A photodiode (PD) is a type of diode that converts light energy into electrical energy, when incident light falls on the PD it generates a current flow directly proportional to the intensity of light. If the intensity of light at the PN junction of the PD is increased, then the reverse current also increases.
Avalanche Photodiodes (APDs) are highly sensitive semiconductor-based photodetectors that can multiply the current generated by the light falling on them via an avalanche process. As with PDs, APDs convert photons into an electrical current, but APDs have a multiplication factor that amplifies the current generated by a single photon to produce a measurable signal. This results in higher sensitivity than PDs and as such APDs are ideal for low-level light detection and photon counting.
Standard APDs are operated with a reverse voltage, which is normally just below breakdown, to achieve the maximum internal gain for the APD. In this region carriers, electrons, and holes, are excited by absorbed photons and strongly accelerated in the strong internal electric field so that they can generate secondary carriers, as shown in the diagram. The avalanche process effectively amplifies the photocurrent by a significant factor enabling APDs to be used for very sensitive detectors, which need less electronic signal amplification and are thus less susceptible to electronic noise.
Common APDs are typically available on Silicon or InGaAs and many configurations are available to provide a wide range of sensitivity and speed options, with some targeting specific wavelengths for particular applications. Applications include:
Silicon-based avalanche photodiodes are sensitive in the wavelength region from 200 to 1100 nm, with the maximum responsivity occurring around 900 nm. Depending on the device and the reverse voltage applied, the multiplication factor of silicon APDs can vary between 50 and 1000. For longer wavelengths, up to roughly 1700 nm, APDs based on germanium or indium gallium arsenide (InGaAs) are used, with typical current multiplication factors of 10 to 40. Normally InGaAs APDs are significantly more expensive than those based on germanium but exhibit superior noise performance and a higher detection bandwidth. The high absorption rate of the InGaAs coefficient allows the use of a rather thin absorbing layer. Other options include germanium/silicon (GeSi) devices, where radiation is absorbed in germanium, and the carriers are transferred into a silicon region for charge multiplication, but these are not popular due to the high dark current of the devices.
InGaAs APDs have been commercially available for several years, with the main market focus being on telecommunication applications. Primarily the application for market for these APDs was due to the ability to operate in the near-infrared optical spectrum and this was critical for applications in physical and life science, that require high sensitivity over the 900-1700 nm wavelength range.
Wavelength absorptions for APDs for various materials.
Phlux has developed a new class of InGaAs APD technology that delivers up to 12X higher sensitivity - more than an order of magnitude lower noise and greater sensitivity than traditional InGaAs APDs, with stable high-temperature performance. Allowing APD gains over greater than 100, we call the technology, “Noiseless InGaAs APD”. The APD has very fast overload recovery, 10X lower temperture drift and stable high-temperature performance The breakthrough was achieved by designing a new InGaAs APD structure and incorporating antimony (Sb) alloys into the epitaxial structure.
Several of the characteristics of high-performance (or good) APDs are all interrelated. Often there is a compromise between speed, sensitivity, noise, linearity, optical damage threshold, and APD gain. Phlux has developed a technology that allows for leading-edge performance for the three main characteristics (noise, sensitivity, and gain) required for high-performance LIDAR, Ranging, and Optical Time Domain Reflectometry (OTDR) systems.
A high-gain APD enables the detection of lower light levels or a smaller number of photons. This is achieved by multiplying the small amount light to a signal that is above the noise floor of the transimpedance amplifier (TIA). Current APDs have a useful gain of between 10 - 40 before the noise of the APD becomes dominant. Phlux APDs can operate with a gain of over 100. Typically, high gain is close to the breakdown voltage of the APD, however with Noiseless InGaAs™ APD technology the APD can achieve high gain well away from the breakdown voltage, enabling stable operation.
Receiver sensitivity is the lowest received signal power at the detector at which the signal can be decoded or recognised satisfactorily, so the lower the sensitivity the more sensitive the detector is to light. One way of measuring this sensitivity is by using Noise Equivalent Power (NEP). The NEP is the amount of optical power applied at the detector which will equal the noise of the detector or system, or in other words, the NEP is the optical power that results in an Signal-to-Noise Radio (SNR) of 1.
Low noise is essential in being able to amplify the photons/light at low levels. If the noise is such that it is higher or the same as the photons being detected, then it will just be amplified in the system and the light detection will not be possible. Therefore, low noise must be inherent in the APD if the gain can be utilised.
This backgrounder explains the basics of APDs and outlines the benefits that Phlux’s Noiseless InGaAs™ APD technology provides. The technology enables the development of extremely low noise, improved sensitivity, and high-gain detectors and with these benefits, facilitates a step change in the performance of laser range finders, LiDAR, and optical time domain reflectometry (OTDR) systems.