Study On AlGaN/GaN/ALGaN Based Millimeter-wave Double-heterostructure HEMT

In the application of millimeter wave AlGaN/GaN HEMT,the frequency characteristic of the device is very important.The cutoff frequency and the oscillation frequency of the device are two of most important parameters which represent the quality of device.It can be see from the principles of HEMT that transconductance,resistance and parasitic capacitance mostly influence cutoff frequency.Transconductance and resistance,part of the decision is from structure of the device,due in part to material and process,most of the parasitic capacitance is decided by the structure of the device,the main factors influencing the oscillation frequency is a parasitic capacitance and resistance device.In the aspect of device structure design,shortening the gate length is the most direct way to reduce the gate resistance,however,the short channel effect is inevitable when the gate length decreases,which directly leads to the deterioration of dc characteristics of the device.In order to deal the contradiction,we found that adding AlGaN back barrier layer on the basis of the original device can effectively improve the limit of 2DEG,suppressing the short channel effect and improving transconductance of the device,and thus improving the frequency of the device.By means of device simulation and fabrication,the factors affecting the frequency characteristics of the device are discussed in this paper from various aspects,and the accuracy of the simulation is verified.In the aspect of material growth,we will not do a lot of discussion.Firstly,through optimizing the AlGaN barrier layer of the device by using the Atlas module in Silvaco simulation tool,including the thickness of the barrier layer and the Al component,and then extracted the transfer output curve,breakdown curve and small signal characteristics of the device at different thickness and Al group of the barrier layer.Through simulation,we analyzed the internal causes of the influence of different barrier thickness and Al components on dc and small signal characteristics of the device from the aspects of carrier distribution,energy level distribution and potential barrier.Secondly,the device with AlGaN back barrier layer was designed and the dc characteristic curve and small signal characteristic curve of the device were extracted.When the back barrier layer was added,the peak transmittance of the device increased by 26.4%,the leakage potential barrier decreased by 29%,and the cutoff frequency increased by 23.8%.The barrier layer mainly determine the concentration and mobility of two-dimensional electronic gas,and also have a certain impact on the grid leakage and breakdown voltage of the device.With the addition of the back barrier layer,the breakdown voltage of the device is increased,and the gate leakage is significantly reduced.Through the simulation,T-gate was designed in grid,grid cap and grating feet all aspects and the influence of the gate on the frequency of the device are analyzed.At the same time,designing the different spacing of source to drain and passivation layer.Not only does T-gate reduce resistance of the device,but also improve the parasitic capacitance possibility,the passivation layer suppresses the current collapse and improve the breakdown characteristic of the device and also increases the parasitic capacitance of the device.Finally,T-gate devices with different thickness was made through the flow sheet.The results of simulation found that as the barrier layer thinned,the threshold voltage of the device was positively drifting and the transconductance increased,the gate leakage increased and the frequency characteristics of the thin-barrier device had a great advantage over the conventional devices.As the gate length of the device increases,the transconductance of the device decreases and the breakdown voltage increases.By the simulation,we also analyzed the internal causes of this phenomenon through simulation.Comparing with the straight gate structure,the frequency of the T-gate structure and the cutoff frequency of the device increased by 33%.