Beyond Light

That SiTek makes the best PSDs available for sensing the position of a lightspot is well known. That they also function well with other types of radiation than light is a little less known. Sitek's detectors convert light energy to electrical signals that contain information about the position and intensity of the energy received.

One way of looking at light radiation is to say that light is delivered in the form of particles called "photons". Depending on how much energy a photon has, it can be classified in different spectral regions.
The photons in all of these regions have the same electromagnetic nature, but because of their varying energies, interact with matter very differently. For example, the photons within the visible region of the spectrum (approximately 400 - 700 nm wavelength) interact with the retina in the human eye to create the sensation of light in our brains. In a PSD (which is based on a Si- PIN- photodiode) that is exposed to energy greater than 1.12 eV (wavelength less than 1100 nm) electron-hole pairs are created. The p-n junction field separates these photogenerated carriers and a current proportional to the number of electron-hole pairs created is divided up between the detector electrodes and flows through an external circuit.

Unfortunately at shorter wavelengths (and thus higher energies) the photons find it hard to penetrate the Si base material to create carriers. This means that the number of carriers generated per impinging photon will decrease at these short wavelengths. Hence the output current will drop as the wavelength goes down. A normal PSD will respond down to wavelength of about a 400 nm.
The trick is to bring the p-n junction as close to the sensing surface of the PSD as possible. This means that the region (called the "dead layer" or "window thickness") that the radiation has to cross to reach the p-n junction field is as thin as possible. Dead layers as thin as 50 nm have successfully been accomplished in our processes, and as our customers will tell you, been used with great success in position detectors for higher energy radiation such as X-ray or gamma ray photons.

Theoretically the detection efficiency should be close to 100 % at 10 keV falling to approximately 1% at 150 keV. For energies above approximately 60 keV, photons interact almost entirely through Compton scattering. For this reason the overall detection efficiency is maintained at a fairly constant 1% over a wide range of photon energies.

Recently SiTek introduced the ES (Enhanced Sensitivity) components. These components have a built in amplification which gives more current output for a given energy input compared to the standard PIN-photodiodes. The new components are really large area phototransistors but the interesting thing is that this technology can also be applied to all our UV and "Nuclear" detectors boosting the sensitivity about 50 times. Thus a response value of 10 A/W at a wavelength of 200 nm is quite possible to reach. The same goes for detectors used at higher energies. The ES technology can be applied to PSDs as well as to ordinary photodiodes.

J. Räisänen, O. Harju, G. Tjurin, I. Riihimäki and L. Lindholm: " Electron Detection by Resistive Charge Division Based Position Sensitive Detectors".
To be published in November 2001 issue of Review of Scientific Instruments journal M Lindroos and Ö Skeppstedt: "A Position Sensitive Photon Detector Used as a Charged Particle Detector".
Nucl. Instr. and Meth. A 306 (1991) 225.
P.Röjsel: "Position-Sensitive Detector for Synchroton Radiation".
Nucl. Instr. and Meth. in Physics Research. A 290 (1990) 603A.L. Hanson, W. Kwiatek and K.W. Jones: "The Use of a SiTek Position Sensitive Detector for Synchroton Radiation Beam Monitoring and Alignment". Nucl. Instr. and Meth. in Physics Research. A 260 (1987) 529.

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