Y. C. Choi

The Effect of an Fe-doped GaN Buffer on off -State Breakdown Characteristics in AlGaN/GaN HEMTs on Si Substrate

An Fe-doped GaN buffer layer was employed in the growth of AlGaN/GaN high-electron mobility transistors (HEMTs) on Si substrates. In order to investigate the effects of an Fe-doped GaN buffer on OFF-state breakdown characteristics, HEMT devices with an Fe-doped GaN buffer on Si substrates were fabricated along with conventional devices utilizing an unintentionally doped GaN buffer on Si substrates. The device characteristics were compared. While HEMT devices with the conventional structure showed an extremely unstable OFF-state breakdown behavior due to punchthrough to the Si substrate, it was demonstrated that an Fe-doped GaN buffer layer on a Si substrate successfully suppressed the premature failure caused by Si-induced breakdown. As a result, the AlGaN/GaN HEMTs with an Fe-doped GaN buffer on Si substrates exhibited much more consistent and enhanced breakdown voltages, when compared with the conventional devices. Consequently, it is highly desirable that AlGaN/GaN HEMTs on Si substrates have an Fe-doped GaN buffer layer in order to achieve stable and robust OFF-state breakdown characteristics

Fabrication and characterization of pre-aligned gallium nitride nanowire field-effect transistors

We report on the fabrication of gallium nitride (GaN) nanowire field-effect transistors (FETs) with both bottom-gate and top-gate structures, with very high yield using a unique pre-alignment process. The catalyst positions were chosen to be aligned with the source/drain position, and Ni catalysts with a diameter of 200 nm were deposited selectively at these pre-determined positions. Electrostatic analysis was performed for the bottom-gate devices to estimate the nanowires electrical characteristics. Comparison of the bottom-gate and the top-gate structures indicated that better performance, in terms of saturation and breakdown characteristics, can be obtained using the top-gate structure. For the top-gate nanowire FETs, temperature-dependent characteristics were investigated up to the current saturation regime, and memory effects were observed at room temperature.

Recurrent renal cell carcinoma after 45 years

Late recurrence of renal cell carcinoma (RCC), arbitrarily defined as > 10 years post nephrectomy, is rare. The longest known clinical disease-free interval of 36 years was reported by Walter and Gellespie in 1960. We report a case of recurrent RCC presenting 45 years after nephrectomy.

High breakdown voltage C-doped GaN-on-sapphire HFETs with a low specific on-resistance

High-quality C-doped GaN buffers grown on sapphire substrates were employed for the fabrication of high-power AlGaN/GaN heterojunction field effect transistors (HFETs). The fabricated device exhibited a very high breakdown voltage (BV) over 1350 V and low specific on-resistance (ARDS(ON)) of 3.4 mΩ cm2. This result is very close to the 4H-SiC limit and a record achievement for GaN-based HFETs realized on sapphire substrates, to the best of our knowledge.

Fabrication and characterization of high breakdown voltage AlGaN∕GaN heterojunction field effect transistors on sapphire substrates

High-quality C-doped GaN buffers with a very low doping concentration were grown on 2in. c-plane sapphire substrates, and high-power AlGaN∕GaN heterojunction field effect transistors (HFETs) on sapphire substrates for high-power switching applications were fabricated using a self-align process. The fabricated devices with gate-drain spacing (Lgd) of 16μm exhibited a high breakdown voltage (BV) over 1100V and low specific on resistance (ARDS(on)) of 4.2mΩcm2, with no additional photolithography process for a field plate design. This result approaches the SiC theoretical limit and is a record achievement for GaN-based HFETs on sapphire substrates, to the best of our knowledge. Based on the investigation of the influence of Lgd on device characteristics, it was shown that Lgd had a strong effect on ARDS(on) and BV while no noticeable change in maximum transconductance (gm,max) and maximum drain current (IDS,max) was observed when Lgd was varied. The ARDS(on) of a device [1.5μm gate length (Lg)] with Lgd>7μm ...

C-doped semi-insulating GaN HFETs on sapphire substrates with a high breakdown voltage and low specific on-resistance

High breakdown voltage (BV) AlGaN∕GaN heterojunction field effect transistors (HFETs) with a low specific on-resistance (ARDS(on)) were successfully fabricated using intentionally C-doped semi-insulating GaN buffers with a high resistivity on sapphire substrates. With the improvement of not only the resistivity of a C-doped GaN buffer but also the layout design near the gate feeding region, the fabricated devices exhibited a high BV of ∼1600V and low ARDS(on) of 3.9mΩcm2. This result even reaches the 4H-SiC theoretical limit and the best ever reported for the high-power GaN-based HFETs realized on sapphire substrates to the best of our knowledge.

Gate field emission induced breakdown in power SiC MESFETs

The breakdown mechanism of SiC MESFETs has been analyzed by careful investigation of gate leakage current characteristics. It is proposed that gate current-induced avalanche breakdown, rather than drain avalanche breakdown, is the dominant failure mechanism for SiC MESFETs: thermionic-field emission and field emission are dominant for the ON state (above pinch-off voltage) and the OFF state (below pinch-off voltage), respectively. The effect of Si/sub 3/N/sub 4/ passivation on breakdown voltage has been also investigated. Si/sub 3/N/sub 4/ passivation decreases the breakdown voltage due to higher electric field at the gate edge compared to edge fields before passivation. A reduction in surface trapping effects after passivation results in the higher electric field because the depletion region formed by trapped electrons is reduced significantly.

A new 4H-SiC normally off lateral channel vertical JFET with extremely low power losses: source inserted double-gate structure with a supplementary highly doped region

A novel silicon carbide (SiC) normally off lateral channel vertical junction field-effect transistor (LC-VJFET), namely a source-inserted double-gate structure with a supplementary highly doped region (SHDR), was proposed for achieving extremely low power losses in high-power switching applications. The proposed architecture was based on the combination of an additional source electrode inserted between two adjacent surface gate electrodes and a unique SHDR in the vertical channel region. Two-dimensional numerical simulations for the static and resistive switching characteristics were performed to analyze and optimize the SiC LC-VJFET structures for this purpose. Based on the simulation results, the excellent performance of the proposed structure was compared with optimized conventional structures with regard to total power losses. Finally, the proposed structure showed about a 20% reduction in on-state loss (P/sub on/) compared to the conventional structures, due to the effective suppression of the JFET effect. Furthermore, the switching loss (P/sub sw/) of the proposed structure was found to be much lower than the results of the conventional structures, about a 75% /spl sim/ 95% reduction, by significantly reducing both input capacitance (C/sub iss/) and reverse transfer capacitance (C/sub rss/) of the device.

High Breakdown Voltage and Low Specific On-resistance C-doped GaN-on-sapphire HFETs for Low-loss and High-power Switching Applications

High-quality C-doped GaN buffers grown on sapphire substrates were employed for the fabrication of high-power AlGaN/GaN heterojunction field effect transistors (HFETs). The fabricated device exhibited a very high breakdown voltage (BV) over 1350 V and low specific on-resistance (ARDS(ON)) of 3.4 m¿m2. This result is very close to the 4H-SiC theoretical limit and a record achievement for GaN-based HFETs realized on sapphire substrates, to the best of our knowledge.

Effects of an Fe-doped GaN Buffer in AlGaN/GaN Power HEMTs on Si Substrate

AlGaN/GaN power high electron mobility transistors (HEMTs) with a Fe-doped GaN buffer on a Si substrate were presented for high power switching applications. In order to investigate the effects of an Fe-doped GaN buffer on device characteristics, HEMT devices with an Fe-doped GaN buffer on Si were fabricated alongside with the conventional devices utilizing an unintentionally doped (UID) GaN buffer on Si, and their device characteristics were compared. It was shown that the AlGaN/GaN HEMT with a conventional structure was not suitable for realizing a high breakdown voltage (BV) due to the unstable off-state breakdown behavior. On the other hand, the AlGaN/GaN HEMT with a Fe-doped GaN buffer on Si exhibited much more stable and higher BVs by successfully suppressing the premature failure caused by Si breakdown. As a result, a BV of 295 V and the specific on-resistance (ARDS(ON) ) of 2.44 mOmegacm2 was achieved