In recent years, an electronic device using a wide gap semiconductor such as silicon carbide (SiC) and gallium nitride (GaN) has been vigorously developed to apply it to power electronics.
As a device using a nitride semiconductor typified by gallium nitride (GaN), a horizontal device using a non-doped AlGaN/GaN heterojunction, for example, has been vigorously developed.
This is characterized in that a conductive layer formed of a non-doped, but two-dimensional electron gas (hereinafter abbreviated to 2DEG) is generated on the GaN side near a junction interface by the influence of a large band offset, a natural polarization generated on a heterojunction interface, and a strong piezopolarization.
Since the 2DEG conductive layer has a high electron mobility and a high electron concentration (approximately 1013 cm−2), a high electron mobility transistor (HEMT) device using an AlGaN/GaN heterostructure has been mounted on a DC-DC converter circuit for power electronics and commercialized in recent years.
A horizontal diode using the heterostructure has also been developed on assumption that it is applied to power electronics. An attempt to increase current density per unit area viewed from the upper has been made such that the heterostructure is multi-layered to improve a forward direction characteristic, stacking a plurality of conductive layers made of the 2DEG in the vertical direction (in the perpendicular direction with respect to the principal plane of a substrate).
Japanese Unexamined Patent Application Publication No. 2009-117485 discusses a horizontal diode having a multi-layered heterojunction in which anode and cathode electrodes are formed at the side face portion of the heterojunction to lower an access resistance to a 2DEG conductive layer positioned in the lower layer.
“Phys. Status Solid B 247, No. 7, 2010, T. Ueda et al.” discusses a technique in which anode and cathode electrodes are formed at the side face portion of a three-layered 2DEG conductive layer in the recess structure to obtain an on-resistance of 52 mΩcm2 and a reverse breakdown voltage of 9400V.