S. Heikman

Growth of Fe doped semi-insulating GaN by metalorganic chemical vapor deposition

Iron doped GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) using ferrocene as the Fe precursor. Specular films with concentrations up to 1.7×1019 cm−3, as determined by secondary ion mass spectrometry, were grown. The Fe concentration in the film showed a linear dependence on the precursor partial pressure, and was insensitive to growth temperature, pressure, and ammonia partial pressure. Memory effects were observed, similar to Mg doping of GaN by MOCVD. The deep acceptor nature of Fe was used for growth of semi-insulating GaN films on sapphire substrates. A 2.6-μm-thick GaN film with a resistivity of 7×109 Ω/sq was attained when the first 0.3 μm of the film was Fe doped. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that Fe doping did not affect the structural properties of the film. Fe doping allows for growth of semi-insulating GaN on sapphire without the high threading dislocation densities and/or high carbon leve...

AlGaN/AlN/GaN high-power microwave HEMT

In this letter, a novel heterojunction AlGaN/AlN/GaN high-electron mobility transistor (HEMT) is discussed. Contrary to normal HEMTs, the insertion of the very thin AlN interfacial layer (/spl sim/1 nm) maintains high mobility at high sheet charge densities by increasing the effective /spl Delta/E/sub C/ and decreasing alloy scattering. Devices based on this structure exhibited good DC and RF performance. A high peak current 1 A/mm at V/sub GS/=2 V was obtained and an output power density of 8.4 W/mm with a power added efficiency of 28% at 8 GHz was achieved.

AlGaN/GaN high electron mobility transistors with InGaN back-barriers

A GaN/ultrathin InGaN/GaN heterojunction has been used to provide a back-barrier to the electrons in an AlGaN/GaN high-electron mobility transistor (HEMT). The polarization-induced electric fields in the InGaN layer raise the conduction band in the GaN buffer with respect to the GaN channel, increasing the confinement of the two-dimensional electron gas under high electric field conditions. The enhanced confinement is especially useful in deep-submicrometer devices where an important improvement in the pinchoff and 50% increase in the output resistance have been observed. These devices also showed excellent high-frequency performance, with a current gain cut-off frequency (f/sub T/) of 153 GHz and power gain cut-off frequency (f/sub max/) of 198 GHz for a gate length of 100 nm. At a different bias, a record f/sub max/ of 230 GHz was obtained.

High breakdown GaN HEMT with overlapping gate structure

GaN high electron mobility transistors (HEMTs) were fabricated using an overlapping-gate technique in which the drain-side edge of the metal gate overlaps on a high breakdown and high dielectric constant dielectric. The overlapping structure reduces the electric field at the drain-side gate edge, thus increasing the breakdown of the device. A record-high three-terminal breakdown figure of 570 V was achieved on a HEMT with a gate-drain spacing of 13 /spl mu/m. The source-drain saturation current was 500 mA/mm and the extrinsic transconductance 150 mS/mm.

High breakdown voltage AlGaN-GaN HEMTs achieved by multiple field plates

High-voltage Al/sub 0.22/Ga/sub 0.78/N-GaN high-electron mobility transistors have been fabricated using multiple field plates over dielectric passivation layers. The device breakdown voltage was found to increase with the addition of the field plates. With two field plates, the device showed a breakdown voltage as high as 900 V. This technique is easy to apply, based on the standard planar transistor fabrication, and especially attractive for the power switching applications.

AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy

We report on an extensive study of the two-dimensional electron gas (2DEG) structures containing AlN layers. It is shown that the presence of large polarization fields in the AlN barrier layer in AlN/GaN heterostructures results in high values of the 2DEG sheet density of up to 3.6×1013 cm−2. Room-temperature sheet resistance of 180 Ω/□ is demonstrated in the AlN/GaN structure with a 35 A AlN barrier. As a result of reduced alloy disorder scattering, low-temperature electron mobility is significantly enhanced in AlN/GaN heterostructures in comparison to AlGaN/GaN structures with similar values of the 2DEG sheet density. The growth of GaN cap layers on top of AlN/GaN structures with relatively thick (∼35 A) AlN barriers is found to lead to a significant decrease in the 2DEG sheet density. However, inserting a thin (∼10 A) AlN layer between AlxGa1−xN and GaN in the AlxGa1−xN/GaN (x∼0.2–0.45) 2DEG structures does not affect the 2DEG sheet density and results in an increase of the low-temperature electron mob...

A 97.8% Efficient GaN HEMT Boost Converter With 300-W Output Power at 1 MHz

A 175-to-350 V hard-switched boost converter was constructed using a high-voltage GaN high-electron-mobility transistor grown on SiC substrate. The high speed and low on-resistance of the wide-band-gap device enabled extremely fast switching transients and low losses, resulting in a high conversion efficiency of 97.8% with 300-W output power at 1 MHz. The maximum efficiency was 98.0% at 214-W output power, well exceeding the state of the art of Si-based converters at similar frequencies.

Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9–1.0 eV for GaN, and an interface polarization charge density of 1.6×1013–1.7×1013 cm−2, were extracted from the simulations.

Effect of carbon doping on buffer leakage in AlGaN/GaN high electron mobility transistors

Carbon doping via CBr4 in AlGaN/GaN high electron mobility transistors grown by rf-plasma-assisted molecular beam epitaxy on 4H–SiC (0001) was investigated as a means to reduce buffer leakage. For carbon doping in the first 400 nm of the structure, a significant decrease in buffer leakage was observed with increasing overall carbon concentration. A carbon doping scheme in which the level of doping is tapered from 6×1017 cm−3 down to 2×1017 cm−3 was found to result in sufficiently low drain-source leakage currents. The effect of thickness of the GaN:C layer was explored as well as the effect of thickness of the subsequent unintentionally doped GaN layer. For structures with reduced leakage, rf I–V and power measurements revealed better performance in structures in which the two-dimensional electron gas was spaced at a large distance from the GaN:C layer. Possible sources and locations of unintentional free carriers contributing to leakage in these structures are discussed in light of the results.

High-power polarization-engineered GaN/AlGaN/GaN HEMTs without surface passivation

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.