With its very broad portfolio of discrete detectors and arrays, AMS Technologies offers optical detector solutions for every wavelength range, uncooled and cooled, as a single detector, PSD or detector array – complemented by assemblies with filters or amplifiers as well as complete detection subsystems that can be configured by the user.
Our exceptionally large portfolio of silicon detectors and arrays includes general-purpose silicon photodiodes as well as diodes with particularly high speed or fast response behaviour, variants with wavelength-enhanced sensitivity or silicon avalanche photodiodes (APDs). Beyond single silicon diodes, the product range comprises duo- and tetra-lateral detectors as well as silicon multi-element photodiodes and arrays. In addition to the photodiodes, our silicon detector assemblies also include filters, transimpedance amplifiers or multiple diodes in a sandwich design.
AMS Technologies carries a broad variety of detectors and arrays that are based on indium gallium arsenide (InGaAs), including general-purpose and PIN InGaAs photodiodes, as well as large active area, segmented, back-illuminated diodes or InGaAs photodiode arrays. For high-performance applications, we also provide InGaAs avalanche photodiodes (APDs). In addition to the photodiode devices, our InGaAs detector assemblies include photodiode-amplifier hybrids, mini-DIL optical receivers or pigtail assemblies.
A broad variety of NIR (near infrared) and MWIR (midwave infrared) detectors and arrays based on lead sulfide (PbS) or lead selenide (PbSe) is available, covering a wide spectral range with wavelengths from 1 to 5 µm. The portfolio encompasses PbS near-infrared (NIR) detectors, available as single-pixel, multi-single-pixel and multi-pixel versions, PbS line array detectors, available as bare chips or encapsulated modules, uncooled or TEC-cooled single-pixel PbSe detectors as well as bare chip PbSe NIR detectors, protected by a unique thin-film encapsulation for longer lifetime and highest detectivity at room temperature.
Complementing the ranges of Silicon, InGaAs, PbS and PbSe detectors and arrays, AMS Technologies offers a comprehensive array of other discrete detectors and arrays based on materials like GaAs, HgCdTe, InAs/InAsSb, GaP or AlGaAs, but also balanced photodetectors, integrated optical receiver modules or infrared detection modules.
Guiding and focusing light onto optical detectors can be done using AMS Technologies’ broad portfolio of optics components like optical lenses, optical filters or optical windows, as well as our optical fibers.
Optical detectors and detector arrays usually consist of photodiodes, i.e. semiconductor light sensors with a p-n junction or PIN junction (active area) at which incident light in the ultraviolet, visible or infrared wavelength range generates an electric current due to the internal photovoltaic effect.
If photons of sufficient energy (higher than that of the semiconductor material’s bandgap, e.g. more than 1.1 eV for silicon) hit the material of the diode, charge carriers (electron-hole pairs) are generated, which quickly drift apart in the depletion region and lead to a photocurrent. This photocurrent is linearly proportional to the light’s radiation power over many orders of magnitude until saturation. Given suitable layout and circuitry, the reaction time of a photodiode to the incidence of light is very short; it can be as short as a fraction of a nanosecond.
The main distinguishing property of detectors and arrays is the semiconductor material used – as this material or rather its bandgap (see above) determines the range of spectral sensitivity of the detector. Some materials and their spectral sensitivity:
|Silicon||190 to 1100 nm|
|GaAs||400 to 850 nm|
|InGaAs||800 to 2600 nm|
|PbS/PbSe||1000 to 3500 nm|
|HgCdTe||400 to 14000 nm|
|InAs/InAsSb||2000 to 5500 nm|
|CdTe||5000 to 20000 nm|
Lower bandgap results in more charge carriers (electron-hole pairs) being generated “spontaneously” (without any incident light) from thermal molecular movement. In order to reduce this photocurrent noise in applications with very low light intensity, some photodetectors are cooled by either liquid nitrogen or multiple-stage thermoelectric coolers (TECs).
In addition to a regular photodiode’s p- and n-doped layers, PIN photodiodes feature an additional, intermediate intrinsic layer. As a result, PIN photodiodes generally have a lower junction capacitance, a higher maximum reverse voltage and a higher bandwidth. Typical cut-off frequencies of PIN photodiodes are over 1 GHz, compared to around 10 MHz for p/n photodiodes.
The basic principle of operation for avalanche photodiodes (APDs) is also the internal photovoltaic effect creating charge carriers, but in addition, these diodes use the avalanche effect for internal amplification and thus are particularly sensitive and fast detectors. However, the high sensitivity or amplification simultaneously leads to higher noise. APDs are used for detecting very low radiant powers, down to single photons, with achievable cut-off frequencies up to the gigahertz range.
A photodiode’s output signal can be measured as voltage or current, with the current measurement being far more advantageous in terms of linearity, offset and bandwidth. The generated output photocurrent is proportional to the incident light power. Due to the high output impedance of a photodiode, a transimpedance amplifier is usually applied to convert this current into a voltage signal for further processing.
Photoconductive or Photovoltaic?
Depending on the requirements of the application, photodiodes can be operated with or without bias voltage. These operating modes are called "photoconductive mode" (unbiased) and "photovoltaic mode" (biased).
In photoconductive mode with a reverse bias voltage applied, the bandwidth and linearity of photodiodes can be considerably improved, due to the reverse voltage widening the semiconductor’s depletion area and thus reducing its junction capacitance. However, applying a bias voltage also leads to higher dark current (a small amount of photocurrent present even in full darkness) and noise levels.
If the photodiode is used in applications with low frequencies up to a few hundred kHz or for the detection of extremely low light intensities, engineers usually prefer the photovoltaic mode without any reverse bias voltage applied. In this configuration, the output current of the detector shows a lower temperature dependence, and the circuit is less complex to realize.
Multi-element Detectors, Arrays and PSDs
Multi-element detectors and detector arrays are made of multiple, separate photodiodes, lined-up on a carrier, with a pitch that can for instance be aligned to a multi-channel fiber ribbon. Linear configurations are available as well as two-dimensional arrays of e.g. 2x2 or 4x4 photodiode elements.
Position sensing detectors (PSDs) consist of a common sensor surface that is either segmented or utilizes the lateral effect to determine the exact position at which a light beam hits the sensor. While segmented PSDs carry several detector segments divided by a gap or dead region, lateral-effect PSDs consist of a single planar diffused photodiode area without gaps or dead zones.
Alternative Terms: Optical Detector; Optical Array; Avalanche Photodiode; APD; PSD; Detector Module