We present an AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor （MIS-HEMT） with an NbAIO high-k dielectric deposited by atomic layer deposition （ALD）. Surface morphology of samples are observed by atomic force microscopy （AFM）, indicating that the ALD NbA10 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 NbA10 on A1GaN. 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 NbA10 can be considered to be a potential gate oxide comparable to other dielectric insulators.
Photoluminescence （PL） measurement is used to study the point defect distribution in a GaN terahertz Gunn diode, which is able to the degrade high-field transport characteristic during further device operation. PL, secondary ion mass spectroscopy （SIMS）, transmission electron microscope （TEM）, and capacitance-voltage （C-V） measurements are used to discuss the origin of point defects responsible for the yellow luminescence in structures. The point defect densities of about 1011 cm-2 in structures are extracted by analysis of C-V characterization. After thermal annealing treatment, diminishments of point defect densities in structures are efficiently demonstrated by PL and C-V results.
A high power density monolithic power amplifier operated at Ku band is presented utilizing a 0.3μm AlGaN/GaN HEMT production process on a 2-inch diameter semi-insulating（SI） 4H-SiC substrate by MOCVD. Over the 12-14 GHz frequency range,the single chip amplifier demonstrates a maximum power of 38 dBm（6.3 W）, a peak power added efficiency（PAE） of 24.2%and linear gain of 6.4 to 7.5 dB under a 10%duty pulse condition when operated at V_（ds） = 25 V and V_（gs） = -4 V.At these power levels,the amplifier exhibits a power density in excess of 5 W/mm.
The current voltage （IV） characteristics are greatly influenced by the dispersion effects in A1GaN/CaN 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.
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 0.35μm 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 aτ-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.
This paper deduces the expression of the Schottky contact capacitance of AlGaN/A1N/GaN high electron mobility transistors （HEMTs）, which will help to understand the electron depleting process. Some material parameters related with capacitance-voltage profiling are given in the expression. Detailed analysis of the forward-biased capacitance has been carried on. The gate capacitance of undoped AlGaN/AlN/GaN HEMT will fall under forward bias. If a rising profile is obviously observed, the donor-like impurity or trap is possibly introduced in the barrier.
A high power X-band hybrid microwave integrated voltage controlled oscillator（VCO） based on Al-GaN /GaN HEMT is presented.The oscillator design utilizes a common-gate negative resistance structure with open and short-circuit stub microstrip lines as the main resonator for a high Q factor.The VCO operating at 20 V drain bias and-1.9 V gate bias exhibits an output power of 28 dBm at the center frequency of 8.15 GHz with an efficiency of 21%.Phase noise is estimated to be -85 dBc/Hz at 100 kHz offset and -128 dBc/Hz at 1 MHz offset.The tuning range is more than 50 MHz.The dominating effect of GaN HEMT＇s flicker noise on oscillator phase noise performance has also been discussed.The measured results show great promise for AlGaN/GaN HEMT technology to be used in high power and low phase noise microwave source applications.
