Knowledgebase

What is the Output Signal of a Photodiode?

Photodiode operates as a voltage source as well a current source in response to the incident light in the wavelength range of 200 nm to 1100 nm. The current measurement is preferred since the output current changes linearly with incident light power. The voltage output, however, changes logarithmically with incident light power.

How Linear Is The Output Photocurrent In The Current Source Mode?

Typically it is linear from a few pico-amps up to few milli-amps.

What is the Dynamic Range of a Typical Silicon Photodiode?

The dynamic range is the range of incident light power over which the current output from the photodiode is linearly related to the input power and is sometimes expressed in decibels: This range for a typical device is from 1 picowatt to 10 milliwatts, or 100 dB

Is the Outside of the Active Area completely insensitive to light?

No. Irradiated light on the non-active area adjacent to an active area may generate a small photocurrent in the detector. The magnitude of this signal is dependent of many parameters such as the wavelength of the radiation, the applied bias, and the amount of incident light on the active area as well as distance from the active area.

How Can The Light Absorption In The Non-Active Area Be Reduced?

The silicon non-active area also absorbs light and contributes to the total photocurrent. If this contribution is not desired, a metal shield and/or a black polyamide layer can be put on the non-active area as a part of the semiconductor wafer process.

What us the difference between the Photoconductive (PC) and Photovoltaic (PV) modes?

In the photovoltaic mode, the photodiode is neither operated with nor biasing. It is simply acting like a solar call, which converts light into electricity. In the Photoconductive mode, however, the photodiode can be Reversed Biased by voltages up to the specified maximum reverse voltage.

Which mode (PV Or PC) should I use for my application?

Applying a reverse bias in the PC mode introduces additional noise current to the generated photocurrent, therefore, reducing the signal to noise ratio. Hence, consider using operating the photodiode in PC mode for high speed applications (greater than 350 kHz) and/or in applications that require a wide dynamic range. For weak signal detection the PV mode is the preferred mode of operation.

How do you reverse bias a Photodiode?

Reverse biasing a photodiode is accomplished by setting the cathode of the detector at a higher electric potential than the anode's. In another words, applying a negative voltage to anode.

What is current non-linearity?

A photodiode is non-linear when an increase or decrease in the amount of incident light power does not generate an equal amount of photocurrent in the same photodiode with a tolerance of +/- 1%. This is the range where responsivity remains constant within +/-1%. That is: Above this range, also called dynamic range, the device may saturate completely at any incident light power level.

What is the maximum light power I can irradiate on the detector before the output becomes non-linear? At what level does a typical photodiode saturate?

The maximum light power receivable by the detector before it saturates depends on the photodiode itself, the reverse voltage applied Vbias (Volts), and the resistance R (Ohms) of the circuit in which the detector is used and the wavelength used. The maximum photocurrent created in the linear range is Imax = Vbias / R (A mperes) and the corresponding Optical power is given by: Pmax = Imax / Rl (Watts) where Rl is the responsivity of the photodiode and varies with the wavelength. In the Photovoltaic (PV) mode, the previous definition is still correct but the voltage to consider is the silicon built-in value Built-in = 0.3 Volt. In the Photoconductive (PC) mode, the reverse voltage can reach up to 30 Volts (depending on the device). It is easy to understand that PC mode photodiodes have a wider dynamic range. To give an idea, Max is between 1 and 100 milliwatts.

How can the light intensity received by the Photodiode be reduced?

To reduce the light intensity received by the photodiode, one could use optical instruments such as beam splitters, filters or polarizers placed on the optical path between the source and the detector.

How does Responsivity change with temperature?

In silicon, the absorption coefficient increases with the temperature. If the temperature goes up, the absorption depth of light decreases. As a consequence, the responsivity for short wavelengths has a negative temperature coefficient, since a significant amount of carriers could recombine and get lost in the heavily doped p+ layer. For the longer wavelengths, responsivity has a positive temperature coeffcient, since more carriers will be generated in or near the depletion region of the pn junction, and participate in the photocurrent by increasing it.

How does Dark Current (Shunt Resistance) change with temperature?

Drift current is the dominating current (dark current) in Photoconductive (PC) mode and varies directly with the temperature. In Photovoltaic (PV) mode, the diffusion current is the dominating current, which determines the Shunt resistance. It varies as square of the temperature. Thus the change in temperature affects the photodetector more in photovoltaic mode than in photoconductive mode of operation. In general, in PC mode of operation, dark current may approximately double for every 10°C increase in temperature. In PV mode of operation, however, shunt resistance may approximately double for every 6°C decrease in temperature. The exact change is dependent on additional parameters such as the applied reverse voltage, the bulk resistivity as well as the thickness of the bulk substrate.

What about the other characteristics of the Photodiodes?

Since noise currents are generated as result of dark current (shunt resistance), the higher the temperature, the higher the noise in the detector. As a consequence, VB (breakdown voltage) decreases as the temperature increases.

Can Advanced Photonix provide Photodiodes with anti-reflective coating on the glass?

Yes, anti-reflective coatings can be applied to either one or both sides of the glass to reduce glass reflection for a certain wavelength.

What is NEP and how do I use it in my design?

The Noise Equivalent Power or NEP, is the incident optical power, which generates a photocurrent equal to the noise of the photodiode. In other words, it is the optical power for which the Signal-to-Noise Ratio is equal to one. This value is used as the minimum detectable incident power.

What is the lowest optical power a Photodiode can detect?

Theoretically there is no lower limit for the optical power that a photodiode can detect. However, the dark current limits the minimum detectable optical power by reducing the signal to noise ratio to less than one.

How does Dark Current (Shunt Resistance) relate to noise?

Pn the photovoltaic mode measurements, the limiting source of noise is the Johnson (or thermal) noise in the source or shunt resistance. In other words shunt or source resistance is a noise-generating resistance. These are the charged particles produced as a result of the thermal energy in the resistance. This relation is given by: where kb is the Boltzman constant, T the temperature in Kelvin, Df the noise measurement bandwidth, and RSH the shunt resistance of the photodiode. In the Photoconductive mode, however, the dominant source of noise is the Shot noise and is related to dark current by: where IP is the photocurrent, ID the dark current, q the elementary charge and Df the noise measurement bandwidth.

What is Noise Bandwidth?

Noise is the random electrical signal that interferes with the measurement of the signal of interest. Since both shot and Johnson noises are distributed over a wide frequency range, reducing the noise bandwidth effectively reduces the noise in the measurement. The high frequency noise cutoff point is approximately equal to the smallest of either: - p/2 times the upper 3 dB frequency limit of the analog DC measuring circuitry or - 0.55/tr where tr is the photodiode's 10%-90% rise time.

What are Position Sensors?

Position Sensing Devices, better known as PSDs, are single substrate photodiodes capable of finding or locating a beam within a defined sensing area. They are used in a variety of applications from human eye movement monitoring, mirrors or machine tool alignment to vibration analysis, beam position control and more.

What are the different types of available PSDs?

Position Sensing Devices are divided into two families: the segmented PSDs and the lateral-effect PSD's.

What are Segmented PSDs?

Segmented PSDs are common substrate photodiodes divided into either two or four segments, separated by a gap or dead region. A symmetrical optical beam generates equal photocurrents in all segments, if positioned in the center.

How uniform is the response between the segments?

The response uniformity between the segments is almost perfect with XX% maximum non-uniformity between the elements.

How does the size of the gap affect the measurement?

Crosstalk between the elements is affected by the gap size between the elements and the wavelength that's being measured. A larger gapsize will result in less cross talk and shorter wavelengths produce less crosstalk. Quantifiable results depend on the specific silicon device structure.

What are Lateral-Effect PSDs?

Lateral PSDs are continuous single element planar diffused photodiodes with no gaps or dead areas. Theses types of PSDs provide direct readout of a light spot displacement across the entire sensing area.

When does one choose Segmented PSDs Vs. Lateral Effect PSDs and vice versa?

Segmented PSDs offer position resolutions better than 0.1mm and accuracy higher than lateral effect PSDs due to superior responsivity match between elements. With S.N.R. (Signal to Noise Ratio) not effecting the resolution, very low light level detection is possible. They exhibit excellent stability over time and temperature and fast response times necessary for pulsed applications. They are excellent for nulling or beam centering applications. Lateral effect PSDs have a wide dynamic range, they can measure the light position all the way to the edge of the sensor. They are independent of the light spot profile and can resolve positions better than 0.5 mm. Their resolution is detector / circuit S.N.R. dependent.

What are the different types of Lateral Effect PSDs?

Advanced Phoenix manufactures two types of lateral effect PSDs. Duo-Lateral and Tetra-Lateral structures. Both structures are available in one and two-dimensional configurations.

What are the primary differences between the Tetra-Lateral and the Duo-Lateral Structure PSDs?

In Duo-Lateral PSDs, there are two resistive layers, one at the top and the other at the bottom of the photodiode for two directional movements of X and Y. This type of structure can resolve light spot movements of less that 0.5 mm. They exhibit excellent position linearity over the entire active area. The Tetra-Lateral PSDs, however, consist of a single resistive layer, in which the photocurrent is divided in two or four parts for one or two-dimensional sensing respectively. They show smaller dark currents and faster response times compared to duo-lateral PSDs.

What is the Position Resolution?

Position resolution is defined as the minimum detectable displacement of a spot of light on the detector active area.

What is the Position Detection Error (PDE)?

Position Detection Error (PDE) or Accuracy is defined as the geometric variation between the actual position and the measured position of an incident spot of light. For all calculations, the zero is defined as the electrical center. This is the point at which I1=I2. The PDE is calculated as follows: where I1 and I2 are the photocurrents at the ends of the PSD, L is the sensing area half-length in mm, and X (mm) is the actual displacement of the light spot from the electrical center. PDE is smaller for a small active area device.

What is Position Linearity?

Position linearity refers to the accuracy of the measured position compared to the actual position of an incident light spot. Good position linearity means low Position Detection Error.

What does the Position Resolution depend on?

The position resolution depends on the light intensity, the detector/electronics noise and bandwidth. In lateral-effect PSDs, the position resolution is limited by the signal to noise ratio of the system. It varies from device to device; however, position resolutions in excess of one part in ten million have been achieved with Advanced Photonix lateral effect PSDs.

Does accuracy (PDE) depend on the spot profile, shape or intensity?

In the lateral-effect PSDs, the input light spot profile and intensity do not affect the accuracy of the devices. The incoming beam may be any size and shape, since the position of the centroid of the light spot is measured which provides output signals proportional to the displacement from the center.

How about spot size?

In lateral effect PSDs, neither position resolution nor accuracy is dependent on the spot size. However, in segmented PSDs, the light spot has to overlap all segments at all times and it can not be smaller than the gap between the segments. Furthermore, in segmented PSDs, the position linearity is dependent on beam profile uniformity.

I notice a drift in the Position Detection over a period time. What is going on?

It could be related to fluctuations of the ambient temperature or instability of the power supply source.

Does Sesolution and/or Position Detection error change with the Reverse Bias?

Position Detection Error (PDE) does not change whether the device is operated in PV mode or PC mode. However, since Position resolution is SNR dependent, the additional dark current (shot noise) due to biasing does in fact reduce the resolution.

Is the Position Detection error for Position Sensitive Detectors repeatable?

It is repeatable, but only within the maximum percentage specified for the device.

Why do I need to Bias the PSD?

Similar to a regular photodiode applying a reverse bias reduces the rise time. Furthermore, it increases the high end of dynamic range.

How does Accuracy and Resolution vary with Shot Noise?

Since in segmented photodiodes the relative position of the spot is determined by the ration between the photocurrents, the accuracy and resolution does not depend on the photodiode dark current, as long as the signal to noise ratio is above one.

What is an Avalanche Photodiode? What are the differences between AODs and PIN devices?

In very low light level applications, since a very large value feedback resistor is needed, there is Johnson current noise associated with the resistor, which reduces signal to noise ratio, which is undesirable. Silicon Avalanche Photodiodes (APDs) make use of internal multiplication instead to achieve gain. The gain is due to impact ionization. APDs are also useful when the amplifier noise is limiting.

Where does the word "Avalanche" come from?

TPDs internal gain is triggered by a high reverse bias (~200-300V usually). Due to high bias the resulting strong electric field energize the electrons with enough kinetic energy to generate more electrons by collision ionization and starting a ''chain reaction'', or ''avalanche'' process.

What are the advantages of having the Op-Amp in the same package with the Photodiode?

By having a signal amplified as close to the detector as possible, the effects of noise picked by the pins, wire leads, and other PCB conductor traces can be minimized. This is in addition to saving a lot of space on your board or any other substrate.

How do I determine what values for Rf and Cf should be chosen?

The feedback resistor, Rf, directly determines the gain in a transimpedance amplifier configuration; e.g. a 10kW resistor will amplify the signal, including noise, by a factor 10,000. The feedback capacitor, Cf, will determine the -3dB-frequency response as following: The Gain Bandwidth Product (GBP) of the op-amp limits the desired frequency response of the detector/amplifier combination. Rf and Cf should be chosen such that the -3dB frequency response to be less than the maximum frequency of the op-amp: where CA is the amplifier's input capacitance and CJ is the photodiode's junction capacitance. A large value RF, even though achieving a large gain, will reduce the bandwidth as well as introducing additional noise due to the thermal noise in the resistor. An increase of Cf, however, will increase the stability of the system, but at the expense of a slower response time.

How does the TIA bandwidth vary with load resistance?

The Transimpedance Amplifier (TIA) bandwidth is related to the load resistance per following relationship: where RL is the load resistance.

Are any of the fiber optic devices available and connected?

All of Advanced Photonix fiber optic detectors are available with a flat window, spherical micro lens (fish eye) window as well as any of the standard receptacles such as SMA, ST, FC, etc., for direct optical fiber coupling. Contact our Applications Group for further information.

What different types of packaging are Standard Photodiodes offered?

Advanced Photonix standard photodiodes are offered in three different types of packaging as well as chip only (solderable series):- Metal packages, including standard TO-18, TO-46, TO-5 and TO-8. - Plastic packages and - Mold packages.

What is the chip centering tolerance with respect to the OD of header?

Our standard products are usually centered within +/- 0.010 inches.

What is the rotational tolerance of the Photodiode Chip with respect to the header tab?

Usually +/- 1º.

How can the glass windows be cleaned?

Since most of Advanced Photonix standard photodiodes are either silicon or quartz, they can be cleaned with isopropyl alcohol and a soft optical grade pad.

Can the Photodiode be damaged by very high incident light power on the Active Area?

Once the Saturation point is reached, the behavior of the photodiode becomes more and more non-linear until the Damage threshold point is reached. At this point the photodiode is no longer able to turn excess in incident optical power into a photocurrent output and all the extra energy received by the device would be absorbed, typically as heat. This heat inside the device can easily damage it irreversibly as it endangers the fragile wire bonds on the surface of the photodiode.

What is the operating life of Silicon Photodiodes?

Photodetectors last for an indefinite period of time when used properly and within the specified specifications. However, certain applications may put the photodiodes through optical, electrical, mechanical and/or thermal stresses beyond the specified ranges, and therefore limit their useful life.

Does Responsivity change over time?

In hermetically sealed detectors, such as TO metal packages, the responsivity is not expected to change over time. In non-hermetically sealed devices, however, the atmospheric contaminants as well as humidity may be diffused into the active area and result in trapping centers causing a short in the junction.

Does Dark Current change over time?

The surface dark current component can change over time due to ambient moisture. It is also susceptible to surface cleanness, surface contamination, i.e. sodium from hand grease could increase the dark current significantly. The bulk dark current should not increase over time.

What is the responsivity matching from device to device?

Responsivity matching from device to device for our catalog items is specified with 10%.

Can Advanced Photonix provide responsivity-matched Photodiodes?

Advanced Photonix, for an additional fee, can provide responsivity matching photodiodes within a specified tolerance at a particular wavelength. Contact the Applications Group for your specific requirements.

Are the Photodiodes susceptible to EMI?

If not properly shielded, electromagnetic energy can be picked up at the pins of a device. Also, to a lesser extent, the same energy can come through unshielded windows.

What are the concerns associated with exposed or open devices, such as solderable series?

Advanced Photonix standard photodiodes are offered in three different types of packaging as well as chip only (solderable series):- Metal packages, including standard TO-18, TO-46, TO-5 and TO-8. - Plastic packages and - Mold packages.

Are there any Photodiodes available for direct measurement of X-Rays over 17.6 KeV?

No. For energies over 17.6 keV, an indirect measurement method by using scintillating crystals is recommended. You may contact our Applications Group for further information about your specific application.