  We present characterization results of two silicon photomultipliers; the
Hamamatsu LVR-6050-CN and the Ketek PM3325 WB. With our measurements of the
bias dependence of the breakdown probability we are able to draw conclusions
about the location and spatial extension of the avalanche region. For the KETEK
SiPM we find that the avalanche region is located close to the surface. In the
Hamamatsu SiPM the high-field region is located $0.5\,\mu$m below the surface,
while the volume above is depleted almost until the surface. Furthermore, for
the Hamamatsu SiPM we find that charge carriers produced by optical-crosstalk
photons enter a cell below the avalanche region as opposed to an earlier device
where most of the photons enter a cell from above. In the here tested Hamamatsu
device the crosstalk photons probably absorb in the bulk and the generated
holes diffuse into the active volume of the cell within 2\,ns and initiate a
breakdown. The present paper is an attempt to spur further interest in the use
of the bias dependence of the breakdown probability and establish it as a
standard tool not only to determine the location of the high-field region but
also to determine the origin of charge carriers relative to the high-field
region. With the knowledge of where the charges come from it should be possible
to further improve the optical crosstalk, dark count, and afterpulsing
characteristics of SiPM.
