Patent Description:
Single photon avalanche diodes (SPADs) are semiconductor devices capable of detecting light. A photon impinging on a detection region of a SPAD generates an electron and hole pair via the photoelectric effect. The SPAD is reverse-biased with a high voltage magnitude such that when the electron/hole carriers are generated, the electric field applied across the detection region causes the carriers to be accelerated to a relatively high velocity according to the strength and direction of the applied field. If the kinetic energy of the accelerated carriers is sufficient, additional carriers will be generated from the semiconductor lattice, which are in turn accelerated by the field, and may liberate further carriers in an exponentially increasing fashion. Thus, when a sufficiently high electric field is applied across the detection region, a single impinging photon may generate an avalanche of carriers, resulting in an output current 'pulse', where the current output is proportional to the number of photons detected.

The minimum voltage required to cause an avalanche of carriers, and thus allow the device to operate as a SPAD, is known as the breakdown voltage. If the voltage applied across the SPAD is too low, i.e. below the breakdown voltage, then the device does not produce any output. However if the voltage applied across the SPAD is too high, then it is possible that the electric field generated may be sufficient to cause a carrier avalanche even when there are no photons impinging on the SPAD, resulting in a false output current. This false output is known as a "dark current".

<CIT> discloses a photodiode array, a recommended operating voltage determining method for determining a recommended operating voltage of a reverse bias voltage to be applied to the photodiode array, and a reference voltage determining method for determining a reference voltage for determination of the recommended operating voltage.

<CIT> discloses a technique of adjusting the sensitivity of a SPAD and expanding the dynamic range by changing the power supply voltage applied to the SPAD according to the intensity of the ambient light.

<CIT> discloses an automatic avalanche photodiode bias setting system based on unity-gain noise measurement.

According to a first aspect, there is provided a method for controlling a voltage across a single photon avalanche diode according to claim <NUM>.

According to a second aspect, there is provided an apparatus for controlling a voltage applied across a single photon avalanche diode according to claim <NUM>.

Some embodiments may provide circuitry for controlling the voltage applied across a SPAD. In order to aid understanding of some of the embodiments described herein, a representation of a typical SPAD circuit is shown in <FIG>.

In operation, a voltage VHV <NUM> may be applied to the cathode of the SPAD <NUM>. The anode is connected via a resistor <NUM> to ground, and to an input terminal of a comparator <NUM>. When a photon impinges on the detection region of the SPAD <NUM>, a current is generated in the SPAD, and electrons collected at the anode of the SPAD. The current may be output to the comparator <NUM>. If the current generated is above a threshold value, then an output pulse is produced by the comparator, indicating the detection of a photon. The circuit may also comprise reset circuitry (not shown) for resetting the SPAD ready for the next detection event. After a SPAD detection event, where the anode voltage increases to reduce the current flowing through the SPAD the anode may be discharged to ground via a resistor. The resistor may be replaced by a MOS device with a fixed gate bias. With either resistor or MOS with fixed bias, the SPAD is said to operate in active recharge mode.

The SPAD circuit may also comprise a switch to a positive voltage on the anode, allowing the voltage across the SPAD to be reduced, thereby taking the SPAD out of the avalanche region.

In order to better illustrate the operation of a SPAD, reference is now made to <FIG>.

<FIG> shows a representation of the current flowing through the SPAD as a function of the voltage applied across the SPAD. Initially, when there is a zero voltage difference, there is no current flowing, as denoted by the flat line region <NUM> in <FIG>.

As the magnitude of the voltage difference between the cathode and the anode increases (i.e. the voltage is made more negative), the voltage difference reaches an initial value <NUM> where the diode will start to conduct and current will begin to flow. This is known as the breakdown voltage, VBD.

As the voltage applied is made more negative, the voltage reaches a value which represents the minimum voltage required to cause an output pulse to be generated when a photon impinges upon the SPAD. This is the voltage difference required to accelerate the photo-generated carriers to sufficiently cause an avalanche within the SPAD. This is the minimum operation voltage <NUM>, VHV<NUM>.

The voltage difference may then be made more negative with respect to value <NUM>, which represents an operating voltage of the SPAD which is of a sufficient magnitude so as to generate a current which is easily detectable by the SPAD and trigger an output pulse.

However, if the voltage becomes too negative, i.e. the voltage magnitude becomes too high, then the electric field applied may reach a value at which electron-hole pairs are generated spontaneously (i.e. without the device receiving a photon), resulting in dark current.

In some applications, such as optical communications or light detection and ranging (LIDAR), a device comprises both a light emitter and a SPAD detector. The light emitter may for example be a vertical cavity surface emitting laser (VCSEL). Other embodiments may use other suitable light sources.

In the example of a LIDAR system, light may be emitted by a VCSEL. When the emitted light reaches a target within the field of view of the device, a portion of the light may be reflected back to the SPAD detector. If the voltage applied to the SPAD is too low, then it is not possible to determine whether no light is being received is a result of the SPAD being improperly biased, or if there is no target within the field of view.

Equally, if the voltage difference applied to the SPAD is too high, it is not possible to determine if the current output is a result of a target being present, or if the current output is dark current generated by a high electric field.

It has been proposed, in some devices, to provide a reference light path in the device. An example LIDAR system incorporating a reference light path is shown in <FIG>. The system comprises a VCSEL <NUM>, a reference SPAD <NUM> detector and a main SPAD detector <NUM>. The VCSEL <NUM> may emit light across a wide angle, or field of view of the system, denoted by the dotted lines in <FIG>. A portion of the light generated by the VCSEL <NUM> may be blocked by the device housing <NUM>, while the remainder of the light is emitted.

When a portion of the emitted light strikes a target <NUM>, part of the light may be reflected back to the main SPAD detector <NUM>, which receives the reflected light and counts the number of photons received. The number of counts, and timing information may be used to determine and locate objects within the field of view of the device.

A solid partition <NUM> is located between the VCSEL <NUM> and the main SPAD detector <NUM> to ensure no stray light emitted from the VCSEL <NUM> is detected by the main SPAD detector <NUM> without it being reflected from the target <NUM>.

The reference light path <NUM> is formed by a portion of light emitted from the VCSEL <NUM> being reflected off the interior of the device housing <NUM>, which is detected by the reference SPAD detector <NUM>. Thus the reference light path <NUM> is engineered to always receive a portion of the light emitted by the VCSEL <NUM>.

With the reference SPAD detector <NUM>, the applied voltage may be gradually changed until a certain count threshold within a set integration period is achieved. This voltage is considered to be the minimum voltage required to cause the SPAD to emit an output pulse indicating a photon detection.

The voltage applied to the main SPAD detector <NUM> may then be set to a value comprising the minimum threshold determined using the reference SPAD detector <NUM> plus an amount to enter into an assumed robust region of operation of the main SPAD detector <NUM>.

This requires a reference light path to be built into the device, which requires additional space and results in an increased cost to manufacture. The time taken to perform the detection and calibration may not be insignificant - for example, it may be of the order of <NUM>, which is time in which data could be gathered.

For some applications, it is desirable to control the voltage applied to a SPAD automatically and/or continuously, without the need for a reference light path.

A representation of an apparatus according to some embodiments is shown in <FIG>.

In <FIG>, the apparatus comprises a SPAD <NUM>, measurement circuitry <NUM>, and voltage setting circuitry <NUM>.

The SPAD <NUM> is supplied with a cathode voltage VHV <NUM> and an anode voltage <NUM>. The anode of the SPAD <NUM> outputs a current to the measurement circuitry <NUM>. The measurement circuitry <NUM> comprises circuitry suitable for measuring the current output from the SPAD <NUM>.

The measurement circuitry <NUM> is connected to the voltage setting circuitry <NUM>. The voltage setting circuitry <NUM> comprises circuitry suitable for receiving an input from the measurement block <NUM>, and controlling at least one of the cathode voltage VHV <NUM> and the anode voltage <NUM> applied to the SPAD based on the received input.

Some example circuits according to some embodiments are shown in <FIG> and <FIG>. lt should be understood that the circuits shown in <FIG> are non-limiting example implementations of the apparatus described above with reference to <FIG>. It should be understood that in some embodiments, different circuitry to that provided in <FIG> may be used.

The circuit shown in <FIG> comprises a SPAD <NUM>, first n-type FET transistor <NUM>, a second n-type FET transistor <NUM>, and an op-amp <NUM>. The SPAD is biased by applying voltage VHV <NUM> to the SPAD cathode. The anode of the SPAD <NUM> is connected to source of the first transistor <NUM>. The anode of the SPAD <NUM> is also connected to the non-inverting input terminal of op-amp <NUM>. The inverting input terminal of the op-amp receives a biasing voltage VBG <NUM>. The output of the op-amp <NUM> is connected to the gate of the first transistor <NUM> and the gate of the second transistor <NUM>. The drain of the first transistor <NUM> and the second transistor <NUM> are connected together.

By varying the bias voltage VBG <NUM>, the current passing through the first n-type FET transistor <NUM> may be varied. Thus the bias voltage VBG <NUM> determines the anode voltage of the SPAD <NUM>.

The arrangement of the circuit shown in <FIG> results in the first transistor <NUM> and the second transistor <NUM> forming a current mirror. Thus the current flowing through the SPAD <NUM>, is input into the source of the first transistor <NUM> and is mirrored by the second transistor <NUM> and output at <NUM> to a current comparator <NUM>. The current comparator <NUM> compares the current <NUM> to a threshold value VREF_ICMP <NUM>.

In some embodiments, at least one of the cathode voltage <NUM> and the bias voltage <NUM> may be dependent on the output of the current comparator <NUM>.

That is to say, the measurement circuitry may comprise a current mirror. The current mirror may be connected to a current comparator. The current mirror may comprise a first transistor <NUM> and a second transistor <NUM>, wherein a source terminal of the first transistor <NUM> is connected to an anode <NUM> of the single photon avalanche diode, and a drain terminal of the first transistor <NUM> is connected to a drain terminal of the second transistor <NUM>. The drains of the first and second transistors may be held at a ground voltage. The source terminal of the second transistor <NUM> may be connected to a current comparator <NUM>.

The voltage setting circuitry may comprise an op-amp. The op-amp may receive a bias voltage VBG <NUM> at the inverting input terminal, and the output of the SPAD <NUM> at the non-inverting input terminal. The output of the op-amp may be connected to the gate of the first transistor <NUM> and the gate of the second transistor <NUM>. The source of the first transistor may be connected to the anode of the SPAD <NUM>. The bias voltage VBG <NUM> may be dependent on the output of the current comparator.

In some embodiments, the circuit shown in <FIG> may be used. The circuit in <FIG> is substantially the same as the circuit shown in <FIG>, except that the op-amp <NUM> has been removed, and the gate of the first transistor <NUM> and the second transistor <NUM> may be connected to the output of the SPAD <NUM>.

In some embodiments, alternatively or additionally, the cathode voltage VHV <NUM> may be dependent on the output of the current comparator.

It should be understood that the use of n-type FETs in the circuits shown in <FIG> and described above is only one such example. It would be understood by those skilled in the art that any other suitable transistor may be used. For example, a p-type FET may be used. It should be appreciated that alternatively or additionally, different types of transistor may be used such as bipolar transistors or MOSFETs.

With reference to <FIG> and <FIG>, the circuit of <FIG> may provide a more accurate control of the voltage across the SPAD <NUM>. The circuit of <FIG> may be smaller, and thus easier and cheaper to manufacture.

It should be appreciated that the examples shown in <FIG> are only two ways in which some embodiments may be implemented. Other embodiments may be implemented using different circuitry.

Some methods of some embodiments of controlling the voltage applied to a SPAD will now be described with reference to <FIG>.

According to some embodiments, the voltage VHV applied to the SPAD is fixed at a first value such that the voltage is well above the breakdown voltage VBD. The operation of setting the SPAD voltage to above VBD is shown in <FIG> by step <NUM>.

The voltage applied across the SPAD may then be reduced. The operation of reducing the voltage applied across the SPAD is shown in <FIG> by step <NUM>.

The current flowing through the SPAD may then be compared to a threshold value. The operation of comparing the current flowing through the SPAD to a threshold value is shown in <FIG> by step <NUM>.

If the current flowing through the SPAD is above the threshold value, the method repeats step <NUM>, where the voltage applied across the SPAD may be reduced, and the current flowing through the SPAD again determined and compared to the threshold value in step <NUM>.

When the comparison in step <NUM> results in the current flowing through the SPAD being below the threshold value, the voltage applied across the SPAD may be increased by a fixed amount VEB. The subsequently applied voltage is determined to be the operating threshold voltage of the SPAD.

The applied excess bias voltage VEB may be large enough to ensure that a current output from the SPAD exceeds a threshold current of the SPAD readout circuitry for all SPAD pixels. Increasing the excess bias voltage above this value may result in increasing the sensitivity of the SPAD - that is to say, an incident photon may be likely to trigger an avalanche in the SPAD when a large excess bias is applied. Increasing the excess bias may, however, also result in increased dark count rate (the dark count rate being outputs from the SPAD pixel which occur without the presence of a photon due to defects within the silicon).

In some embodiments the applied excess bias voltage VEB may be determined by a trade-off between the achieved sensitivity and dark count rate. Additionally or alternatively, a further factor which may be considered is based on the ability to turn off the SPADs when they are not in operation.

In some embodiments it may be possible to have some SPADs within an array enabled while others are disabled. For a SPAD configured with a high voltage VHV on the cathode and zero volts on the anode during normal operation, the SPAD may be disabled by pulling the anode voltage to a higher voltage. The anode voltage may be set by the supply voltage. The anode voltage being increased may cause the voltage across the SPAD to be lower than the breakdown voltage VBD, moving the SPAD out of its avalanche region. In some embodiments, the applied excess bias voltage may be smaller than the voltage used to disable the SPAD to enable the SPAD to be disabled.

In some embodiments, the increase in the voltage may be dependent on the one or more other factors. The one or more other factors may be any other suitable factors such as temperature, operating conditions, application and/or the like. Employing an increased voltage may result in increased probability of detecting a photon at the expense of increased Dark Count Rate.

The operation of increasing the voltage applied across the SPAD by VEB is shown in <FIG> by step <NUM>.

According to another embodiment, the voltage VHV applied to the SPAD is fixed at a second value well below the breakdown voltage VBD. The operation of setting the voltage applied across the SPAD to above VBD is shown in <FIG> by step <NUM>.

The voltage applied across the SPAD may then be increased. The operation of increasing the voltage applied across the SPAD is shown in <FIG> by step <NUM>.

If the current flowing through the SPAD is below the threshold value, the method repeats step <NUM>, where the voltage applied across the SPAD may be increased, and the current flowing through the SPAD again compared to the threshold value in step <NUM>.

When the comparison in step <NUM> results in the current flowing through the SPAD being above the threshold value, the voltage applied across the SPAD may be increased by a fixed amount VEB. The subsequently applied voltage is determined to be the operating threshold voltage of the SPAD.

In some embodiments, the increase in the voltage may be dependent on the current voltage being used.

In some embodiments, the increase in the voltage may be dependent on the one or more other factors. The one or more other factors may be any other suitable factors such as temperature, operating conditions, application and/or the like.

In one modification, some embodiments may carry out steps <NUM> to <NUM> of <FIG> and steps <NUM> to <NUM> of <FIG>. The voltage which triggers the current to be below the threshold in <FIG> and the voltage which triggers the current to be above the threshold in <FIG> are both used to determine an operating bias voltage for the SPAD.

Some embodiments may provide a method comprising determining a current flowing through a single photon avalanche diode, and controlling the voltage applied to the single photon avalanche diode in dependence on the determined current.

In some embodiments, determining the current flowing through the SPAD may comprise setting the voltage applied to the SPAD with respect to a threshold voltage, adjusting the voltage applied to the SPAD, and comparing the current flowing through the SPAD to a threshold current.

In some embodiments, setting the voltage applied to the SPAD with respect to a threshold voltage comprises setting the voltage applied to the SPAD above the threshold value. In some embodiments, setting the voltage applied to the SPAD with respect to a threshold voltage comprises setting the voltage applied to the SPAD below the threshold value.

In some embodiments, adjusting the voltage applied to the SPAD comprises reducing the voltage applied to the SPAD. In some embodiments, adjusting the voltage applied to the SPAD comprises increasing the voltage applied to the SPAD.

In some embodiments, the comparison is successful if the current flowing through the single photon avalanche diode is below a threshold current. In some embodiments, the comparison is successful if the current flowing through the single photon avalanche diode is above a threshold current.

In some embodiments, controlling the voltage applied to the SPAD in dependence on the determined current may comprise increasing the voltage applied to the SPAD by a set value in response to the comparison being successful.

In some embodiments there may be a single SPAD. In other embodiments, there may be an array of SPADs. Where there is more than one SPAD, each SPAD may have its voltage individually controlled such as described previously. In some embodiments, where there is more than one SPAD, one or more SPADs may act as reference SPADs for the determining of a voltage. The determined voltage for these one or more SPADs may be used to control the voltage applied to one or more other SPADs. It should be appreciated that these so-called reference SPADs may be configured to be a detecting SPAD.

The apparatus and method described above may be implemented in any device or apparatus which utilises single photon avalanche detectors. For example, the apparatus and method described above may be implemented in automotive LIDAR, medical systems (fluorescence lifetime imaging microscopy for example), industrial ranging, light sensing and in communications. It should be understood that these non-limiting implementations are only exemplary, and the apparatus and method may be implemented in any manner of other light-detecting applications.

It should be appreciated that the above described arrangements may be implemented at least partially by an integrated circuit, a chip set, one or more dies packaged together or in different packages, discrete circuitry or any combination of these options.

Various embodiments with different variations have been described here above. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations.

Claim 1:
A method for controlling a voltage across a single photon avalanche diode (<NUM>), the method comprising:
providing, by a current mirror (<NUM>, <NUM>), a first current dependent on a second current flowing through the single photon avalanche diode (<NUM>);
comparing, by a current comparator (<NUM>), the first current to a threshold; and
decreasing or increasing the voltage applied across the single photon avalanche diode (<NUM>) in dependence on an output of the current comparator (<NUM>);
wherein the steps of providing said first current, comparing the first current to a threshold and decreasing or increasing the voltage across the single photon avalanche diode are repeated until the first current is respectively below or above the threshold.