Supressing leakage in nitride diodes
Leakage currents in Schottky barrier diodes plummet by seven orders of magnitude when the barrier composition is shifted from Al0.21Ga0.79N to Al0.11Ga0.89N.
ENGINEERS from Central Research Lab, Hitachi, have shown that reverse leakage currents in AlGaN/GaN Schottky barrier diodes can plummet through reductions in sheet carrier density.
High leakage currents are an Achilles heel for today’s GaN diodes, which hold much commercial promise thanks to their combination of high temperature, high frequency and high power.
It is widely believed that the dominant cause of leakage is structural defects, especially dislocations, which can result from strain in the AlGaN layer.
However, initial studies by the Hitachi researchers indicated that sheet carrier density also influences leakage current.
This finding motivated these engineers to carry out further work involving three AlGaN/GaN heterostructures that shared a 3.6 μm, highly-resistive GaN buffer layer and a 5 nm GaN cap.
In these samples, barrier compositions were Al0.11Ga0.89N, Al0.16Ga0.84N and Al0.21Ga0.79N, and corresponding roomtemperature two-dimensional electron gas densities determined by Hall measurements were 3.6 x 1012 cm-2, 6.6 x 1012 cm-2 and 1.0 x 1013 cm-2, respectively.
“We prepared those three samples with our own MOCVD [tool] under precisely controlled conditions,” says lead author Akihisa Terano. “We think that the material quality of our three samples is almost the same.”
Figure 1. Schottky diodes built by Hitachi feature 1000 μm by 200 μm cathodes and anodes, separated by 200 μm
All three epiwafers were processed into planar Schottky barrier diodes with contacts formed by electron beam evaporation (see Figure 1).
Measurements on the devices revealed that a three-fold fall in the density of the two-dimensional electron gas results in a reduction in leakage current by seven orders of magnitude (see Figure 2).
Figure 2. Massive changes in leakage current result from relatively small changes in the density of the two dimensional electron gas
Unfortunately, slashing leakage current with this approach has its downsides.
“A reduction in the sheet density is thought to lead to some increase in the resistance of the drift layer, resulting in some decrease of the forward current,” explains Terano.
He and his co-workers are now planning to tackle this issue head-on, developing devices that combine a sufficiently high forward current with a low leakage.
A. Terano et al. Electron. Lett. 48 274 (2012)
