The Research On Low Temperature Ohmic Contact And Threshold Voltage Of AlGanN/GaN HFET

AlGaN/GaN heterostructure has attracted a lot of attentions from all over the world due to its large bandgap, high breakdown voltage, high thermal conductivity, high electron mobility and large electron saturation velocity. AlGaN/GaN heterostructure field-effect transistors (HFETs) were one kind of electronic devices which commonly used in high power and high frequency area. Self-aligned structure can largely decrease the access resistance between the gate and source/drain, which can improve the device performance. However, using the self-aligned structure need fabricate the gate electrode before source/drain electrodes. So, the gate electrode needs annealed with the source/drain electrodes. The high annealing temperature may damage or even destroy the gate schottky contact. Therefore, the low temperature ohmic contact was proposed. But the contact resistance of this ohmic contact is large. It needs optimize. In this thesis, we optimized the annealing condition to decrease the contact resistance. At the same time, we also applied this new ohmic contact for device fabrication to confirm its electronic performance. Moreover, for decrease the power consumption of the AlGaN/GaN HFET and avoid fail-safe problem, realize the threshold voltage of the AlGaN/GaN HFET over 0 V was needed. The AlGaN etching structure was used to improve the threshold voltage. The influence of the off-state current was also researched.First, the low temperature ohmic contact was realized by generate N vacancies which induced by ICP etching. By optimizing the annealing temperature and annealing time, an ohmic contact with 0.52 flmm contact resistance can be formed after annealing at 575℃ for 1 min. Compared with the conventional annealing temperature, the low temperature ohmic contact kept the contact resistance at the same level and decreased the annealing temperature approximate 300℃. We also researched the thermal stability of this ohmic contact by thermal treatment for different time and different temperature.Next, we introduced the self-aligned gate structure and gate-first structure. The requests of low annealing temperature and thermal stable gate electrode for appling these two structures also be introduced. Then, by test the TiN shottky diode with different annealing temperature, we confirmed the TiN can be used to replace the Ni/Au gate for high temperature application. Moreover, we fabricated two gate-first AlGaN/GaN HFET with low temperature ohmic contact and one gate-first AlGaN/GaN HFET with conventionalTi/Al/Ti/Au ohmic contact. Depending on the electronic performance test, we confirmed a three magnitude decreasing of gate leak current by appling the low temperature ohmic contact. Combine these two results, the two requests for appling the self-aligned gate structure and gate-first structure can be realized.Last, we use the AlGaN etching structure fabricated some HFETs to evaluate the change of the HFET’s threshold voltage. We found etching the AlGaN to 8 nm, the threshold voltage can be increased to-0.04 V. By research the Id-Vg and Ig-Vg characteristics, we found and researched the phenomenon that AlGaN etching can increase the drain current even the HFET was under the off-state.After all, the researched methods can be used to improve the electronic performance of the low temperature ohmic contact and the threshold voltage of AlGaN/GaN HFET. These methods did not complicated and they can be used for device fabrication.