JPH03111724A - Synchronous charge accumulating type photodetector - Google Patents

Abstract

PURPOSE:To make it possible to detect light at high sensitivity and low noises by removing all load resistances for a detector, reading a voltage appearing at the internal impedance of an element with a FET amplifier, and providing a resetting FET. CONSTITUTION:A photodetector 1 comprises a photodiode wherein internal impedance is high and dark currents are extremely few. The cathode terminal of the photodetector is connected to the drain of a resetting FET 2 and the gate of a reading FET 3. A virtual capacitance C is present between the gate and the drain of the FET 2. When an ON pulse (-3V) is inputted, positive charge is accumulated in the capacitor C, and the part between the drain and the source conducts. Therefore, the charge accumulated in the stray capacitor of the photodetector 1 flows to the ground. When an OFF pulse (+1V) is inputted, a positive voltage is applied. Therefore, the accumulated positive charge is discharged, and the part between the drain and the source becomes the OFF state. Since any load resistor is connected to the photodetector 1. Johnson noise is determined with only the internal resistance of a detector, and the highly sensitive detection can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a photodetection device that detects extremely weak light in the infrared region, particularly in the near-infrared region, with high sensitivity and low noise.

[Conventional technology]

Conventionally, a single photon counting method using a photomultiplier tube is known as a method for detecting extremely weak light. However, in the near-infrared region, especially in the band from 900 to 1600 nm, the quantum efficiency of the photocathode decreases significantly, so Ge, In, GaAs, In
Higher sensitivity can be obtained by using a semiconductor detection element such as GaAs or InGaAsP.

Among these semiconductor detection elements, an avalanche photodiode (A
PD), single photon counting is possible in principle. However, there is no detection element with a large light-receiving surface, and the dark count is quite high at 10' with the currently available one with a light-receiving surface of 80 μm in diameter.
Furthermore, disturbances in wave height distribution may occur in low-temperature regions, which is thought to be due to instability of the avalanche effect.
It is extremely difficult to achieve stable operation using the single photon counting method using PD.

The photodetection method currently used in the near-infrared region is to amplify the photocurrent generated by a semiconductor element by some method so that it can be handled by general measurement equipment.

This kind of preamplifier amplifies the signal from the element while suppressing the noise of the amplifier itself as much as possible, and it is necessary to reduce the noise figure and maintain the time response of the signal as necessary. .

Generally, highly sensitive detectors are often used as preamplifiers, such as the voltage amplification type shown in Figure 10(a).
There is a current-voltage conversion type as shown in FIG. 10(b).

In addition, FIG. 10(C) is an equivalent circuit of a voltage amplification type detector,
FIG. 10 (Company) is an equivalent circuit of a current-voltage conversion type. The detector 31 has an internal resistance Rd and a stray capacitance T connected to the signal source is.
, can be represented as connected in parallel, and the first
In FIG. 10(C), this internal impedance is connected in parallel to the signal source, and in FIG. 10(d), this internal impedance is connected in series with the signal source.

In the voltage conversion type, only the load resistor RL is connected to the detector, and no external resistance is connected to the amplifier 32, so it has a narrow bandwidth determined by the internal resistance and capacitance of the detector. Johnson noise is relatively small, but the charge accumulation time when light is received is short because the time constant determined by the internal impedance is short, so the drawback is that high sensitivity cannot be achieved.

On the other hand, in the current-voltage conversion type, the feedback resistor R3 and the feedback capacitor CL are connected in parallel to the amplifier to form an integrating circuit, so it has a wide bandwidth determined by the external resistance and capacitance.
Although there is a problem that the S/N ratio decreases due to the presence of Johnson noise from both the internal resistance of the detector and the external resistance of the amplifier, the sensitivity can be increased because it allows a longer time for the charge to accumulate when light is received. It is possible to do so.

For example, as shown in FIG. 11, a feedback impedance 44 is connected to an amplifier 43 to form an integrating circuit, and the light receiving element 4
When we actually measured the current-voltage conversion type in which the signal detected in step 1 is read out by the differential amplifier 42 and amplifier 43,
The results shown in FIG. 12 were obtained.

As can be seen from the figure, there is considerable noise in the output, indicating that the S/N ratio is low. As described above, conventional detection devices have been unable to detect extremely weak light with high precision.

The present invention is intended to solve the above problems, and eliminates all load resistance for the detector and converts the voltage appearing in the internal impedance of the element into a high-impedance, low-noise FET.
Direct reading by amplifier and reset FET
It is an object of the present invention to provide a synchronous charge accumulation type photodetection device that can perform photodetection with high sensitivity and low noise by discharging accumulated charges at an appropriate period.

[Means to solve the problem]

To this end, the present invention provides a photodetection device that has high internal impedance, extremely low dark current, and reads out a semiconductor light-receiving element to which no load resistance is connected using a voltage amplification type preamplifier to which no external resistance is connected. The cathode of the light-receiving element is connected to the drain terminal, and a reset FET is provided which is ON/OFF controlled by a reset pulse applied to the gate to discharge the charge accumulated in the stray capacitance of the light-receiving element. A detection device is used to detect the incident light through the chopper, and the reset FET is turned on with a reset signal synchronized with the opening/closing cycle of the chopper.
N10 F F is operated to discharge the charge in the light receiving element, and an addition/subtraction counter is operated in synchronization with the opening and closing of the chopper, and the background is removed by subtracting the signals when the chopper is open and closed. It is characterized by

[Effect]

The photodetection device of the present invention is composed of two FETs and a semiconductor detection element, and the readout FET directly reads out the charge accumulated in the stray capacitance of the high impedance semiconductor detection element, and the reset FET is turned ON/OFF. It is designed to discharge the accumulated charge and return to the initial state.
Bandwidth is determined only by internal impedance, and by using a semiconductor detection element with extremely high internal impedance and low dark current, charge storage linearity is good, sensitivity can be increased, and no external resistor is connected. Therefore, the S/N ratio can be greatly improved, and by applying reset at appropriate intervals, it becomes possible to perform detection according to the periodicity of the measurement target.

〔Example〕

The present invention will be explained in detail below.

Fig. 1 is a diagram showing the configuration of the detection device of the present invention, Fig. 2 is a diagram for explaining the principle of charge accumulation, Fig. 3 is a waveform diagram in signal detection, and Fig. 4 is an example of the detection device. Figure, 5th
The figure shows the mounting structure of the detector. In the figure, 1 is a light receiving element, 2 is a reset FET, and 3 is a readout FE.
T, 4 is a resistor, 5 is a differential amplifier, 6 is a Zener diode, 7 is a resistive element, 8 is an operational amplifier, 9 is a low-pass filter, IO is a detector, 11 is a sensor, 12 is a mounting device, 13.14 is a FET, 15 is a resistance element, 16 is a metal block, 17 is a window, and 18 is a terminal.

The light receiving element of the present invention is Si, Ge, In, GaAs, In
It is made of a semiconductor detection element such as GaAsP, which has high internal impedance and extremely low dark current.As shown in FIG. It has become. The detection signal can be taken out from the output terminal 18 side via a preamplifier consisting of FETs 13 and 14, a resistive element 15, and the like. In addition, FET13.14
is cooled with liquid nitrogen, and the detection element is cooled with liquid nitrogen or liquid helium to reduce noise.

In FIG. 1, a light receiving element 1 is comprised of a photodiode with high internal impedance and extremely low dark current, and is normally turned off by applying a reverse bias voltage to it. The cathode terminal of the light receiving element is connected to the drain of the reset FET 2 and the gate of the read FET 3.

As shown in Figure 2(a), +I is applied to the gate of FET2.
V and -3V are applied, +IVt"OFF.

It is designed to turn on at 3V. There is a virtual capacitance C between the gate and drain of FET2, as shown in Figure 2(b).
As shown in the figure, when an ON pulse (-3V) is applied manually, positive charge accumulates in the capacitor C, and conduction occurs between the drain and source, so the charge accumulated in the stray capacitance of the light-receiving element flows to the ground side.

As shown in Figure 2 (C), when an OFF pulse (+IV) is applied manually, a positive voltage is applied to the capacitor C, so the positive charge accumulated here is discharged, and the drain-source is in the OFF state. Become. In the detection device of the present invention, the light receiving element 1
Since no load resistance is connected to the sensor, Johnson noise is determined only by the internal resistance of the detector, and the charge will accumulate unless a reset pulse is applied, allowing for a long accumulation time and enabling highly sensitive detection. .

As shown in Figure 1, when FET3 is operated as a source follower and read out, the output vour is as shown in Figure 3(a).
The voltage changes as shown in Figure 3 (C)), V
, and V2. FIG. 4 is a diagram showing an actual circuit example of the detection device of the present invention shown in FIG. 1. A constant voltage is input from the Zener diode 6 to one input terminal, and the accumulated voltage of the photodiode is input to the other input terminal.The signal read out by the source follower 3 is amplified by the differential amplifier 5 and the operational amplifier 8, and then filtered by the low-pass filter 9. Signals in a predetermined band can be detected.

FIG. 6 shows the ON/OFF state at a predetermined period by the detection device of the present invention.
It is a figure which shows the detection signal when it is made to turn FF.

In Fig. 6(a), the detection element is reset at a cycle of 10 seconds,
This is the signal waveform when the ON/OFF of light to the detection element is set to several times the reset period, and its details are shown in Figure 6 (b).
As shown in the figure, a signal output with extremely low noise and good linearity can be obtained.

Note that FIG. 6(C) shows the output when the light source is turned off, and shows the dark current. Since the background is considered to be constant over time, Figure 6 (C) is obtained from the values in Figure 6, etc.
By subtracting the value of , a signal from which the background has been removed can be detected.

FIG. 7 shows the photocurrent with S/N=1 as a function of integration time for the current-voltage conversion type detector of the present invention and the conventional current-voltage conversion type detector. It can be seen that sufficient detection is possible even if the photocurrent is small.

FIG. 8 is a diagram showing an embodiment of a detection device using the detector of the present invention, and FIG. 9 is a waveform diagram.

In the figure, extremely weak incident light from a light source 21 is chopped by a chopper 22 and detected by a detector 23 of the present invention. At this time, a reset signal is output from the control circuit 26 in synchronization with the switching signal of the chopper 22, and the detector 23
Reset. At the same time, the addition/subtraction counter 35 is controlled by a gate control signal output from the control circuit 26 in synchronization with the switching signal of the chopper 22, and the
Add the D-converted signals when the light source is on,
The dark current when the light source is turned off is subtracted and the result is output by a digital printer/analog recorder 27.

As shown in Fig. 9(a), while the chopper is open, the sum of the signal S and the noise N is obtained, and while the chopper is closed, the background noise N is obtained. . Each signal of S and N is integrated at the application period of the reset signal shown in Fig. 9 (C, etc.).
), while the chopper is open, addition is performed by the addition gate signal, and while the chopper is closed, the addition is performed by the addition gate signal, as shown in Figure 9 (d).
As shown in FIG. 3, the addition/subtraction counter 35 is controlled so as to be subtracted by the subtraction gate signal. As a result, since noise has the property of appearing constantly over all periods, it is possible to remove noise from the output of the addition/subtraction counter 35 and detect only the signal S.

〔Effect of the invention〕

As described above, according to the present invention, a voltage amplifying preamplifier that does not connect an external resistor has high internal impedance, and dark current is extremely low.By reading out the output of a light receiving element that does not connect a load resistor, the accumulation time can be increased. By increasing the sensitivity and improving the S/N ratio, it is possible to detect extremely weak light with low noise and high sensitivity.In addition, the accumulated charge is discharged at a predetermined timing, so it can be used in various ways. It becomes possible to detect extremely weak light at a periodicity that corresponds to the measurement target.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of the detection device of the present invention, Fig. 2 is a diagram for explaining the principle of charge accumulation, Fig. 3 is a waveform diagram in signal detection, and Fig. 4 is an example of the detection device. Figure, 5th
The figure shows the mounting structure of the detector, Figure 6 shows the detection signal when the detection device of the present invention turns on 10 F F at a predetermined cycle, and Figure 7 shows the difference between the present invention and the conventional example. Current -
For the voltage conversion type, the photocurrent for S/N = 1 is shown as a function of integration time. Figure 8 is a diagram showing an embodiment of the present invention, Figure 9 is a waveform diagram, and Figure 10 is a conventional diagram. FIG. 11 is a diagram showing the configuration of a conventional voltage-current conversion type detection device, and FIG. 12 is a diagram showing detection results by the device of FIG. 11. 1... Photodiode, 2... FET for reset
. 3... Readout FET, 12... Mounting device,
13.14...FET0 Applicant New Technology Development Corporation (2 others)

Claims (4)

[Claims] (1) A photodetection device that has high internal impedance, extremely low dark current, and reads out a semiconductor photodetector without connecting a load resistor using a voltage amplification type preamplifier without connecting an external resistor. A synchronous charge storage type characterized by having a reset FET whose cathode is connected to the drain terminal and which is ON/OFF controlled by a reset pulse applied to the gate to discharge the charge accumulated in the stray capacitance of the light receiving element. Photodetection device. (2) The detection device according to claim 1, which detects the incident light through the chopper and turns the reset FET ON/OFF using a reset signal synchronized with the opening/closing cycle of the chopper;
The synchronous charge is characterized by discharging the charge in the light receiving element, operating an addition/subtraction counter in synchronization with the opening and closing of the chopper, and subtracting the signals when the chopper is open and closed to remove the background. Storage type photodetector. (3) The light receiving element is made of InGaAs, InGaAsP, In
2. The photodetection device according to claim 1, wherein the detection element is made of Sb, InAs, or Si. (4) The photodetection device according to claim 1, wherein the reset FET and readout FET are cooled with liquid nitrogen, and the light receiving element is cooled with liquid nitrogen or liquid helium.