  The ideal diode is a theoretical concept that completely conducts the
electric current under forward bias without any loss and that behaves like a
perfect insulator under reverse bias. However, real diodes have a junction
barrier that electrons have to overcome and thus they have a threshold voltage
$V_T$, which must be supplied to the diode to turn it on. This threshold
voltage gives rise to power dissipation in the form of heat and hence is an
undesirable feature. In this work, based on half-metallic magnets and
spin-gapless semiconductors we propose a diode concept that does not have a
junction barrier and the operation principle of which relies on the
spin-dependent transport properties of the HMM and SGS materials. We show that
the HMM and SGS materials form an Ohmic contact under any finite forward bias,
while for a reverse bias the current is blocked due to spin-dependent filtering
of the electrons. Thus, the HMM-SGS junctions act as a diode with zero
threshold voltage $V_T$, and linear $I-V$ characteristics as well as an
infinite on:off ratio at zero temperature. However, at finite temperatures,
non-spin-flip thermally excited high-energy electrons as well as low-energy
spin-flip excitations can give rise to a leakage current and thus reduce the
on:off ratio under a reverse bias. Furthermore, a zero threshold voltage allows
one to detect extremely weak signals and due to the Ohmic HMM-SGS contact, the
proposed diode has a much higher current drive capability and low resistance,
which is advantageous compared to conventional semiconductor diodes. We employ
the NEGF method combined with DFT to demonstrate the linear $I-V$
characteristics of the proposed diode based on two-dimensional half-metallic
Fe/MoS$_2$ and spin-gapless semiconducting VS$_2$ planar heterojunctions.

Materials Science;Mesoscale and Nanoscale Physics