APD gain dependence on applied bias voltage
The gain dependence on the bias voltage was measured and then the voltage at which the APD achieves the nominal gain M = 30 at the constant temperature was determined. Behavior of output signal depending on applied bias voltage for 173 APDs was examined at constant temperature T = 25 �C. For each APD about 15 measurements were made at different bias voltages. The quantity of measurements depends on each APD characteristics: for APDs with higher breakdown voltage more measurements were needed.
Collected data were analyzed with ROOT program. The dependence of the avalanche gain on applied bias voltage exhibits an exponential growth. As an example, picture below shows the gain curve at the fixed temperature T = 25 �C.
After an initial plateau between 20 and 100 V, which determines the unitary gain, the curve increases exponentially with bias voltage. The curve can be well fitted to an exponential function plus a constant:
M(V) = (p0) + (p1) exp((p2) V), where M(V) is the gain at the voltage V.
The results of analysis showed considerable differences in the value of individual APD gain at the same reverse bias voltage: at a bias voltage of 380 V, the distribution of gains spans M = 20 to M = 90.
APD gain dependence on temperature.
The study of the gain dependence on temperature is another key feature of the APD testing activity. It is known that the gain of the avalanche photodiodes is strongly dependent on the temperature: since the avalanche multiplication depends on the mean free path of electrons between ionizing collisions, which is temperature dependent, the APD gain decreases with temperature. Whereas the PHOS spectrometer will cool its APDs down to minus 25 degrees Celsius to work at higher gains, the EMCal will be operated at ambient temperature. For this reason the gain dependence on temperature was studied in the range of temperatures from 19C to 29C. 4 APDs were measured at 6 different temperatures and at 14 values of bias voltage. The results for the single APD are shown on picture below.
The gain undergoes strong variations when the temperature changes and this effect is more evident for higher values of the bias voltage. The relation between gain and temperature is approximately linear at a fixed value of bias voltage. The dependence on temperature has an important effect on the stability of the EMCal response: the ambient temperature inside the ALICE magnet could continuously undergo small variations and it is important to predict and take into account the corresponding change in the APD gain. An example of the APD's gain dependence on the temperature is shown on the next picture.
