# Patent

## Title
Method and apparatus for providing non-linear, passive quenching of avalanche currents in Geiger-mode avalanche photodiodes

## Abstract

A method and apparatus for providing non-linear, passive quenching of avalanche currents in Geiger-mode avalanche photodiodes (APDs) is provided. A non-linear, passive, current-limiting device is connected in series with the APD and a bias source. The non-linear, passive, current-limiting device rapidly quenches avalanche currents generated by the APD in response to an input photon and resets the APD for detecting additional photons, using a minimal number of components. The non-linear, passive, current-limiting device could comprise a field-effect transistor (FET), as well as a junction FET (JFET) a metal-oxide semiconductor FET (MOSFET), or a current-limiting diode (CLD) connected in series with the APD and the bias source.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the quenching of avalanche currents in photodiodes. More particularly, the present invention relates to a method and apparatus for providing non-linear, passive quenching of avalanche currents in Geiger-mode avalanche photodiodes.
2. Related Art
In many applications in the optoelectronic arts, it is often beneficial to detect individual photons with an optoelectronic device. Single-photon detection can be accomplished using devices such as photomultiplier tubes and avalanche photodiodes (APDs). Photon counting with APDs is typically accomplished by operating the APD in the so-called “Geiger” mode, wherein the APD is biased above its zero-frequency breakdown voltage to produce an average internal gain on the order of one million or higher. Under such conditions, a readily-detectable avalanche current can be produced in response to a single input photon, thereby allowing the APD to be utilized to detect individual photons.
When a current avalanche is triggered in a Geiger-mode APD in response to a single input photon, the avalanche current continues as long as the bias voltage remains above the breakdown voltage of the APD. Thus, in order to detect the next photon, the avalanche current must be “quenched” and the APD reset. Quenching the avalanche current and resetting the APD involves a two-step process, wherein the APD bias is reduced below the APD breakdown voltage to quench the avalanche current as rapidly as possible, and the APD bias is then raised to a voltage above the APD breakdown voltage so that the next photon can be detected. During this process, the APD is incapable of detecting photons, thereby resulting in a “dead” time period. Therefore, it is beneficial to quench the avalanche current and reset the APD as quickly as possible to reduce dead time. Additionally, to minimize increases in the dark count rate (“after-pulsing”) that can occur with high photon arrival rates, it is also beneficial to limit the avalanche current to a minimum.
Various passive and active circuits have in the past been developed for quenching avalanche currents generated by Geiger-mode APDs. For example, the most basic passive quenching circuit is a resistor connected in series with a high-voltage bias applied to an APD. While such a circuit has the advantage of simplicity, this circuit typically results in detector dead times of many tens of microseconds. Due to the long resistor-capacitor (RC) time constant of this circuit, the bias across the APD varies continuously during the reset time, which results in undesirable variations in the photon detection probability of the APD. Various active quenching circuits overcome these limitations by employing a fast transistor circuit with numerous transistors to switch the bias voltage of the APD rapidly between voltages above and below the APD breakdown voltage. However, such circuits are often complex, require numerous components in addition to the transistors, and cannot easily be integrated into large photon-counting arrays.
Accordingly, what would be desirable, but has not yet been provided, is a simple and effective method and apparatus for quenching of avalanche currents in Geiger-mode APDs, wherein avalanche currents are rapidly quenched, detector dead time is minimized, and accurate photon detection is provided using a small number of components.
SUMMARY OF THE INVENTION
The present invention relates to method and apparatus for providing non-linear, passive quenching of avalanche currents in Geiger-mode avalanche photodiodes. The apparatus comprises a Geiger-mode avalanche photodiode (APD); a bias source; and a non-linear, passive, current-limiting device connected in series with the bias source and the APD for quenching avalanche currents generated by the APD. The non-linear, passive, current-limiting device could comprise a field-effect transistor (FET), a junction FET (JFET), a metal-oxide semiconductor FET (MOSFET), or a current-limiting diode (CLD) connected in series with the APD and the bias source. The non-linear, passive, current-limiting device rapidly quenches avalanche currents generated by the APD in response to an input photon and resets the APD for detecting additional photons, using a minimal number of components.
The method of the present invention for quenching avalanche currents in a Geiger-mode APD comprises the steps of connecting a non-linear, passive, current-limiting device in series with an APD and a bias source; after an avalanche current is triggered by an input photon received by the APD, reducing a bias voltage of the APD with the non-linear, passive, current-limiting device to a voltage below a breakdown voltage of the APD to quench the avalanche current; and after quenching the avalanche current, increasing the bias voltage of the APD with the non-linear, passive, current-limiting device to the breakdown voltage of the APD. The method could be practiced using a FET, JFET, MOSFET, or CLD connected in series with the APD and the bias source.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other important objects and features of the invention will be apparent from the following Detailed Description of the Invention, taken in connection with the accompanying drawings, in which:
FIG. 1 is a simplified schematic diagram showing an embodiment of the quenching apparatus of the present invention, wherein quenching is achieved using a junction field-effect transistor (JFET).
FIG. 2 is a simplified schematic diagram showing an alternate embodiment of the quenching apparatus of the present invention, wherein quenching is achieved using a metal-oxide semiconductor field-effect transistor (MOSFET).
FIG. 3 is a simplified schematic diagram showing an alternate embodiment of the quenching apparatus of the present invention, wherein quenching is achieved using a current-limiting diode (CLD).
FIG. 4 is a graph showing quenching of a single Geiger-mode avalanche current pulse achieved by the present invention.
FIG. 5 is a graph showing quenching achieved by the present invention of APD output currents in a non-gated mode.



DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method and apparatus for providing non-linear, passive quenching of avalanche currents in Geiger-mode avalanche photodiodes (APDs). In one embodiment, the present invention comprises a junction field-effect transistor (JFET) connected in series with an APD, wherein the drain terminal of the JFET is connected to the APD or bias source and the gate and source terminals are connected together. In another embodiment, the present invention comprises a metal-oxide semiconductor field-effect transistor (MOSFET) connected in series with an APD, wherein the drain terminal of the MOSFET is connected to the APD or bias source and the gate and source terminals are connected together. In another embodiment, the present invention comprises a current-limiting diode (CLD) connected in series with an APD. In response to an avalanche current generated by an input photon received by the APD, the series impedance of the JFET, MOSFET, or CLD increases rapidly, thereby reducing the bias voltage across the APD to a level below the breakdown voltage of the APD to quench the avalanche current. In response to the sharp current decrease resulting from the drop in voltage across the APD, the series impedance of the JFET, MOSFET, or CLD then decreases to its original value, thereby returning the bias voltage across the APD to its original value above the breakdown voltage and resetting the APD so that additional input photons can be detected. In this manner, the present invention minimizes detector dead time and provides efficient photon detection using a small number of components.
FIG. 1 is a schematic diagram showing an embodiment of the quenching apparatus of the present invention, wherein quenching is achieved using a junction field-effect transistor (JFET). The circuit shown in FIG. 1 includes an avalanche photodiode (APD) D1 which could comprise any APD suitable for Geiger-mode operation. A JFET Q1 is connected in series with the APD D1 and a bias voltage source. The JFET Q1 could be connected to either the anode or the cathode of the APD D1, so long as the indicated polarity of both components is maintained and the JFET and APD are in series with the bias source. The JFET Q1 could comprise any suitable, commercially-available, depletion-mode JFET. The gate terminal G and drain terminal S of JFET Q1 are connected together to form a two-terminal device. A bias voltage Vbias is applied across APD D1 and JFET Q1 (illustratively, at connection points A and B).
During operation of the circuit shown in FIG. 1, JFET Q1 operates as a constant-current source, limiting the current supplied to the APD D1 to the saturated drain-source current (Idss) of the JFET Q1. If JFET Q1 is fabricated such that Idss is approximately equal to the APD dark current obtained when Vbias is equal to the breakdown voltage of the APD D1, then Q1 will successfully quench avalanche currents generated by the APD D1 operated in Geiger mode in response to an input photon. During application of a constant or gated bias voltage Vbias exceeding the breakdown voltage of the APD and prior to the triggering of an avalanche current, the impedance of the JFET will be low (e.g., on the order of 10 kΩ or less), because the current magnitude will be less than Idss. When an avalanche current is produced in response to an input photon or a thermally-generated free carrier, the JFET impedance will rise rapidly and prevent the current from exceeding Idss, thereby quenching the avalanche current. Thereafter, the circuit impedance will drop rapidly, thereby successfully “resetting” the APD D1 to allow it to detect the next photon. The response time from the initiation to quenching of the avalanche current is limited by the internal resistance-capacitance (RC) time constant of the JFET Q1, and could be as low as 1 nanosecond, or even less. Accordingly, the circuit shown in FIG. 1 combines the simplicity and low component count of passive quenching devices with the fast response, short dead time, and APD bias stability of active quenching devices. Importantly, the present invention requires significantly fewer components than conventional active quenching circuits, thereby providing a significant manufacturing advantage and allowing the present invention to be easily fabricated in one- or two-dimensional monolithic or hybrid arrays of photon-counting, Geiger-mode APDs.
FIG. 2 is a schematic diagram showing an alternate embodiment of the quenching apparatus of the present invention, wherein quenching is achieved using a metal-oxide semiconductor field-effect transistor (MOSFET). The circuit shown in FIG. 2 is substantially similar to the circuit shown in FIG. 1, except that the JFET Q1 of FIG. 1 is substituted with MOSFET Q2. Similar to the JFET Q1 of FIG. 1, the drain terminal D of MOSFET Q2 is connected in series with APD D1 and the gate and source terminals G and S of MOSFET Q2 are connected together to form a two-terminal device. MOSFET Q2 could comprise any suitable, commercially available MOSFET operated in depletion mode. During operation, MOSFET Q2 operates as a constant-current source, limiting the current supplied to the APD D1 to the Idss of MOSFET Q2.
FIG. 3 is a schematic diagram showing an alternate embodiment of the quenching apparatus of the present invention, wherein quenching is achieved using a current-limiting diode (CLD). The JFET and MOSFET embodiments of the present invention, discussed earlier with respect to FIGS. 1 and 2, are optimal for integrated applications (e.g., providing current avalanche quenching in Geiger-mode APD arrays). However, for non-integrated applications (e.g., providing current avalanche quenching for single APD sensors) where Idss values as low as 10 μA are required, a CLD can perform the same function as a JFET or MOSFET. CLDs are two-terminal devices that are functionally equivalent to depletion-mode JFETs and MOSFETs with their gate and source terminals connected, but have design and fabrication parameters optimized for current-regulating functions. As shown in FIG. 3, CLD D2 replaces the JFET and MOSFET transistors of FIGS. 1 and 2, and is connected in series with APD D1. CLD D2 could be any suitable, commercially-available CLD having a typical current limit of 35 μA or lower, with precise control of the current-limiting value and voltage compliance (breakdown voltage) of 100 volts or more. A typical commercially-available CLD rated at approximately 35 μA has a DC impedance of approximately 3 kΩ at a current of one μA, and a DC impedance of 500 kΩ at a current of 50 μA. The inherent response time of a discrete CLD with such ratings is on the order of a few nanoseconds, which is sufficiently fast for APD pulse quenching.
It should be noted that the present invention could be implemented using enhancement-mode transistors as well as depletion-mode transistors. Such an implementation would require additional components for saturating the transistors. Further, the present invention can be implemented using commercially-available IC fabrication processes. Additionally, the present invention can be used as a non-gated Geiger-mode photon counter, wherein the APD is cooled to a sufficiently low temperature so that the mean dark pulse generation rate of the APD is lower than the expected mean photon arrival rate. In such a configuration, Vbias is set to a DC value of approximately 1-10% above the breakdown voltage of the APD with no pulsed (“gate”) bias. With each avalanche current generated by the APD, the present invention quenches the avalanche and resets the APD within approximately a few nanoseconds to a few tens of nanoseconds. Therefore, the present invention can be operated without any triggered gate function, in approximately the same manner as a photomultiplier tube.
FIG. 4 is a graph showing quenching of an avalanche current achieved by the present invention. The graph is a single-shot oscilloscope trace of APD current generated in response to a single input photon, wherein the magnitude axis represents the voltage across a 50 ohm load resistor produced by an APD avalanche current. The spike in the graph corresponds to the avalanche current generated in response to the photon. Initially, the APD is biased to a voltage of about 10 mV. At around 40 nanoseconds, an avalanche current is generated and rises rapidly to approximately 100 mV in response to the input photon. Thereafter, the present invention rapidly quenches the current and resets the APD to its initial bias voltage. The APD dead time is approximately 40 nanoseconds.
FIG. 5 is a graph showing quenching achieved by the present invention of APD output currents in a non-gated mode. The present invention can be operated as a non-gated photon counter, whereby no trigger gate function is required to initiate photon detection. In such an arrangement, the present invention automatically quenches and resets the APD within a few nanoseconds to tens of nanoseconds, allowing for the rapid detection of photons. The graph shows an oscilloscope trace of APD current in response to photons detected in non-gated mode, wherein the magnitude axis represents the voltage across a 50 ohm load resistor produced by an APD avalanche current. The spikes shown in the graph correspond to avalanche currents generated in response to photons received by the APD, as well as background noise due to random, thermally-generated carriers in the APD absorption layer. As can be seen in the graph, each avalanche current is rapidly quenched by the invention.
Having thus described the invention in detail, it is to be understood that the foregoing description is not intended to limit the spirit and scope thereof. What is desired to be protected by Letters Patent is set forth in the appended claims.

## Claims

1. An apparatus for quenching avalanche currents generated by Geiger-mode avalanche photodiodes comprising:
a Geiger-mode avalanche photodiode (APD);
a bias source; and
a non-linear, passive, current-limiting device connected in series with the bias source and the APD for quenching avalanche currents generated by the APD.

2. The apparatus of claim 1, wherein the non-linear, passive, current-limiting device comprises a field-effect transistor (FET) connected in series with the APD and the bias source, the FET having gate and source terminals connected together.

3. The apparatus of claim 2, wherein the FET comprises a junction field-effect transistor (JFET).

4. The apparatus of claim 3, wherein the JFET comprises a depletion-mode JFET.

5. The apparatus of claim 2, wherein the FET comprises a metal-oxide semiconductor field-effect transistor (MOSFET).

6. The apparatus of claim 1, wherein the non-linear, passive, current-limiting device comprises a current-limiting diode connected in series with the APD and the bias source.

7. The apparatus of claim 1, wherein the apparatus is formed in a wafer fabrication process.

8. An apparatus for quenching avalanche currents generated by a Geiger-mode avalanche photodiode, comprising:
a Geiger-mode avalanche photodiode;
a field-effect transistor (FET) having a source terminal, a drain terminal, and a gate terminal;
a direct electrical connection between the gate terminal and the source terminal of the FET; and
a series connection between the FET and the Geiger-mode avalanche photodiode.

9. The apparatus of claim 8, wherein the FET comprises a junction field-effect transistor (JFET).

10. The apparatus of claim 9, wherein the JFET comprises a depletion-mode JFET.

11. The apparatus of claim 8, wherein the FET comprises a metal-oxide semiconductor field-effect transistor (MOSFET).