Robert Coffie

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.

Metalorganic chemical vapor deposition of GaN on Si(111): Stress control and application to field-effect transistors

Two schemes of nucleation and growth of gallium nitride on Si(111) substrates are investigated and the structural and electrical properties of the resulting films are reported. Gallium nitride films grown using a 10–500 nm-thick AlN buffer layer deposited at high temperature (∼1050 °C) are found to be under 260–530 MPa of tensile stress and exhibit cracking, the origin of which is discussed. The threading dislocation density in these films increases with increasing AlN thickness, covering a range of 1.1 to >5.8×109 cm−2. Films grown using a thick, AlN-to-GaN graded buffer layer are found to be under compressive stress and are completely crack free. Heterojunction field effect transistors fabricated on such films result in well-defined saturation and pinch-off behavior with a saturated current of ∼525 mA/mm and a transconductance of ∼100 mS/mm in dc operation.

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.

Realization of wide electron slabs by polarization bulk doping in graded III–V nitride semiconductor alloys

We present the concept and experimental realization of polarization-induced bulk electron doping in III–V nitride semiconductors. By exploiting the large polarization charges in the III–V nitrides, we are able to create wide slabs of high-density mobile electrons without introducing shallow donors. Transport measurements reveal the superior properties of the polarization-doped electron distributions than comparable shallow donor-doped structures, especially at low temperatures due to the removal of ionized impurity scattering. Such polarization-induced three-dimensional electron slabs can be utilized in a variety of device structures owing to their high conductivity and continuously changing energy gap.

Power and linearity characteristics of field-plated recessed-gate AlGaN-GaN HEMTs

Record power density and high-efficiency operation with AlGaN-GaN high-electron mobility transistor (HEMT) devices have been achieved by adopting a field-plated gate-recessed structure. Devices grown on SiC substrate yielded very high power density (18.8 W/mm with 43% power-added efficiency (PAE) as well as high efficiency (74% with 6 W/mm) under single-tone continuous-wave testing at 4 GHz. Devices also showed excellent linearity characteristics when measured under two-tone continuous-wave signals at 4 GHz. When biased in deep-class AB (33 mA/mm, 3% I/sub max/) device maintained a carrier to third-order intermodulation ratio of 30 dBc up to a power level of 2.4 W/mm with 53% PAE; increasing bias current to 66 mA/mm (6% I/sub max/) allowed high linear operation (45 dBc) up to a power level of 1.4 W/mm with 38% PAE.

Origin of etch delay time in Cl2 dry etching of AlGaN/GaN structures

The etch delay time commonly found during dry etching of AlGaN and GaN has been experimentally proven to be due to the presence of hard–to–etch surface oxides. A BCl3 deoxidizing plasma, followed by a Cl2 etching plasma, was found to give dead-time-free aluminum-mole-fraction-independent etch rates. No selectivity between GaN and AlGaN has been observed up to an aluminum mole fraction of 35%. The aluminum-mole-fraction-dependent etch rates commonly reported in literature have been related to the different dead-times associated with dissimilar surface oxides, disproving the more common explanations in terms of the higher binding energy of AlN compared to GaN and/or the lower volatility of AlClx compared to GaClx.

High linearity and high efficiency of class-B power amplifiers in GaN HEMT technology

A 36-dBm high-linearity single-ended common-source class-B monolithic-microwave integrated-circuit power amplifier is reported in GaN high electron-mobility transistor technology. We also describe the design and simulation of highly linear and highly efficient common-source and common-drain class-B power amplifiers. Single-ended class-B amplifiers with bandpass filtering have equivalent efficiency and linearity to push-pull configurations. The common-source class-B circuit demonstrates high linearity, greater than 35 dBc of third-order intermodulation (IM3) suppression and high power-added efficiency (PAE) of 34%. Simulations of common-drain class-B designs predict a PAE of 54% with a superior IM3 suppression of more than 45 dBc over a wider range of bias due to the strong series-series negative feedback offered by the load resistance.

p-capped GaN-AlGaN-GaN high-electron mobility transistors (HEMTs)

A novel p-capped GaN-AlGaN-GaN high-electron mobility transistor has been developed to minimize radio-frequency-to-dc (RF-DC) dispersion before passivation. The novel device uses a p-GaN cap layer to screen the channel from surface potential fluctuations. A low-power reactive ion etching gate recess combined with angle evaporation of the gate metal has been used to prevent gate extension and maintain breakdown voltage. Devices with gate lengths of 0.7 /spl mu/m have been produced on sapphire. Current-gain cutoff frequencies (f/sub /spl tau//) of 20 GHz and maximum frequencies of oscillation (f/sub max/) of 38 GHz have been achieved. Unpassivated devices demonstrated a saturated output power of 3.0 W/mm and peak power-added efficiency of 40% at 4.2 GHz (V/sub DS/ = +20 V).

Systematic characterization of Cl 2 reactive ion etching for improved ohmics in AlGaN/GaN HEMTs

Pre-metal-deposition reactive ion etching (RIE) was performed on an Al/sub 0.3/Ga/sub 0.7/N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 /spl Omega/mm for unetched ohmics to a minimum of 0.27 /spl Omega/mm for 70 /spl Aring/ etched ohmics. The initial thickness of the AlGaN layer was 250 /spl Aring/. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs.

Systematic characterization of Cl 2 reactive ion etching for gate recessing in AlGaN/GaN HEMTs

High electron mobility transistors (HEMTs) with different gate recess depth were fabricated on an Al/sub 0.33/Ga/sub 0.67/N/GaN heterostructure, utilizing low power Cl/sub 2/ reactive ion etching. An increase in extrinsic transconductance and a positive threshold shift were observed with an increase of etching time. The etch depth was measured by atomic force microscopy (AFM) and determined to be nonlinear with etching time. The two terminal gate-drain leakage increased from about 0.005 mA/mm to 0.05 mA/mm. The destructive three-terminal breakdown voltage was about 120 V for all devices, etched and un-etched. Power measurements were performed in class A/B at a frequency of 8 GHz. The output power varied between 2.5 and 4.5 W/mm with the increase of bias voltage from 25 to 50 V. Independently of etch depth, there was no evidence of device failure even for the highest bias. The low increase in leakage, and no change in breakdown voltage support that low power RIE etching is a viable solution for low damage gate recess etch.