A one-way valve for electric current, and the junction that makes the classic one. Join a p-type semiconductor, rich in holes, to an n-type one, rich in electrons, and carriers spill across the boundary and cancel until a built-in voltage stops them, leaving a thin strip with no free carriers at all. Push current one way and the barrier drops, so it flows easily; push the other way and it rises, so almost nothing flows. The same junction turns current into light in a light-emitting diode and light into current in a solar cell or photodiode. In 2D materials it can be two different monolayers stacked, or one monolayer made p-type on one side and n-type on the other by gates.
Thin films that spread heat away from a hot processor across the body of a phone, laptop or LED module, so the chip does not have to slow itself down to stay cool.
For Josephson-diode measurements, sweep both current directions with careful field compensation, because tiny residual fields also produce non-reciprocity.
A p-type semiconductor carries its current mostly as holes, an n-type one as electrons. Put them in contact and electrons diffuse into the p side and holes into the n side, where they meet and cancel. What they leave behind near the boundary is a strip with no free carriers – the depletion region – holding the fixed charges of the dopant atoms, negative on the p side and positive on the n side. Those charges set up a built-in voltage that stops further spilling.
An external voltage of one sign lowers that barrier and the current grows exponentially with it; of the other sign it raises the barrier and only a tiny current leaks through. That one-way behaviour, rectification, is what makes a diode. A metal on a semiconductor can do the same through a Schottky barrier, and the p–n junction is also the core of every bipolar transistor, solar cell and light-emitting diode.
In silicon a junction is defined by where dopants were implanted. In 2D materials there are two other ways. Stacking a p-type monolayer on an n-type one makes a junction across the van der Waals gap: MoS2 on WSe2 rectifies current and responds to light, and both can be tuned with a gate – but with each side just one unit cell thick, what decides the current is electrons and holes tunnelling across and recombining between the layers, not drift and diffusion across a depletion region (Lee and colleagues, 2014). Sandwiching the junction between graphene layers collects the light-generated carriers better.
Or a single monolayer can be made p-type in one place and n-type in the next by gates underneath. Split gates under WSe2 on thin boron nitride make a lateral junction that emits light with a thousand times less current and a tenth the linewidth of earlier MoS2 devices, and turning up the bias moves the emission between excitons bound to impurities, charged excitons and neutral ones (Ross and colleagues, 2014).
Not every diode is a plain p–n junction. When the bands of two materials overlap – a broken-gap alignment – electrons can tunnel straight through, and the current first rises, then falls as the voltage grows: negative differential resistance, the mark of an Esaki tunnel diode. Black phosphorus on tin diselenide, with a thin insulating barrier between, does this at room temperature, with a peak-to-valley ratio of 1.8, rising to 2.8 at 80 K (Yan and colleagues, 2015).
The name has also spread to superconductors. A Josephson junction of NbSe2, a thin layer of Nb3Br8 and NbSe2 again carries supercurrent in one direction while behaving as a normal resistor in the other, with no magnetic field, and rectifies a square wave with a ratio of about 104 (Wu and colleagues, 2022) – a superconducting diode for low-power circuits.
A rectifying two-terminal device. In a p–n junction, diffusion of majority carriers leaves a depletion region of ionised dopants and a built-in potential; forward bias lowers the barrier and the current rises exponentially, I = I0[exp(qV/nk_BT) − 1], while reverse bias leaves a small saturation current. Recombination under forward bias gives electroluminescence, and illumination separates photocarriers to give a photovoltaic response.
In van der Waals MoS2/WSe2 junctions one unit cell thick per side, gate-tunable rectification and photoresponse are set by tunnelling-assisted interlayer recombination of majority carriers rather than by drift and diffusion across a depletion region. Split gates under monolayer WSe2 make lateral junctions that emit from neutral, charged or bound excitons; broken-gap black phosphorus/SnSe2 junctions are room-temperature Esaki tunnel diodes with negative differential resistance; and NbSe2/Nb3Br8/NbSe2 makes a field-free Josephson diode, superconducting in one direction only.
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