Fabrication Of High-voltage GaN-based Power Devices And The Effects Of Traps On The Electrical Characteristics Of Devices

Commercial GaN-based power devices have been widely used for their excellent performance in high-speed switching,as well as high efficiency and low switching loss.However,the voltage rating for commercially available lateral GaN devices is still below650 V.Till date,several techniques such as GaN on GaN?current aperture vertical electron transistor,Trench MOSFET?and GaN on a polycrystalline AlN substrate have been adopted to extend the voltage rating of these devices.However,it is still difficult to produce a device with a high breakdown voltage(V_BR)while having a low on-resistance(R_on)and a low leakage current.The reliability issues caused by thermal effects and the effects of traps in the epitaxy on the transport characteristics should be investegated for the devices with high V_BR.Based on previous research results,AlGaN/GaN/AlGaN double heterostructure?DH?is used to improve V_BR and high-temperature performance.Compared to AlGaN/GaN single heterostructure high electron mobility transistors?SH-HEMTs?,DH-HEMTs is beneficial to improve carrier confinement and reduce buffer leakage.However,it is still difficult to achieve an AlGaN back barrier layer with a high crystalline quality,and the grown AlGaN back barrier layer has a high trap density,which seriously deteriorates device performance and results in reliability issues.Frequency-dependent capacitance and conductance measurements have been successfully employed to investigate trap states in III-N heterostructures.However,high temperature characteristics of trap state in DH-HEMTs have not been studied systematically.How to suppressing the off-state leakage and the off-state power dissipation while improving the V_BR is one of the key issues in this dissertation.In order to control the off-state power dissipation,the off-state leakage current of 1?A/mm is becoming a common criterion for devices with a V_BR of more than 1 kV.However,the GaN HEMTs with a V_BR of more than 1.5 kV and an off-state leakage current of 1?A/mm have rarely been reported.In order to extend these applications to the high voltage field,the AlGaN-channel HEMTs are reported.It shows great promise for high voltage applications due to an increased critical electric-field in AlGaN materials compared to GaN.However,it is extremely difficult to obtain AlGaN-channel HEMTs with high Al composition?>0.5?and excellent crystalline quality on a hetero-substrate owing to significant lattice and thermal mismatches between the AlGaN buffer layer and the hetero-substrate.Meanwhile,the breakdown voltage sees negligible improvement due to the poor crystalline quality that results from high Al composition.In addition,the reported breakdown field strengths of AlGaN channel HEMTs are much less than their expected critical field strength.To the authors'best knowledge,currently no reports of a breakdown voltage exceeding 2000 V for AlGaN-channel HEMTs exists.GaN-on-Si has become one of the most suitable platforms for commercialization because the cost can be reduced by using a Si substrate with a large diameter up to 200 mm and a CMOS-compatible process flow.The normally-off GaN-based power devices are studied for safe operation of the power electronic systems.However,there exist still great challenges in managing the large lattice and thermal mismatch.The initial AlN nucleation layer is key to obtain AlGaN/GaN epitaxy with good crystalline quality.Moreover,the large density of threading dislocations in the AlN layer have a certain impact on the transport and breakdown of AlGaN/GaN transistors.Another issue is the in-diffusion of Al acceptors into the Si substrate as far as 50-100 nm during MOCVD growth.Consequently,a p⁺silicon layer was formed near the interface,which can act as a parasitic leakage path.Alternatively,GaN-on-SOI technology is also attractive for power systems because Silicon-on-Insulator?SOI?is an important platform to enable GaN monolithic integration.Due to the strain partitioning effect and possibly other secondary effects,the Si device layer in SOI does not necessarily behave the same as in a Si bulk substrate.Therefore,it is important to study the defect status of the Si layer in SOI and the effect of the defects on leakage mechanisms in the devices fabricated on an SOI substrate.In view of the above issues,high-temperature characteristics of trap states in SH-HEMTs and DH-HEMTs and the effect of the traps on leakage current of the devices are studied in this dissertation.Then,high breakdown-voltage AlGaN-channel HEMTs and high performance AlGaN/GaN/AlGaN DH-HEMTs are fabricated.Based on research results on GaN on sapphire substrate and p-GaN technology,the normally-off p-GaN/Al_0.2Ga_0.8N/GaN and p-GaN/Al_0.2Ga_0.8N/Al_0.05Ga_0.95N HEMTs on Si substrate are investigated.Finally,the forward and reverse leakage mechanisms in AlN nucleation layers on p-Si and p-SOI substrates are analyzed.The detailed work and research results are summarized as follows:1.Frequency-dependent capacitances and conductance measurements are adopted to investigate high-temperature characteristics of trap states in AlGaN/GaN/AlGaN double heterostructure high electron mobility transistors?DH-HEMTs?.It is found that fast and slow trap states are present in DH-HEMTs,while only fast traps exist in AlGaN/GaN single heterostructure?SH?HEMTs.In the former,the fast trap state density ranges from4.6×10¹² cm^-2eV^-1to 1.9×10¹³cm^-2eV^-1 located at an energy below the conduction band between 0.273 eV and 0.277 eV,and the slow deep trap state density decreases from2.4×10¹³cm^-2eV^-1 to8.7×10¹²cm^-2eV^-1 located at an energy ranging from 0.384 eV to 0.423eV in DH-HEMTs with a 14 nm GaN channel layer.These active trap energy levels in DH-HEMTs become deeper as the thickness of the channel layer decreases.In addition,the active trap energy levels in SH-and DHHEMTs gradually become deeper as the measurement temperature increases.Also,the change in amplitude of the active trap energy levels in DH-HEMTs is larger than that in SH-HEMTs,which indicates that DH is efficient in suppressing the reverse gate leakage current at elevated temperatures.2.A high breakdown voltage of more than 2200 V in high-electron-mobility transistors?HEMTs?with AlGaN channel and a novel ohmic/Schottky-hybrid drain contact is achieved,which is the record breakdown voltage ever achieved on AlGaN-channel HEMTs.The fabricated device exhibits a high on/off ratio of 7×10⁹ and a low subthreshold swing of 64mV/decade,enabled by the AlGaN channel and wet treatment.Furthermore,it exhibits excellent high-temperature output characteristics and dynamic I_D-V_D characteristics.Even though both the AlGaN channel and the ohmic/Schottky-hybrid drain have certain impact on the on-state resistance because of the higher sheet resistance and drain contact resistance,these results indicate the significance and potential of AlGaN-channel HEMTs with a hybrid drain architecture in high-voltage applications.3.We presented a high performance AlGaN/GaN/AlGaN double heterostructure HEMT with a 10 nm channel layer and an Al_0.1Ga_0.9N back barrier layer.The fabricated devices exhibited an extremely low off-state drain leakage current of<10^-10 A/mm.An ON/OFF current ratio(I_ON/I_OFF)of up to 10¹⁰ and a subthreshold swing?SS?of 63 mV/dec were attained due to the reduced leakage current and enhanced electron confinement.The presented devices with a gate-drain spacing(L_GD)of 22?m exhibited a soft breakdown voltage(V_BR)of 1860 V with the criteria of 0.1?A/mm and a hard V_BR of 2009 V combined with a specific on-resistance(R_on,sp)of 10.6 m??cm².Moreover,excellent high-temperature values for I_ON/I_OFF,SS,and V_BR were also attained.These results demonstrate a promising potential of the proposed devices for high power applications.4.Based on the research results of high-performance GaN-based power devices on sapphire substrate,two normally-off devices,i.e.,p-GaN/Al_0.2Ga_0.8N/GaN and p-GaN/Al_0.2Ga_0.8N/Al_0.05Ga_0.95N on Si substrate were fabricated.Higher threshold voltage and breakdown voltage were attained for the normally-off AlGaN channel HEMT.The carrier transport mechanisms in AlN nucleation layers grown by MOCVD on p-Si and p-Silicon-on-Insulator?SOI?substrates were investigated by using electrical characterizations at various electric fields and temperatures on Ni-Au/AlN/Si Metal-Insulator-Semiconductor?MIS?capacitors.It was found by Capacitance-Voltage measurements that an inversion channel related to Shockley-Read-Hall?SRH?minority carrier generation in the silicon bulk and SOI substrates was formed.It showed that the AlN on p-SOI platform exhibits the same leakage mechanisms as for the AlN on Si substrate.The contact scheme mainly affects the low forward and reverse bias characteristics,by adding a surface leakage contribution.At increasing forward?positive?gate bias,the carrier transport process becomes in turn ohmic,Variable-Range-Hopping?V-R-H?,SRH generation,and Fowler-Nordheim?FN?direct tunneling.Moreover,at reverse bias,the carrier transportation is in turn dominated by Schottky emission,Poole-Frenkel?PF?conduction,and FN direct tunneling with increasing bias.Anyhow,the research in this work is beneficial to the future study of the vertical carrier transport and breakdown mechanisms in full AlGaN/GaN stacks on p-Si and p-SOI substrates.