Improvement of breakdown characteristics in AlGaN/GaN/Al_xGa_1−xN HEMT based on a grading Al_xGa_1−xN buffer layer

Abstract

To improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/AlxGa1−xN double heterostructure (DH‐HEMTs) were designed and fabricated by replacing the semi‐insulating GaN buffer with content graded AlxGa1−xN (x = x1 → x2, x1 > x2), in turn linearly lowering the Al content x from x1 = 90% to x2 = 5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive AlxGa1−xN epilayer suppresses the parasitic conduction in the GaN buffer, and the band edge discontinuity limits the channel electrons spillover, thereby reducing leakage current and drain current collapse. In comparison with the conventional HEMT that use a semi‐insulating GaN buffer, the fabricated DH‐HEMT device with the same size presents a remarkable enhancement of the breakdown voltage.

Barrier Strain and Carbon Incorporation‐Engineered Performance Improvements for AlGaN/GaN High Electron Mobility Transistors**

“…The off‐state leakage current at 200 V (the maximum limit of our measurement system) of sample A and sample B are 0.61 mA and 0.41 mA, respectively. Both structures demonstrate breakdown voltages much higher than that of the reference devices . While the HEMTs with only the AlN buffer, (structure A) show a degradation of the maximum drain current and the transconductance, the HEMTs devices based on structure B, with the HT AlN IL and the HLHT AlN, show that the maximum drain current and the transconductance are comparable with the published literature .…”

“…Several solutions have been developed to reduce the buffer leakage, such as doping intentional/unintentional acceptors into the GaN buffer, or considering a p‐GaN as a back‐barrier, however it is very likely that there are several kinds of traps or defects that may adversely affect the reliability of the device in AlGaN/GaN HEMT . Ozbay and co‐workers have reported that highly resistive GaN layers can be achieved by the growth of a thick HT AlN (0.5 − 1.0 µm) buffer layer on sapphire substrates, however the breakdown voltage of their device was only 60 V. The low breakdown voltage of AlGaN/GaN HEMTs with HT AlN may be due to residual donors in GaN (such as O and Si) incorporated during high‐temperature growth. Because it is necessary to compensate for the residual n‐type conductive GaN buffer to further improve the breakdown voltage of AlGaN/GaN HEMTs on both sapphire substrates and SiC substrates, AlGaN is also widely used to replace GaN buffers to enhance the breakdown .…”

“…Ozbay and co‐workers have reported that highly resistive GaN layers can be achieved by the growth of a thick HT AlN (0.5 − 1.0 µm) buffer layer on sapphire substrates, however the breakdown voltage of their device was only 60 V. The low breakdown voltage of AlGaN/GaN HEMTs with HT AlN may be due to residual donors in GaN (such as O and Si) incorporated during high‐temperature growth. Because it is necessary to compensate for the residual n‐type conductive GaN buffer to further improve the breakdown voltage of AlGaN/GaN HEMTs on both sapphire substrates and SiC substrates, AlGaN is also widely used to replace GaN buffers to enhance the breakdown . Since, however, AlGaN has poor thermal conductivity, it is probably difficult to achieve both good performances for HEMTs and very high insulating performance of GaN buffers grown on sapphire substrates.…”

Comparative study of buffer designs for high breakdown voltage AlGaNGaN HFETs

“…At high drain-source bias electrons are injected under the gate depletion region creating a current path in the buffer. The punch-through effect can be suppressed by using a double heterojunction structure (DHFET), where the electron channel is confined between the upper barrier and a low Al-content Al x Ga 1-x N buffer layer [1,2].…”

Enhancement of channel conductivity in AlGaN/GaN heterostructure field effect transistors by AlGaN:Si back barrier

“…The conduction band off-set (DE c ) between Al 0.05 Ga 0.95 N and the nid GaN channel of about 0.06 eV can prevent electrons from penetrating into the GaN:C buffer. 18 The conduction band of the AlGaN back barrier doped with Si (structure C) is shown to be depleted. However, the conduction band offset is still present.…”