Ivor Guiney

High Performance GaN High Electron Mobility Transistors on Low Resistivity Silicon for

This letter reports the RF performance of a 0.3-μm gate length AlGaN/AlN/GaN HEMT realized on a 150-mm diameter low-resistivity (LR) (σ <; 10 Ω · cm) silicon substrate. Short circuit current gain (f_T) and maximum frequency of oscillation (f_MAX) of 55 and 121 GHz, respectively, were obtained. To our knowledge, these are the highest f_T/f_MAX values reported to date for GaN HEMTs on LR silicon substrates.

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We report the modification and control of threshold voltage in enhancement and depletion mode AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors through the use of in-situ fluorine doping of atomic layer deposition Al2O3. Uniform distribution of F ions throughout the oxide thickness are achievable, with a doping level of up to 5.5 × 1019 cm−3 as quantified by secondary ion mass spectrometry. This fluorine doping level reduces capacitive hysteretic effects when exploited in GaN metal-oxide-semiconductor capacitors. The fluorine doping and forming gas anneal also induces an average positive threshold voltage shift of between 0.75 and 1.36 V in both enhancement mode and depletion mode GaN-based transistors compared with the undoped gate oxide via a reduction of positive fixed charge in the gate oxide from +4.67 × 1012 cm−2 to −6.60 × 1012 cm−2. The application of this process in GaN based power transistors advances the realisation of normally off, high power, high speed devices.

-Band Applications

Dispersion in capacitance and conductance measurements in AlGaN/GaN high-electron mobility transistors is typically interpreted as resulting from interface states. Measurements on varying gate-length devices and a model of an interface-trap-free device are used to demonstrate that the distributed-resistance-induced dispersion is significant for 1-MHz measurements if the gate length exceeds ~10 μm. Hence, interface state density measurements using the conductance technique need to use shorter gate-length devices in order to avoid this artefact.

Control of threshold voltage in E-mode and D-mode GaN-on-Si metal-insulator-semiconductor heterostructure field effect transistors by in-situ fluorine doping of atomic layer deposition Al2O3 gate dielectrics

Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates (σ<;40 Ω·cm). Transmission losses (S21) of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si.

Interface State Artefact in Long Gate-Length AlGaN/GaN HEMTs

Enhancement-mode AlInN/GaN metal–insulator–semiconductor heterostructure field-effect transistors on silicon are reported. A fluorine-based plasma treatment and gate dielectric are employed, and the devices exhibit a threshold voltage of +3 V. A drain current density of 295 mA/mm for a gate bias of +10 V is measured. An excellent off-state blocking voltage capability of 630 V for a leakage current of 1 µA/mm, and over 1000 V for 10 µA/mm are achieved on a 20-µm-gate–drain separation device at gate bias of 0 V. The dynamic on-resistance is ~2.2 times the DC on-resistance when pulsing from an off-state drain bias of 500 V.

Novel Shielded Coplanar Waveguides on GaN-on-Low Resistivity Si Substrates for MMIC Applications

The effective mass, sheet carrier concentration, and mobility of electrons within a two-dimensional electron gas in an AlGaN/GaN heterostructure were determined using a laboratory-based terahertz cyclotron resonance spectrometer. The ability to perform terahertz cyclotron resonance spectroscopy with magnetic fields of up to 31 T was enabled by combining a high-field pulsed magnet with a modified asynchronous optical sampling terahertz detection scheme. This scheme allowed around 100 transmitted terahertz waveforms to be recorded over the 14 ms magnetic field pulse duration. The sheet density and mobility were measured to be 8.0 × 1012 cm−2 and 9000 cm2 V−1 s−1 at 77 K. The in-plane electron effective mass at the band edge was determined to be 0.228 ± 0.002m0.

Enhancement-mode metal–insulator–semiconductor GaN/AlInN/GaN heterostructure field-effect transistors on Si with a threshold voltage of +3.0 V and blocking voltage above 1000 V

Electron mobility of AlGaN/GaN HEMTs is studied using a gate admittance-based technique. This analysis extends to electron densities as low as 4×1010 cm-2 with good accuracy. Zero lateral electric field is applied, in contrast to conventional methods. At these low electron densities, the mobility can be a factor of ~50 less than that in the ON-state. We reveal a regime at low electron densities where the screening of the two dimensional electron gas (2-DEG) becomes negligible causing the mobility to be independent of electron concentration, suggesting percolative transport. This region defines the rate at which the channel depletes and is a strong indicator of the epitaxial control of the impurities in the GaN channel.

Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an asynchronous optical sampling technique

The authors acknowledge financial support from the Engineering and Physics Sciences Research Council (EPSRC) under EP/K014471/1 (Silicon Compatible GaN Power Electronics).

Subthreshold Mobility in AlGaN/GaN HEMTs

Charge trapping and transport in the carbon doped GaN buffer of an AlGaN/GaN-on-Si high electron mobility transistor (HEMT) have been investigated. Back-gating and dynamic Ron experiments show that a high vertical leakage current results in significant long-term negative charge trapping in the buffer leading to current collapse under standard device operating conditions. Controlling current-collapse requires control of not only the layer structures and its doping, but also the precise balance of leakage in each layer.

Enhancement mode operation in AlInN/GaN (MIS)HEMTs on Si substrates using a fluorine implant

In this paper, viable transmission media technology has been demonstrated for the first time on GaN on low-resistivity silicon) substrates (ρ <; 40 Ω·cm) at H-band frequencies (220-325 GHz). The shielded-elevated coplanar waveguide (CPW) lines employ a standard monolithic microwave integrated circuit compatible air bridge process to elevate the CPW traces above a 5-μm layer of benzocyclobutene on shielded metalized ground plates. An insertion loss of less than 2.3 dB/mm was achieved up to 325 GHz, compared with 27 dB/mm for CPW fabricated directly on the substrate. To prove the efficiency of the technology, a short-circuited stub filter with a resonant frequency of 244 GHz was used. The filter achieved an unloaded Q-factor of 28, along with an insertion loss of 0.35 dB and a return loss of - 34 dB. To our knowledge, these results are the best reported to date for GaN-based technology.