Li Chengzhan

High performance 1 mm AlGaN/GaN HEMT based on SiC substrate

This is our first report on the high performance 1 mm AlGaN/GaN high electron mobility transistor (HEMT) which was developed using home-made AlGaN/GaN epitaxy structures based on SiC substrate. Metal-organic chemical vapor deposition (MOCVD) was used to generate the epitaxy layers. Corresponding experiments show that the device has a gate length of 0.8 μm exhibiting drain current density of 1.16 A/mm, transconductance of 241 ms/mm, a gate-drain breakdown voltage larger than 80 V, maximum current gain frequency of 20 GHz and maximum power gain frequency of 28 GHz. In addition, the power gain under the continues wave condition is 14.2 dB with a power density of 4.1 W/mm, while under the pulsed wave condition, power gain reaches 14.4 dB with power density at 5.2 W/mm. Furthermore, the two-port network impedance characteristics display great potential in microwave application.

Annealing before gate metal deposition related noise performance in AlGaN/GaN HEMTs

For a further improvement of the noise performance in AlGaN/GaN HEMTs, reducing the relatively high gate leakage current is a key issue. In this paper, an experiment was carried out to demonstrate that one method during the device fabrication process can lower the noise. Two samples were treated differently after gate recess etching: one sample was annealed before metal deposition and the other sample was left as it is. From a comparison of their Ig–Vg characteristics, a conclusion could be drawn that the annealing can effectively reduce the gate leakage current. The etching plasma-induced damage removal or reduction after annealing is considered to be the main factor responsible for it. Evidence is given to prove that annealing can increase the Schottky barrier height. A noise model was used to verify that the annealing of the gate recess before the metal deposition is really effective to improve the noise performance of AlGaN/GaN HEMTs.

Improvements to the extraction of an AlGaN/GaN HEMT small-signal model ∗

The accurate extraction of AlGaN/GaN HEMT small-signal models, which is an important step in large-signal modeling, can exactly reflect the microwave performance of the physical structure of the device. A new method of extracting the parasitic elements is presented, and an open dummy structure is introduced to obtain the parasitic capacitances. With a Schottky resistor in the gate, a new method is developed to extract Rg. In order to characterize the changes of the depletion region under various drain voltages, the drain delay factor is involved in the output conductance of the device. Compared to the traditional method, the fitting of S 11 and S 22 is improved, and fT and fmax can be better predicted. The validity of the proposed method is verified with excellent correlation between the measured and simulated S -parameters in the range of 0.1 to 26.1 GHz.

An on-chip temperature compensation circuit for an InGaP/GaAs HBT RF power amplifier

A new on-chip temperature compensation circuit for a GaAs-based HBT RF amplifier applied to wireless communication is presented. The simple compensation circuit is composed of one GaAs HBT and five resistors with various values, which allow the power amplifier to achieve better thermal characteristics with a little degradation in performance. It effectively compensates for the temperature variation of the gain and the output power of the power amplifier by regulating the base quiescent bias current. The temperature compensation circuit is applied to a 3-stage integrated power amplifier for wireless communication applications, which results in an improvement in the gain variation from 4.0 to 1.1 dB in the temperature range between −20 and +80 °C.

Impact of UV/ozone surface treatment on AlGaN/GaN HEMTs

Surface treatment plays an important role in the process of making high performance AlGaN/GaN HEMTs. A clean surface is critical for enhancing device performance and long-term reliability. By experimenting with different surface treatment methods, we find that using UV/ozone treatment significantly influences the electrical properties of Ohmic contacts and Schottky contacts. According to these experimental phenomena and X-ray photoelectron spectroscopy surface analysis results, the effect of the UV/ozone treatment and the reason that it influences the Ohmic/Schottky contact characteristics of AlGaN/GaN HEMTs is investigated.

Structure Optimization of Field-Plate AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) on 6H-SiC with varying field-plate length and gate-drain spacing were fabricated and analyzed. The classical small signal FET model and the well-known ColdFET method were used to extract the small signal parameters of the devices. Though the devices with field-plates exhibited lower better f T characteristic, they did demonstrate better fmax, MSG and power density performances than the conventional devices without field-plate. Besides, no independence of DC characteristic on field-plate length was observed. With the increase of the field-plate length and the gate-drain spacing, the characteristic of fT and fmax degraded due to the large parasitic effects. Loadpull method was used to measure the microwave power performance of the devices. Under the condition of continuous wave at 5.4GHz, an output power density of 4.69W/mm was obtained for device with field-plate length of 0.5mum and gate-drain length of 2mum