Patent Application
20230120951
The present invention relates to an electronic circuit that converts current to voltage.
It is known that in order to measure the current, a target current is converted to a voltage and is measured using a voltmeter. To accurately read a small current, it is necessary to convert the current to a voltage using a low-noise electronic circuit. In order to realize this, a method of reducing thermal noise by using a current-to-voltage converter at a low temperature is used. For current signals in the ultra-long to short wave band (1 kHz to 30 MHz), a current-to-voltage converter using low-power field-effect transistors (FETs) that operate at low temperatures has been reported (NPL 1).
Hashisaka et. al., “Cross-correlation measurement of quantum shot noise using homemade transimpedance amplifiers”, 2014, Rev. Sci. Instrum. 85, 054704
PAM-XIAMEN GaAs HEMT Epi wafer product catalog page, [online], retrieved on Mar. 6, 2020, Internet <URL: https://www.powerwaywafer.com/gaas-hemt-epi-wafer.html
However, the open-loop gain of a signal amplification unit is not sufficient in the conventional current-to-voltage converter using FETs capable of operating at room temperature. Very small signals are measured in a current-to-voltage converter when, for example, measuring cosmic rays, quantum device signals, or “quantum fluctuations” of a current, or observing physical phenomena at low temperatures. In order to measure such small currents, it is necessary to operate the current-to-voltage converter at a very low temperature, at least at the temperature of liquid nitrogen (77K) to closer to absolute zero. The FETs used in the conventional technology current-to-voltage converter are based on the premise of operating at room and low temperatures. For this reason, even if these FETs are operated at cryogenic temperatures, the current-to-voltage conversion efficiency is insufficient, and sensitive current measurement is difficult. Further, a desired low-temperature environment cannot be realized with a cooling system due to power consumption of the current-to-voltage converter or heat inflow from outside when used in a cooled state.
The present invention has been made in view of the foregoing problems, and an object of the invention is to provide a means for sensitively measuring small currents in extremely low-temperature conditions.
To achieve the above object, one embodiment of the present invention is a current-to-voltage converter including: an amplification unit having at least three stages each including an electronic element and configured to convert a target current, which is fed to a first stage, to a voltage while feeding back an output signal of a final stage to the first stage; and a buffer unit connected to the amplification unit and configured to output the converted voltage, wherein the electronic element is a field-effect transistor (FET) adapted to operation at a temperature of 150 K or less, a gate of the FET is connected to a ground potential, and a single common power supply is fed to each FET.
A means for sensitively measuring small currents in extremely low-temperature conditions is provided.
The following disclosure relates to a current-to-voltage converter that sensitively measures small currents even in extremely low-temperature conditions. A current-to-voltage converter of the present disclosure uses elements optimized specifically for low-temperature operation (e.g., HEMTs) as electronic elements for current-to-voltage conversion. This configuration makes it possible to achieve significantly more excellent current-to-voltage conversion characteristic than those of the conventional technique even if the current-to-voltage converter is operated at a low temperature of 150 K or less or in cryogenic temperature conditions close to absolute zero. Further, the configuration of power supply to the current-to-voltage conversion circuit is simplified, and the heat inflow from outside the cooling device is suppressed, thus reducing the load on the cooling device.
Referring to
However, the current-to-voltage conversion characteristics of FETs operating at both room and low temperatures are inadequate in situations where the FETs are cooled to near absolute zero to measure small currents, such as cosmic rays, quantum device signals, or “quantum fluctuations” of a current. The inventors conceived that if the electronic elements used in the conventional current-to-voltage converter were specialized for operation and performance at low temperatures, more favorable low-noise characteristics could be obtained in low-temperature conditions.
The cooling device can take various forms, but one example is a dilution refrigerator. The dilution refrigerator is a cylindrical can having a diameter of 0.5 to 1 m×a height of about 2 m that contains the above-described current-to-voltage conversion circuit 10 and has a mechanism for circulating helium inside the can. The dilution refrigerator may also include external mechanisms such as a pump and a compressor for helium circulation, which are not shown in
In order to sensitively measure small currents using a current-to-voltage converter circuit, it is assumed to be used in combination with a cooling device as shown in
“Cooling capacity” is known as an important indicator of a cooling device, and indicates how many watts or less of heat generation in the cooling device is required to maintain a certain temperature. In other words, it means the maximum amount of heat generated that can be tolerated with respect to a cooling target in the cooling device. For example, in the case of a cooling device (dilution refrigerator) for achieving a low temperature of about milli-Kelvin, the typical cooling capacity is about 500 μW at 100 mK in the most advanced device. This means that in order to maintain a low temperature of 100 mK, the power consumption generated in the cooling device needs to be or 500 μW or less. Therefore, the indicator “cooling capacity” may be higher as the temperature set by the cooling device is higher. For example, if the temperature is maintained at 200 mK, the heat generated by a cooling object, i.e., the power consumption in the cooling system may be twice as large as that in the case of 100 mK.
In general, the lower the temperature of an electronic circuit, it is possible to further suppress thermal noise and achieve low noise. It is also advantageous to use a current-to-voltage converter in an environment at as low a temperature as possible for the realization of the ultimate low-noise measurements. Accordingly, it is necessary to suppress the power consumption of the current-to-voltage converter as much as possible to a small level relative to the index “cooling capacity” of the cooling device. The conventional current-to-voltage converter (NPL 1) shown in
The current-to-voltage converter of the present disclosure simultaneously solves the aforementioned problems of insufficient open-loop gain during low-temperature operation and limited capacity of the cooling device by using electronic elements (FETs) specifically configured for low-temperature operation and by the unique configuration of the current-to-voltage conversion circuit.
The output-stage source follower unit 102 is not included in the conventional current-to-voltage conversion circuit 10 in
When measuring a current at room temperature at which current consumption is not limited, a large current can be applied to the source follower (H4) of the current-to-voltage conversion unit 101. However, it was found that a single-stage source follower was not sufficient when a current-to-voltage converter is used at cryogenic temperatures based on the premise of the use of a cooling device. There is a limit to the power consumption that can be allocated to the source follower FET, and the output impedance of the source follower cannot be reduced sufficiently. Therefore, the output-stage source follower unit 102 is further provided, making it possible to prevent deterioration of the frequency characteristics in the current-to-voltage conversion characteristics due to the cable stray capacitance on the downstream side. By using later-described low-current FETs specialized for low-temperature operation, which will be discussed later, the increase in the number of amplification stages (from 4 to 5) can be compensated for, and smaller current consumption can also be achieved in the entire current-to-voltage conversion circuit. As a result, the load on the cooling device in cryogenic measurement is reduced, while at the same time wide-band small current measurement is realized.
One of the characteristic points of the current-to-voltage converter of the present disclosure is that the power supply voltage is fed to the gates of the FETs at the amplification stages H1 to H5 using the self-bias method. That is, the gate of each FET is connected to ground by a gate ground resistor 106, and the gate is fixed at OV. Since there is also resistance between the source and ground, if a voltage is fed to the drain from the power supply terminal 105, the source potential is positive. Accordingly, when n-type HEMTs are used, it is equivalent to applying a negative voltage between the gate and the source. If appropriate values are selected for the gate and source resistors, an appropriate power supply voltage is fed to each FET in the self-bias method by providing only one power supply terminal 105.
In the conventional current-to-voltage conversion circuit 10 shown in
The amount of heat inflow per wire can be estimated from the metal material used for the wires and the length of each wire determined by the structure of the refrigerator. Materials commonly used for the wires at low temperatures are copper-nickel alloys, stainless steel, superconducting wires, or the like. A superconducting wire has the lowest thermal conductivity. However, since the wires are only used at cryogenic temperatures of 4 K or less, estimation is made here for the case of using a coaxial cable made of cupronickel (a copper-nickel alloy), which is widely used. In the case of a cable with a thickness (0.86 mmφ) that is easy to use and commonly used, the thermal conductivity is about 0.07 mW·m/K, for example. If it is assumed that the temperature is cooled to 4 K, and a 1-m wire is used while taking an example of a standard cooling device size, the amount of heat inflow per wire is about 0.2 mW. The amount of heat inflow of 0.2 mW is too large to ignore compared to the power consumption of 0.75 mW in the current-to-voltage converter itself.
In contrast, according to the current-to-voltage conversion circuit 100 of the present disclosure shown in
Another characteristic point of the current-to-voltage converter of this disclosure lie in that FETs with a configuration specialized for low-temperature operation are used as the FETs (H1 to H5) of the current-to-voltage conversion circuit in
Here, a more specific description will be given to characteristics between the FET that operates at both room and low temperatures used in the conventional current-to-voltage converter and the FET specialized for low-temperature operation used in the current-to-voltage converter of the present disclosure. The HEMTs (FETs) used in the current-to-voltage conversion circuit 100 shown in
Accordingly, the current-to-voltage converter of the present disclosure can be implemented as a device that includes the amplification unit 101 that has at least three stages each including an electronic element, and converts a target current, which is fed to the first stage, to a voltage while feeding back the output signal of the final stage to the first stage, and the buffer unit 102 that is connected to the amplification unit and outputs the converted voltage. Here, the electronic element is a field-effect transistors (FETs) adapted for operation at a temperature of 150 K or less, the gate of the FET is connected to a ground potential, and a single common power supply 105 is fed to each FET.
In HEMTs, the configuration of the channel portion is related to the detection sensitivity for small currents. Considering the cross-sectional structure of the HEMTs, the channel is formed between the drain and the source. A current at the channel is controlled by an input signal to the gate. In the case of a HEMT that operates at room temperature, the thickness d of the gate insulating layer needs to be sufficiently large in order to reduce leakage current between the channel and the gate. Accordingly, the HEMT capable of operating at both room and low temperatures usually has an insulating layer thickness d of 100 nm or more. Meanwhile, the larger the thickness d, the smaller the response to a change in gate voltage, and the lower the detection sensitivity to an input signal to the gate.
In the GaAs—AlGaAs HEMT with a channel width of 3 mm specialized for low-temperature operation used in the current-to-voltage converter of the present disclosure, the insulating layer thickness d at a temperature 4 K is set to be 100 nm or less; more specifically, 55 nm. When this HEMT is used as an amplifier element, the electrical resistance between the gate and the channel is 200 kQ/mm in actual measurement at room temperature. Thus, this HEMT cannot be used due to its large leakage behavior. In contrast, for example, at liquid helium temperature (4.2K), the electrical resistance between the gate and the channel is 1 GΩ/mm or more, and thus the leakage current can be ignored. By abandoning normal operation at room temperature and using a HEMT specialized for low-temperature operation, the current detection sensitivity of the HEMT serving as the current-to-voltage conversion circuit for cryogenic temperature can be greatly improved.
In HEMTs for room-temperature operation, it is important in general to suppress leakage between the gate and the channel. In GaAs—AlGaAs HEMTs, the gate and the channel are naturally insulated since the Schottky barrier is formed. However, the insulating layer needs to be made thick to some extent. A configuration of a commonly available GaAs—AlGaAs HEMT is disclosed, for example, in NPL 2, where the thickness of the insulating layer is 210 nm, although the amount of doping is not mentioned. Although different materials require different insulating layer thicknesses, a thickness of 100 nm or more is generally considered to be common in the case of GaAs—AlGaAs. The current-to-voltage converter of the present disclosure adopts a configuration specialized for low-temperature operation, with a gate insulating layer having a thickness of 100 nm or less, which cannot be selected for room temperature operation, and simplifies the configuration of power supply to the current-to-voltage conversion circuit to achieve current-to-voltage conversion characteristics that are significantly superior to those of conventional technique.
Here, the configuration of the HEMT specialized for low-temperature operation will be mentioned further. In a current-to-voltage conversion circuit, the shorter the distance between the gate and the channel and the thinner the gate insulating layer, the better in order to increase the current detection sensitivity, as mentioned above. Further, the larger the amount of change in channel current (transconductance) with respect to the gate voltage, the better. Thus, the larger the amount of doping, the higher the current detection sensitivity.
However, the two conditions of the gate insulating layer thickness and the doping amount can only be optimized within the range where no carrier is generated in the gate insulating layer. It is known that beyond this range, a gate leakage current occurs at room temperature, and parallel conduction reduces mobility and degrades HEMT characteristics. If carriers are generated in the gate insulating layer of the HEMT and a gate leakage current flows, the HEMT cannot be used as a current-to-voltage conversion circuit, or even as an electronic element as it does not have the basic operation and performance at room temperature.
In order to ensure the aforementioned basic operation as an electronic element, most of the commercially available HEMTs have a barrier layer, which is a part of the gate insulating layer, with a thickness of 100 nm or more, for example. According to NPL 2, the barrier layer is 180 nm, and the total gate thickness of the three-layer structure is 210 nm. A HEMT with a configuration having such a thick gate insulating layer is a barrier to highly sensitive measurements conducted at low temperatures.
In the current-to-voltage conversion circuit of the present disclosure, a current-to-voltage conversion circuit was prototyped using HEMTs with a gate insulating layer thickness of 55 nm to which delta doping (6×1011 cm−2) was performed twice (equivalent to a channel carrier density of 4×1011 cm−2), and excellent noise performance was confirmed. These HEMTs have a gate resistor with an electrical resistance of 200 kΩ/mm in actual measurement at room temperature, and cannot be used as HEMTs at room temperature due to leakage current. However, by using the above-described HEMT specialized for low-temperature operation at very low temperatures and simplifying the configuration of power supply to the current-to-voltage conversion circuit as shown in
As described above in detail, small current measurement with excellent sensitive can be realized in extremely low-temperature conditions by the current-to-voltage converter of the present disclosure. Industrial Applicability
The present invention can be used in highly sensitive measurement of small currents.
10, 100 Current-to-voltage conversion circuit
11, 103 Current input terminal
12, 13, 105 Power supply terminal
14, 104 Voltage output terminal
15, 107 Feedback resistor
20 Current-to-voltage converter
21 Case
22 Cooling stage
108 Drain output voltage
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/012150 | 3/18/2020 | WO |
