Biased Photodetectors Lab Facts

Dark currents were measured over temperatures of 10 °C to 50 °C for three representative packaged detectors: the Si-based DET100A (previous generation), the Ge-based DET50B (previous generation), and the InGaAs-based DET10C (previous generation).

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Figure 1.1  The Nested Metal Box Test Fixture with Covers Removed and DET10C Installed
Black insulating foam lines the interiors of both boxes. 
A: Outer Aluminum Box; B: Inner Aluminum Box; C: Thermistor; D: DET10C; E: BNC-to-Triax Feedthrough; F: BNC-to-BNC Feedthrough 

Experimental Setup
The experimental setup was designed to ensure a constant reverse-bias voltage across the photodiode, control the temperature of the detector, block light from reaching the detector, and establish an electrical path between the detector and the Keithley 6487 ammeter that isolated the measured current from extraneous electromagnetic interference (EMI) noise sources. Dark currents for these detectors can be on the order of nA, which made it important to control these conditions to ensure accurate measurements.

As the dark current depends to some degree on the magnitude of the reverse-bias voltage, care was taken to ensure the reverse-bias voltage remained constant over the duration of each measurement set. For these detectors, the source of the reverse-bias voltage was a battery. Before each data set was acquired, the detector's A23 battery was replaced with a fresh one. The battery's voltage was also measured before and after each test to ensure the bias voltage remained constant over the course of the measurement.

Thermistors were used to monitor the temperatures of each photodiode continuously during the experiment. A thermistor was held in contact with the outer surface of the TO can housing using a piece of thermal tape. With the thermistor in place, it was not possible to use the detector cap to block light from reaching the photodiode.

The detector enclosure consisted of a pair of nested aluminum metal boxes, which are shown with their covers off in Figure 1.1. Only one box would be required to create a light-tight environment for the photodiode, but an outer box was included in the setup to shield the inner box from EMI related noise. The detector was placed inside the inner box, which was equipped with a BNC-to-triax adapter feedthrough; the BNC end was accessible from the inside of the box, and the triax end was accessible from the outside of the box. The BNC connector on the detector was attached directly to the BNC end of the adapter, which advantageously eliminated the need to use a BNC cable. BNC cables are poorly shielded, and using one could introduce noise from EMI sources to the signal read from the detector. This inner metal box was placed inside of an outer metal box, which possessed a triax-to-triax feedthrough. A triax cable was used to route the detector signal between the feedthroughs on the inner and outer boxes. A triax cable was also used to connect the Keithley 6487 ammeter to the end of the Triax-to-Triax feedthrough accessible on the outside of the outer box. The outer metal box served to isolate the inner metal box from EMI, and the triax cables shielded the detector signal from EMI. After covering two boxes with their respective lids, the nested box set was placed inside an ESPEC ESX-3CW temperature chamber. The ammeter was located outside of the chamber.

The electrical connection between the thermistor and TSP01 temperature logger was performed using BNC cables, BNC-to-BNC bulkhead feedthroughs to route the signal out of the nested boxes, and a custom BNC-to-phono jack cable to connect to the temperature logger.

Experimental Results
The data curves plotted in Figure 1.2 are the dark currents measured for the Si-based DET100A, the Ge-based DET50B, and the InGaAs-based DET10C. Data were acquired continuously while the temperature was within the specified operating range of the photodiodes: between 10 °C and 50 °C (shaded region of Figure 1.3). Figure 1.3 is representative of the temperature of the photodiode's TO can, measured by the thermistor during the experiment. The temperature profile of the chamber intentionally included higher and lower temperatures to ensure that the measurements could be taken over the full temperature operating range of the photodiodes. The data plotted in Figure 1.2 include dark current measurements acquired as the temperature was both increasing and decreasing; the two data sets overlay one another for each diode. 

The data curves plotted in Figure 1.2 show the InGaAs-based detector exhibited the least amount of dark current and the Ge-based detector exhibited the most, with the levels of the latter being approximately 5 orders of magnitude higher. In all cases, the dark current increased with the temperature of the photodiode, as expected. The individual points on this graph, plotted as diamonds, are the values of the dark current specified for each detector at 25 °C. These points specify a maximum value of dark current at 25 °C; each diode's dark current must be equal to or less than this value at 25 °C, but the dark current may exceed this specification at higher temperatures, as is the case for the measured DET50B detector.

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Figure 1.2  Dark Current Data Measured for Three Packaged Photodiodes
The discrete data points, plotted as diamonds, are values of the dark current specified for each detector at 25 °C.

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Figure 1.3  Temperature of a Representative Photodiode Controlled by the Conditions in the Environmental Chamber
Dark current measurements were acquired between 10 °C and 50 °C (indicated by the shaded region) and are plotted in Figure 1.2 .

Experimental Limitations
Measurements were performed using a single representative of each detector type, as these data were intended to illustrate general trends. These data should not be taken as specific for a particular diode. The measured dark current is a function of voltage bias, temperature dependence of resistive loads, and other effects. Efforts were made to suppress their influence on these measurements, including replacing each detector's battery with a fresh one prior to each measurement and using an ammeter to measure the dark current. Using the ammeter removed the need to use a load resistor, which may exhibit its own temperature dependence. The thermistor was placed as close to the semiconductor sensor as was possible, but it was not placed in direct contact with the sensor. Because of this, there may have been a difference between the measured temperature and the temperature of the semiconductor material. The dark current was measured while the temperature of the environmental chamber was continuously varied. Measurements were not made during steady-state temperature conditions, but rather during a slow temperature ramp. Temperatures below 10 °C and above 50 °C were not measured, and the effects of quickly-varying changes in temperature were not investigated. Humidity was not controlled during this experiment.

Dark currents were measured while three representative packaged detectors, the Si-based DET100A (previous generation), the Ge-based DET50B (previous generation), and the InGaAs-based DET10C (previous generation), were held at 24 °C and their reverse-bias voltages were varied between 0 and 10 V. Either a battery or an external power supply, such as the DET2B Power Adapter Bundle, can be used to provide the required reverse-bias voltage. The measurements described below were made to demonstrate the change in dark current as the voltage supplied by the battery decreases from battery drainage.

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Figure 1.4  The Nested Metal Box Test Fixture with Covers Removed and DET10C Installed
Black insulating foam lines the interiors of both boxes. 
A: Outer Aluminum Box; B: Inner Aluminum Box; C: Thermistor; D: DET10C; E: Power Adapter of the DET1B Power Supply; F: BNC-to-BNC Feedthrough for the DET1B, G: BNC-to-Triax Feedthrough for the Detector Signal; H: BNC-to-BNC Feedthrough for the Thermistor

Experimental Setup
The experimental setup was designed to measure the dark current of the photodiodes as a function of the reverse bias voltage while mitigating the contributions from noise sources. The setup controlled the temperature of the detector, blocked light from reaching the detector, and established an electrical path between the detector and the Keithley 6487 ammeter, which isolated the measured current from extraneous electromagnetic interference (EMI) noise sources. Dark currents for these detectors can be on the order of nA, which made it important to control these conditions to ensure accurate measurements.

The detector enclosure consisted of a pair of nested aluminum metal boxes, which are shown with their covers off in Figure 1.4. Only one box would be required to create a light-tight environment for the photodiode, but an outer box was included in the setup to shield the inner box from EMI related noise. The detector was placed inside the inner box, and electrical connections were made through it using BNC and BNC-to-triax adapter feedthroughs.

The measurement signal from the detector was routed through the BNC-to-triax adapter feedthrough. The BNC connector on the detector was attached directly to the BNC end of the adapter, which advantageously eliminated the need to use a BNC cable. BNC cables are poorly shielded, and using one could introduce noise from EMI sources to the signal read from the detector. This inner metal box was placed inside of an outer metal box, which possessed a triax-to-triax feedthrough. A triax cable was used to route the detector signal between the feedthroughs on the inner and outer boxes. A triax cable was also used to connect the Keithley 6487 ammeter to the end of the Triax-to-Triax feedthrough accessible on the outside of the outer box. The outer metal box served to isolate the inner metal box from EMI, and the triax cables shielded the detector signal from EMI. After covering the two boxes with their respective lids, the nested box set was placed inside an ESPEC ESX-3CW temperature chamber. The ammeter was located outside of the chamber.

A DET1B (previous generation, current generation DET2B), which includes a power supply and a power adapter designed to be inserted into the battery port of the DET series detectors, was used to supply the reverse-bias voltage, which was routed through BNC feedthroughs. The power adapter was inserted into the detector, and the opposite end of the adapter was connected to a BNC feedthrough fitted into the wall of the inner box. A BNC cable and another feedthrough were used to extend the electrical connection to the outside of the outer box, and the voltage output of the Keithley 6487 was connected to that. A computer was interfaced with the Keithley, which allowed the computer to simultaneously control the reverse-bias voltage while measuring the dark current.

As the dark current depends strongly on temperature, thermistors were used to monitor the temperatures of each photodiode continuously during the experiment. A thermistor was held in contact with the outer surface of the TO can housing using a piece of thermal tape. With the thermistor in place, it was not possible to use the detector cap to block light from reaching the photodiode. The electrical connection between the thermistor and TSP01 temperature logger was performed using BNC cables, BNC-to-BNC bulkhead feedthroughs to route the signal out of the nested boxes, and a custom BNC-to-phono jack cable to connect to the temperature logger.

Experimental Results
The data curves plotted in Figures 1.5, 1.6, and 1.7 are the dark currents measured for the Si-based DET100A, the Ge-based DET50B, and the InGaAs-based DET10C, respectively.

The Si-based DET100A does not contain an integrated voltage regulator, while both the Ge-based DET50B and the InGaAs-based DET10C do. Voltage regulators integrated into photodiodes are used to maintain a stable reverse bias voltage across the photodiode.

In general, voltage regulators are applied in a range of applications to automatically maintain a stable voltage level: the regulator ensures that the output voltage level is approximately constant as long as the voltage input falls within a specified range. When the input voltage decreases below the voltage regulator's dropout voltage, which marks the lower end of the range, the regulator is no longer able to regulate the output voltage. As the input voltage continues to decrease from the dropout voltage, the output voltage decreases until a threshold is reached and the regulator becomes non-functional. While in this off state, the output voltage is negligible. In the case of the Ge-based DET50B and the InGaAs-based DET10C, the output voltage provided by the voltage regulator is used to reverse bias the photodiode.

The absence of a voltage regulator in the Si-based DET100A results in its measured dark current, plotted in Figure 1.5, continuing to increase as the voltage supplied by the Keithley 6487 increases. The dark current measured for the DET100A increases exponentially up to a reverse-bias voltage of 0.5 V, and then it increases linearly for reverse-bias voltages between approximately 0.5 V and 10 V. A fresh battery supplies ≥10 V.

Both the Ge-based DET50B, shown in Figure 1.6, and the InGaAs-based DET10C, shown in Figure 1.7, exhibit no dark current until the reverse-bias voltage reaches a threshold (approximately 1 V for the DET50B and 1.5 V for the DET10C). The voltage range up to this threshold value corresponds to the off state of the voltage regulator. After this threshold is exceeded, the dark currents measured for these detectors increase, approximately exponentially, until the voltage supplied by the Keithley 6487 reaches the dropout regulator voltage of 5.5 V. For supplied voltages between 5.5 V and 10 V, the voltage regulator maintains a constant 5.5 V reverse-bias across the photodiodes, which results in an approximately constant values of dark current for supplied voltages over this range. As a fresh battery supplies ≥10 V, the voltage regulator has the effect of ensuring the dark current level remains approximately constant over much of the battery's lifetime, as well as protecting the photodiode from exposure to excessive reverse-bias voltages in general.

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Figure 1.5  Dark current data measured for the Si-based DET100A packaged photodiode, which does not include an integrated voltage regulator, is plotted as a function of the voltage supplied by the Keithley 6487.

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Figure 1.6  Dark current data measured for the Ge-based DET50B packaged photodiode, which includes an integrated voltage regulator, is plotted as a function of the voltage supplied by the Keithley 6487.

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Figure 1.7  Dark current data measured for the InGaAs-Based DET10C packaged photodiode, which includes an integrated voltage regulator, is plotted as a function of the voltage supplied by the Keithley 6487.

Experimental Limitations
Measurements were performed using a single representative of each detector type, as these data were intended to illustrate general trends. These data should not be taken as specific for a particular diode. The measured dark current is a function of voltage bias, temperature dependence of resistive loads, and other effects. Efforts were made to suppress their influence on these measurements, including performing the measurement in a temperature controlled chamber and using an ammeter to measure the dark current. Using the ammeter removed the need to use a load resistor, which may exhibit its own temperature dependence. Humidity was not controlled during this experiment.