Description Silicon Avalanche Photodiodes PerkinElmer Type C30902E avalanche C30902E, C30902S, C30921E, C30921S EVERYTHING High Speed Solid State Detectors for Fiber Optic and Very Low Light-Level Applications IN A photodiode utilizes a silicon detector chip fabricated with a double-diffused "reachthrough" structure. This structure provides NEW high responsivity between 400 and 1000 nm as well as extremely fast rise and fall times LIGHT. at all wavelengths. Because the fall time characteristics have no "tail”, the responsivity of the device is independent of modulation frequency up to about 800 MHz. The detector chip is hermetically-sealed behind a flat glass window in a modified TO18 package. The useful diameter of the photosensitive surface is 0.5 mm. PerkinElmer Type C30921E utilizes the same silicon detector chip as the C30902E, but in a package containing a lightpipe which allows efficient coupling of light to the detector from either a focussed spot or an optical fiber up to 0.25 mm in diameter. The internal end of the lightpipe is close enough to the detector surface to allow all of the illumination exiting the lightpipe to fall within the active-area of the detector. The hermetically-sealed TO-18 package allows fibers to be epoxied to the end of the lightpipe to minimize signal losses without fear of endangering detector stability. The C30902E and C309021E are designed for a wide variety of uses including optical communications at data rates to 1 GBit/second, laser range-finding, and any other applications requiring high speed and/or high responsivity. Features • High Quantum Efficiency 77% Typical at 830 nm • C30902S and C30921S in Geiger Mode: Single-Photon Detection Probability to 50% Low Dark-Count Rate at 5% Detection Probability - Typically 15,000/second at +22°C 350/second at -25°C Count Rates to 2 x 106/second • Hermetically Sealed Package • Low Noise at Room Temperature C30902E, C30921E - 2.3 x 10-13 A/Hz1/2 C30902S, C30921S - 1.1 x 10-13 A/Hz1/2 • High Responsivity - Internal Avalanche Gains in Excess of 150 • Spectral Response Range - (10% Points) 400 to 1000 nm • Time Response - Typically 0.5 ns • Wide Operating Temperature Range - -40°C to +70°C C30902E, C30902S, C30921E, C30921S The C30902S and C30921S are selected C30902E and C30921E photodiodes having extremely low noise and bulk dark-current. They are intended for ultra-low light level applications (optical power less than 1 pW) and can be used in either their normal linear mode (VR < VBR) at gains up to 250 or greater, or as photon counters in the "Geiger" mode (VR > VBR) where a single photoelectron may trigger an avalanche pulse of about 108 carriers. In this mode, no amplifiers are necessary and single-photon detection probabilities of up to approximately 50% are possible. Photon-counting is also advantageous where gating and coincidence techniques are employed for signal retrieval. Figure 1. Typical Spectral Responsivity at 22°C Optical Characteristics C30902E, C30902S (Figure 13) Photosensitive Surface: Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Circular Useful area . . . . . . . . . . . . . . . . . . . . . . . . . .0.2 mm2 Useful diameter . . . . . . . . . . . . . . . . . . . . . . .0.5 mm Field of View: Approximate full angle for totally illuminated photosensitive surface . . . . . . . . .100 deg C30921E, C30921S (Figure 14) Numerical Aperture of Light Pipe . . . . . . . . . . . . .0.55 Refractive Index (n) of Core . . . . . . . . . . . . . . . . .1.61 Lightpipe Core Diameter . . . . . . . . . . . . . . . . . . . .0.25 mm Figure 2. Typical Quantum Efficiency vs. Wavelength Maximum Ratings, Absolute-Maximum Values (All Types) Reverse Current at 22°C: Average value, continuous operation 200 µA Peak value (for 1 second duration, non-repetitive) 1 mA Forward Current, IF at 22°C: Average value, continuous operation 5 mA Peak value (for 1 second duration, non-repetitive) Maximum Total Power Dissipation at 22°C 50 mA 60 mW Ambient Temperature: Storage, Tstg -60 to +100°C Operating, TA -40 to +70°C Soldering (for 5 seconds) 200°C Figure 3. Typical Noise Current vs. Gain C30902E, C30902S, C30921E, C30921S Electrical Characteristics1 at TA = 22°C C30902E, C309021E Min Typ Max Breakdown voltage, VBR Temperature Coefficient of VR for Constant Gain Gain Responsivity: At 900 nm At 830 nm Quantum Efficiency: At 900 nm At 830 nm Dark Current, Id Noise Current, in: 2 f = 10 kHz, ∆f = 1.0 Hz Capacitance, Cd Rise Time, tr: RL = 50Ω, λ = 830 nm, 10% to 90% points Fall Time: RL = 50Ω, λ = 830 nm, 90% to 10% points Geiger Mode (See Appendix) Dark Count Rate at 5% Photon Detection Probability 3 (830 nm): 22°C -25°C Voltage Above VBR for 5% Photon Detection Probability 3 (830 nm) (See Figure 8) Dead-Time Per Event (See Appendix) After-Pulse Ratio at 5% Photon Detection Probability (830 nm) 22°C 4 C30902S, C30921S Min Typ Max Units - 225 - - 225 - V 0.5 - 0.7 150 0.8 - 0.5 - 0.7 250 0.8 - V/°C 55 70 65 77 - 92 117 108 128 - A/W A/W - 60 77 1.5x10-8 3x10-8 (Figure 6) - 60 77 1x10-8 3x10-8 (Figure 6) % % A - 2.3x10-13 5x10-13 (Figure 3) 1.6 2 - 1.1x10-13 2x10-13 (Figure 3) 1.6 2 A/Hz1/2 - - pF - 0.5 0.75 - 0.5 0.75 ns - 0.5 0.75 - 0.5 0.75 ns - - - - - - - - 2 - V - - - - 300 - ns - - - - 2 15 % 15,000 30,000 350 700 cps cps Note 1. At the DC reverse operating voltage VR supplied with the device and a light spot diameter of 0.25 mm (C30902E, S) or 0.10 mm (C30921E, S). Note that a specific value of VR is supplied with each device. When the photodiode is operated at this voltage, the device will meet the electrical characteristic limits shown above. The voltage value will be within the range of 180 to 250 volts. Note 2. The theoretical expression for shot noise current in an avalanche photodiode is in = (2q (Ids + (IdbM2 + PoRM) F) Bw)1/2 where q is the electronic charge, Ids is the dark surface current, Idb is the dark bulk current, F is the excess noise factor, M is the gain, Po is the optical power on the device, and Bw, is the noise bandwidth. For these devices F = 0.98 (2-1/M) + 0.02 M. (Reference: PP Webb, RJ McIntyre, JJ Conradi, "RCA Review", Vol. 35 p. 234, (1974)). Note 3. The C30902S and C30921S can be operated at a substantially higher Detection Probabilities. See Appendix. Note 4. After-Pulse occurring 1 microsecond to 60 seconds after main pulse. C30902E, C30902S, C30921E, C30921S Figure 4. Typical Responsivity at 830 nm vs. Operating Voltage Figure 5. Typical Gain-Bandwidth Product vs. Gain Figure 6. Typical Dark Current vs. Operating Voltage (V < VBR) Figure 7. Avalanche Photodiode Response to a 100 ps Laser Pulse as Measured with a 350 ps Sampling Head. (Horizontal Axis: 200 ps/Division) Figure 8. Gelger Mode, Photoelectron Detection Probability vs. Voltage Above VBR (VR > VBR) Figure 9. Passively Quenched Circuit and Resulting Pulse Shape C30902E, C30902S, C30921E, C30921S Figure 10. Load Line for C30921S in the Geiger Mode Figure 11. Typical Dark Count vs. Temperature at 5% Photon (830 nm) Detection Efficiency Figure 12. Chance of an After-Pulse within the Next 100 ns vs. Delay-Time in an Actively Quenched Circuit. (Typical for C30902S, C30921S at VBR + 25) Modified TO-18 Package. Note: Optical distance is defined as the distance from the surface of the silicon chip to the front surface of the window. Figure 13. Dimensional Outline - C30902E, C30902S, C30921E, C30921S Figure 14. Cutaway of the C30921E, C30921S Dimensions in millimeters. Dimensions in parentheses are in inches. Appendix Operation of the C30902S and C30921S in the Geiger Mode Passive-Quenching Circuit The simplest, and in many cases a perfectly adequate method Introduction of quenching a breakdown pulse, is through the use of a current limiting load resistor. An example of such a "passive" When biased above the breakdown voltage, an avalanche quenching circuit is shown in Figure 9. The load-line of the photodiode will normally conduct a large current. However, circuit is shown in Figure 10. To be in the conducting state at if the current is such that the current is limited to less than VBR two conditions must be met: a particular value (about 50 µA for these diodes), the current is unstable and can switch off by itself. The explanation of this behavior is that the number of carriers in the avalanche region at any one time is small and fluctuating wildly. If the number happens to fluctuate to zero, the current must stop. It subsequently remains off 1. The avalanche must have been triggered by either a photoelectron or a bulk-generated electron entering the avalanche region of the diode. (Note: holes are inefficient at starting avalanches in silicon.) The probability of an avalanche being initiated is discussed above. until the avalanche pulse is retriggered by a bulk- or photo- 2. To continue to be in the conducting state a sufficiently large generated carrier. current, called the latching current ILATCH, must be passing through the device so that there is always an electron or hole in The C30902S and C30921S are selected to have small bulk-generated dark-current. This makes them suitable for low-noise operation below VBR or of photon-counting above VBR in the Geiger mode. In this so-called Geiger mode, a single photoelectron (or thermally-generated electron) may trigger an avalanche pulse which discharges the photodiode from its reverse voltage VR to a voltage slightly below VBR. The probability of this avalanche occurring is shown in Figure 8 as the "Photoelectron Detection Probability" and as can be seen, it increases with the avalanche region. Typically in the C30902S and C30921S, ILATCH =50 µA. For currents (VB-VBR)/RL, much greater than ILATCH, the diode remains conducting. If the current (VRVBR)/RL, is much less than ILATCH, the diode switches almost immediately to the non-conducting state. If (VB-VBR)/RL, is approximately equal to ILATCH, then the diode will switch at an arbitrary time from the conducting to the non-conducting state depending on when the number of electrons and holes in the avalanche region statistically fluctuates to zero. reverse voltage VR. For a given value of VR-VBR, the Photoelectron Detection Probability is independent of When RL is large, the photodiode is normally nonconducting, and the operating point is at VR - IdsRL in the non-conducting temperature. To determine the Photon Detection state. Following an avalanche breakdown, the device recharges Probability, it is necessary to multiply the Photoelectron to the voltage VR - IdsRL with the time constant CRL where C is the total device capacitance including stray capacitance. Detection Probability by the Quantum Efficiency, which is shown in Figure 2, the Quantum Efficiency also is relatively independent of temperature, except near the 100 nm cutoff. The C30902S and C30921S can be used in the Geiger mode using either "passive" or "active" pulse quenching circuits. The advantages and disadvantages of each are discussed below. Using C = 1.6 pF and RL = 200.2 KΩ a recharge time constant of 0.32 microseconds is calculated, in reasonable agreement with observation as shown in Figure 9. As is also evident from Figure 9, the rise-time is fast, 5 to 50 ns, decreases as VRVBR increases, and is very dependent on the capacitances of the load resistors, leads, etc. The jitter at the half-voltage point is typically the same order of magnitude as the rise-time. For timing purposes where it is important to have minimum jitter, the lowest possible threshold of the rising pulse should be used. C30902E, C30902S, C30921E, C30921S Active-Quenching Circuit Dark Current Until the C30902S or C30921S is recharged, the probability Both the C30902S and C30921S have been selected to have a of detecting another incoming photoelectron is relatively low dark-count rate. Cooling to -25°C can reduce this by a factor low. To avoid an excessive dead-time when operating at a of 50, since the dependence of dark-count rate on temperature large voltage above VBR, an "actively quenched" circuit can is exponential. be used. The circuit temporarily drops the bias voltage for a fraction of a microsecond following the detection of an The dark-count increases with voltage following the same curve avalanche discharge. This delay time allows all electrons as the Photoelectron Detection Probability until a voltage where and holes to be collected, including most of those after-pulsing is responsible for a feedback mechanism which temporarily "trapped" at various impurity sites in the silicon. dramatically increases the dark-count rate. This maximum When the higher voltage is reapplied, there are not voltage is circuit dependent, and is not warranted other than the electrons in the depletion region to trigger another values listed on page 3. In most cases, with a delay time of 300 avalanche or latch the diode. Recharging can now be very ns, the diode can be used effectively at VR up to VBR + 25V. rapid through a small load resistor. Alternatively, the bias voltage can be maintained but the load resistor is replaced by a transistor which is kept off for a short time after an avalanche, and then turned on for a period sufficient to The C30902S and C30921S should not be forward biased or, when unbiased, exposed to strong illumination. These conditions result in a greatly enhanced dark-count which requires up to 24 hours to return to its nominal value. recharge the photodiode. After-Pulsing An after-pulse is an avalanche breakdown pulse which follows a photon-generated pulse and is induced by it. An after-pulse is usually caused by one of the approximately 108 carriers which pass through the diode because of the first avalanche. This electron or hole is captured and trapped at some impurity site in the silicon, as previously described. When this charge-carrier is liberated, usually in less than 100 ns but sometimes several milliseconds later, it may start another avalanche. The probability of an after-pulse occurring more than one microsecond later is typically less than 2% at 2 volts above VBR, using the circuit shown in Figure 9. After-pulsing increases with bias voltage. If it is necessary to reduce afterpulses, it is recommended that one keep VR-VBR low, use an actively-quenched circuit with a long delay-lime (See Figure 12), or a passively-quenched circuit with a long RLC constant. Stray capacitances must also be minimized. Electronic gating of the signal can be performed in certain situations. Should afterpulses be a serious complication in a particular application, operation below VBR with a good amplifier might be considered. For more information e-mail us at [email protected] or visit our web site at www.perkinelmer.com/opto All values are nominal; specifications subject to change without notice. © 2000 PerkinElmer, Inc. All rights reserved. 0700 is a registered trademark of PerkinElmer, Inc. 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