Research On AlGaN /GaN Based High Electron Mobility Transistors And The Piezotronic Modulation

As the representative material of the third-generation semiconductors,Ga N has excellent properties,such as large band gap width,higher breakdown electric field,higher thermal conductivity,corrosion resistance,radiation resistance,etc.It is also suitable for making high-frequency,high-temperature,high-voltage and high-power electronic devices,and short wavelength,high-power optoelectronic devices.High electron mobility transistors(HEMTs)based on AlGaN/Ga N heterojunction structure have significant advantages including high carrier concentration,high electron mobility,and large breakdown voltage.And they will be widely used in radar and communication systems,power converters,microwave systems,artificial intelligence and other fields.Commonly,Ga N crystal has a hexagonal wurtzite structure(non-centrosymmetric),and shows piezoelectric characteristics,which belongs to piezoelectric semiconductors.Piezotronic effect is a new physical effect that coupling piezoelectric and semiconductor properties in piezoelectric semiconductors,and this interdisciplinary frontier field has developed rapidly and made many important advances.At present,many novel Ga N-based semiconductor devices with one-dimensional or two-dimensional structure under piezotronic effect have been demonstrated in many important fields,such as micro-nano energy harvesting,logic circuits,memory,wearable electronic devices,and so on.However,traditional Ga N-based semiconductor devices are mostly prepared by epitaxial growth on hard substrates(Si,Si C,sapphire,etc),which seriously affects the in-depth study of piezotronic effect in the devices.Hence,we fabricated flexible HEMT devices by thinning Si substrates by backside dry etching method,and then applied external stress to investigate the piezotronic modulation on the performance of HEMTs.The main research contents and results are as follows:(1)The fabrication process of AlGaN/Ga N heterojunction HEMT devices was studied.The key processes for the fabrication of AlGaN/Ga N heterojunction HEMT devices are explored in detail,including UV lithography,isolation of active region,preparation of Ohmic contact of source and drain electrodes,preparation of dielectric layer,preparation of gate Schottky contact,etc.The parameters and optimization schemes of various processes were obtained,and the AlGaN/Ga N heterojunction HEMT with better performance was successfully prepared.(2)The effect of backside dry etching of Si substrate on the properties of AlGaN/Ga N HEMTs was investigated.The physical properties were characterized by scanning electron microscope(SEM),Raman spectra,and X-ray diffraction(XRD).After the dry etching process,the peak red-shift of Ga N E₂-high mode indicates an increase of tensile stress,and the XRD rocking curve of Ga N film shows a certain extent decrease of the dislocation density.Furthermore,the maximum saturation current density and maximum transconductance of the HEMT are improved by 21.1%and 25.5%,respectively.It proves that the approach of backside dry etching for thinning Si substrate would contribute to the optimization of Ga N heterojunction-based devices,and also provide inspiration for the development of flexible and robust power devices.(3)The piezotronic modulation on the performance of AlGaN/Ga N HEMTs was studied.The electrical characteristics(including DC output characteristics,transconductance,pulse output characteristics,and the gate leakage etc)of the AlGaN/Ga N HEMTs can be regulated effectively by applying external stress to the HEMTs through a customized stress fixture device.When the Ga N layer is subjected to tensile stress,the DC and pulse output characteristics of HEMTs show a significant decreased trend.In addition,a three-dimensional stress model was introduced to further analyze the external stress conditions,and a self-consistent model was used to explain the regulating mechanism of the piezotronic effect on the two-dimensional electron gas(2DEG)concentration in AlGaN/Ga N heterojunction.