In-situ SiN combined with etch-stop barrier structure for high-frequency AlGaN/GaN HEMT

The etch-stop structure including the in-situ SiN and AlGaN/GaN barrier is proposed for high frequency applications.The etch-stop process is realized by placing an in-situ SiN layer on the top of the thin AlGaN barrier.F-based etching can be self-terminated after removing SiN,leaving the AlGaN barrier in the gate region.With this in-situ SiN and thin barrier etch-stop structure,the short channel effect can be suppressed,meanwhile achieving highly precisely controlled and low damage etching process.The device shows a maximum drain current of 1022 mA/mm,a peak transconductance of 459 mS/mm,and a maximum oscillation frequency(fmax)of 248 GHz.     

High-frequency enhancement-mode millimeterwave AlGaN/GaN HEMT with an fT/fmax over 100 GHz/200 GHz

Ultra-thin barrier (UTB) 4-nm-AlGaN/GaN normally-off high electron mobility transistors (HEMTs) having a high current gain cut-off frequency (f_T) are demonstrated by the stress-engineered compressive SiN trench technology. The compressive in-situ SiN guarantees the UTB-AlGaN/GaN heterostructure can operate a high electron density of 1.27×10¹³cm^-2, a high uniform sheet resistance of 312.8 Ω /□, but a negative threshold for the short-gate devices fabricated on it. With the lateral stress-engineering by full removing in-situ SiN in the 600-nm SiN trench, the short-gated (70 nm) devices obtain a threshold of 0.2 V, achieving the devices operating at enhancement-mode (E-mode). Meanwhile, the novel device also can operate a large current of 610 mA/mm and a high transconductance of 394 mS/mm for the E-mode devices. Most of all, a high f_T/f_max of 128 GHz/255 GHz is obtained, which is the highest value among the reported E-mode AlGaN/GaN HEMTs. Besides, being together with the 211 GHz/346 GHz of f_T/f_max for the D-mode HEMTs fabricated on the same materials, this design of E/D-mode with the realization of f_max over 200 GHz in this work is the first one that can be used in Q-band mixed-signal application with further optimization. And the minimized processing difference between the E- and D-mode designs the addition of the SiN trench, will promise an enormous competitive advantage in the fabricating costs.     

High power-added-efficiency AlGaN/GaN HEMTs fabricated by atomic level controlled etching

An atomic-level controlled etching(ACE)technology is invstigated for the fabrication of recessed gate AlGaN/GaN high-electron-mobility transistors(HEMTs)with high power added efficiency.We compare the recessed gate HEMTs with conventional etching(CE)based chlorine,Cl₂-only ACE and BCl³/Cl₂ACE,respectively.The mixed radicals of BCl₃/Cl₂were used as the active reactants in the step of chemical modification.For ensuring precise and controllable etching depth and low etching damage,the kinetic energy of argon ions was accurately controlled.These argon ions were used precisely to remove the chemical modified surface atomic layer.Compared to the HEMTs with CE,the characteristics of devices fabricated by ACE are significantly improved,which benefits from significant reduction of etching damage.For BCl₃/Cl₂ACE recessed HEMTs,the load pull test at 17 GHz shows a high power added efficiency(PAE)of 59.8%with an output power density of 1.6 W/mm at Vd=10 V,and a peak PAE of 44.8%with an output power density of 3.2 W/mm at Vd=20 V in a continuous-wave mode.     

The degradation mechanism of an AlGaN/GaN high electron mobility transistor under step-stress

Step-stress experiments are performed in this paper to investigate the degradation mechanism of an AlGaN/GaN high electron mobility transistor(HEMT).It is found that the stress current shows a recoverable decrease during each voltage step and there is a critical voltage beyond which the stress current starts to increase sharply in our experiments.We postulate that defects may be randomly induced within the AlGaN barrier by the high electric field during each voltage step.But once the critical voltage is reached,the trap concentration will increase sharply due to the inverse piezoelectric effect.A leakage path may be introduced by excessive defect,and this may result in the permanent degradation of the AlGaN/GaN HEMT.     

Negative bias-induced threshold voltage instability and zener/interface trapping mechanism in GaN-based MIS-HEMTs

We investigate the instability of threshold voltage in D-mode MIS-HEMT with in-situ SiN as gate dielectric under different negative gate stresses.The complex non-monotonic evolution of threshold voltage under the negative stress and during the recovery process is induced by the combination effect of two mechanisms.The effect of trapping behavior of interface state at SiN/AlGaN interface and the effect of zener traps in AlGaN barrier layer on the threshold voltage instability are opposite to each other.The threshold voltage shifts negatively under the negative stress due to the detrapping of the electrons at SiN/AlGaN interface,and shifts positively due to zener trapping in AlGaN barrier layer.As the stress is removed,the threshold voltage shifts positively for the retrapping of interface states and negatively for the thermal detrapping in AlGaN.However,it is the trapping behavior in the AlGaN rather than the interface state that results in the change of transconductance in the D-mode MIS-HEMT.     

Improved device performance of recessed-gate AlGaN/GaN HEMTs by using in-situ N2O radical treatment

The N₂O radicals in-situ treatment on gate region has been employed to improve device performance of recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). The samples after gate recess etching were treated by N₂O radicals without physical bombardment. After in-situ treatment (IST) processing, the gate leakage currents decreased by more than one order of magnitude compared to the sample without IST. The fabricated HEMTs with the IST process show a low reverse gate current of 10^-9 A/mm, high on/off current ratio of 10⁸, and high f_T×L_g of 13.44 GHz· μm. A transmission electron microscope (TEM) imaging illustrates an oxide layer with a thickness of 1.8 nm exists at the AlGaN surface. X-ray photoelectron spectroscopy (XPS) measurement shows that the content of the Al-O and Ga-O bonds elevated after IST, indicating that the Al-N and Ga-N bonds on the AlGaN surface were broken and meanwhile the Al-O and Ga-O bonds formed. The oxide formed by a chemical reaction between radicals and the surface of the AlGaN barrier layer is responsible for improved device characteristics.     

Mechanism of improving forward and reverse blocking voltages in AlGaN/GaN HEMTs by using Schottky drain

In this paper, we demonstrate that a Schottky drain can improve the forward and reverse blocking voltages(BVs)simultaneously in AlGaN/GaN high-electron mobility transistors(HEMTs). The mechanism of improving the two BVs is investigated by analysing the leakage current components and by software simulation. The forward BV increases from72 V to 149 V due to the good Schottky contact morphology. During the reverse bias, the buffer leakage in the Ohmicdrain HEMT increases significantly with the increase of the negative drain bias. For the Schottky-drain HEMT, the buffer leakage is suppressed effectively by the formation of the depletion region at the drain terminal. As a result, the reverse BV is enhanced from-5 V to-49 V by using a Schottky drain. Experiments and the simulation indicate that a Schottky drain is desirable for power electronic applications.     

High linearity AlGaN/GaN HEMT with double-Vth coupling for millimeter-wave applications

We demonstrated an AlGaN/GaN high electron mobility transistor(HEMT)namely double-Vthcoupling HEMT(DVC-HEMT)fabricated by connecting different threshold voltage(Vth)values including the slant recess element and planar element in parallel along the gate width with N;O plasma treatment on the gate region.The comparative studies of DVC-HEMT and Fin-like HEMT fabricated on the same wafer show significantly improved linearity of transconductance(Gm)and radio frequency(RF)output signal characteristics in DVC-HEMT.The fabricated device shows the transconductance plateau larger than 7 V,which yields a flattened fT/fmax-gate bias dependence.At the operating frequency of 30 GHz,the peak power-added efficiency(PAE)of 41%accompanied by the power density(Pout)of 5.3 W/mm.Furthermore,the proposed architecture also features an exceptional linearity performance with 1-d B compression point(P1 d B)of 28 d Bm,whereas that of the Fin-like HEMT is 25.2 d Bm.The device demonstrated in this article has great potential to be a new paradigm for millimeter-wave application where high linearity is essential.     

基于粉浆法提高功率型白光LEDs的提取效率

讨论了基于水溶性感光胶的荧光粉的涂层技术,利用粉浆法,在蓝光LED芯片的表面获得了荧光粉平面涂层。证实了得到的白光LED的光提取效率与粉浆中各成份的浓度和相应的比例有关,并在实验的基础上,采用了一些切实切可行的措施来增加器件的光提取效率。得到的白光LED在700 mA加速老化168 h后,其光能量为初始时的95.95%。     

生物质热解产物中焦油的催化裂解

对生物质热解产物（粗煤气）中焦油的催化裂解过程进行了实验研究。实验装置主体由一个常压鼓泡床热解反应器和一个固定床催化裂解反应器组合而成,生物质原料为玉米杆屑,催化剂分别选用煅烧石灰石和煅烧白云石,实验结果表明,当催化裂解反应器内温度在800－850℃之间时,两种催化剂对粗煤气中焦油的催化裂解效果均非常明显,焦油裂解率均可达到90％以上,虽然煅烧石灰石的催化活性比煅烧白云石高,其失活速率也相对大一些。