Yuan Tingting

Optimized power simulation of AlGaN/GaN HEMT for continuous wave and pulse applications*

An optimized modeling method of 8 × 100 μ m AlGaN/GaN-based high electron mobility transistor (HEMT) for accurate continuous wave (CW) and pulsed power simulations is proposed. Since the self-heating effect can occur during the continuous operation, the power gain from the continuous operation significantly decreases when compared to a pulsed power operation. This paper extracts power performances of different device models from different quiescent biases of pulsed current-voltage ( I-V ) measurements and compared them in order to determine the most suitable device model for CW and pulse RF microwave power amplifier design. The simulated output power and gain results of the models at V gs = -3.5 V, V ds = 30 V with a frequency of 9.6 GHz are presented.

Gate-structure optimization for high frequency power AlGaN/GaN HEMTs

The influence of gate-head and gate-source-spacing on the performance of AlGaN/GaN HEMTs was studied. Suggestions are then made to improve the performance of high frequency power AlGaN/GaN HEMTs by optimizing the gate-structure. Reducing the field-plate length can effectively enhance gain, current gain cutoff frequency and maximum frequency of oscillation. By reducing the field-plate length, devices with 0.35 μm gate length have exhibited a current gain cutoff frequency of 30 GHz and a maximum frequency of oscillation of 80 GHz. The maximum frequency of oscillation can be further optimized either by increasing the gate–metal thickness, or by using a τ-shape gate (the gate where the gate-head tends to the source side). Reducing the gate–source spacing can enhance the maximum drain-current and breakdown voltage, which is beneficial in enhancing the maximum output power of AlGaN/GaN HEMTs.

AlGaN/GaN Based Diodes for Liquid Sensing

The characteristics of AlGaN/GaN Schottky diodes as polar liquid sensors are reported. Circular structures, with a gate metal diameter of 200 μm, are designed and fabricated by using a optical lithography process. Ni/Au and Ti/Al/Ni/Au metals are used as the Schottky contact and the ohmic contact, respectively. The Schottky diodes exhibit large changes in reverse leakage current at a bias of −20 V in response to the surface exposed to various polar liquids, such as acetone and ethanol. The effective Schottky barrier height of the diodes is also changed with the polar liquids. The polar nature of the liquids leads to a change of surface charges, producing a change in surface potential at the semiconductor/liquid interface. The effect of the SiNx passivation layer thickness on the liquid sensing is also discussed. The results demonstrate that the AlGaN/GaN heterostructures are promising for polar liquids, combustion gas, biological, and strain sensing applications.

Breakdown mechanisms in AlGaN/GaN high electron mobility transistors with different GaN channel thickness values

In this paper, the off-state breakdown characteristics of two different AlGaN/GaN high electron mobility transistors (HEMTs), featuring a 50-nm and a 150-nm GaN thick channel layer, respectively, are compared. The HEMT with a thick channel exhibits a little larger pinch-off drain current but significantly enhanced off-state breakdown voltage (BVoff). Device simulation indicates that thickening the channel increases the drain-induced barrier lowering (DIBL) but reduces the lateral electric field in the channel and buffer underneath the gate. The increase of BVoff in the thick channel device is due to the reduction of the electric field. These results demonstrate that it is necessary to select an appropriate channel thickness to balance DIBL and BVoff in AlGaN/GaN HEMTs.In this paper,the off-state breakdown characteristics of two different AlGaN/GaN high electron mobility transistors（HEMTs）,featuring a 50-nm and a 150-nm GaN thick channel layer,respectively,are compared.The HEMT with a thick channel exhibits a little larger pinch-off drain current but significantly enhanced off-state breakdown voltage(SVoff).Device simulation indicates that thickening the channel increases the drain-induced barrier lowering（DIBL） but reduces the lateral electric field in the channel and buffer underneath the gate.The increase of BVoff in the thick channel device is due to the reduction of the electric field.These results demonstrate that it is necessary to select an appropriate channel thickness to balance DIBL and BVoff in AlGaN/GaN HEMTs.

Multi-bias capacitance voltage characteristic of AlGaN/GaN HEMT

The method of multi-bias capacitance voltage measurement is presented. The physical meaning of gate—source and gate—drain capacitances in AlGaN/GaN HEMT and the variations in them with different bias conditions are discussed. A capacitance model is proposed to reflect the behaviors of the gate—source and gate—drain capacitances, which shows a good agreement with the measured capacitances, and the power performance obtains good results compared with the measured data from the capacitance model.

A Ku band internally matched high power GaN HEMT amplifier with over 30% of PAE

We report a high power Ku band internally matched power amplifier (IMPA) with high power added efficiency (PAE) using 0.3 μm AlGaN/GaN high electron mobility transistors (HEMTs) on 6H-SiC substrate. The internal matching circuit is designed to achieve high power output for the developed devices with a gate width of 4 mm. To improve the bandwidth of the amplifier, a T type pre-matching network is used at the input and output circuits, respectively. After optimization by a three-dimensional electromagnetic (3D-EM) simulator, the amplifier demonstrates a maximum output power of 42.5 dBm (17.8 W), PAE of 30% to 36.4% and linear gain of 7 to 9.3 dB over 13.8–14.3 GHz under a 10% duty cycle pulse condition when operated at Vds = 30 V and Vgs = −4 V. At such a power level and PAE, the amplifier exhibits a power density of 4.45 W/mm.

A Ka-band 22 dBm GaN amplifier MMIC

A Ka-band GaN amplifier MMIC has been designed in CPW technology, and fabricated with a domestic GaN epitaxial wafer and process. This is, to the best of our knowledge, the first demonstration of domestic Ka-band GaN amplifier MMICs. The single stage CPW MMIC utilizes an AlGaN/GaN HEMT with a gate-length of 0.25 μm and a gate-width of 2 × 75 μm. Under Vds = 10 V, continuous-wave operating conditions, the amplifier has a 1.5 GHz operating bandwidth. It exhibits a linear gain of 6.3 dB, a maximum output power of 22 dBm and a peak PAE of 9.5% at 26.5 GHz. The output power density of the AlGaN/GaN HEMT in the MMIC reaches 1 W/mm at Ka-band under the condition of Vds = 10 V.

High performance AlGaN/GaN HEMTs with 2.4 μm source–drain spacing

This paper describes the performance of AlGaN/GaN HEMTs with 2.4 μm source–drain spacing. So far these are the smallest source–drain spacing AlGaN/GaN HEMTs which have been implemented with a domestic wafer and domestic process. This paper also compares their performance with that of 4 μm source–drain spacing devices. The former exhibit higher drain current, higher gain, and higher efficiency. It is especially significant that the maximum frequency of oscillation noticeably increased.

A 4?9 GHz 10 W wideband power amplifier

A 4–9 GHz wideband high power amplifier is designed and fabricated, which has demonstrated saturated output power of 10 W covering 6–8 GHz band, and above 6 W over the other band. This PA module uses a balance configuration, and presents power gain of 7.3 ± 0.9 dB over the whole 4–9 GHz band and 39% power-added efficiency (PAE) at 8 GHz. Both the input and output VSWR are also excellent, which are bellow −10 dB.

Dispersion effect on the current voltage characteristic of AlGaN/GaN high electron mobility transistors

The current voltage (IV) characteristics are greatly influenced by the dispersion effects in AlGaN/GaN high electron mobility transistors. The direct current (DC) IV and pulsed IV measurements are performed to give a deep investigation into the dispersion effects, which are mainly related to the trap and self-heating mechanisms. The results show that traps play an important role in the kink effects, and high stress can introduce more traps and defects in the device. With the help of the pulsed IV measurements, the trapping effects and self-heating effects can be separated. The impact of time constants on the dispersion effects is also discussed. In order to achieve an accurate static DC IV measurement, the steady state of the bias points must be considered carefully to avoid the dispersion effects.