预辐射甲基丙烯酸甲酯形成过氧化物的结构及引发活性研究

乙烯类单体,在氧存在下经电离辐照形成具有强引发能力的过氧化物.离开辐射场,在一定条件下,过氧化物分解为自由基立即引发聚合进行.这一过程称为预辐射聚合.前人曾研究了预辐射聚合的反应机制和动力学过程.但对于辐照产生的过氧化物的组成、结构及其行为,只有推测,而无任何研究工作.本文报道以甲基丙烯酸甲酯(MMA)为单体,研究预辐照产物的结构和引发活性的初步结果.     

The effect of a HfO2 insulator on the improvement of breakdown voltage in field-plated GaN-based HEMT

A GaN/Al_0.3Ga_0.7N/AlN/GaN high-electron mobility transistor utilizing a field plate (with a 0.3 μm overhang towards the drain and a 0.2 μm overhang towards the source) over a 165-nm sputtered HfO₂ insulator (HfO₂-FP-HEMT) is fabricated on a sapphire substrate. Compared with the conventional field-plated HEMT, which has the same geometric structure but uses a 60-nm SiN insulator beneath the field plate (SiN-FP-HEMT), the HfO₂-FP-HEMT exhibits a significant improvement of the breakdown voltage (up to 181 V) as well as a record field-plate efficiency (up to 276 V/μm). This is because the HfO₂ insulator can further improve the modulation of the field plate on the electric field distribution in the device channel, which is proved by the numerical simulation results. Based on the simulation results, a novel approach named the proportional design is proposed to predict the optimal dielectric thickness beneath the field plate. It can simplify the field-plated HEMT design significantly.     

Effect of SiN:H_x passivation layer on the reverse gate leakage current in GaN HEMTs

This paper concentrates on the impact of SiN passivation layer deposited by plasma-enhanced chemical vapor deposition(PECVD) on the Schottky characteristics in GaN high electron mobility transistors(HEMTs). Three types of SiN layers with different deposition conditions were deposited on GaN HEMTs. Atomic force microscope(AFM), capacitance-voltage(C-V), and Fourier transform infrared(FTIR) measurement were used to analyze the surface morphology, the electrical characterization, and the chemical bonding of SiN thin films, respectively. The better surface morphology was achieved from the device with lower gate leakage current. The fixed positive charge Q_f was extracted from C-V curves of Al/SiN/Si structures and quite different density of trap states(in the order of magnitude of 1011-1012 cm~(-2)) was observed.It was found that the least trap states were in accordance with the lowest gate leakage current. Furthermore, the chemical bonds and the %H in Si-H and N-H were figured from FTIR measurement, demonstrating an increase in the density of Q_f with the increasing %H in N-H. It reveals that the effect of SiN passivation can be improved in GaN-based HEMTs by modulating %H in Si-H and N-H, thus achieving a better Schottky characteristics.     

Neutron irradiation effects on A1GaN//GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors(HEMTs) were exposed to 1 MeV neutron irradiation at a neutron fluence of 1 × 10 15 cm 2.The dc characteristics of the devices,such as the drain saturation current and the maximum transconductance,decreased after neutron irradiation.The gate leakage currents increased obviously after neutron irradiation.However,the rf characteristics,such as the cut-off frequency and the maximum frequency,were hardly affected by neutron irradiation.The AlGaN/GaN heterojunctions have been employed for the better understanding of the degradation mechanism.It is shown in the Hall measurements and capacitance-voltage tests that the mobility and concentration of two-dimensional electron gas(2DEG) decreased after neutron irradiation.There was no evidence of the full-width at half-maximum of X-ray diffraction(XRD) rocking curve changing after irradiation,so the dislocation was not influenced by neutron irradiation.It is concluded that the point defects induced in AlGaN and GaN by neutron irradiation are the dominant mechanisms responsible for performance degradations of AlGaN/GaN HEMT devices.     

Hot carrier degradation and a new lifetime prediction model in ultra-deep sub-micron pMOSFET

The hot carrier effect (HCE) of an ultra-deep sub-micron p-channel metal-oxide semiconductor field-effect transistor (pMOSFET) is investigated in this paper. Experiments indicate that the generation of positively charged interface states is the predominant mechanism in the case of the ultra-deep sub-micron pMOSFET. The relation of the pMOSFET hot carrier degradation to stress time (t), channel width (W ), channel length (L), and stress voltage (Vd ) is then discussed. Based on the relation, a lifetime prediction model is proposed, which can predict the lifetime of the ultra-deep sub-micron pMOSFET accurately and reflect the influence of the factors on hot carrier degradation directly.     

High-electric-field-stress-induced degradation of SiN passivated AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated by employing SiN passivation, this paper investigates the degradation due to the high-electric-field stress. After the stress, a recoverable degradation has been found, consisting of the decrease of saturation drain current I_Dsat, maximal transconductance g_m, and the positive shift of threshold voltage V_TH at high drain-source voltage V_DS. The high-electric-field stress degrades the electric characteristics of AlGaN/GaN HEMTs because the high field increases the electron trapping at the surface and in AlGaN barrier layer. The SiN passivation of AlGaN/GaN HEMTs decreases the surface trapping and 2DEG depletion a little during the high-electric-field stress. After the hot carrier stress with V_DS=20 V and V_GS=0 V applied to the device for 10⁴ sec, the SiN passivation decreases the stress-induced degradation of I_Dsat from 36% to 30%. Both on-state and pulse-state stresses produce comparative decrease of I_Dsat, which shows that although the passivation is effective in suppressing electron trapping in surface states, it does not protect the device from high-electric-field degradation in nature. So passivation in conjunction with other technological solutions like cap layer, prepassivation surface treatments, or field-plate gate to weaken high-electric-field degradation should be adopted.     

Development and characteristic analysis of a field-plated Al2O3/AlInN/GaN MOS–HEMT

We present an AlInN/AlN/GaN MOS–HEMT with a 3 nm ultra-thin atomic layer deposition (ALD) Al₂O₃ dielectric layer and a 0.3 μm field-plate (FP)-MOS--HEMT. Compared with a conventional AlInN/AlN/GaN HEMT (HEMT) with the same dimensions, a FP-MOS--HEMT with a 0.6 μm gate length exhibits an improved maximum drain current of 1141 mA/mm, an improved peak extrinsic transconductance of 325 mS/mm and effective suppression of gate leakage in both the reverse direction (by about one order of magnitude) and the forward direction (by more than two orders of magnitude). Moreover, the peak extrinsic transconductance of the FP-MOS--HEMT is slightly larger than that of the HEMT, indicating an exciting improvement of transconductance performance. The sharp transition from depletion to accumulation in the capacitance--voltage (C--V) curve of the FP-MOS--HEMT demonstrates a high-quality interface of Al₂O₃/AlInN. In addition, a large off-state breakdown voltage of 133 V, a high field-plate efficiency of 170 V/μ m and a negligible double-pulse current collapse is achieved in the FP-MOS--HEMT. This is attributed to the adoption of an ultra-thin Al₂O₃ gate dielectric and also of a field-plate on the dielectric of an appropriate thickness. The results show a great potential application of the ultra-thin ALD-Al₂O₃ FP-MOS--HEMT to deliver high currents and power densities in high power microwave technologies.     

Influence of 60Co gamma radiation on fluorine plasma treated enhancement-mode high- electron-mobility transistor

AlGaN/GaN depletion-mode high-electron-mobility transistor(D-HEMT) and fluorine(F) plasma treated enhancement-mode high-electron-mobility transistor(E-HEMT) are exposed to 60Co gamma radiation with a dose of 1.6 Mrad(Si).No degradation is observed in the performance of D-HEMT.However,the maximum transconductance of E-HEMT is increased after radiation.The 2DEG density and the mobility are calculated from the results of capacitance-voltage measurement.The electron mobility decreases after fluorine plasma treatment and recovers after radiation.Conductance measurements in a frequency range from 10 kHz to 1 MHz are used to characterize the trapping effects in the devices.A new type of trap is observed in the F plasma treated E-HEMT compared with the D-HEMT,but the density of the trap decreases by radiation.Fitting of G p /ω data yields the trap densities D T =(1-3) × 10 12 cm-2 · eV-1 and D T =(0.2 0.8) × 10 12 cm-2 ·eV-1 before and after radiation,respectively.The time constant is 0.5 ms-6 ms.With F plasma treatment,the trap is introduced by etch damage and degrades the electronic mobility.After 60Co gamma radiation,the etch damage decreases and the electron mobility is improved.The gamma radiation can recover the etch damage caused by F plasma treatment.     

AlGaN/GaN MIS-HEMT using NbAlO dielectric layer grown by atomic layer deposition

We present an AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MIS-HEMT) with an NbAlO high-k dielectric deposited by atomic layer deposition (ALD).Surface morphology of samples are observed by atomic force microscopy (AFM),indicating that the ALD NbAlO has an excellent-property surface.Moreover,the sharp transition from depletion to accumulation in capacitance-voltage (C-V)curse of MIS-HEMT demonstrates the high quality bulk and interface properties of NbAlO on AlGaN.The fabricated MIS-HEMT with a gate length of 0.5 μm exhibits a maximum drain current of 960 mA/mm,and the reverse gate leakage current is almost 3 orders of magnitude lower than that of reference HEMT.Based on the improved direct-current operation,the NbAlO can be considered to be a potential gate oxide comparable to other dielectric insulators.     

Characteristics in AlN/AlGaN/GaN Multilayer-Structured High-Electron-Mobility Transistors

A new multilayer-structured AlN/AlCaN/CaN heterostructure high-electron-mobility transistor （HEMT） is demonstrated. The AIN/AlCaN/CaN HEMT exhibits the maximum drain current density of 800mA/mm and the maximum extrinsic transconductance of 170 mS/mm. Due to the increase of the distance between the gate and the two-dimensional electron-gas channel, the threshold voltage shifts slightly to the negative. The reduced drain current collapse and higher breakdown voltage are observed on this AIN/AlGaN/CaN HEMT. The current gain cut-off frequency and the maximum frequency of oscillation are 18.5 CHz and 29.0 GHz, respectively.