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P F 3 2 G E N E R A L C H A R A C T E R I S T I C S
Within each pixel of the PF32 sensor, the detection of single photons is performed by the SPAD
active area. This active area is surrounded by the necessary electronics to bias and quench the
SPAD, as well to time and count the detected photons. In a traditional TCSPC setup, each detector
would need its own external electronics to time-tag the photon’s arrival, whereas the PF32 is
intended to provide a compact and highly parallel means of data acquisition for a variety of
applications such as fluorescence lifetime imaging microscopy (FLIM) and 3D active imaging.
The active area is a silicon p-n junction which is reverse-biased beyond its breakdown voltage in
the so-called “Geiger-mode”. In this regime, the absorption of a single photon gives rise to a
detectable current pulse due to impact ionisation events within the semiconductor lattice enabled
by the acceleration of the initial photo-generated carriers in the high electric field. After the onset
of an avalanche, the SPAD is disarmed by reducing the bias to a value below the breakdown
voltage. The SPAD is then re-armed after a short period known as the “dead time”. The dead time
(sometimes called the “hold-off” time) is needed to ensure that any carriers trapped within the
semiconductor structure following an avalanche event are released; if the dead time is too short
and the SPAD is re-armed, any remaining trapped carriers may trigger another avalanche,
referred to as an “afterpulse”. Afterpulsing effectively increases the dark count rate (DCR) of the
SPAD, thus decreasing the signal to noise ratio (SNR). For this reason, the dead time is set to a
sufficiently long period to negate the deleterious effects of afterpulsing.
The DCR itself is a function of temperature and bias voltage; increasing either, or both, of these
parameters also increases the DCR. The final contribution to the DCR is through cross-talk,
whereby during the avalanche process some photons are emitted during the large flow of high-
energy carriers. Due to the large distance between active areas, the contribution of cross-talk is
negligible.
The photon-detection efficiency is also a function of bias voltage as shown in the figure below.
The final important figure of merit is the jitter, or instrument response function (IRF), which
relates to the uncertainty in absolute timing of the stochastic avalanche process in response to
the detection of a photon. The IRF of the SPADs within the PF32 array is ~150 ps.
Photon detection probability vs. wavelength of a PF32 pixel.
300 400 500 600 700 800 900 1000 1100
0
5
10
15
20
25
30
0.2 V excess bias
0.6 V excess bias
1.0 V excess bias
Photon Detection Probability (%)
Wavelength (nm)
