  The standard technique to electrically isolate the $n^+$ implants of
segmented silicon sensors fabricated on high-ohmic $p$-type silicon are
$p^+$-implants. Although the knowledge of the $p^+$-implant dose and of the
doping profile is highly relevant for the understanding and optimisation of
sensors, this information is usually not available from the vendors, and
methods to obtain it are highly welcome. The paper presents methods to obtain
this information from circular MOSFETs fabricated as test structures on the
same wafer as the sensors. Two circular MOSFETs, one with and one without a
$p^+$-implant under the gate, are used for this study. They were produced on
Magnetic Czochralski silicon doped with $\approx 3.5 \times 10^{12}$ cm$^{-2}$
of boron and $\langle 1 0 0 \, \rangle$ crystal orientation. The drain-source
current as function of gate voltage for different back-side voltages is
measured at a drain-source voltage of 50 mV in the linear MOSFET region, and
the values of threshold voltage and mobility extracted using the standard
MOSFET formulae. To determine the bulk doping, the implantation dose and
profile from the data, two methods are used, which give compatible results. The
doping profile, which varies between $3.5 \times 10^{12}$ cm$^{-3}$ and $2
\times 10^{15}$ cm$^{-3}$ for the MOSFET with $p^+$-implant, is determined down
to a distance of a fraction of a $\mu $m from the Si-SiO$_2$ interface. The
method of extracting the doping profiles is verified using data from a TCAD
simulation of the two MOSFETs. The details of the methods and of the problems
encountered are discussed.
