Study Of New Structure AlGaN/GaN Heterostructure Field-Effect Transistors

AlGaN/GaN heterojunction field effect transistors(HFETs)are important representatives of GaN-based electronic devices which benefit from superior characteristics of GaN materials.AlGaN/GaN HFETs play an important role in high-power,high-frequency applications,civil communications(5G base stations),automotive electronics,aerospace and other fields.Polarization Coulomb Field(PCF)scattering is one of important scattering mechanisms in AlGaN/GaN HFET device.During the 13 years of studying PCF scattering,the PCF scattering theoretical model and sets of scattering theoretical systems have been played an important role in guiding device design and device modeling.But these works are based on conventional square gate device,the research of new structure AlGaN/GaN HFET device did not involve.Therefore,this thesis is based on PCF scattering theory system and researching opening gate structure and arc gate structure AlGaN/GaN HFET device.We compared the characteristics of new structure AlGaN/GaN HFET device with conventional square gate AlGaN/GaN HFET device.The specific research contents include:1.The effect of open gate structure on threshold voltage of AlGaN/GaN HFET deviceIn this paper,we prepared central opening gate structure with gate length of 60?m,gate width of 100 ?m,gate-to-source and gate-to-drain spacing of 6 ?m,conventional AlGaN/GaN HFET device.The opening width of opening gate structure AlGaN/GaN HFET device are 3 ?m,4 ?m,5?m.The ?-? output characteristic curves of the device are obtained by testing.When the current of per unit gate width are in the order of 10-8A,the gate-source bias of the device is threshold voltage.We found that opening gate structure changed threshold voltage of devices.The threshold voltage of conventional device is-4.2V,the threshold voltage of opening gate structure devices with opening widths of 3?m,4?m,and 5pm are-5.5V,-7V,-8V.The opening gate structure makes threshold voltage of devices more negative,as the width of opening increases,the threshold voltage of devices become more and more negative.The GaN power amplifier AC input signal power are proportional to the product of gate-source bias change ?VGS and gate-source current change?lGs.The open gate structure increase gate-source bias change ?VGS which will effectively increase the input power of power amplifier AC signal.There is no gate metal in the middle opening of opening gate structure devices.Why the channel current can also be reduced to the order of 10-8A is that channel electron current at the opening are regulated by gate metal voltage on both sides.The reason is that the more negative of gate source bias on both sides,more additional polarized charge under the gate,the stronger PCF scattering of the channel electrons at the opening,reducing the channel electron mobility and drift speed.In addition,the gate-source bias modifies the AlGaN barrier layer under gate mental which could relax to the AlGaN barrier layer at the opening.Therefore,the channel electron concentration at the opening are subject to the negative bias of gate-source modulation.So the output current at the opening decreases with gate-source bias,the lower the negative current means more negative gate-source bias,the larger width of the opening,the larger negative gate bias of the gate metal required to modulate current at the opening.That is why the device with an opening gate structure increases with the width of the opening,threshold voltage become more negative.PCF scattering cause different resistance distribution along gate lengths because of PCF scattering is Coulomb scattering.The gradient of PCF scattering potential changed a lot at one point on the opening,the PCF scattering is stronger at that point.The position of the opening along the center of gate length opposite gate metal on both sides has the strongest symmetry of additional polarized charges,which means the gradient of PCF scattering potential change is small,the PCF scattering are weakest,the resistance corresponding to PCF scattering are smallest.The opening gate structure has the weakest symmetry of the additional polarized charge under gate metal on both sides at opening along gate length,the largest gradient of PCF scattering potential change is the largest,the PCF scattering are the strongest,and the resistance corresponding to PCF scattering are the largest.Opening gate structure AlGaN/GaN HFET device channel conductivity modulation at the opening are different from conventional field effect transistor device.It is of great significance to study the new mechanism of channel conductance modulation.2.The effect of opening gate structure on extrinsic transconductance of AlGaN/GaN HFET deviceWe prepared central opening gate structure with gate length of 60 ?m,gate width of 100 ?m,gate-to-source and gate-to-drain spacing of 6 ?m,and conventional AlGaN/GaN HFET device.The opening widths of opening gate structure are 3 ?m,4?m,5 ?m.Combined the transfer characteristic curves of devices,the extrinsic transconductance under different gate bias voltages are obtained by differential processing.We found that the extrinsic transconductance of opening gate structure device are larger than conventional device under each gate bias,the extrinsic transconductance of opening gate structure device also increase with opening width increase.In a certain extent opening gate structure can control extrinsic transconductance of the devices.The reason why extrinsic transconductance of opening gate structure are large is that under the same gate-source bias,additional polarized charge under gate of opening gate structure device are less than conventional device,PCF scattering of electrons in the channel are relatively weak,and the resistance of gate-source channel Rchs(PCF)corresponding to PCF scattering are small,the source parasitic resistance RS are relatively small,therefore,the extrinsic transconductance of opening gate structure device are relatively large.By adjusting the ratio of the opening width and gate width could modulate the trasnsconductance of opening gate devices.Based on the output currents of opening gate structure device and conventional device,we conduct equivalent calculations on net current at the opening of opening gate structure device.Considering opening width 3 ?m as gate length 60 ?m,gate width 3 ?m devices.By calculating low electron field carrier mobility,we found that the additional charge of opening gate structure device with an opening width of 3 pm are less than conventional device under the same gate-source bias.This indicates there is a strain interaction between AlGaN barrier layer under the gate metal and AlGaN barrier layer at the opening.The strain changes of the AlGaN barrier layer of opening gate structure device are smaller than AlGaN barrier layer strain of conventional AlGaN/GaN HFETs with the gate source bias voltage.Therefore,it proves that the devices with an opening gate structure have weaker PCF scattering of electron in the gate-source channel,resulting in a smaller parasitic resistance.3.Effect of arc gate structure on the characteristics of AlGaN/GaN HFET deviceWe prepared arc gate structure devices with gate width of 80 ?m,gate-to-source and gate-to-drain spacing of 10 ?m,the maximum gate length of 84 ?m,and square gate devices with the same area as the arc gate.Testing I-V characteristic curve and C-V characteristics,we calculate the low electron mobility carrier mobility of each device by theoretical model.The concave arc gate structure device are equivalent to square structure device which has a gate length of 60 ?m,but the gate length of square gate device whose gate area equal to the concave arc is 21 u m.By calculating the low electron mobility carrier mobility the convex arc gate structure device are equivalent to square gate structure device with gate length of 78?m,but the gate length of square gate device whose gate area equal to the concave arc is 66 ?m.It shows that the concave arc gate devices and convex gate devices have different PCF scattering intensity than square gate devices for the channel carriers under gate.By testing transfer characteristic curve of the devices,we calculated the extrinsic transconductance of arc gate and square gate devices.We found that both the extrinsic transconductance of arc gate structure and square gate device rise to the peak and appear to decrease,the extrinsic transconductance value of arc gate structure device are smaller,and the slope from the peak decreases more slowly.It is because that the arc gate structure have different RS than square gate structure,and the arc gate structure can improve the linearity of GaN power amplifiers.