Electron emission from a GaN planar-doped-barrier hot-electron emitter with piezoelectric surface barrier lowering (i.e., no cesiation) was reported. The piezoelectric effect in the InGaN, which is produced by the strain from the pseudomorphic growth of InGaN on top of GaN, was utilized to lower the surface barrier, replacing the cesiation used before which is the major reason for the degradation. The potential drop produced by the piezoelectric field can lower the effective electron affinity. The structure of the planar electron emitter consisted of an In0.1Ga0.9N cap layer to lower the surface barrier, a i-p-i layer to form the planar-doped-barrier, and n+ GaN buffer to provide electrons. The sample was grown by molecular beam epitaxy and was fabricated to a diode-based planar emitter with an air-bridge anode. The emission current about 5×10−5 A/cm2 was measured with efficiency of 5.7×10−7. The low current and efficiency are due to the oxidation of the surface, dislocation-assisted tunneling current, and low breakdown.
We report the first gallium nitride (GaN)-based broad-band power amplifier. The circuit was fabricated on an AlN substrate using AlGaN-GaN power high-electron mobility transistors (HEMTs), grown on sapphire substrates, which were flip-chip bonded for thermal management. The amplifier employed a modified traveling-wave power amplifier (TWPA) topology that eliminated the backward wave of conventional TWPAs. Using four HEMTs each with 0.75-μm gate length and 0.75-mm gate periphery, a small-signal gain of /spl sim/7 dB was obtained with a bandwidth of 1-8 GHz. At mid-band, an output power of 3.6 W was obtained when biased at V/sub ds/=18 V and 4.5 W when biased at V/sub ds/=22 V.
This paper presents the designs and results of two high-efficiency harmonics-tuned microwave power amplifiers (PA): the first one is a 2 GHz class-F PA in monolithic integrated circuit (MMIC) by using GaN HEMT technology, and the other one is a 2.45-GHz inverse class-F PA using packaged GaAs pHEMT devices with PCB technology. In the class-F MMIC PA, field-plated GaN HEMT device is used for high-power performance. The 2.0-GHz class-F MMIC PA achieves a PAE of 50%, 38 dBm output power, and 6.2 W/mm power density. The inverse class-F PA at 2.45 GHz achieves 22.6 dBm output power and 73% PAE at 3 dB compression, and has very low cost
Despite the considerable improvement in GaN-technology and material quality, RF-dispersion is still one of the main issues hampering device progress. RF-dispersion affects device output power and device power added efficiency (PAE) due to a reduction in saturation current and an increase in knee voltage at high frequencies and high biases. Surface passivation, using silicon nitride, has been found to mitigate RF-dispersion and microwave power degradation (B.M. Green et al, IEEE Electron Dev. Lett., vol. 21, pp. 268-270, 2000; S.C. Binari et al, IEEE Trans Electron. Dev., vol. 48, pp. 465-471, 2001; R. Vetury et al, ibid., vol. 48, pp. 560-566, 2001). This paper discusses a novel AlGaN/GaN high electron mobility transistor (HEMT) device structure which has been developed to reduce RF-dispersion prior to silicon nitride passivation. The device structure uses a p-doped GaN cap layer to screen surface potential changes (regardless of origin) from affecting the gate-drain access region resistance, reducing the amount of RF-dispersion in the device. The epilayers of AlGaN/GaN devices were grown by metal organic chemical vapor deposition (MOCVD) on a c-plane sapphire substrate. Sheet electron concentration and electron Hall mobility of the as-grown wafer were /spl sim/1.35/spl times/10/sup 13/ cm/sup 2/ and 1,475 cm/sup 2//V-s at room temperature.
A novel selective area mass transport regrowth technique for n+-GaN, requiring no Ga or Si precursor, was developed for low resistivity contact formation to AlGaN/GaN HEMTs. Masked samples were reactive ion etched followed by a short anneal in NH3 and H2 at 1050–1080 °C. During annealing mass transport occurred from large open areas to mask edges and narrow mask openings. Autodoping from the SiO2 mask provided n-type doping. After depositing Ti/Al/Ni/Au contacts, the contact resistance was measured to ∼0.3 Ω mm, showing a weak dependence on the annealing temperature. Devices with regrown contacts were fabricated, achieving a power density of 4.8 W/mm at 8 GHz. The technique provides a low-cost regrowth process, with applications in particular for high Al composition AlGaN/GaN devices.
A high linearity MMIC RF power amplifier is reported in the AlGaN/GaN HEMT technology. In order to obtain high linearity, a pre-linearization gate diode is added at the input to compensate for the nonlinear input capacitance C/sub gs/ of the GaN HEMT device. Another single-ended class B power amplifier without the gate diode is also designed for comparison. The circuit with the pre-linearization gate diode demonstrates at least 4dB improvement on 3rd order intermodulation distortion (IMD/sub 3/) performance over the one without the diode over the useful power range in two-tone measurement.
This paper presents class-E microwave monolithic integrated circuit (MMIC) power amplifiers at 2.0 GHz, which is based on field-plated GaN HEMT technology. The 2-stage power amplifier consists of a class-F driver stage and a class-E power stage. The circuit schematic, layout and fabrication are described. The amplifier achieves an output power of 37.5dBm into a 50H load, a power added efficiency (PAE) of 50%, and a gain of 18.2dB. A power density of 5.6W/mm is achieved.
Double-channel structures have been used in AlGaN/GaN high electron mobility transistors to reduce the access resistance. Carrier densities as high as 2.9/spl times/10/sup 13/ cm/sup -2/ and mobilities in the 1300 cm/sup 2//V/spl middot/s range have been obtained in the access region. Also, the correct design of the potential barrier between the different channels allowed tailoring the differential access resistance to enhance the linearity of the transistors. This increase in linearity has been measured as a flatter profile of the transconductance and cutoff frequency versus current and as an improvement of more than 2 dB in large-signal two-tone linearity measurements.
In this work, we cover four topics. Three studies are presented on the effect of different epilayer structures on the noise figure of AlGaN/GaN HEMTs in the 4-12 GHz frequency range. The material studies include varying aluminum composition in the barrier, sapphire vs. SiC substrates, and, for the first time, the influence of a thin AlN layer on the noise parameters; all three against frequency and drain current. In addition is a comparison of two equivalent circuit models at 5 GHz.
In this paper, a high-power GaN/AlGaN/GaN high electron mobility transistor (HEMT) has been demonstrated. A thick cap layer has been used to screen surface states and reduce dispersion. A deep gate recess was used to achieve the desired transconductance. A thin SiO/sub 2/ layer was deposited on the drain side of the gate recess in order to reduce gate leakage current and improve breakdown voltage. No surface passivation layer was used. A breakdown voltage of 90 V was achieved. A record output power density of 12 W/mm with an associated power-added efficiency (PAE) of 40.5% was measured at 10 GHz. These results demonstrate the potential of the technique as a controllable and repeatable solution to decrease dispersion and produce power from GaN-based HEMTs without surface passivation.
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