  Transmission of electrons across a rectangular barrier of IV-VI semiconductor
compounds is considered. Conduction electrons arrive at the barrier and are
reflected or transmitted through it depending on the relative values of the
barrier potential $V_b$ and the electron energy $E$. The theory, in close
analogy to the Dirac four component spinors, accounts for the boundary
conditions on both sides of the barrier. The calculated transmission
coefficient $T_C$ is an oscillatory function of the barrier voltage varying
between zero (for full electron reflection) and unity (for full electron
transmission). Character of electron wave functions outside and inside the
barrier is studied. There exists a total current conservation, i. e. the sum of
transmitted and reflected currents is equal to the incoming current. The
transmission $T_C$ is studied for various barrier widths and incoming electron
energies. Finally, the transmission coefficient $T_C$ is studied as a function
of $V_b$ for decreasing energy gaps $E_g$ of different Pb$_{1-x}$Sn$_x$Se
compounds in the range of 150 meV $\geq E_g \geq$ 2 meV. It is indicated that
for very small gap values the behaviour of $T_C$ closely resembles that of the
chiral electron tunneling by a barrier in monolayer graphene. For $E_g$ =0
(Pb$_{0.81}$Sn$_{0.19}$Se) the coefficient $T_C$ reaches the value of 1
independently of $V_b$.
