Study On GaN Based HEMT-like Planar Gunn Diode In Terahertz Band

THz wave is the electromagnetic wave in the range of?0.1-10?THz?wavelength of3000-30?m?,which is between the millimeter wave?sub millimeter wave?and the infrared phase coincidence.Terahertz wave occupies a very special position in the electromagnetic wave spectrum.It has many excellent characteristics,and it has very important academic and applied value.In the region of the semiconductor devices that used at low THz frequency band,the negative differential resistance devices have attracted much attention,such as the Gunn diode,IMPATT,RTD,FET.The GaN-based Gunn diode has excellent high frequency characteristics,and large output power density under high electric field.The planar Gunn diode has significant advantages over the conventional vertical Gunn diode in the possibility of making multiple oscillators on a single chip and the compatibility with monolithic integrated circuits.Moreover,as the operating frequency of planar diode is defined by the metal contact separation,it easily allows the fabrication of different planar devices with different oscillating frequencies on the same wafer.GaN-based HEMT-like planar Gunn diode exhibits more excellent performance.The high-electron-concentration 2DEG channel of GaN heterojunction allows the formation of narrow electron domain,which greatly reduced the possible device channel length.At the same time,more complex electron domain phenomena easily occur in the high-electron-concentration channel,which provides more operation modes for the Gunn diode.Our work has demonstrated that the GaN HEMT-like planar Gunn diode is one of the most potential candidates as a solid state terahertz source device,and it is also the latest research field.This dissertation aims to study the GaN based HEMT-like planar Gunn diode to generate stable and high-power terahertz signals.We have displayed the modeling of the structure and material of the planar Gunn diode,the investigation of the special electron domain phenomena inside the 2DEG channel and the physical explanation.We also proposed several novel design and new materials in the planar Gunn diode in oder to improve its operate performance.The main conclusion is as follows:The first part focuses on the research of multi-domain phenomena in the 2DEG channel of HEMT-like planar Gunn diode based on Silvaco-ATLAS.Firstly we have studied the Gunn effect in the AlGaN/GaN HEMT.Based on the AlGaN/GaN HEMT,we propose the micron-size AlGaN/GaN HETM-like planar Gunn diode for the first time.The phenomenon of multiple domain is observed in the two-dimensional electron gas?2DEG?channel.An adjustment to the doping concentration of the cathode region is proposed to control the number of the domains,the oscillation mode and the operating frequency,aimed at optimizing the multi-domain mode conductive to the device to enter the THz range.The second part focuses on the research of ultra-short AlGaN/GaN HEMT-like planar Gunn diode?HBEAN diode?,which contains composite contacts.We have revised the velocity-field dependence of two-dimensional electron gas?2DEG?channel by taking the velocity overshoot into consideration.We propose a novel composite contact,i.e.,a combination of Schottky-contact and ohmic-contact at anode and cathode sides of GaN HEMT-like Gunn diode.The use of composite contact at the anode side aims to smooth the high electric field,therefore greatly suppresses the impact ionization so as to sustain the propagation of Gunn domain.The use of composite contact at the cathode side aims to form a depletion region of the channel beneath the Schottky extension electrode which acts as a notch-doping layer.It reduces the length“dead zone”.We also study the effect of surface traps on the composite contact.The thrid part focuses on the research of the modulation of multi-domain and harmonic wave in GaN planar Gunn diode by recess layer.In this part,in order to improve the high RF output power associated with the diode operating at high frequencies,we first propose to create multiple nucleate sites in one single GaN planar Gunn diode by means of incorporation of several isosceles trapezoidal recess layers in the AlGaN barrier layer,which aims to enhance harmonic components of Gunn oscillation so as to minimize the use of frequency multipliers for submillimeter-wave and terahertz?THz?operation.Operation of multiple Gunn domains is controllable by adjusting the total number,etching depth and the adjacent distance of the recess layers.The selective reduction of 2DEG density locally under the recess layer successfully induces the electric field fluctuation and acts as an electron domain nucleate site.N recess layers designed in AlGaN barrier introduces N electron domains forming simultaneously inside one oscillation circle.The domains interact with each other during their formation and maturation process and result in the high degree of nonlinearity in the oscillation waveforms.As a result,the harmonic wave component is greatly enhanced,especially at the corresponding N^th harmonic frequency.Our simulation results have demonstrate that the multiple recess layers structure in the planar Gunn diode is a feasible means of increasing the RF output power and frequency in the long-channel diode.The forth part focuses on the research of the multi-channel planar Gunn diode.We proposed the multi-channel Gunn diode,which is realized by containing multiple AlGaN/GaN heterostructures.Several duplicate 2DEG channels are introduced beneath the original channel;therefore the DC current increases by a scaling factor approximately equaling to the number of the 2DEG channels.It is expected that Gunn oscillations generated in each channel of the multi-channel diode can be self-synchronized to each other since dipole domains generate simultaneously inside each channel and transit to the anode side at the same pace.Due to the superposition of the dipole domain in each channel,the output characteristics of the multi-channel diode are greatly improved as compared with the single-channel AlGaN/GaN diode.In addition,the dual-channel AlGaN/GaN diode even generate a higher DC-to-AC conversion efficiency than the AlInN/GaN HEMT-like planar Gunn diode,even though the electron density of the Al InN/GaN is higher than the total electron concentration of the dual-channel HEMT planar Gunn diode.We can conclude that if the single channel diode has a comparable electron density to the dual-channel diode,higher DC-to-AC efficiency is generated in the dual-channel diode,which is to say the superposition of the output characteristics of each channel of the dual-channel diode is superior to the single-channel diode.