Brief Introduction and Guidance of Characterization for Photodiodes

What is a photodiode?

　　Sensors have a wide range of types and applications. They are generally classified according to the physical properties as shown below:

Physical Properties	Types of Sensors
Light	Light Sensor, UV Sensor, Image Sensor
Sound	Ultrasonic Sensor
Electricity	Electricity Sensor
Magnetic	Magnetic Sensitivity Sensors, Magnetoresistive Sensors, Magnetic Sensors
Temperature	Temperature Sensor
Humidity	Humidity Sensor
Gas	Gas Detector
Pressure	Pressure Sensor
Movement	Displacement Sensor, Linear Displacement Sensor, Acceleration Sensor, Collision Sensor, Vibration Sensor

　　With the advent of the AI and 5G era, the demand for optical sensing in application fields such as image recognition, machine vision, and self-driving cars is rapidly increasing. Therefore, this article will focus on the photo sensor, and will especially aim at the introduction of photodiodes.

　　A photodiode is a semiconductor p–n junction device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas.

　　Photodiodes usually have a slower response time as their surface area increases. Traditional solar cell used to generate electric power is a large area photodiode.

　　Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specially as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.

Photodiode Types

　　Refer to the RF Wireless World website, photodiodes are roughly divided into the following four types, namely PIN photodiode, avalanche photodiode, p-n photodiode and Schottky photodiode:

PIN Photodiode: This photodiode type has undoped semiconductor layer (viz. intrinsic) between p-doped and n-doped layers. Hence it is known as PIN photodiode. It is more sensitive than regular PN photodiode. Moreover, it has faster response than PN photodiode. Many of the photodiodes available now-a-days are of PIN type. If the I-layer material is a low-doped P-type semiconductor, the diode can be called a π-type PIN diode; if the I-layer material is a low-doped N-type semiconductor, the diode can be called a ν type PIN diode. In PIN diodes, the P and N layers are usually composed of highly doped semiconductor materials. Due to the existence of the I layer, PIN diodes usually have a wider depletion layer, larger junction resistance and smaller junction capacitance than ordinary diodes. In RF and microwave level circuits, PIN diodes are often used as microwave switches, phase shifters and attenuators.

Schematic diagram of PIN diode

Avalanche Photodiode (APD): When light falls on undoped part of the avalanche photodiode, it triggers generation of electron-hole pairs. The migration of electrons toward avalanche region increases their velocity due to cumulative field strength. As a result they collide with crystal lattice and create further pairs of electrons and holes. Due to this behavior, avalanche photodiode is more sensitive compare to PIN photodiode. However higher sensitivity makes avalanche photodiode vulnerable to electrical noise. Moreover, it is affected by heat. To overcome this drawbacks, guard ring is enclosed around p-n junction of avalanche photodiode and heat sink is used. The APD can obtain an internal current gain of about 100 by using the effect of ionization collisions (avalanche breakdown) after applying a higher reverse bias voltage (typically 100-200 V for silicon materials). Some silicon APDs use a different doping technique than traditional APDs, allowing higher voltages (>1500 V) to be applied without breakdown, resulting in greater gain (>1000). In general, the higher the reverse voltage, the greater the gain. APD is mainly used for laser rangefinders and long-distance optical fiber communication. In addition, it has also begun to be used in fields such as positron tomography and particle physics. APD arrays have also been commercialized.

Schematic diagram of APD

p-n Photodiode: When a P-type semiconductor and an N-type semiconductor are combined to form a p-n junction diode, the holes in the P-type material and the electrons in the N-type material will combine at the junction surface. There is a lack of carriers in the region, forming a depletion region or a space charge region near the junction surface, as shown in the figure below. Don’t think that the electrons in the N-type semiconductor will continue to combine with the holes of the P-type semiconductor through the junction until all the electrons and holes disappear. The actual situation is that the N-type semiconductor near the junction loses some electrons and become positive ions; P-type semiconductors lose some holes and become negative ions. These positive and negative ions will be concentrated near the junction surface, preventing the continued combination of electrons and holes (positive ions repelling holes, negative ions repelling electrons), and reach equilibrium. There are only ions near the surface, no carriers (electrons or holes).

Schematic diagram of p-n photodiode

Schottky Photodiode: The figure depicts Schottky Photodiode As shown thin metal layer replaces either P-region or N-region of the diode. Hence it is known as “metal-semiconductor diode”. Depending upon semiconductor and metal, a barrier is formed at the interface of these two materials. This barrier results into bending of the bands. Due to application of voltage, the bands can be bended more or less. In this region of band bending, electron hole pairs can easily be separated.

Schematic diagram of Schottky Photodiode

How does a photodiode work?

　　Photodiodes are semiconductor devices that convert optical signals into electrical signals. Its core part is also a p-n junction. Compared with ordinary diodes, the difference in structure is that in order to facilitate receiving incident light, the area of the PN junction should be made as large as possible, the electrode area should be as small as possible. The depth of p-n junction is very small, generally less than 1 micron.

　　Wikipedia explains how photodiodes work: A photodiode is a PIN structure or p–n junction. When a photon of sufficient energy strikes the diode, it creates an electron–hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction’s depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. The total current through the photodiode is the sum of the dark current (current that is generated in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.

Photovoltaic mode: When the bias voltage is 0, the photodiode works in the photovoltage mode. At this time, the current flowing out of the photodiode is suppressed, and the potential difference between the two ends accumulates to a certain value.
Photodiode mode: When operating in this mode, the photodiode is often reverse biased, drastically reducing its response time, but the noise increased. At the same time, the width of the depletion layer is increased, thereby reducing the junction capacitance, which also reduces the response time. Reverse bias causes a small amount of current (saturation current), which is in the same direction as the photocurrent. For a specified spectral distribution, the photocurrent is linearly proportional to the incident light illuminance.

　　In general, the work of the photodiode is an absorption process, which converts the light into the reverse current. Photocurrent is the sum of the light-generated current and dark current. Thus minimizing the dark current means improving the sensitivity of the photodiode to light. The intensity of the light is proportional to the photocurrent, so the optical signal can be converted into an electrical signal.

I-V characteristic of a photodiode (From Wikipedia)

How to evaluate the quality of photodiodes? What parameters are usually tested?

　　　When the photodiode exhibits the characteristics of high responsivity, small dark current, short response time, small junction capacitance, and low equivalent noise power, it will be evaluated as a good photodiode. These aforementioned key performance parameters are described as follows:　

Spectral responsivity: Responsivity is defined as the ratio of photocurrent produced in photoconductive mode to excitation illumination, in amperes per watt (A/W). The response characteristic can also be expressed as the quantum efficiency, as shown in the figure below, which is the ratio of the number of carriers generated by illumination to the number of photons excited by the illumination.

EQE spectra of different wavelength-responsive devices

Dark Current: The dark current is the current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.

Various Dark IV curve scanned

Response Time: The response time is the time required for the detector to respond to an optical input. A photon absorbed by the semiconducting material will generate an electron–hole pair which will in turn start moving in the material under the effect of the electric field and thus generate a current. The finite duration of this current is known as the transit-time spread and can be evaluated by using Ramo’s theorem. One can also show with this theorem that the total charge generated in the external circuit is e and not 2e as one might expect by the presence of the two carriers. Indeed, the integral of the current due to both electron and hole over time must be equal to e. The resistance and capacitance of the photodiode and the external circuitry give rise to another response time known as RC time constant (τ =RC). This combination of R and C integrates the photoresponse over time and thus lengthens the impulse response of the photodiode. When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission.

Response Time of Constant-Intensity Pulsed Light

Noise-equivalent Power (NEP): Noise-equivalent power is the minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. NEP is essentially the minimum detectable power. The related characteristic detectivity (D) is the inverse of NEP (1/NEP) and the specific detectivity (D*) is the detectivity multiplied by the square root of the area (A) of the photodetector (D*=DA^0.5) for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system. Although it is traditional to give (D*) in many catalogues as a measure of the diode’s quality, in practice, it is hardly ever the key parameter. When a photodiode is used in an optical communication system, all these parameters contribute to the sensitivity of the optical receiver which is the minimum input power required for the receiver to achieve a specified bit error rate.

Noise-equivalent power for different devices

The detectivity of different devices

Linearity Dynamic Range (LDR): LDR is an important index to evaluate the characteristics of optoelectronic devices. The change of responsivity (mA/W) can be obtained from the test of photocurrent and light intensity, which is a parameter commonly used to characterize the quality of optoelectronic devices.

LDR measurement results

-3 dB Frequency Response: -3 dB means that when the modulation frequency of the light source increases, the device response cannot keep up with the switching change of the light source, and the response photocurrent decreases accordingly. The intensity -3 dB is considered an indicator of photodetector’s performance.

Application of photodiode

p–n photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices (CCD), and photomultiplier tubes. They may be used to generate an output which is dependent upon the illumination (analog for measurement), or to change the state of circuitry (digital, either for control and switching or for digital signal processing).

Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.

Photosensors of all types may be used to respond to incident light or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a light-emitting diode (LED), either to detect the presence of a mechanical obstruction to the beam (slotted optical switch) or to couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance.

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors. They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters.

　　In addition, in response to the rapid rise of demand for image recognition, machine vision, self-driving cars, etc. in the 5G era, the application of photodiodes in the following two categories is very important:

Ambient Light Sensing (ALS): In pursuit of a better user experience, all smartphones and new cars are equipped with ALS as standard. This allows mobile phone panels and car dashboards to automatically adjust the brightness based on ambient light sensing signals. It allows users to clearly read the information displayed on the mobile phone panel or the car dashboard whether they are entering or leaving the room with their mobile phones, or driving in and out of tunnels.

Ambient Light Sensors (ALS) have become standard in modern smartphones and new automotive digital dashboard

2. 3D Sensing: According to research by Yole Development, the market size of 3D Sensing will reach a compound annual growth rate (CAGR) of 20% from 2019 to 2025. Using Direct Time of Fly (dToF) technology with Single Photon Avalanche Diode Array (SPAD Array) is the current mainstream solution.

Estimation of 2019~2025 3D Sensing Market Size (from Yole Development

What are the latest developments in photodiodes? What are the new types of photodiodes?

　　At present, there are three main types of new photodiodes: OPD, QD PD & PPD, which are detailed as follows:

Organic Photodiode (OPD):

　　Organic photodiode devices are built on materials composed of carbon-based molecules or polymers rather than traditional inorganic semiconductors such as silicon. Its manufacturing facility uses simple solutions and inkjet printing technology, so it is cheaper and simpler than equipment for manufacturing conventional electronic devices. Currently, this technology is widely used in displays, solar cells and other devices.

　　The application of polyethyleneimine has shown that the devices produced by it have low levels of dark current, that is, the photodetector can be used to capture the weak signal of visible light.

　　Organic photodiodes are used in the pulse oximeter, which is now used to measure heart rhythm and blood oxygen concentration. Using multiple organic photodiodes working simultaneously, the light intensity is 10 times lower than conventional devices. In this way, wearable health monitors can generate better physiological information and do not require frequent battery changes for continuous monitoring. Other applications include touchless gesture recognition and control of human-machine interfaces.

　　Organic photodiodes can display electronic noise current values in the range of tens of femtoamps, and can display equivalent noise power values in the hundreds of femtowatts. In addition to response time, the main performance indicators of organic photodiodes are comparable to those of silicon, so many researchers continue to work to improve their application prospects.

Quantum Dot Photodiode (QD PD):

　　　Compared with traditional semiconductor infrared multispectral detectors, solution-processable and wide-spectrum tunable colloidal quantum dots (CQDs) are more suitable for fabricating various low-cost and high-performance optoelectronic devices. The double-ended colloidal quantum dot dual-band detector is used to realize dual-band infrared imaging. In this detector, the photodiode is still the key element.

1. Using thin layers of colloidal nanocrystals of Ag₂Te and Bi₂Se₃ to develop a double-ended colloidal quantum dot dual-band detector, which can produce spatially stable p and n doping at the interface. The high-density colloidal quantum dot rectifier photodiode adopts an n-p-n structure with a small hole tunnel barrier in the middle. Two sizes of HgTe quantum dots were selected: short-wave infrared (<2500 nm, SWIR) and mid-wave infrared (3000-5000 nm, MWIR).
2. Able to provide switchable spectral response over two different frequency bandwidths, by changing the polarity and amplitude of the bias voltage, the dual-frequency detector can quickly switch between SWIR mode and MWIR mode (up to 100 kHz). Experiments show that quantum dot photodiodes have excellent performance in dual-frequency infrared imaging and long-distance temperature monitoring.

Perovskite Photodiode (PPD):

　　In recent years, perovskite materials have attracted more and more attention. The most commonly studied materials are Methylammonium lead halide (MAPbX₃, X=Cl, Br, and I) and other perovskite structures. The reason why perovskites have attracted attention is that they have good optoelectronic properties and cheap. PPD can be produced at room temperature, and can save energy loss.

　　The first research team to evaluate the application of halide perovskite materials in photodiodes was Professor Yang Yang from UCLA. In his research, the effect of the interface layer on the device performance was clearly compared. In the study, it was pointed out that adding a Hole Blocking Layer on top of the PCBM will greatly improve the device characteristics. The materials compared include Bathocuproine(BCP) and Poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2, 7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN).

　　Gerwin H. Gelinck, Eindhoven University of Technology, Netherlands, Eric A. Meulenkamp, Netherlands Organization for Applied Scientific Research, et al November 2021: The front panel and the oxide thin film transistor (TFT) back panel of the solution-processed metal halide perovskite photodetector (PPD) constructed a graphic array (VGA; 640×480 pixels) scanner, which could capture full-color images and high-resolution fingerprints. Through the optimization of the PPD front panel and the use of pixel ECL, low dark current density of 10⁻⁶ mA cm⁻², 66% external quantum efficiency (EQE) and high photodetection rate of 1.3×10¹² Jones have been obtained in the visible spectrum. The research team used photodetector characteristic analyzers for analysis. The research results showed that high photodetection can be achieved in the wavelength range of 550 nm to 770 nm when the low noise current was combined with high external quantum efficiency. The researchers also showed that the imager could be used for document scanning and biometric fingerprint recognition. Furthermore, it could be wrapped around objects with radii as small as 0.6 cm.

What are the challenges in measuring new types of photodiodes?

　　Conventional quantum efficiency systems face many testing challenges in novel photodetectors. Such as:

The bias voltage cannot exceed 12V: The traditional quantum efficiency system uses a lock-in amplifier, and its withstand DC voltage cannot be too large. Therefore, in a general quantum efficiency tester, the electric bias cannot be applied to exceed 12V.
Unable to do noise frequency analysis.
NEP and D* cannot be measured directly.

　　Enlitech provides a complete solution for the new generation of photodetectors (PD), named PD-QE. The PD-QE system is a product developed by Enlitech on the basis of the small light spot (power mode) in the past ten years.

　　With the rise and popularity of 5G and mobile devices, more and more new light sensors are being used in our daily life. In order to be better used in mobile devices, the photosensitive area of the components of these advanced light sensors is getting smaller and smaller. However, these applications require higher performance of advanced light sensors. Along with the process of shrinking the photosensitive area, it also brings the challenge of accurate measurement of quantum efficiency. For example, the focal point shift caused by dispersion difference can reach the mm level for small light spots with different wavelengths. It is difficult to focus all photons into a photosensitive area on the micrometer scale. Therefore, it is difficult to accurately measure the full spectrum quantum efficiency curve. APD-QE adopts exclusive beam space homogenization technology and uses the ASTM standard “Irradiance Mode” test method to form a complete full-spectrum quantum efficiency test solution for micron-scale optical sensors with various advanced probe stations. APD-QE has been used in the testing of various advanced light sensors, such as iPhone LiDAR and its various light sensors, blood oxygen light sensor of Apple Watch, TFT image sensor, active pixel sensor (APS), high-sensitivity indirect conversion X-ray sensor, etc.