High breakdown voltage in AlGaN/GaN HEMTs using AlGaN/GaN/AlGaN quantum-well electron-blocking layers
© Lee et al.; licensee Springer. 2014
Received: 6 June 2014
Accepted: 19 August 2014
Published: 27 August 2014
In this paper, we numerically study an enhancement of breakdown voltage in AlGaN/GaN high-electron-mobility transistors (HEMTs) by using the AlGaN/GaN/AlGaN quantum-well (QW) electron-blocking layer (EBL) structure. This concept is based on the superior confinement of two-dimensional electron gases (2-DEGs) provided by the QW EBL, resulting in a significant improvement of breakdown voltage and a remarkable suppression of spilling electrons. The electron mobility of 2-DEG is hence enhanced as well. The dependence of thickness and composition of QW EBL on the device breakdown is also evaluated and discussed.
GaN-based high-electron-mobility transistors (HEMTs) have attracted considerable interests for the high-speed and high-power-switching applications because of their outstanding electronic properties. The high sheet-carrier density of the two-dimensional electron-gas (2-DEG)[1, 2] and large critical breakdown electric field[3, 4] allow the fabricated HEMT devices with unprecedented high drain current density and large breakdown voltage, which are essential for the important applications of power devices[5–9]. However, the high sheet electron density inherently in GaN-based HEMTs will inevitably induce the spillover of transport electrons at high-drain-voltage conditions, and that becomes a growing issue. In general, the confinement of transport electrons to the bottom side of the device is insufficient in the conventional AlGaN/GaN HEMT, due mainly to the insufficient potential height provided by the GaN buffer layer underneath. Consequently, transport electrons supposed to be confined within the 2-DEG channel would easily spill or leak into the buffer layer, causing a rapid increase of subthreshold drain leakage currents, accelerating the device breakdown. The above-mentioned phenomenon is often interpreted as the ‘punchthrough effect,’ hindering the further applications of GaN-based HEMTs. Therefore, methods improving the confinement of transport electrons within the channel layer and alleviating the punchthrough effect are necessary. Over the years, several approaches, such as the introduction of p-type doping to the GaN buffer layer[10–12] and the use of AlGaN/GaN/AlGaN double-heterojunction HEMTs[13–15], have been reported to enhance the breakdown voltage of GaN-based HEMTs. The basic principle is to raise the conduction band of the GaN buffer layer, and thus generates a deeper and narrower potential well for the better confinement of 2-DEG.
In this work, we present an improved bottom confinement of 2-DEG by introducing the AlGaN/GaN/AlGaN quantum-well (QW) electron-blocking layer (EBL) structure. It is shown that the large electric field induced at the interfaces of AlGaN/GaN/AlGaN QW EBL effectively depletes the spilling electrons toward the 2-DEG channel. As compared to previous approaches, the subthreshold drain leakage current becomes less sensitive to the drain voltage (Vds), and that postpones the HEMT breakdown. Meanwhile, our proposed structure not only exhibits the highest electron mobility among other compared HEMT devices but also allows a great tolerance for epitaxial imperfections during the device fabrication. As a result, we conclude that the proposed AlGaN/GaN/AlGaN QW EBL HEMT is viable and highly promising for the high-speed and high-power-switching applications.
Specifically, to calculate the impact ionization in the GaN wurtzite structure, the values of coefficients αn,p and Ecritn,p were set to be 2.60 × 108 cm−1 and 3.42 × 107 V cm−1 for electrons, and 4.98 × 106 cm−1 and 1.95 × 107 V cm−1 for holes, respectively.
Results and discussion
In conclusion, we propose a novel AlGaN/GaN/AlGaN QW EBL structure to alleviate the punchthrough effect that is generally observed on the conventional AlGaN/GaN HEMT. The introduction of AlGaN/GaN/AlGaN QW EBL leads to a better confinement of transport electrons into the 2-DEG channel, resulting in a reduction of subthreshold drain leakage current and a postponement of device breakdown. The large electric field induced at the interfaces of AlGaN/GaN/AlGaN QW EBL, which effectively depletes the spilling electrons toward the 2-DEG channel, is mainly responsible for the improved performances.
The authors gratefully acknowledge financial support from the National Science Council of the Republic of China (ROC) in Taiwan (contract no. NSC–100–2112–M–003–006–MY3), from the Bureau of Energy, Ministry of Economic Affairs in Taiwan, and from the Ministry of Science and Technology in Taiwan (contract no. MOST 103–2112–M–003–008–MY3).
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