Translated Abstract
GaN based HEMT(high electron mobility transistors) become important research projects at radiation microwave and power electronics due to their high thermal conductivity, high corrosion resistance, radiation resistance and their advantages at the case of high voltage, high frequency, high temperature, high radiation and high power. Compared with the GaAs based HEMT devices, the electron mobility of GaN is lower than the GaAs, but the density of the two dimensional electron gas(2DEG) formed at the AlGaN/GaN heterojunction is 3 to 10 times more than InAlAs/InGaAs and AlGaAs/GaAs. At the same time, the saturated velocity of the AlGaN/GaN is higher than AlGaAs/GaAs. All of these advantages of GaN based HEMT excellently compensate for the shortages of the two former generation semiconductive materials, Si and GaAs, and make the GaN HEMT become the frontier of scientific researches.
However, there are many problems remaining to be solved concerned with the basic theories, the material defects, the devices performances and the devices structures, such as the current collapse, always on-state, the leakage current of the gate, the linearity of the transductance, the much lower saturated source&drain current and maximum transductance than the theoretical limitation values. So, many researcher have dedicated themselves into the selection of the semiconductive materials, the growth of the semiconductive materials, the design of the epitaxial structure and the design of the device structure to improve the performance of the GaN based HEMT devices in recent years.
This article is mainly about the special shaped gate and we have designed, fabricated and done some researched about them. First of all, we managed to fabricate the normal GaN HEMT devices through the previous investigation and the optimized fabricating processes. We also represent and analyze the electrical properties of the GaN HEMT devices. Then, we designed and fabricated the ring shaped gate GaN HEMT, representing and analyzing the advantages of the ring shaped gate GaN HEMT after comparing the ring shaped gate GaN HEMT with the striped shaped GaN HEMT. Finally, we managed to fabricate the GaN HEMT with nanohole shaped gate, based on the ring shaped gate GaN HEMT, and represented and analyzed the GaN HEMT with nanohole shaped gate after the comparison with the normal ring shaped gate GaN HEMT.
The article mainly contains the following contents and results:
(1)The common fabricating process about GaN HEMT devices can be divided into three parts: The first part is ICP etching to isolate the mesa. The second part is the metallization of the source and drain contact. The last part is the metallization of the gate contact. After the adjustment and optimization of the fabricating processes, we finally fabricated the common GaN HEMTs devices.
(2)After we master the fabrication of the GaN HEMT and study other concerned papers, we designed the electrodes of the ring shaped gate GaN HEMT again. Due to instinctive advantages of the ring shaped gate, we skip the ICP etching during the fabricating processes, avoiding the etching damage induced by ICP. Moreover, due to the geometrical characteristics of ring shaped gate, ring shaped gate can get higher values of Wg/Lg than striped shaped gate when facing the same craft limitations. Conclusively, the saturated source&drain current of the ring shaped gate GaN HEMT are three times as high as the striped shaped ones. The maximum transductance of the ring shaped gate GaN HEMT is also nearly three times higher than the striped ones.
(3)After having fabricated the ring shaped gate GaN HEMT successfully, we designed and fabricated the brand new GaN HEMT with nanohole shaped gate based on the normal ring shaped gate GaN HEMT. In theory, the contact area at gate region of the GaN HEMT with nanohole shaped gate are much bigger than the GaN HEMT with traditional gate structures or the other special ones. Furthermore, the nanohole shaped gate attain better control on the GaN HEMT due to its surrounding filed effects. In experiments, the threshold voltage of the GaN HEMT with nanohole shaped gate shift positively at 5V compared with the normal ring shaped gate ones and the GaN HEMT with nanohole shaped gate also attain the 15% higher maximum transductance and 20% lower subthreshold swing(ss) than the normal ones.
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