Double-Recessed High-Frequency AlInGaN/InGaN/GaN Metal–Oxide Double Heterostructure Field-Effect Transistors

We demonstrate a low-threshold AlInGaN/InGaN/GaN metal-oxide semiconductor double heterostructure field-effect transistor (MOS-DHFET) for high-frequency operation. A combination of an InGaN channel (for carrier confinement), a DRE process, and a new digital-oxide-deposition technique helped us to achieve MOS-DHFET devices with extremely low subthreshold leakage currents. This reduction in output conductance (short channel effect) resulted in a high cutoff gain frequency f_T of about 65 GHz and a current gain frequency f _max of 94 GHz. The devices exhibited high drain-currents of 1.3 A/mm and delivered RF powers of 3.1 W/mm at 26 GHz with a 35 V drain bias.     

High-power operation of III-N MOSHFET RF switches

We describe a large-signal performance of novel high-power radio frequency (RF) switches based on III-nitride insulated gate metal-oxide semiconductor heterostructure field-effect transistors (MOSHFETs). The maximum switching powers for a single MOSHFET with only 1-mm gate width exceed 50W at 10GHz, more than an order of magnitude higher than those achievable using GaAs transistors. In the ON state, the highest powers are determined by the device peak drain currents, 1-2A/mm for the state-of-the art III-N MOSHFETs; in the OFF state their maximum powers are limited by the breakdown voltage, normally well above 100V. Our experimental data are in close agreement with large-signal simulations and the proposed simple analytical model. We also show that the insulating gate design allows for broader bandwidth and higher switching powers and better stability as compared to conventional Schottky gate transistors.     

Determination of the average channel temperature of GaN MOSHFETs under continuous wave and periodic-pulsed RF operational conditions

We present a method to determine the average device channel temperature of AlGaN/GaN metal–oxide–semiconductor heterostructure field effect transistors (MOSHFETs) in the time domain under continuous wave (CW) and periodic-pulsed RF (radiation frequency) operational conditions. The temporal profiles of microwave output power densities of GaN MOSHFETs were measured at 2 GHz under such conditions and used for determination of the average channel temperature. The measurement technique in this work is also being utilized to determine the thermal time constant of the devices. Analytical temporal solutions of temperature profile in MOSHFETs are provided to support the method. The analytical solutions can also apply to generic field effect transistors (FETs) with an arbitrary form of time-dependent heat input at the top surface of the wafer. It is found that the average channel temperature of GaN MOSHFETs on a 300 μm sapphire substrate with the output power of 10 W/mm can be over 400 °C in the CW mode while the average channel temperature of GaN MOSHFETs on a SiC substrate with the same thickness only reaches 50 °C under the same condition. The highest average channel temperature in a pulsed RF mode will vary with respect to the duty cycle of the pulse and type of the substrate.     

Monolithically integrated high-power broad-band RF switch based on III-N insulated gate transistors

We report on the high-performance monolithically integrated RF switch based on metal-oxide-semiconductor III-N heterostructure field-effect transistors (MOSHFETs). The radio frequency (RF) switch microwave monolithic integrated circuit (MMIC) consists of three submicron-gate MOSHFETs connected into /spl pi/-type configuration. In the 0-10 GHz frequency range, the insertion loss is less than 1dB and the isolation is better than 20 dB. The switching powers well exceed 20 W per 1mm of the active element width. The high performance parameters of the switch are achieved due to unique properties of III-nitride MOSHFET, which combines a low channel resistance and high breakdown voltage features of AlGaN/GaN HFETs and extremely low gate leakage currents, large gate voltage swing and low gate capacitance specific to insulated gate design. The combination of these parameters makes MOSHFETs excellent candidates for high-power switching. The experimental data obtained from the RF switch are in close agreement with the results of simulations.     

Mechanism of current collapse removal in field-plated nitride HFETs

An experimental study of the mechanism of RF current collapse removal in high-power nitride-based HFETs is presented. The results show that the conductivity of the dielectric material under the field plate plays a crucial role in the current collapse removal. Identical geometry field plated HFETs differing only in the FP dielectric conductivity show varying degree of current collapse removal. Devices with semiconducting dielectric layers exhibit perfectly linear RF power - drain bias dependence with the output powers of 20 W/mm at 55 V drain bias with essentially no current collapse. A trapped charge discharging model is presented to explain the removal of current collapse in FPd devices.     

Ohmic Contact to High-Aluminum-Content AlGaN Epilayers

We report on the effect of dry etching and the combination of metal stacks used to form ohmic contacts on silicon-doped high-Al-content (>60%) n-AlGaN layers for deep-ultraviolet light-emitting diodes. The contact characteristics are compared for as-grown and plasma-etched n-AlGaN samples. The Ti/Al/Ti/Au contacts to as-grown n-AlGaN were linear, with a specific contact resistivity of 5 × 10⁻⁵ Ω-cm². The same metallic layer combinations yielded nonlinear contacts on the plasma-etched surface of the n-AlGaN layers. However, when Ni was used as the barrier layer instead of titanium, the contacts to plasma-etched AlGaN surfaces became linear, with a specific contact resistivity of 5 × 10⁻⁴ Ω-cm².     

Silicon Dioxide-Encapsulated High-Voltage AlGaN/GaN HFETs for Power-Switching Applications

In this letter, new approach in achieving high breakdown voltages in AlGaN/GaN heterostructure field-effect transistors (HFETs) by suppressing surface flashover using solid encapsulation material is presented. Surface flashover in III-Nitride-based HFETs limits the operating voltages at levels well below breakdown voltages of GaN. This premature gate-drain breakdown can be suppressed by immersing devices in high-dielectric-strength liquids (e.g., Fluorinert); however, such a technique is not practical. In this letter, AlGaN/GaN HFETs encapsulated with PECVD-deposited SiO₂ films demonstrated breakdown voltage of 900 V, very similar to that of devices immersed in Fluorinert liquid. Simultaneously, low dynamic ON-resistance of 2.43 mOmega ldr cm² has been achieved, making the developed AlGaN/GaN HFETs practical high-voltage high-power switches for power-electronics applications.     

Selectively Doped High-Power AlGaN/InGaN/GaN MOS-DHFET

We describe a novel AlGaN/InGaN/GaN metal-oxide-semiconductor double heterostructure field-effect transistor with peak drain current of 1.67 A/mm and 2-GHz RF power of 15 W/mm at a drain bias as low as 35 V. These high values of peak currents and high RF powers at relatively low drain bias resulted from an additional selective area doping of the access regions during the device fabrication. The RF-output power of 12.5 W/mm (at a drain bias of V_D=30 V) was stable within a 0.5-dB variations during a 100-h continuous-wave stress test     

GaN/AlGaN p-channel inverted heterostructure JFET

A novel GaN/AlGaN p-channel inverted heterostructure junction field-effect transistor (HJFET) with a n/sup +/-type gate is proposed and demonstrated. A new superlattice aided strain compensation techniques was used for fabricating high quality GaN/AlGaN p-n junction. The p-channel HJFET gate leakage current was below 10 nA, and the threshold voltage was 8 V, which is close to that of typical n-channel HFETs. This new HJFET device opens up a way for fabricating nitride based complimentary integrated circuits.     

Power Stability of AlGaN/GaN HFETs at 20 W/mm in the Pinched-Off Operation Mode

High power-added efficiency (PAE) (ap74%) and rf-power (20 W/mm) operation of Schottky and insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) is reported at 2 GHz. In the pinched-off mode of operation, the PAE increases from a value of 55% to 74% when the drain bias is changed from 35 to 60 V. While both the Schottky and the insulated HFETs show high powers and PAE values, only the insulated-gate devices are stable at 20-W/mm output powers during a 60-h continuous wave rf-stress test. Their power drop of less than 0.1 dB is much smaller than the 0.8-dB drop for identical geometry Schottky-gate HFETs. The superior stability of the insulated-gate HFETs is attributed to the low forward gate currents