Photoemissive and semiconductor detectors are able to follow rapid changing radiation levels, due to extremely rapid direct photo-electron interaction. Compared to thermal detectors, detectivity is typically better, but over a limited range of wavelengths. All detection mechanisms of photoemissive and semiconductor detectors are wavelength dependent.
Photomultiplier Tubes (PMTs)
A photoemissive detector involves interaction of light with the electrons in the detector material. An absorbed photon releases an electron, while the excess energy is transformed into kinetic energy of an electron. Electrons having adequate kinetic are expelled from the surface, generating the cathode photocurrent in photomultiplier tubes.
The application of voltage drives the electrons towards the anode, generating a current over six to eight orders of magnitude proportional to light intensity. The photocurrent is amplified by the electron multiplier through secondary emission. The sensitivity of PMTs are higher than any other detector in the visible and near UV regions.
A schematic representation of an end-on tube is illustrated in Figure 1. The end-on tubes have larger and more homogeneous photosensitive areas. The Oriel side-on tubes contain similar components in a tight configuration, thus enabling easy packaging and eliminating certain environmental sensitivities of these detectors.
Figure 1. Schematic of a photomultiplier tube.
With faster risetimes, side-on tubes can achieve higher responsivities with opaque photocathodes, which eliminates the optical losses experienced by the semitransparent photocathodes of the end-on tubes. The fastest Oriel side-on tubes employ a semitransparent photocathode for achieving even closer packaging of the electron-multiplying dynode chain.
Oriel Instruments offers magnetic shielding in its housings to achieve significant improvement in measurement reproducibility. Narrow bandwidth AC detection in the Oriel Instruments’ Merlin™ Digital Lock-in Amplifier further reduces their, already low, noise equivalent power (NEP) levels. Photon counting mode of operation enables achieving NEP levels of 10-19 W Hz-1/2.
Junction Detectors or Photodiodes
Junction detectors (or photodiodes) consist of a p-n junction, and can be utilized in the photoconductive mode with the application of a reverse bias, or in the photovoltaic mode without a bias. Photocurrent amplification is possible with an avalanche ionization process in properly designed structures, when back biased at near breakdown voltage.
In the photovoltaic mode, a voltage is produced due to migration of the electron-hole pairs towards opposite sides of the junction. In the photoconductive mode, the application of a reverse bias across the junction facilitates the light to generate electron-hole pairs, which considerably improve the conductance. The current generated by the bias, and free carriers, is in proportion to the light intensity over a broad range.
Silicon photodiodes are the widely used light detectors in instrumentation, with a spectral range covering the visible, UV, and near infrared regions. It is easy to get a signal, thanks to the excellent inearity and dynamic range. The typical structure of a silicon photodiode is depicted in Figure 2. Electrons and holes are produced near the junction when photons are passed through the thin top layer. The junction drives the electrons into the n material, and holes into the p material, causing a voltage difference between the two regions, or generating a current when the two regions are linked through external circuitry.
Figure 2. Model of a silicon photodiode. The junction between the p+ and depletion regions give this detector its name.
The photovoltaic mode has better NEP at low frequencies, due to increased 1/f noise with bias. Operation with reverse bias (photoconductive mode) lowers junction capacitance and improves the response rate of the diode. The photoconductive mode is suitable for pulsed detectors. In the photovoltaic mode, a linear output can be achieved using a low-impedance load resistor. This makes the optimal output voltage very low, to attain a wide dynamic range in the use of an oscilloscope as a monitor.
A transimpedance amplifier is able to address most of the linearity limitations by yielding near ‘zero’ load impedance, generating higher output voltage, and also limiting the high frequency response of the system. Achieving a wide range linear performance is much easier in the back-biased mode of operation. Oriel Instruments offers 6V back-bias from durable lithium batteries for its biased detectors.
GaN and InSb Photodiodes
GaN diodes are wide-band gap diodes delivering UV sensitivity with the rejection of VIS-IR background, thereby enabling measurements in this difficult wavelength range. InSb diodes are liquid nitrogen cooled detectors employed in the Oriel FT-IR instruments, yielding unprecedented performance in the 2-5µm range.
Ge, InGaAs, and Extended InGaAs Photodiodes
Ge, InGaAs, and extended InGaAs photodiodes have found most applications in the NIR range of 0.8-2.5µm. Their noise performance level is inferior to silicon diodes. For that reason, Oriel Instruments offers TE cooled versions of them, in addition to the room temperature versions.
HgCdZnTe photodiodes are small-band gap IR photodetectors. The current biased versions of HgCdZnTe photodiodes show more compatibility than back biased junction diodes, for use with the PbS and PbSe photoconductors. Oriel Instruments offers these detector for the wavelength range of 2-12µm, with optical immersion, for most of them to make use of the detector active area effectively.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.