Research On Ohmic Contact Method Based On AlGaN/GaN HEMT Device

Based on the material advantages of the third-generation semiconductors,AlGaN/GaN high electron mobility transistors(HEMT)have great potential for applications in microwave communications,power control,radar and aerospace.The ohmic contact is the channel connecting the device and the external circuit.The low ohmic contact resistance can reduce the on-resistance and power loss of the device to make the device have better electrical and thermal characteristics,better reliability.However,the wide band gap and large surface state density of Al Ga N make it difficult to fabricate low-resistance ohmic contacts based on AlGaN/GaN HEMT devices.Therefore,in order to further increase the application advantages of AlGaN/GaN HEMT devices,at the same time improve the reliability of the device,and exert its full potential as a high-frequency high-power device.The uniform,stable,and low resistance based on AlGaN/GaN HEMT devices is of great significance.Based on this,with the goal of reducing the ohmic contact resistance on AlGaN/GaN HEMT devices and improving the ohmic contact characteristics in this article.Different ohmic barrier layer etching schemes are proposed to explore the best ohmic contact mode.And The reliability for different ohmic contact schemes under electrical stress has been studied.In this paper,a thin Si layer is inserted into the conventional Ti/Al/Ni/Au ohmic metal.The introduction of the thin Si layer can reduce the ohmic contact resistance,and achieve annealing at a relatively low temperature of 650?to form ohmic contacts.Afterwards,the effects of the three etching methods of overall etching,comb hole etching and array square hole etching on the barrier layer of the ohmic region to improve the ohmic contact characteristics of the AlGaN/GaN HEMT device.Among them,in order to improve the ohmic contact in the square hole,for the larger depth of the array square hole etching,the second etching method of performing overall etching and then square hole etching is used to achieve.The experimental results show that after etching through the ohmic barrier layer array square hole,the ohmic contact resistance obtained at the annealing temperature of 650?is the lowest,reaching 0.16?·mm,the square resistance is 360.24?/?,and the specific contact resistivity It is 2.4×10^-6?·cm²,exhibiting good ohmic contact characteristics.By etching the barrier layer of the ohmic region with an array of square holes of different apertures,realizing ohmic contact samples with resistance as low as 0.08?·mm.It is found that the increasement in the ohmic contact area by the barrier layer etching is the main reasonfor the improvement of the ohmic contact characteristics.In order to implement the array square hole etching scheme well on the actual AlGaN/GaN HEMT device,this paper proposes an improved secondary etching method to minimize the damage of the barrier layer etching process to the device.The test results show that the device characteristics are improved compared with the barrier layer etching directly.In this paper,experiments on the comparison of positive and negative electrical stress for different graphical ohmic contact schemes were carried out.Through the analysis of ohmic characteristics during the stress process,the degradation law and mechanism of ohmic contact under electrical stress were explored.Combined with the common degradation effects of HEMT devices,it is found that the degradation of ohmic contact under forward electrical stress is mainly caused by the electron trapped by the new trap of the barrier layer due to the reverse piezoelectric effect.Under long-term large forward stress,the reverse piezoelectric effect will also occur in the barrier layer near the ohmic contact.The main defect affecting the ohmic contact of the device under negative electrical stress is the native traps of the Al Ga N barrier layer.Under certain negative voltage stress,these traps will release electrons and improve the ohmic contact characteristics of the device.These research results are of great significance for improving the reliability of HEMT devices under electrical stress.