In this thesis, AlGaN/GaN HEMTs with high breakdown voltage by employing the metal-insulator-semiconductor (MIS) structure have been fabricated. $SiN_x$ and $Al_2O_3$ are used for the insulators.
Fabrication processes of the AlGaN/GaN HEMT are optimized. Mesa isolation is achieved by $BCl_3$ (20 sccm)/$Cl_2$ (20 sccm)/Ar (6 sccm) mixed gases based transformer coupled plasma (TCP) etcher with RF power of 350 W and an etching rate of 3,000 Å/min for GaN material. Ohmic contact for source and drain are formed using the Ti/Al/Ni/Au (200/1,500/350/500 Å), which are evaporated, and annealed at 800 °C for 30 sec in $N_2$ ambient. Contact resistance of 0.24 Ω·mm is obtained from a standard TLM measurement. The Schottky contact for gate is formed by evaporating Ni/Au (300/2,500 Å), which shows an ideality factor of 1.33.
Using the optimized fabrication processes, the AlGaN/GaN HEMTs are fabricated and good characteristics, such as maximum drain current of 600 mA/mm, transconductance of 160mS/mm, current-gain cutoff frequency of 12.75 GHz, maximum oscillation frequency of 30 GHz and off-state breakdown voltage of 120 V, are obtained. The breakdown voltage of the fabricated HEMTs has been significantly improved with an increase of the gate-drain length $L_gd$. In addition, to reduce the dispersion problem, the passivated AlGaN/GaN HEMTs are fabricated and measured by means of DC-pulse characteristics to investigate the effects of passivation. To improve the breakdown characteristic, a field plate (FP) structure is fabricated and investigated. AlGaN/GaN HEMTs with a field plate have demonstrated better breakdown characteristics.
Next, the AlGaN/GaN MIS-HEMTs are fabricated and the performance characteristics are compared with those of the previous AlGaN/GaN HEMTs. The MIS-HEMTs have shown good characteristics, such as maximum drain current of 581 mA/mm, transconductance of 116.4 mS/mm and current-gain cutoff frequency of 8.62 GHz and maximum oscillation frequency of 21.67 GHz, are obtained. The gate leakage current is significantly reduced (30 times) by employing the MIS structure. The breakdown voltage characteristics are improved by 25 % and the RF power performances are improved more than 25 % compared to conventional HEMT.
The results demonstrate that the MIS-HEMT structure is very promising for high power microwave applications.
본 논문에서는 최근 microwave power amplification에서 low frequency high voltage switching까지 널리 사용되는 high power AlGaN/GaN HEMT와 MIS-HEMT에 관해 다루었다. 먼저 2장에서는 AlGaN/GaN HEMT의 RF power 향상과 Breakdown voltage 향상을 위해 Issue가 되고 있는 RF dispersion, field plate HEMT, MIS-HEMT에 대해 언급하고, 3장에서는 AlGaN/GaN HEMT의 각 공정 단계에 따른 최적화된 공정과 향상된 단위 공정 성능을 측정하여 비교하였으며 단위 공정의 안정화를 이루었다. 또한 최적화된 단위 공정을 통한 소자의 공정 및 breakdown voltage의 향상을 측정하여 비교하였다. 4장에서는 3장에서 최적화된 공정을 통해 현재 Issue가 되고 RF Dispersion 문제 해결을 위한 passivation과 breakdown characteristics 향상을 위해 field plate 구조를 소자에 적용하여 제작 및 측정을 하여 passivation& field plate effect를 확인하였다. 또한 $L_gd$ 길이에 따른 소자의 제작 및 특성 분석을 하여 소자 library를 구축하였다. 5장에서는 AlGaN/GaN HEMT의 단점인 높은 gate leakage current를 감소시키고 high breakdown characteristics을 갖도록 gate insulator로 $SiN_x$와 $Al_2O_3$를 이용하여 MIS(Metal Insulator Semiconductor) structure를 제작하였고 insulator에 따른 특성 분석 및 측정을 하였다. 또한 MIS-HEMT를 제작하여 HEMT와의 특성 분석 및 leakage current characteristics의 향상, Breakdown voltage의 증가와 RF power characteristics의 향상을 확인하였다.
앞으로 해야 할 일은 $SiN_x$와 $Al_2O_3$를 gate insulator로 이용한 MIS-HEMT 공정의 안정화와 더 자세한 분석이 필요하다. MIS-HEMT의 drain current의 감소와 small signal characteristics의 예상보다 큰 성능 저하에 대한 추후 연구가 필요하며, RF Power 최대 출력의 측정 역시 필요하다.
MIS-HEMT 공정이 안정화되면 더 좋은 high breakdown, low leakage, high RF power characteristics을 가지는 MIS-HEMT 소자가 될 것이다.
