  Pyro-current measurements have been widely used to study ferroelectric
properties in multiferroic materials. However, determination of intrinsic
polarization by this method is not straightforward because of leakage current
and trapped charge carriers. Here, we demonstrate the formation of internal
electric field due to thermally stimulated charge carriers and its influence on
ferroelectric polarization in a polycrystalline sample of the well known
multiferroic TbMnO$_3$. While an electric field ($E_{ext}$) poling across the
ferroelectric transition ($T_C$ $\sim$ 26 K) is essential to obtain
depolarization current at $T_C$, the sample poled only in the paraelectric
state ($T_{pole}$ = 130 $-$ 50 K) also exhibits a pyro-current peak at $T_C$
but with the same polarity ($-$ $I_{pyro}$) as that of the external field ($-$
$E_{ext}$). We demonstrate that these unusual behavior of pyro-current are
caused by a positive internal electric field ($+$ $E_{int}$) which in turn is
created by thermally stimulated free charge carriers during the poling process
in the paraelectric state. We also show that a combination of DC-biased current
and pyro-current measurements is a promising method to study the intrinsic
ferroelectric properties in multiferroic materials.
