As wireless communication technology moves forward, the power and linearity performance requirements placed upon the power amplifiers in these systems grow more difficult to meet. Currently, silicon LDMOS power transistors are the output devices of choice for base-station power amplifiers, but as the limits of operability of these devices are reached, there will be a need for a semiconductor material that can fulfill the high-frequency and high-power requirements of the third generation of wireless technology. Gallium nitride and its related alloys are widely acknowledged as prime candidates for high-power microwave applications due to their high breakdown field (5 MV/cm), high electron saturation velocity (2.5x10 cm/s) and high operating temperature.
In this thesis, an AlGaN/GaN high electron mobility transistor (HEMT) based on a novel inner field-plate (IFP) structure has been proposed and demonstrated, which is implemented by using III-V semiconductor fabrication facilities in KAIST.
In order to improve the device power performance, an improved breakdown voltage of the GaN HEMT is necessary. For the higher breakdown voltage characteristics, various device structures, such as a multiple field-plate (MFP), a single field-plate (SFP) and a double field-plate (DFP), have been proposed and developed. Especially, the SFP structure has been widely used to reduce the gate leakage current and electric field at the gate-drain edge. In this thesis, the conventional HEMT, single field-plate HEMT and newly proposed inner field-plate (IFP) HEMT are compared.
To study the optimal device structure parameters, the new inner field-plate (IFP) HEMT structure has been simulated by using the two-dimensional device simulator (SILVACO Atlas). The reduced peak electric field and gate leakage current lead to higher breakdown voltages for the improved RF power performance of the device. To achieve these performances, the inner field-plate is inserted between the gate and drain sides of the SFP HEMT structure. At an optimum bias condition, the breakdown voltage of the proposed IFP-HEMT is 88.9\% higher than that of SFP HEMT.
Finally, the conventional HEMT, single field-plate HEMT and proposed inner field-plate (IFP) HEMT have been fabricated and their gate-leakage, small signal and large signal characteristics are compared. The proposed IFP HEMT structure has been designed to suppress the gate-drain leakage current more effectively compared to the conventional HEMT and SFP HEMT. As compared to the SFP HEMT structure, the optimized IFP HEMT can improve the gate leakage current characteristics by more than 145\%. The suppressed leakage current can lead to higher breakdown voltages for the improved RF power performance of the device.
The simulated and measured results indicate that the proposed IFP structure is very promising for high power microwave applications.
최근 base station 대역에서 RF power 소자로써 LDMOS가 널리 사용되고 있다. 최근 선진국에서는 높은 breakdown field, electron saturation velocity와 열적 특성이 우수한 GaN기반의 소자가 많이 연구되고 있다. 이 GaN은 Si기반의 소자보다 power 소자로써 우수한 성능을 지니고 있어 추후 LDMOS를 대체할 것으로 예상된다. 본 논문에서는 KAIST 반도체동 장비를 사용하여 GaN 기반의 물질에 새로 제안된 inner field-plate 구조를 사용한 AlGaN/GaN HEMT를 제작하였다. 제안된 구조의 최적화를 위해 2-D 시뮬레이션을 수행하였다. 시뮬레이션과 결과는 제안된 구조가 기존의 구조대비 높은 항복전압을 갖고 주파수특성이 우수하여 고출력용에 유망함을 보여준다. 다음으로 직접 소자를 제작하여 제안된소자의 특성을 살펴보았다. 제작된 소자의 측정결과, 제안된 Inner field-plate(IFP) HEMT구조에서 IFP단자의 인가전압에 따라 소자의 특성이 변화되며 최적인 IFP 단자 인가전압에서 breakdown 특성이 우수함을 알았다. 비록 공정의 최적화가 이루어지지않아 시뮬레이션 결과와 모두 일치 하지는 않았지만 경향성은 파악하였기때문에 추후 공정의 최적화를 이룬다면 고출력용에 유망한 소자가 될것이다.
