  Solid semiconductor sensors are used as detectors in high-energy physics
experiments, in medical applications, in space missions and elsewhere. Minimal
knowledge of the electric field inside the elementary cells of these sensors is
highly important for their performance understanding. The field governs the
charge propagation processes and ultimately determines the size and quality of
the electronic signal of the cell. Hence, the simulation of these sensors as
detectors in different analyses relies strongly on the field knowledge. For a
certain voltage applied to the cell, the field depends on the specifics of the
device's growth and fabrication. The information about these is often
commercially protected or otherwise very difficult to encode in
state-of-the-art technology computer-aided-design (TCAD) software. In this
work, we show that by taking the top-down approach, combining public beam-test
data and a very limited public TCAD knowledge, we are able to effectively
approximate the 3D electric field function in the pixel cell of one important
and widely used example, namely the ALPIDE sensor, for simulating the charge
propagation processes. Despite its broad usage worldwide, the ALPIDE field is
not available to the community. We provide an effective field function, that
adequately describes the sensor behaviour without trying to reconstruct further
details about the device or the details behind its processing. We comment on
the process by which the effective field function is derived with the help of
the Allpix$^2$ software, and on how similar work can be performed for other
devices, starting from the same grounds.
