the error amplifier 6 is larger than the pulse width of the pulsed light L input to the photodiode 4. When the pulse width of the pulsed light L is 100 nanoseconds, the frequency band of the error amplifier 6 is, for example, 17 kHz or less.

[0043] The current extraction 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, the reference potential line (GND line) of the optical detection circuit 3. The current extraction circuit 7 is also connected to the output terminal 6c of the error amplifier 6, and according to the output voltage from the output terminal 6c of the error amplifier 6, a current is extracted from the photodiode 4 and made to flow to the second wiring 92. When the light absorption layer 44 of the photodiode 4 is an InAsSb layer and the absolute value of the reverse bias voltage Vb is 0.1 V, the magnitude of the current extracted by the current extraction circuit 7 is, for example, on the order of 1 mA.

[0044] Figure 5 It is a circuit diagram of the optical detection circuit 3 including a specific structural example of the current extraction circuit 7. Figure 5 The shown current extraction circuit 7 includes 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.

[0045] 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.

[0046] The smoothing capacitor 8 is connected between the node N1 between the output terminal 6c of the error amplifier 6 and the current extraction 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 smooths the output voltage from the error amplifier 6.

[0047] Regarding the effects obtained by the optical detection device 1 and the optical detection circuit 3 of the present embodiment described above, together with the technical problems of the current optical detection circuit, an explanation will be given. As described above, the parallel resistance of the photodiode 4 having sensitivity to wavelengths of 3.0 μm or more is on the order of several tens of Ω to several kΩ, so it is difficult to increase the gain of the amplifier. Therefore, it is desired to improve the sensitivity of the photodiode 4 itself. For example, the sensitivity can be improved by applying a reverse bias voltage Vb to the photodiode 4.

[0048] Figure 6This is a circuit diagram of the optical detection circuit 100A representing the charge amplifier method. In the optical detection circuit 100A, a bias voltage Vb1 is applied to the cathode of the photodiode 104, and this bias voltage Vb1 is input to the non-inverting input terminal 105a of the amplifier 105. The signal current J3 output from the anode of the photodiode 104 is input to the inverting input terminal 105b of the amplifier 105. A capacitor 106 is connected between the inverting input terminal 105b and the output terminal 105c of the amplifier 105. In addition, a switch 107 for resetting the charge accumulated in the capacitor 106 is connected in parallel with the capacitor 106.

[0049] In this way, in the charge amplifier method, a circuit structure (auto-zero method) for suppressing the reverse bias voltage applied to the photodiode 104 to the minimum is adopted. Therefore, applying a reverse bias voltage to the photodiode 104 can be said to deviate from the purpose of the charge amplifier method. In addition, in the charge amplifier method, since the gain of the amplifier 105 is large, it is difficult for the signal current from a photodiode having a small parallel resistance, for example, a photodiode sensitive to wavelengths longer than the near-infrared region, to flow into the amplifier 105. Therefore, if the optical detection circuit 100A of the charge amplifier method is used for a photodiode having a small parallel resistance, the sensitivity of the entire optical detection circuit 100A is reduced.

[0050] Figure 7 This is a circuit diagram of the optical detection circuit 100B representing the TIA method. In the optical detection circuit 100B, a feedback resistor 108 is provided in place of the capacitor 106 and the switch 107 of the optical detection circuit 100A of the charge amplifier method. According to the TIA method, the gain of the amplifier 105 is smaller than that of the charge amplifier method. 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. Thus, the signal current from the photodiode can be read out with high sensitivity. However, in the case where the parallel resistance of the photodiode 104 is small, if a reverse bias voltage is applied to the photodiode 104 in the optical detection circuit 100B, a large steady component (offset component) is included in the output voltage from the amplifier 105 due to the dark current generated in the photodiode 104. Therefore, in the case of applying a reverse bias voltage to the photodiode 104, it is difficult to detect only the signal current in the TIA method shown Figure 7 as above.

[0051] Figure 8It is a graph showing the simulation results related to the output voltage waveform from amplifier 105 when a reverse bias voltage is applied to photodiode 104 in the optical detection circuit 100B in TIA mode. Here, assuming that the light absorption layer of photodiode 104 is an InAsSb layer, the parallel resistance of photodiode 104 is set to 100 Ω, and the resistance value of feedback resistor 108 is set to 1 kΩ. In Figure 8 it shows graph G11 when the absolute value of the reverse bias voltage is 0.00 V, graph G12 when the absolute value of the reverse bias voltage is 0.01 V, graph G13 when the absolute value of the reverse bias voltage is 0.05 V, and graph G14 when the absolute value of the reverse bias voltage is 0.1 V. In these graphs G11 - G14, the pulse waveform PL is the pulse waveform assuming the incidence of pulsed light L. Referring to Figure 8 , it can be seen that the larger the absolute value of the reverse bias voltage, the greater the dark current of photodiode 104, and the more the steady-state component of the output voltage from amplifier 105 deviates from the reference voltage (2.5 V). In the case where the absolute value of the reverse bias voltage is 0.1 V (graph G14), only the steady-state component of the output voltage is 1.0 V, approaching the saturation voltage VT of the TIA. In such a 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.

[0052] Regarding the above problem, the optical detection circuit 3 of this embodiment operates as follows. When periodic pulsed light L is incident on photodiode 4, the periodic signal current J1 generated in photodiode 4 is input to TIA 5 and converted into a periodic signal component in the output voltage from TIA 5. The frequency of this signal component, that is, the repetition frequency of the pulsed light L input to photodiode 4, is greater than the frequency band of error amplifier 6. Therefore, amplification of this signal component by error amplifier 6 is suppressed. On the other hand, photodiode 4 has a light absorption layer 44 that is an InAsSb layer or has photosensitivity to wavelengths of 3 μm or more. Therefore, a relatively large dark current J2 is generated in photodiode 4 by the reverse bias voltage Vb applied from the first wiring 91. This dark current J2 is also input to TIA 5 and converted into a steady-state component in the output voltage from TIA 5. The frequency of this steady-state component is included in the frequency band of error amplifier 6, so the steady-state component is amplified by error amplifier 6 and input to current extraction circuit 7. Current extraction circuit 7 extracts a current corresponding to the magnitude of the steady-state component, that is, dark current J2, from the anode of photodiode 4. As a result of such a feedback operation, the dark current J2 flowing from photodiode 4 to TIA 5 decreases. Therefore, according to the optical detection circuit 3 of this embodiment, even if the dark current J2 generated in photodiode 4 increases, the signal component caused by the pulsed light L input to photodiode 4 can be detected with high precision.

[0053] Figure 9 It is a graph showing the result of designing and prototyping the optical detection circuit 3 of this embodiment and measuring the output voltage from the TIA5. In this prototyping, a photodiode having an InAsSb optical absorption layer was used as the photodiode 4, and the resistance value of the feedback resistor 52 was set to 1 kΩ. In Figure 9 graphs G21 when the absolute value of the reverse bias voltage Vb is set to 0.01 V, graph G22 when the absolute value of the reverse bias voltage Vb is set to 0.10 V, graph G23 when the absolute value of the reverse bias voltage Vb is set to 0.15 V, graph G24 when the absolute value of the reverse bias voltage Vb is set to 0.20 V, and graph G25 when the absolute value of the reverse bias voltage Vb is set to 0.25 V are shown. In the figure, the graph G26 represents the trigger signal waveform, and the double arrow W represents the irradiation period (pulse width 100 nanoseconds) of the pulsed light L. Referring to Figure 9 it can be seen that due to the action of the current extraction circuit 7, regardless of the magnitude of the reverse bias voltage Vb, the steady-state component of the output voltage (the component when the pulsed light L is not incident) is approximately constant at 2.5 V. Furthermore, referring to Figure 9 it can be seen that the higher the reverse bias voltage Vb, the higher the sensitivity of the optical detection circuit 3.

[0054] The preferred 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 photosensitivity to wavelengths of 3 μm or more (or has an InAsSb optical absorption 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 photosensitivity is about 300 Ω even if it is large (refer to Figure 4 ). It was confirmed by simulation that if the feedback resistor 52 is 1 kΩ or less, all the signal current from the photodiode 4 flows to the feedback resistor 52, but if the feedback resistor 52 becomes 2 kΩ or more, the signal current flowing into the feedback resistor 52 decreases. Therefore, when the photodiode 4 has photosensitivity to wavelengths of 3 μm or more (or has an InAsSb optical absorption layer), the feedback resistor 52 can also be less than 2 kΩ (for example, 1 kΩ).

[0055] As described above, the photodiode 4 can also have photosensitivity to wavelengths of 5 μm or more. In this case, the dark current J2 due to the application of the reverse bias voltage Vb further increases. Therefore, the structure of the optical detection circuit 3 of this embodiment is more effective.

[0056] As described above, the optical absorption layer 44 can also contain In, As, and Sb. For example, in such a case, the photodiode 4 can have photosensitivity to wavelengths of 3.0 μm or more or 5.0 μm or more.

[0057] As in the present embodiment, the current extraction circuit 7 may also include: 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 extraction circuit 7 can be simply configured.

[0058] As in the present embodiment, the current extraction circuit 7 may further include: a second transistor 72 connected in series with the first transistor 71, and the first current terminal 72b is diode-connected to the control terminal 72a. Compared with a photodiode having sensitivity to a wavelength less than 3.0 μm (for example, a photodiode having an InGaAs light absorption layer), in the photodiode 4 of the present embodiment having sensitivity to a wavelength of 3.0 μm or more (or having an InAsSb light absorption layer), the dark current J2 generated when a reverse bias voltage Vb is applied is significantly larger. In the opinion of the inventors of the present invention, the dark current J2 of the photodiode 4 having an InAsSb light absorption layer is more than 1000 times that of a photodiode having an InGaAs light absorption layer. If such a large current is extracted only by a single first transistor 71, the size of the first transistor 71 will increase. By connecting the second transistor 72 with the first current terminal 72b diode-connected to the control terminal 72a in series with the first transistor 71, the size of the entire transistor circuit can be reduced compared to the case where only a single first transistor 71 is provided.

[0059] As in the present embodiment, the light detection circuit 3 may also include a smoothing capacitor 8 connected to a node N1 between the output terminal 6c of the error amplifier 6 and the current extraction circuit 7. In this case, the periodic signal component included in the output voltage from the error amplifier 6 can be further reduced, and only the dark current J2 can be accurately extracted.

[0060] [Modification Example]

[0061] Figure 10It is a circuit diagram showing the structure of a photodetection circuit 3A which represents a modification of the above-described embodiment. In addition to the structure of the photodetection circuit 3 of the above-described embodiment, the photodetection circuit 3A of this modification further includes a switch 10. One end of the switch 10 is connected to the node N1 and is connected to the current extraction circuit 7 via the node N1. The other end of the switch 10 is connected to the output terminal 6c of the error amplifier 6. The switch 10 switches the connection state between the node N1 and the output terminal 6c of the error amplifier 6, in other words, between the current extraction circuit 7 and the output terminal 6c of the error amplifier 6. That is, when the switch 10 is in the connected state, the output terminal 6c of the error amplifier 6 is electrically connected to the node N1. When the switch 10 is in the non-connected state, the output terminal 6c of the error amplifier 6 is electrically separated from the node N1. The switch 10 can be a mechanical switch such as an electromagnetic relay or a semiconductor switch such as a transistor.

[0062] When the switch 10 is in the connected state, the photodetection circuit 3A performs the same operation as the photodetection circuit 3 of the above-described 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 the voltage across the smoothing capacitor 8 is equal to the output voltage from the error amplifier 6. Then, the switch 10 is switched from the connected state to the non-connected state. Then, the voltage across the smoothing capacitor 8, which is equal to the output voltage from the error amplifier 6 at the switching moment of the switch 10, is continuously input to the current extraction circuit 7. Therefore, the current extraction circuit 7 continuously extracts the dark current J2, which is the steady component from the photodiode 4. The smoothing capacitor 8 is not an essential element, and the same operation can also be performed by additionally providing an element for storing charge. For example, the gate capacitance of the first transistor 71 (see Figure 5 ) constituting the current extraction circuit 7 stores charge and the gate voltage is also maintained. Therefore, even when the switch 10 is in the non-connected state, the current extraction circuit 7 continuously extracts the dark current J2, which is the steady component from the photodiode 4. In addition, according to this modification, the error amplifier 6 is separated from the current extraction circuit 7 by the switch 10. Therefore, even when a signal current J1 having a pulse width larger than the period corresponding to the maximum frequency in the frequency band of the error amplifier 6 is output from the photodiode 4 and a voltage obtained by amplifying the voltage caused by the signal current J1 is output from the error amplifier 6, the signal current J1 is not extracted by the current extraction circuit 7 but is amplified by the TIA 5. Thus, according to the photodetection circuit 3A of this modification, by switching the switch 10 as needed, not only can a signal current J1 with a small pulse width be amplified, but also a signal current J1 with a large pulse width can be amplified.

[0063] The optical detection circuit and the optical detection device of the present disclosure are not limited to the above-described embodiments and can be variously modified. For example, in the above embodiment, a photodiode having an InAsSb light absorption layer is illustrated, but the light absorption layer of a photodiode having photosensitivity to a wavelength of 3.0 μm or more may also be an InAs layer or an InSb layer. When the signal component of the pulsed light L is sufficiently reduced in the error amplifier 6, the smoothing capacitor 8 may be omitted.

[0064] In the above embodiment, as the current extraction circuit 7, a current extraction circuit having a first transistor 71 and a second transistor 72 is illustrated, but the current extraction circuit 7 may also have only a single first transistor 71. As long as the current extraction circuit 7 has a function of extracting current from the photodiode according to the output voltage from the error amplifier, it is not limited to Figure 5 the structure shown and can have various structures.

[0065] Symbol Explanation

[0066] 1... Optical detection device, 2... Light source, 3, 3A... Optical detection circuit, 4... Photodiode, 5... Transimpedance amplifier (TIA), 6... Error amplifier, 6a... First input terminal, 6b... Second input terminal, 6c... Output terminal, 7... Current extraction 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... Optical 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 photodetection circuit, wherein: Comprising: 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 to apply a reverse bias voltage to the photodiode; A second wiring having a potential lower than that of 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, and a period corresponding to the maximum frequency in the frequency band being greater than the pulse width of the pulsed light having a periodicity of the light absorption layer of the photodiode; And A current extraction circuit connected between the anode of the photodiode and the second wiring to extract current from the photodiode according to the output voltage from the output terminal of the error amplifier.

2. A photodetection circuit, wherein: Comprising: A photodiode having a light absorption layer, a cathode and an anode, and having photosensitivity to a wavelength of 3.0 μm or more; A first wiring connected to the cathode of the photodiode to apply a reverse bias voltage to the photodiode; A second wiring having a potential lower than that of 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, and a period corresponding to the maximum frequency in the frequency band being greater than the pulse width of the pulsed light having a periodicity of the light absorption layer of the photodiode; And A current extraction circuit connected between the anode of the photodiode and the second wiring to extract current from the photodiode according to the output voltage from the output terminal of the error amplifier.

3. The photodetection circuit according to claim 2, wherein: The photodiode has photosensitivity to a wavelength of 5.0 μm or more.

4. The photodetection circuit according to claim 2 or 3, wherein: The light absorption layer contains In, As and Sb.

5. The photodetection circuit according to any one of claims 1 to 4, wherein: The current extraction circuit has: a first transistor comprising: 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. The photodetection circuit according to claim 5, wherein: The current extraction circuit further includes: a second transistor, which is connected in series with the first transistor and has a current terminal and a control terminal, and the current terminal and the control terminal are diode-connected.

7. The optical detection circuit according to any one of claims 1 to 6, wherein it further includes: a smoothing capacitor, which is connected to a node between the output terminal of the error amplifier and the current extraction circuit.

8. The optical detection circuit according to claim 7, wherein it further includes: a switch, which has one end and the other end, the one end is connected to the node, and the other end is connected to the output terminal of the error amplifier, and switches the connection state between the node and the output terminal of the error amplifier.

9. The optical detection circuit according to any one of claims 1 to 6, wherein it further includes: a switch, which has one end and the other end, the one end is connected to the current extraction circuit, and the other end is connected to the output terminal of the error amplifier, and switches the connection state between the current extraction circuit and the output terminal of the error amplifier.

10. The optical detection circuit according to any one of claims 1 to 9, wherein the second wiring is a reference potential line of the optical detection circuit.

11. An optical detection device, wherein it includes: the optical detection circuit according to any one of claims 1 to 10; and a light source, which outputs the periodic pulsed light, the periodic pulsed light from the light source is input to the light absorption layer of the photodiode, a period corresponding to the maximum frequency in the frequency band of the error amplifier is greater than the pulse width of the periodic pulsed light.

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