Yulong Fang

AlGaN/GaN Polarization-Doped Field-Effect Transistors With Graded Heterostructure

The design, fabrication, and characterization of AlGaN/GaN polarization-doped field-effect transistors (PolFETs) with graded heterostructure are presented in this paper. The 3-D profiles of carriers are obtained across the graded AlGaN/GaN heterostructure grown on a sapphire substrate, which brings about some novel features. The dc transfer and frequency characteristics exhibit bias-insensitive throughout the low- and high-voltage operating regions, demonstrating the potential for high linearity applications. In addition, dynamic I-V measurement was carried out to analyze the trapping behaviors. Negligible current collapses were observed in the unpassivated PolFETs with graded heterostructure, which can be explained in the aspect of unique energy band profiles of AlGaN/GaN graded heterostructures.

Enhanced effect of strain-induced polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

Using the measured Capacitance-Voltage and Current-Voltage characteristics of the rectangular AlN/GaN Heterostructure Field-Effect Transistors (HFETs) with different Schottky areas, we found that after device processing the polarization Coulomb field (PCF) scattering is induced and has an important influence on the two-dimensional electron gas electron mobility. Moreover, it was also found that PCF scattering has an enhanced influence on the mobility in AlN/GaN HFETs compared to that in AlGaN/AlN/GaN HFETs. This is attributed to the large lattice mismatch between AlN and GaN necessitating a thinner AlN barrier layer, which gives rise to a stronger converse piezoelectric effect.

Improved performance of scaled AlGaN/GaN HFETs by recessed gate

The performance of scaled AlGaN/GaN HFETs was significantly improved by using gate-recess technology and regrown n⁺-GaN Ohmic contacts. Source-to-drain distance (Z<inf>sd</inf>) was scaled to 600 nm by employing regrown n⁺-GaN Ohmic contacts. Low-damage gate recess was taken before 60 nm T-shaped gate metallization. The drain current curve becomes flat over knee voltage, and the short-channel effects were suppressed obviously after gate recessing. The peak transconductance (g<inf>m</inf>) increased from 607 mS/mm to 764 mS/mm. Moreover, after gate recessing, the value of maximum oscillation frequency (f<inf>max</inf>) increases from 192 GHz to 263 GHz, while there is almost no decrease in the value of unity current gain cut-off frequency (f<inf>T</inf>). The devices with gate recess exhibit a record-high value of √f<inf>T</inf>·f<inf>max</inf> in AlGaN/GaN HFETs. This indicates that the AlGaN/GaNHFETs still have the potential for D-band application with further optimization.

High-Uniformity and High Drain Current Density Enhancement-Mode AlGaN/GaN Gates-Seperating Groove HFET

In this paper, we report on a novel E-mode AlGaN/GaN gates-seperating groove heterostructure field-effect transistor (GSG HFET). The current turn-on/off is controlled by changing gate voltage to regulate the horizontal energy band between the double gates. A threshold voltage of 0.23 V and a high drain current density of 851 mA/mm are obtained in GSG HFET. Compared with the proposed depletion-mode device, the maximum drain current of the GSG HFET deceases slightly (about 7%). It is noteworthy that the threshold voltage is less sensitive to the etching time. The devices show high-uniformity threshold voltage of 0.23 V within wide etching time range from 5 to 9 min.

High linearity step-graded AlGaN/GaN heterojunction field effect transistor

We designed and fabricated the AlGaN/GaN HFETs with step graded heterostructure in this work. Compared with the AlGaN/GaN HFETs with linearly graded layer, the AlGaN/GaN HFETs with step graded layer exhibited similar device performance, including the notable wide flat transconductance characteristics. For the AlGaN/GaN step graded heterostructure, each step layer is individual and can be controlled separately during the epi-growth. Especially for the uniformity optimization, the individual growth mode for the AlGaN/GaN step graded heterostructure guarantee the targeted optimization for each layer. It is hoped that this step-graded structure would promote the industrialization of GaN devices in wireless communications applications.

Modified model of gate leakage currents in AlGaN/GaN HEMTs*

It has been reported that the gate leakage currents are described by the Frenkel–Poole emission (FPE) model, at temperatures higher than 250 K. However, the gate leakage currents of our passivated devices do not accord with the FPE model. Therefore, a modified FPE model is developed in which an additional leakage current, besides the gate (I II), is added. Based on the samples with different passivations, the I II caused by a large number of surface traps is separated from total gate currents, and is found to be linear with respect to ( B−V g)0.5. Compared with these from the FPE model, the calculated results from the modified model agree well with the I g–V g measurements at temperatures ranging from 295 K to 475 K.

W-band GaN MMIC PA with 257 mW output power at 86.5 GHz*

A three-stage W-band GaN monolithic microwave integrated circuit power amplifier (MMIC PA) is reported. In order to manage coupling effects between all the parts of the W-band MMIC, all matching and bias networks have been first optimized using circuit simulating software and then systematically simulated on 3D full-wave electromagnetic simulator. The fabricated MMIC PA achieves a 257 mW output power at 86.5 GHz in continuous-wave mode, with an associated power added efficiency of 5.4% and an associated power gain of 6.1 dB. The power density is 459 mW/mm. Moreover, the MMIC PA offers over 100 mW in the 83-90 GHz bandwidth. Those performances were measured at drain bias of 12 V.