Research Of Characteristic Contrast Between GaN HEMTs And GaAs PHEMTs

In recent decades,human society has witnessed three industrial revolutions driven by semiconductor materials.Silicon material together with germanium material of the first generation,the GaAs material in company with the InP material of the second generation and the GaN material together with the SiC material of the third generation,respectively,led the development of microelectronics,having achieved three leaps already.With wide bandgap,high breakdown electric field and other characteristics,GaN High Electron Mobility Transistors(HEMTs)have stood out among Field Effect Transistors(FETs),leading to widespread applications irreplaceable in defense,communications,lighting,electricity,aerospace and so on.As a material with wide bandgap,A GaN HEMT unfolds unique effects named spontaneous polarization and piezoelectric polarization,resulting in an inherent two-dimensional electron gas(2DEG)without intentional doping,which is a superior-ity relative to other FETs.Meanwhile,the two polarizations also engender a sort of distinctive scattering,which is entitled the polarization Coulomb field scattering(PCF scattering).The PCF scattering derives from the non-homogeneous distribution of strain in AlGaN barrier between source and drain.It leads to the non-homogeneous distribution of polarization charge along the heterojunction interface in AlGaN/GaN,resulting in the PCF scattering.This is a vital scattering mechanism in GaN HEMT devices.The existing PCF scattering studies are restricted to GaN material itself,and there are no deepgoing researches between different materials on PCF scattering.This thesis supplies the gap.Like GaN,GaAs materials are direct-gap semiconductors.As a semiconductor ma-terial of second generation,GaAs still has its own advantages.Unlike GaN,GaAs pseudomorphic HEMTs(PHEMTs)do not manifest spontaneous polarization and pi-ezoelectric polarization.Therefore,to yield carrier,they have to be doped.Barrier doping of AlGaAs layer supplies 2DEG to the channel.In this thesis,both structures are HEMT devices,regulating the transport of channel carriers under gates.Conse-quently,PHEMTs of GaAs are desirable contrast objects.By analyzing the difference between them,we can clear the distinct effect of the PCF scattering in GaN HEMTs further,and lay foundations for optimizing characters in GaN HEMTs.In this dissertation,GaN HEMT devices and GaAs PHEMT devices(Lds=20?m and 100?m)were fabricated,and the centered gate devices were studied as follows:1.A contrastive study of the mobility between GaN HEMTs and GaAs PHEMTs.Mobility in GaN HEMTs and GaAs PHEMTs were obtained by testing C-V charac-teristic curves,I-V output characteristic curves and diode characteristic curves.By contrasting the difference of variation tendency of mobility between GaN HEMTs and GaAs PHEMTs,the inherence principles were analyzed and studied.In GaN HEMTs,it was found that the varying trend with gate voltage increasing depends on the ratio of gate length(Lg)to the distance between source and drain(Lsd).The PCF scattering grew more and more significant when the ratio of Lg to Lsd decreased.With the same ratio of Lg to Lsd,PCF scattering came to be strengthened when Lg decreased.In shorter channels,non-homogeneous strain distribution strengthens in AlGaN barrier,leading to more conspicuous effects.It was a decline after the knee point as the volt-age drain to source increased and an ascent before the knee point in evolvement ten-dency of the mobility calculated in different GaAs PHEMTs of different gate lengths.The law is applicable despite alteration of gate length or the ratio of Lg to Lsd.Back-ground ionized impurity scattering dominates when gate voltage Vg is low.Coulomb screening intensifies between 2DEG to ionized impurity scattering along with the in-crease of 2DEG density and mobility increases until the second subband commences filling.Mobility initiates decreasing with gate voltage increases after voltage reached the knee point.It is due to the change of dominant scattering mechanism that polar optical phonon(POP)scattering replaces the former one,the ionized impurity scat-tering,with 2DEG decreasing when gate voltage increases after the knee point.Dis-tinct discrepancy was manifested due to the PCF scattering between GaN HEMTs and GaAs PHEMTs.2.A contrastive study of parasitic source access resistance characteristics between GaN HEMTs and GaAs PHEMTs.Parasitic source access resistance(Rs)characteris-tics directly determine the device transconductance.Testing Rs characteristics by the gate probe method,trend of Rs with increasing gate voltage was analyzed.Distinct discrepancy was manifested in Rs characters between GaN HEMTs and GaAs PHEMTs.Steady ascent of Rs variation with gate current Ig increasing was observed in GaN HEMTs and a descend in GaAs PHEMTs.The variation rate of Rs in GaN HEMTs is much larger than that in GaAs PHEMTs.The difference derived from the PCF scattering in GaN HEMTs.The trend in GaAs PHEMTs derived from ionized impurity scattering.It was unintentional doped in GaN HEMTs leading to nothing of ionized impurity scattering.Therefore,nothing but the PCF scattering could result in variation of Rs with increasing gate current in GaN HEMTs.3.A contrastive study of transconductance characteristics between GaN HEMTs and GaAs PHEMTs.Transconductance characteristics were acquired by differentia-tion from transfer characteristic curves.Transconductance characteristic in each de-vice varied almost the same,amplifying with lower increasing gate voltage first and attenuating with higher increasing gate voltage.In diminishment interval,negative ratios of growth of transconductance in GaAs PHEMTs were greater than the ones in GaN HEMTs.The POP scattering lead to an intensification of Rs and a decline of transconductance and the PCF scattering retarded the declining extent.To make use of character which retards the decline of transconductance with increasing gate voltage,it is beneficial for solving the problem of linear distortion in power amplifier applica-tions in GaN HEMTs.