A normally off GaN MOSFET was proposed by utilizing an extremely high 2-D electron-gas density (> 1014 / cm2) at an AlGaN/GaN heterostructure as source and drain, which can be obtained by controlling the tensile stress accompanied with the growth of GaN on silicon substrate. The fabricated MOSFET with an Al2O3 gate insulator exhibited excellent device performance, such as a threshold voltage of 2 V, drain current of 353 mA/mm, extrinsic transconductance of 98 mS/mm, and field-effect mobility of 225 cm2/V·s.

Normally Off GaN MOSFET Based on AlGaN/GaN Heterostructure With Extremely High 2DEG Density Grown on Silicon Substrate

This paper reports on state-of-the-art millimeter-wave power performance of N-polar GaN-based metal–insulator–semiconductor high-electron-mobility transistors at 30 and 94 GHz. The performance is enabled by our N-polar deep recess structure, whereby a GaN cap layer is added in the access regions of the transistor to simultaneously enhance the access region conductivity while mitigating dc-to-RF dispersion. The impact of lateral scaling of the drain access region length is examined using the tradeoff between breakdown voltage and small-signal gain. Load-pull measurements are presented at 94 GHz, corresponding to the target device operating frequency in W-band, where the device demonstrated a peak power-added efficiency (PAE) of 28.8% at 16 V and record-high maximum output power density of 8 W/mm at 20 V. Additional load-pull measurements at 30 and 10 GHz demonstrate the viability of this device across a wide frequency range where the peak power remained constant at 8 W/mm and with peak PAEs of 56% and 58%, respectively.

Demonstration of Constant 8 W/mm Power Density at 10, 30, and 94 GHz in State-of-the-Art Millimeter-Wave N-Polar GaN MISHEMTs

Enhancement-mode devices are in the centre of current research on group-III nitride transistors. The realisation of high-performance enhancement-mode transistors via gate recessing requires damage-free processing. We report on enhancement-mode AlGaN/GaN-on-Si heterostructure field-effect transistors (HFETs) fabricated with a damage-free digital etch technique. The threshold voltage (Vth) achieved is as high as +0.5 V. For AlGaN/GaN-on-Si HFETs, a record extrinsic transconductance (gm) of 420 mS/mm and a record maximum drain current Idmax of 500 mA/mm have been demonstrated. Furthermore, proper turn-off characteristics have been realised. Pulsed I–V characteristics reveal nearly no current collapse.

Recessed-Gate Enhancement-Mode AlGaN/GaN Heterostructure Field-Effect Transistors on Si with Record DC Performance

A novel N-Polar GaN cap (MIS)high electron mobility transistor demonstrating record 6.7-W/mm power density with an associated power-added efficiency of 14.4% at 94 GHz is presented. This state-of-the-art power performance is enabled by utilizing the inherent polarization fields of N-Polar GaN in combination with a 47.5-nm in situ GaN cap layer to simultaneously mitigate dispersion and improve access region conductivity. These excellent results build upon past work through the use of optimized device dimensions and a transition from a sapphire to a substrate for reduced self-heating.

N-Polar GaN Cap MISHEMT With Record Power Density Exceeding 6.5 W/mm at 94 GHz

We present GaN-based high electron mobility transistors (HEMTs) with a 2-nm-thin InAlN/AlN barrier capped with highly doped n++ GaN. Selective etching of the cap layer results in a well-controllable ultrathin barrier enhancement-mode device with a threshold voltage of +0.7 V. The n++ GaN layer provides a 290-Omega/\square sheet resistance in the HEMT access region and eliminates current dispersion measured by pulsed IV without requiring additional surface passivation. Devices with a gate length of 0.5-mum exhibit maximum drain current of 800 mA/mm, maximum transconductance of 400 mS/mm, and current cutoff frequency fT of 33.7 GHz. In addition, we demonstrate depletion-mode devices on the same wafer, opening up perspectives for reproducible high-performance InAlN-based digital integrated circuits.

Ultrathin InAlN/AlN Barrier HEMT With High Performance in Normally Off Operation

By combining a low damage chlorine based gate-recess etching and a sophisticated technology for AlGaN/GaN depletion-mode high electron mobility transistors (HEMTs) we fabricated high performance recessed enhancement-mode HEMTs. A comparative investigation of depletion- and enhancement-mode devices prepared by this technique shows excellent DC and RF properties. A transconductance of 540 mS/mm and cut-off frequencies fT of 39 GHz and fmax of 74 GHz were obtained for 0.25 µm gate enhancement-mode HEMTs. Large-signal power measurements at 2 GHz reveal an output power density of 4.6 W/mm at 68% PAE conclusively demonstrating the capability of our enhancement-mode devices.

https://iopscience.iop.org/article/10.1143/JJAP.48.04C083/pdf

Gate-Recessed AlGaN/GaN Based Enhancement-Mode High Electron Mobility Transistors for High Frequency Operation

A high efficiency digital MMIC amplifier for mobile communication switch-mode concepts was designed by utilizing a 0.25 μm GaN HEMT technology with fT of 32 GHz. A comparative investigation of two different driver concepts for a 1.2 mm GaN HEMT PA is shown. The MMICs were on-wafer evaluated for class-D and class-S operation. A drain efficiency of 70% for an output power of 4.4 W for a band pass delta-sigma (BPDS) class-S input signal at a bit rate of 3.6 Gbps equivalent to a 0.9 GHz fundamental was obtained. For the first time the operating mode up to 8 Gbps (2 GHz) is shown with an efficiency of 62%, demonstrating the prospect of future use of GaN HEMTs for switch mode amplifier concepts.

High Efficiency Digital GaN MMIC Power Amplifiers for Future Switch-Mode Based Mobile Communication Systems

This letter describes the millimeter-wave operation of III-N dual-gate MMICs based on a complete mm-wave MMIC technology suitable for operation up to 110 GHz. The GaN HEMTs have a gate length of 100 nm, yield high maximum transconductance, and very low parasitic capacitances. The cutoff frequency fT is above 80 GHz at an operation bias of 15 V in a fully passivated device. Dual-gate devices were developed for high gain at high gate widths and for substantial improvements in gain per stage on MMIC level. Complete III-N MMICs in grounded coplanar passive technology were designed. A single-stage dual-gate MMIC at 60 GHz yields 150 mW (840 mW/mm) of output power. A second MMIC shows a linear gain of greater than 10 dB at 94 GHz. It further yields an output power of 22.8 dBm (190 mW or 520 mW/mm) in CW-operation to a 50 Ω load with a maximum PAE of 7% at 94 GHz. The letter demonstrates the advantage of GaN dual-gate devices in power gain over common-source devices while maintaining essential improvements in power density.

Dual-Gate GaN MMICs for MM-Wave Operation

We report on recent results from our GaN transistor and circuit technology. Epitaxial growth can be performed on either SiC or Si substrates in order to provide high-quality AlGaN/GaN heterostructures. These heterostructures are then utilized in order to realize transistors and integrated circuits ranging from high-voltage transistors for voltage conversion in efficient power switches, L/S-band power bars and X-band MMICs for next-generation communication systems, and finally W-band MMICs for radar applications. The technology is characterized by state-of-the-art performance, good uniformity and high yield as well as excellent long-term stability. In combination with the space compatibility we believe that this technology is ideal for space. X-band MMICs from Fraunhofer IAF are scheduled to have the first in-orbit demonstration of European GaN within the Proba-V mission which is planned to be launched in spring 2013.

https://iopscience.iop.org/article/10.1088/0268-1242/28/7/074010/pdf

GaN HEMTs and MMICs for space applications

In this paper we report on the development of high transconductance GaN-based high electron mobility transistors (HEMTs) to improve the performance at W-Band frequencies. At first, the influence of the barrier thickness on the maximum transconductance (gm,max) was investigated by using two different technologies: growth of thin barrier layers and deep gate recess. Second, the effect of a gate length reduction down to 100 nm on gm,max was examined. The reduction of the barrier thickness results in a strong increase of the extrinsic transconductance up to 600 mS/mm. The technology was then used to fabricate HEMTs, with a cut-off frequency of 110 GHz, which are compatible to a MMIC technology (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

C. Haupt

Papers

Gate-Recessed AlGaN/GaN Based Enhancement-Mode High Electron Mobility Transistors for High Frequency Operation

High Efficiency Digital GaN MMIC Power Amplifiers for Future Switch-Mode Based Mobile Communication Systems

Dual-Gate GaN MMICs for MM-Wave Operation

GaN HEMTs and MMICs for space applications

Development of a high transconductance GaN MMIC technology for millimeter wave applications