"How much can a PSD take?"

Several things will happen when you put a small very intense light spot onto a Position Sensing Detector (PSD). The PSD can be divided into two parts. One part transforms light into current, in other words a "photo diode". The other part has to do with the determination of the position of the incident light.

Photo Diode Characteristics:

Local saturation is photo diode related. This means that it is the photo diode that surrenders. Above saturation, the photocurrent remains constant with increasing incident light power. Theoretically, this should not affect the position sensing part of the PSD.

To avoid saturation too early, the PSD should have as high a bias as possible. The maximum bias for Sitek PSDs is set to 35 volts but our measurements has shown that many of them can sustain a 110 volts bias. The best way to find out how much bias a PSD can take is trial and error. At the maximum bias there will be a "knee" where the dark current starts to increase dramatically. Stay below that knee. Thus, the saturation behavior of the light generating part of a PSD is very similar to an ordinary photodiode.

At very high light levels, thermal effects will ruin the photodiode and eventually burn a hole through it. Temperature should not be above 100 degrees centigrade during operation. The PN junction will stop operating around that temperature.

Position Sensing Characteristics:

The PSD has rather large resistances (sheet resistivities between electrodes) that are used for current division in order to find the spot position. There will be a rather large voltage drop over these resistances for a large photocurrent. This voltage drop is the real culprit that will seriously affect the PSD operation. The voltage drop will be subtracted from the bias which could reduce the bias to zero at the point of light incidence.

The lack of bias will make the PSD position response very nonlinear (and of course seriously affect the "photodiode part" of the PSD). The generated carriers will not eliminated from the device but will pile up around the PN junction making the sheet resistances very inhomogeneous. Also, the step response will slow down due to the abundance of carriers floating around. Here again, the remedy is a high bias (higher than the maximum allowed 35 volts).

We use a extremely high resistivity material for our PSDs, which means that the lifetime of the carriers is very long (in the order of seconds). We have a special process (the SV process) where we reduce some of that lifetime, thus reducing the number of carriers accumulating at any given moment in order for the components to endure more light.

If the light has sufficient energy to raise the temperature at the point of incident above 150 - 200 degrees centigrade, the sheet resistances will be permanently damaged resulting in a non linear PSD. It is really the power density that counts so the same energy in a small spot will generate more heat.

The rule of thumb is a maximum 3W/cm square or less than 0.25 mW in a 100 u spot (but by increasing the bias you can get far beyond these values).

If you need any further information please contact us directly.