Current oscillation in GaN-HEMTs with p-GaN islands buried layer for terahertz applications

Wen-Lu Yang(楊文璐), Lin-An Yang(楊林安), Fei-Xiang Shen(申飛翔), Hao Zou(鄒浩),Yang Li(李楊), Xiao-Hua Ma(馬曉華), and Yue Hao(郝躍)

State Key Discipline Laboratory of Wide Band-gap Semiconductor Technology,School of Microelectronics,Xidian University,Xi’an 710071,China

Keywords: p-GaN island,high electron mobility transistor(HEMT),AlGaN/GaN,electron domain

1. Introduction

For applications of terahertz(THz)technology,much research has been devoted to finding efficient solid-state terahertz sources at room temperature.[1,2]As a typical structure based on heterojunction, HEMTs have gained widespread attention on account of their excellent properties such as high breakdown electric field,high electron velocity and concentration,and is usually studied as an amplifier.[3,4]In recent years,with the progress of material growth and device manufacturing technology, HEMTs have been considered as a possible THz source.

There are two types of mechanisms for generating THz oscillation in HEMTs. One is the plasma instability in the two-dimensional electron gas(2DEG)channel,which can result in high-frequency oscillations above 1 THz. The other is the Gunn effect.Under the action of a high electric field,a part of the electrons transition into the high-energy valley,thereby increasing effective mass and decreasing mobility. This contributes to part of the electron accumulation, thus forming Gunn domains.[2]Gunn domains cause an oscillating output current and some experiments and simulations have reported current instability in HEMTs, in which the current oscillation frequency even reaches the THz range.[5]Compared to the vertically structural Gunn diodes,HEMTs advantageously are compatible with traditional planar device process.[6,7]For GaN-based HEMTs,the 2DEG with a high electron concentration induced by the polarization effect is very beneficial for the formation of electronic domains and makes devices have good combined frequency–power performance.[8]Studies on Gunn oscillation in GaN-based planar Gunn devices have achieved significant results, and the frequency of the devices has approached the THz range.[9–11]

However, there are still some challenges in using GaNbased HEMTs as Gunn oscillation sources. The high electric field region at the edge of the gate is too narrow, making it difficult to obtain a stable oscillating current in traditional GaN-based HEMTs. The method of applying a large gate bias voltage to expand the high electric field region to trigger the formation of Gunn domains may cause large leakage currents and increase the risk of device damage. In addition,to achieve terahertz oscillation, the oscillation channel length of these Gunn devices is usually less than 1 μm. This not only reduces the breakdown voltage of the device but also increases the difficulty of the process,especially the deposition of ohmic electrodes.[12]

In this paper, a novel GaN-based HEMT with a p-GaN islands buried layer(PIBL)is proposed based on the Silvaco-TCAD simulation. By introducing the p-GaN island in the buffer under the channel of the gate–drain region,the electric field in the 2DEG channel can be redistributed to make it more conducive to trigger the formation of electronic domains. We investigate the mechanism of generation and regulation of the formation of electronic domains in the channel, particularly the influence of the position of p-GaN island and the appliedVgson the electronic domain and oscillation frequency.

2. Device structure and parameters for simulation

The schematic diagrams of AlGaN/GaN HEMTs with the PIBL structure used in our simulation are shown in Fig.1,and detailed device parameters are listed in Table 1. The thickness and length of the p-GaN island affect the electric field strength and the length of the domain-forming region and thereby the formation of electronic domains. In order to obtain a stable oscillating current,we set the p-GaN island to have the lengthLp=0.3 μm.

Table 1. Device structure parameters in the simulation.

Fig.1. Schematic of AlGaN/GaN HEMTs with PIBL.

The source and drain electrodes are vertical ohmic contact electrodes located on both sides of the device and are in contact with the 2DEG channel. This type of source/drain electrodes fabricated by re-growth technology is commonly used in high-frequency GaN-based HEMTs[13–15]since it can provide excellent control of the channel electric field. The significant reduction of the on-resistance can also promote the generation and propagation of electronic domains. In our simulation,the ohmic contact resistance is set as 0.4 Ω/mm. The p-GaN island is located in the GaN buffer layer below the gate edge and has floating potential.The p-GaN island-shaped buried layer structure in this paper can be achieved by using the method reported with the GaN-based current aperture vertical electron transistor (CAVET).[16–18]Firstly, dry etching technology is applied to selectively etch the p-GaN layer to form the island at the designated position. Then, the GaN layer is grown to fill the etched trench via the use of re-growth technology. Finally,low-temperature molecular beam epitaxy(MBE)technology is adopted to grow an AlGaN/GaN heterojunction.The p-GaN island region is uniformly doped,and the thickness of the p-GaN island is defined as 10 nm in our numerical simulation. In realistic processing,some methods can be used to reduce the influence of Mg diffusion into the 2DEG channel, such as regrowing the AlGaN/GaN layer above the p-GaN island in a low-temperature environment using MBE technology[19,20]or increasing the distance between the PIBL and the heterojunction interface.

Donor-like defects/impurities inevitably exist at the interface of the p-GaN island and surrounding GaN. The state at the p-GaN/GaN interface, mainly introduced by ion implantation and secondary epitaxy,should be considered in the simulation.[21,22]In the simulation, we set the trapped charge density to be 2.8×1012cm-2with location at the p–n junction interface (E-Ev=3.2 eV).[23,24]A previous article reports that optimizing the interface cleaning and growth conditions can reduce the interface state density.[25]

Non-local transport effects such as the velocity overshoot have great influence on sub-micron devices and should be considered in simulations. Therefore, we adopt the energybalance(E-B)transport model instead of the traditional drift–diffusion (D–D) transport model in the simulation, the E-B model being a non-local model widely used in high-frequency and small-sized devices. Due to the buried p–n junction in the device,Shockley–Read–Hall(SRH)model is also included in our simulation.

Table 2. Transport model parameters at room temperature (T =300 K).[9,26,27]

Fig.2. Velocity versus electric field dependence of 2DEG and bulk GaN.

To reasonably characterize 2DEG in the AlGaN/GaN heterostructure, we employ the fitting parameters of the mobility model obtained from Monte Carlo simulations and experiments in the simulation. The effects of the interface state,velocity overshoot and carrier scattering mechanisms are all taken into account.[26]The velocity–electric field dependence parameters at room temperature(T=300 K)used in our simulation are listed in Table 2.[9,26,27]Figure 2 plots the velocityversus-electric field dependence of 2DEG. Compared with bulk GaN, 2DEG shows a higher peak drift velocity and a higher saturation drift velocity.

In addition,we use the method of transient simulation to generate a stable current oscillation waveform instead of the circuit-level simulation,mounting the device into the RLC network,where the maximum time step is 0.1 ps and the ramping rate is 33 V/ns. The fast Fourier algorithm is used to analyze the oscillating current waveform obtained at each voltage bias point.

3. Simulation results and discussion

Figures 3 and 4 show the transient simulation results of conventional HEMTs and PIBL HEMTs under the same voltage bias conditions. The distribution of the electric field and electron concentration in the channel are shown in Figs.5 and 6, where the gate bias voltageVgs=0.6 V and the drain bias voltageVds=12.5 V.Figure 3 gives a comparison of the transient drain current for GaN-HEMTs with and without a PIBL structure. In the proposed HEMT, an evident oscillation appears like a Gunn oscillation, while the conventional HEMT maintains a stable output current.A lower drain current for the PIBL structural HEMT is attributed to the PIBL structure reducing the local electron concentration in the 2DEG channel.The frequency spectrum of the oscillation current calculated by fast Fourier analysis is shown in Fig. 4, yielding an available fundamental frequency at around 377 GHz,accompanied by the ratio of the oscillation current amplitude of the fundamental component to the average componentIf1/Iavg≈3.84%.The frequency is far higher than thefTandfMAXof the equivalent conventional GaN HEMT with a gate length of 0.2 μm.[3]In previous articles,the p-GaN buried layer was often used in CAVET as a current blocking layer to adjust the distribution of the bulk electric field.[14–16,28,29]In this paper,however,PIBL is used to adjust the local electric field distribution which aims to form a perturbation field at the edge of the gate.

The p-GaN region and the surrounding GaN material form a p–n junction, thus depleting part of the 2DEG. The p-GaN island reduces the electron concentration in the 2DEG channel above it, thereby forming a local high resistance region. Most of the drain voltage is applied to this high resistance region so that a relatively wide high electric field region is formed in the channel, which makes it easier for electrons to transition into a high-energy valley, thereby triggering the formation of Gunn domains, as shown in Fig. 5. Due to the low mobility of electrons in the high energy valley, the electron drift velocity is low, which causes the accumulation of electrons. The electrons behind the domain arrive at a higher velocity, making the domain grow. Simultaneously, the drift velocity of electrons within the domain and the electric field outside the domain continue to decrease. When the velocity of the domain(νdomain)is equal to the drift velocity of electrons outside the domain,the domain stops growing. When the domain is absorbed, the output current will fluctuate, and new domains will be generated,thus forming periodic oscillations.

Fig.3. Transient behavior of drain terminal current.

Fig.4. Frequency spectrum of the oscillation drain current obtained by fast Fourier analysis.

The electron concentration distributions for both HEMTs are drawn in Fig.6. The generation and movement of domains caused changes in the electron concentration in the PIBL region. For the PIBL structural HEMT, the gate acts as a hot electron injector that reduces the length of the “dead zone”and easily triggers the formation of electron domains. The results of the transient simulation show that it can generate a stable oscillation current and achieve the frequency of the fundamental wave up to 377 GHz,far higher than thefTandfMAXof conventional 0.2 μm gate GaN HEMTs.

The influence ofNpon the fundamental frequency of the device and the minimumVdsrequired for generating stable oscillation is depicted in Fig. 7. With the increase ofNpfrom 2.5×1018cm-3to 3×1018cm-3, the electric field strength of the PIBL region in the channel is more likely to meet the conditions required for domain generation, so that the minimumVdsrequired for device operation decreases from 11.8 V to 10 V. In addition, it can be observed that the fundamental frequency decreases from 377 GHz to 344 GHz. AsNpincreases, the p–n junction composed of the p-GaN island and the n-GaN buffer layer above the island depletes more 2DEG.The reduction of the channel electron concentration above the island forms a local high resistance region,which changes the electric field distribution in the channel. The electric field is mainly concentrated on the drain side of the PIBL structure.This changes the nucleation sites of the electronic domains and the distance from the electron domain nucleation point to the drain side of PIBL structure, which affects the device frequency.

Fig. 5. Electric field distribution in 2DEG channel, where the time step is 0.1 ps for transient simulation. Solid lines are obtained from HEMT with PIBL,and dashed lines from HEMT without PIBL.

Fig. 6. Electron concentration distribution in the 2DEG channel, where the time step is 0.1 ps. Solid lines are obtained from HEMT with PIBL, and dashed lines from HEMT without PIBL.

The distanceDpbetween the p-GaN island and the heterojunction interface is a critical processing parameter of device fabrication since it significantly affects the performance of the device. In order to investigate the influence ofDpon the channel electric field and the oscillation characteristics, we selectDpfrom 5 nm to 15 nm for simulation. The results regardingDpare shown in Fig. 8 under the bias ofVgs=0.6 V,Vds=12.5 V.AsDpincreases from 5 nm to 15 nm,the extracted oscillation frequency decreases from 400 GHz to 344 GHz. WhenDp=5 nm,excessively closing the PIBL region to the channel would make the island have a very strong depletion effect on the 2DEG channel.Although the high electric field above the PIBL region facilitates the transition of electrons from the low energy valley to the high energy valley, the too low electron concentration in the channel makes it difficult for the PIBL region to meet the conditions of electron domain formation. As a result, the oscillation frequency of 400 GHz andIrf/Iavgof 3% are obtained due to the high electron domain velocity and narrow electron domain width.Similarly, for the sample withDp=10 nm, the higher electron concentration in the channel above PIBL contributes to electron accumulation,which increases the width of the electron domain and reduces the electron domain rateνdomain,thus reducing the fundamental frequency of the oscillating current but increasing theIrf/Iavg. Also, a too-smallDpwould cause Mg-dopant diffusing into the 2DEG channel and impact the operation of HEMT.Conversely,a too-largeDpsuppresses the p-GaN island modulating the electric field of the channel, so that it is difficult to trigger the stable oscillation of the current.Therefore,it is necessary to adjustDpaccording to the device process.

Fig.7. Influence of Np on the fundamental frequency and the minimum Vds required for generating stable oscillation.

We also investigated the oscillation characteristics of HEMTs with two PIBLs, which are aimed at enhancing the higher-order harmonic components of the output current. The second p-GaN island is located between the first one and the drain.It obviously requires higher gate and drain biases to sustain a sufficient electric field strength and perturbation. Simulation results show that the device’s operating mode is consistent with the aforementioned HEMT with one PIBL whenVgsis lower than 5 V.This means that the adjustment of the second island to the electric field of 2DEG channel is weak. AsVgsincreases to 5 V–6 V accompanied byVdsapproaching 29 V,the formation of Gunn domains appears at the position above the second island. In this case, the fundamental frequency of the oscillation current is around 100 GHz; concurrently, the fifth harmonic component exhibits a significant enhancement.However,an excessively higher gate voltage will cause a large leakage current,which negatively influences normal operation,even leading to a high risk of breakdown for HEMTs. Thus,it is difficult to use in actual circuits.

Fig.8. Influence of Dp on the fundamental oscillation frequency and Irf/Iavg,where Vgs=0.6 V and Vds=12.5 V.

4. Conclusion

In this paper, we propose GaN-HEMTs with a PIBL structure and the oscillation mechanism of a drain current by using the numerical simulation on the Silvaco-TCAD platform.The simulation results show that the 0.2 μm gate HEMT with one PIBL can generate the stable oscillation of the drain current with a fundamental frequency up to 377 GHz at the low gate bias ofVgs=0.6 V.The electric field distribution in HEMTs with a PIBL structure is more conducive to the formation of Gunn-like domains. The position of the PIBL structure significantly affects the oscillation frequency and the RF output power of the device. By appropriately increasingDp, the influence of Mg diffusion on the 2DEG in the actual device can be reduced,although this reduces the oscillation frequency of the output current. At present,the RF output power of the device is relatively low,and it is difficult to meet the application requirements of actual circuits,but this kind of HEMT may reduce the requirements for device size and simplify the circuit scale of the THz system, therefore exhibiting great potential for THz applications compared with vertically structural Gunn diodes.

Acknowledgments

Project supported by the National Natural Science Foundation of China (Grant Nos. 61974108 and 61674117), the National Natural Science Foundation for Young Scholars of China (Grants No. 61804119), and the Postdoctoral Science Foundation of China(Grants No.2018M643576).