Hi guys, I hope this is the last time for the following reversed biased LED as Spad thread. (We've got to do a newsletter then manual, so I need to at least wave my hands at the details.) I should also apologize as this is mostly just my ?thinking out loud? to the SED. So again here's the circuit.
+------+ | | (+) | Vbias - LED (-) ^ | | | +-----opa314-->out | | buffer GND 100k | GNDVbias is ~24V (device dependent) When light is shinned at the LED I see pulses. the number of which changes linearly with light intensity.
There are two details for me to understand. First I've been thinking about the data in terms of 'breakdown channels' in the LED. I assume they are independent, though I can imagine that some samples might have overlapping channels. As any one channel breaks down it starts to discharges the entire LED capacitance. Also each channel has a different threshold voltage. When the voltage across the LED falls below that threshold then the breakdown stops. Does this make sense geometrically? The depletion width is much less than the square root of the area. (Is depletion width the right term?)
The data for this model is manifold. First here's a 'scope 'histogram' of single photon events, 1 second persistence. ~5kHz count rate.
I sent the same data into a comparator with adjustable reference voltage and get a stair case type distribution of count rate vs voltage. (data posted on request)
If you trigger up high, then you just see one channel. The peak height is approximately equal to the bias voltage minus some 'Vt' (threshold voltage). Here's a bunch of 'scope shots with different bias volatges. Vt =~23.1V
I?m not sure how to explain why each channel has a different threshold voltage.
The other thing I don?t understand is the turn on waveform. At low bias voltage and for early times with higher bias voltage the turn on looks linear with a slope that is related to the voltage peak... it takes about 50 ? 100ns to turn on. (See the data above.) Now what?s weird is this turn on time seems to be independent of circuit following the LED. It?s independent of the quenching resistor value (100k ohm for the circuit shown) And also independent of the capacitance to ground. The R and C loading of the LED has an effect at later times and with higher bias voltages. Here?s a screen shot where I added 12pF to ground in parallel with the
100k ohm resistor.Anyway thanks for reading and any thoughts are most welcome.
George H.