  The Picosecond Avalanche Detector is a multi-junction silicon pixel detector
based on a $\mathrm{(NP)_{drift}(NP)_{gain}}$ structure, devised to enable
charged-particle tracking with high spatial resolution and picosecond
time-stamp capability. It uses a continuous junction deep inside the sensor
volume to amplify the primary charge produced by ionizing radiation in a thin
absorption layer. The signal is then induced by the secondary charges moving
inside a thicker drift region. A proof-of-concept monolithic prototype,
consisting of a matrix of hexagonal pixels with 100 $\mu$m pitch, has been
produced using the 130 nm SiGe BiCMOS process by IHP microelectronics.
Measurements on probe station and with a $^{55}$Fe X-ray source show that the
prototype is functional and displays avalanche gain up to a maximum electron
gain of 23. A study of the avalanche characteristics, corroborated by TCAD
simulations, indicates that space-charge effects due to the large primary
charge produced by the conversion of X-rays from the $^{55}$Fe source limits
the effective gain.
