| Terahertz technology shows great potential for applications in radar systems,biomedicine,detection,and communications.As an important part of terahertz technology,terahertz radiation source is an international key research topic,and terahertz solid-state source based on semiconductor materials is one of the research hotspots.Gallium nitride(Ga N),the representative of the third-generation semiconductor material,has attracted much attention in the terahertz field due to its unique qualities such as high electron mobility and large band gap,which are suitable for high frequency and high power.At present,studies have shown the existence of Gunn oscillation in Ga N-based HEMT devices.Although there are still few related studies and the device has a narrow frequency modulation range,it shows the possibility of Ga N-based HEMT as terahertz radiation sources.2DEG in Al Ga N/Ga N heterojunctions has extremely high concentrations and low ionized impurity scattering.The introduction of the gate disturbs the electric field and promotes the transfer of electrons between energy valleys.These characteristics provide favorable conditions for the HEMT device to generate Gunn oscillation.Based on this,the improvement of the traditional HEMT structure can make the Ga N-based HEMT a potential oscillation source in the terahertz field.In this thesis,a field plate structure(FP)with a thin passivation layer is introduced and an insulated-gate are introduced based on the traditional Ga N-based HEMT.Adjusted,these measures promote electron transfer between energy valleys to form Gunn electron domains.The Ga N-based FP MIS-HEMT was modeled on the Silvaco ATLAS simulation platform.The hydrodynamic model is used in the simulation,and the velocity field relationship model including energy relaxation time and lattice temperature is established to describe the negative resistance characteristics in Ga N heterojunctions.Based on this,the influence of the structure on the formation of stable electronic domains and the oscillation frequency was studied.This thesis focuses on the effects of external bias voltages such as gate bias and drain bias,as well as key parameters of device structure such as field plate length(LP),passivation layer thickness(DP)and gate dielectric thickness(dx),on the formation of electronic domains in the device.It is proposed that the gate voltage of the device can control the number and position of the nucleation sites of the device under the appropriate size.In addition,a new frequency modulation mode of drain-controlled electronic domain working mode isproposed to realize wide-range frequency modulation.Meanwhile,a special working mode with multiple nucleation sites on the drain side of the gate and the drain side of the field plate is also found in the research,and this mode has obvious effect on harmonic enhancement.Firstly,the external bias voltage is studied.The introduction of Si N gate-insulating makes it possible to apply a higher bias voltage to the device.The discussion of frequency modulation of gate voltage mainly starts from the single nucleation site near the drain side of the gate,the single nucleation site of the field plate near the drain side,and the separate dual nucleation sites mode of the gate near the drain and the field plate near the drain side.It is found that the fundamental frequency decreases with the increase of gate voltage in the above modes.Meanwhile,it was found that a small variation of the drain bias voltage in a specific structure can make the dipole domains have different operating modes,so that the frequency can be adjusted in the wide range of 959-2120 GHz,and the corresponding RF-DC current component ratio Irf/Iavg is in the range of 0.6%-4.8%.Secondly,the enhancement effect of the FP MIS structure on the nonlinear properties of Ga N-based HEMTs is studied.By reasonably designing of structural parameters,the FP MIS-HEMT can generate high oscillation frequency.The effects of LP,DP and dx on the formation mechanism of electron domains were investigated respectively.The simulation results show that with the increase of LP or the increase of dx,the fundamental frequency of the device shows a downward trend.The fundamental frequency decreases with increasing DP in the separated dual-nucleation sites mode.In the nucleation site mode on the drain side of the gate,the fundamental frequency increases with the increase of Dp.Under a reasonable size,the device will form a special multi-nucleation sites mode of the gate near the drain side and the field plate near the drain side.The device performance is better in this mode,and the harmonic frequency is greatly enhanced.When the device parameters are LP = 375 nm,DP = 60 nm,and dx = 5 nm,the fundamental frequency is 648 GHz,the Irf/Iavg is 3.3%,the second harmonic can reach 1.3 THz,and the corresponding Irf/Iavg is as high as 3.19%.The simulation found that under the appropriate bias voltage,when the device parameters are LP = 350 nm,DP = 70 nm,dx = 3 nm,the fundamental frequency of 626 GHz can be generated and the Irf/Iavg can reach 6.84%;The second harmonic is 1.23 THz and the Irf/Iavg is also relatively high up to 2.04%.The above characteristics are all caused by the fact that the electric field distribution of the channel contains two electric field peaks,which can cause multiple disturbances to the inter-valley transition of electrons.Compared with the reported RBL HEMT oscillator sources,this structure can achieve higher frequency and wider frequency modulation range.At the same time,the number of gate-controlled nucleation sites and the drain-controlled electronic domain working modes have become a new frequency modulation method,which is of great practical and research value. |
