Research On GaN HEMTs With Enhanced Linearity

Due to superior properties including wide bandgap,high electron velocity,as well as high electron mobility,GaN high electron mobility transistors(HEMTs)are promising in the field of microwave power applications.However,GaN HEMTs usually suffer from weak linearity.The transconductance of GaN HEMTs will roll-off soon after reaching its peak as gate bias increases.As a result of this nonideal effect,the application of these devices in communication field is strictly limited.Therefore,device-level linearization techniques are pressingly desired to fulfil the theoretical potential of GaN HEMTs.In this work,a research on the linearity of GaN HEMTs is presented.Firstly,a review of the latest development in GaN HEMTs linearity is offered.The main cause of the nonlinearity is considered to be the nonideal effects that occur under high electric fields.For GaN HEMTs used for high-frequency applications,device size is generally limited,thus these nonideal effects could frequently occur.These nonideal effects include the increase of source access resistance as current rises,as well as the degradation of electron velocity and mobility under high electric fields.To realize highly linear GaN HEMTs,several ideas have been proposed.these methods include nanowire channels,polarization field-effect transistors,electric field relieving structures,transconductance compensation technique,etc.Secondly,a novel AlGaN/GaN HEMT with enhanced linearity has been designed and fabricated.The enhancement in device linearity is realized by selectively injecting fluorine ions into the barrier along the gate-width direction.The fabricated two types of devices feature gate voltage swing of 3.3 V and 3.1 V,respectively,73.6% and 63.1% higher than that of conventional devices.Besides,the novel device also shows greater suppression of nonideal harmonics,as the 1st-and 2nd-order derivatives of transconductance in which are much flatter than in conventional devices.Finally,the effect of several structure and processing parameters on device characteristics are discussed.To begin with,the effect of source-to-drain spacing on device output characteristics,transfer characteristics and linearity seems insignificant.However,an obvious dual-peak transconductance curve is recognised as fluorine-treatment time is decreased from 150 s to 120 s.The first peak of the transconductance curve is caused by the untreated parts of the device while the second transconductance peak corresponds to the regions that have been treated with fluorine ions.After that,the effect of the ratio of the treated/untreated size is investigated.It is found that the higher this ratio is,the higher the second transconductance peak gets,and the lower the first transconductance peak will be.Based on these measurement results,an almost linear dependence of the ratio on device transconductance is proposed and verified.Finally,the effect of the device width is also taken into consideration.As the self-heating effect gets suppressed under lower device widths,the devices with lower widths show enhanced properties,including an increase in gate voltage swing of 22.5%,a rise in transconductance peak value of 24.6%,as well as an enhancement in the output current density of up to 30.2%.This work can contribute to further studies on either the mechanisms of the nonlinearity in GaN HEMTs or the development of device-level linearization techniques.