Study On The Novel Structures And Characteristics Of AlGaN/GaN Heterojunction Power Transistors

Si-based power semiconductor devices have approached their performance limits after decades of development.In order to meet the needs of the rapid development of power electronic technology,the third generation of wide bandgap semiconductors represented by GaN has become the focus of research.Heterojunction transistors represented by AlGaN/GaN High-Electron-Mobility-Transistors(HEMTs)are regarded as the ideal power semiconductor devices in 30-1800V voltage range.However,many key technologies,such as high blocking voltage,enhancement mode and high reliability are still needed to be further broken through for AlGaN/GaN heterojunction power transistors,many mechanisms that affect the device characteristics need to be studied more deeply.In the dissertation,breakdown mechanism,trap states,novel high blocking voltage structures and novel enhancement mode structure of AlGaN/GaN heterojunction power transistors are studied.The main work and outcomes are as follows:1.Threshold characteristics of recessed-gate enhancement-mode AlGaN/GaN HEMTs are investigated.It is shown that the threshold voltage of the AlGaN/GaN HEMTs move in the positive direction by increasing the recessed-gate depth or the work function of the gate metal.The maximum transconductance of the device increases as the depth of the recessed-gate increases,while the work function of gate metal has little impact on the maximum transconductance of the device.Enhancement-mode device with high threshold voltage can be obtained by combining high work function gate metal with recessed-gate structure.The recessed Pt-gate HEMT showed a high Vth of 0.7V and its maximum transconductance is 542mS/mm when the length and depth of the recessed-gate are 1?m and 15nm,respectively.2.Frequency-dependent conductance measurements were carried out to investigate the trap states induced by reactive ion etching in AlGaN/GaN HEMTs quantitatively.It is shown that lots of trap states with shallow energy levels are induced by the gate recess etching for the conventional HEMT.The induced shallow trap states can be changed into deep trap states by 350? annealing process.As a result,there are two different types of trap sates,fast and slow,in the annealed HEMT.The gate leakage currents are changed by the etching and following annealing process,and this change can be explained by the analysis of the trap states.For the DH HEMT,lots of trap states are induced by the gate recess etching.The trap states is located at energy levels in a range of 0.30-0.33eV for the recessed DH HEMT,this energy range is located between the energy levels of two types of traps for nonrecessed DH HEMT.3.The effects of gate length LG on breakdown voltage VBR are investigated in AlGaN/GaN HEMTs.With the increase of LG,VBR is first increased,and then saturated at LG=3?m for both conventional HEMTs and DH HEMTs.It is shown that the breakdown for the HEMTs with LG<3?m is induced by the buffer leakage current,and for the HEMTs with LG?3?m is induced by the gate leakage current.The HEMTs with LG>3?m can alleviate the buffer-leakage-induced impact ionization,and the suppression of the impact ionization is the reason for improving the breakdown voltage.For the HEMTs with LG>3?m,the high electric field region is almost completely depleted,and the electrons from the source are difficult to be injected into the high electric field region,so there is no obvious difference in breakdown voltage among the HEMTs.4.The influence of gate length LG on the DIBL effect of AlGaN/GaN HEMTs and its physical mechanism are investigated.It is shown that the DIBL effect decreases with the increase of the gate length for conventional HEMTs,when the gate length LG increases from 1?m to 20?m,the corresponding DIBL drops from 13.6mV/V to 3.8mV/V.This is due to the increased depth of depletion region below the gate as the gate length increases.As a result,with the increase of drain voltage,the larger the gate length is,the smaller the relative change of the depletion region depth is,and the smaller impact on the depletion area is.When the gate length is small,the electron barrier formed below the gate is narrower,and the barrier near the source side is easy to be pulled down by the bias of the drain terminal.At this time,the DIBL effect of the device is more significant,and vice versa.For the DH HEMTs,the DIBL effect is not affected by the gate length,which result in the DIBL effect of HEMTs with different gate length is almost zero.This is because the 2DEG of DH HEMTs are confined between two AlGaN layers,which improves the confinement of 2DEG,and better confinement results in a lower DIBL effect for DH HEMTs.The effect of deep level doping on the DIBL effect of the HEMTs is also investigated.It is shown that deep level doping can effectively suppress the DIBL effect of HEMTs.For the material structure with low leakage current in buffer layer,the DIBL effect is very small,and it is not easy to be affected by the gate length.5.The concept of AlGaN/GaN heterojunction power transistors based on bulk electric field modulation is proposed.Based on this concept,two novel structures of high voltage HEMTs,DBPL AlGaN/GaN HEMT and PIBL AlGaN/GaN HEMT are designed,and the physical mechanism of the increase of breakdown voltage is analyzed and explained.For the DBPL HEMT,the average breakdown electric field between gate and drain reaches 2.55MV/cm,while the specific on-resistance is only 1.3m?·cm2,and the calculated FOM is as high as 3198MW·cm-2.For the PIBL HEMT,the average breakdown electric field between gate and drain reaches 1.7MV/cm,while the specific on-resistance is only 1.8m?·cm2,and the calculated FOM is as high as 1606MW·cm-2.It is shown that thinner P-GaN buried layer should help to a more significant electric field modulation,and lead to a higher breakdown voltage.Because the DBPL structure has more powerful electric field modulation capability than the PIBL structure,it means the surface electric field distribution between gate and drain is more uniform.As a result,the average breakdown electric field and FOM of DBPL HEMTs are 50%and 100%higher than those of PIBL HEMTs,respectively.6.Dielectric/P-AlGaN stack gate structures are proposed to improve the threshold voltage of enhancement-mode AlGaN/GaN HEMTs,and the mechanism of the increase of threshold voltage is analyzed and explained.It is shown that the addition of the dielectric layer above the P-AlGaN gate is responsible for the increase of the threshold voltage of the device due to partial gate voltage is applied to the additional dielectric layer.Lowering the dielectric constant of the dielectric layer or increasing the thickness of the dielectric layer can increase the threshold voltage of the device,meanwhile,the maximum output current of the device is almost unaffected.The thickness and doping concentration of P-AlGaN are fixed to 100nm and 1 X 1018cm-3 respectively.When the 10nm thick Al2O3 is used as the dielectric layer of the stacked gate structure,the threshold voltage of the device is 2.1V.When the 30nm thick SiO2 is used as the dielectric layer of the stacked gate structure,the threshold voltage of the device is further increased to 8.6V,and this value is very high for the enhancemet-mode AlGaN/GaN HEMTs.