Grigory Simin

III-nitride transistors with capacitively coupled contacts

AlGaN∕GaN heterostructure field-effect transistor design using capacitively coupled contacts (C3HFET) is presented. Insulated-gate [C3 metal-oxide-semiconductor HFET (C3MOSHFET)] has also been realized. The capacitively coupled source, gate, and drain of C3 device do not require annealed Ohmic contacts and can be fabricated using gate alignment-free technology. For typical AlGaN∕GaN heterostructures, the equivalent contact resistance of C3 transistors is below 0.6Ωmm. In rf-control applications, the C3HFET and especially the C3MOSHFET have much higher operating rf powers as compared to HFETs. C3 design is instrumental for studying the two-dimensional electron gas transport in other wide band gap heterostructures such as AlN∕GaN, diamond, etc., where Ohmic contact fabrication is difficult.

5-TERMINAL THzGaN BASED TRANSISTOR WITH FIELD- AND SPACE-CHARGE CONTROL ELECTRODES

We present a novel approach to achieve terahertz-range cutoff frequencies and maximum frequencies of operation of GaN based heterostructure field-effect transistors (HFETs) at relatively high drain voltages. Strong short-channel effects limit the frequency of operation and output power in conventional geometry GaN HFETs. In this work, we propose a novel device with two additional independently biased electrodes controlling the electric field and space-charge close to the gate edges. As a result, the effective gate length extension due to short channel effects is diminished and electron velocity in the device channel is increased. Our simulations show that the proposed five-terminal HFET allows achieving fT=1.28 THz and fmax= 0.815 THz at the drain voltages as high as 12 V. Hence, this device opens up a new approach to designing THz transistor sources.

Simulation of gate lag and current collapse in GaN heterojunction field effect transistors

We present results from numerical simulations of the current collapse phenomenon in GaN heterostructure field effect transistors. Gate lag simulation results show that current collapse can be explained by an enhanced trapping under the gate edges. Hot electrons play an instrumental role in the collapse mechanism. The simulation results also linked collapse with electrons spreading into the substrate, and confirmed that better electron localization, as in a double heterostructure field effect transistor, can dramatically reduce current collapse.

Edge trapping mechanism of current collapse in III-N FETs

Simulations of GaN HFETs using the device simulator DESSIS show, in agreement with our experimental data, that enhanced trapping at both gate edges is responsible for the current collapse. These simulations also show a reduction of the collapse in DHFETs with an InGaN channel, in agreement with our gated transmission line measurements. The results demonstrate that hot electrons play an instrumental role in the collapse process.

NOVEL APPROACHES TO MICROWAVE SWITCHING DEVICES USING NITRIDE TECHNOLOGY

III-Nitride heterostructure field-effect transistors (HFETs) demonstrated a new paradigm in microwave switching and control applications due to unique combination of extremely low channel resistance (leading to low loss), very high RF power, low off-state capacitance, broad range of operating temperatures, chemical inertness and robustness. The paper reviews novel approaches and recent advances in III-Nitride technology for RF switching devices leading to higher operating frequencies and even lower insertion loss.

500 °C operation of AlGaN/GaN and AlInN/GaN Integrated Circuits

High-temperature technology platform has been developed utilizing planar III-nitride heterostructures approach. The record high electron concentration and mobility in 2DEG channel of III-nitride devices result in very high operation speed and are remarkably stable within a broad temperature range, allowing device operation above 500 °C. The developed IC technology is based on three key elements: (1) exceptional quality III-nitride heterostructure with very high carrier concentration and mobility that enables IC fast operation in a broad temperature range; (2) heterostructure field effect transistor approach that provides fully planar IC structure which is easy to scale and to combine with the other high temperature electronic components; (3) robust design with self-compensating 2DEG load resistors, advance metallization and high-k passivation/gate dielectrics, specially developed for high temperature operation. The feasibility of technology was demonstrated by modeling, design and fabrication of inverter ...

GaN-based MESFETs and DC-MOSFETs

We present experimental results, which show that GaN MESFET and MOSFET technology can demonstrate the performance comparable to that of GaN-AlGaN HFETs for highly doped narrow channel devices. The device structures were grown by low-pressure MOCVD over [0001] sapphire substrates. PECVD 10-15 nm thick SiO/sub 2/ was used as an insulating layer for doped channel GaN-based MOSFETs. The threshold voltage for MESFETs and DC-MOSFETs ranged from 1.5 V to 10 V, and from 4 V to 20 V, respectively. The maximum drain currents up to 300 mA/mm and transconductances up to 60 mS/mm were measured for 100 /spl mu/m wide devices. The Schottky gate turn-on voltage for MESFET devices was close to 1 V, which is approximately two times lower than for AlGaN-GaN HEMTs. The gate leakage current in DC-MOSFETs was more than three orders of magnitude lower than in MESFETs. The long-channel GaN MESFETs that we fabricated exhibited a cut-off frequency-gate length product of 11.6 GHz-/spl mu/m. This number is comparable with the 16.4 GHz-/spl mu/m value demonstrated recently for AlGaN-GaN MOS-HFETs on SiC substrates and 18.2 GHz-/spl mu/m demonstrated for AlGaN-GaN HFETs on sapphire substrates. The cut-off frequency improves with increasing channel doping. Experimental results and model predictions show that GaN MESFETs and GaN DC-MOSFETs might find applications for power devices in X-band and above.

High Magnetic Field Studies of AlGaN/GaN Heterostructures Grown on Bulk GaN, SiC, and Sapphire Substrates

We present the results of the high magnetic field studies of properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements, cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies as well as room-temperature characteristics of High Electron Mobility Transistors fabricated on all these substrates. The room temperature high field measurements allow us to clearly separate the contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures. The magnetotransport measurements are performed in the magnetic fields up to 30 Tesla for temperatures between 50mK-300K. This high magnetic field in combination with very high mobilities (over 60.000 cm2/Vs) in the sample on the bulk GaN substrates allow us to observe features related both to cyclotron resonance and spin splitting. The temperature dependence of this splitting determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance and tilted field experiments, allows us to determine the complete energy structure of 2DEG conduction band. We also present the first experimental results showing so called “the exchange enhancement” of the energy gaps between spin Landau levels.

Simulation of AlGaN/GaN Heterostructure Field Effect Transistors

In the last decade, AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) and Metal Oxide Semiconductor HFETs (MOSHFETs) have gained wide recognition as potential devices of choice for ultra high-power microwave systems and power electronics. However, current collapse effects are an important road block for practical applications of these devices and impede realization of their full potential. The elimination of current collapse as well as the ability to predict precise device DC and RF behavior, require understanding of physical phenomena involved. Physicsbased device simulations complementing experimental measurements are the key for gaining qualitative and quantitative insight into the above mentioned phenomena.

Quaternary AlInGaN MQWs for Ultraviolet LEDs

We report a pulsed atomic layer epitaxy (PALE) growth technique for quaternary AlInGaN films for ultraviolet optoelectronic applications. Using the PALE approach high quality quaternary AlInGaN/AlInGaN multiple quantum wells (MQWs) were successfully grown over sapphire substrates. From X-ray diffraction, atomic force microscopy, and photoluminescence study, a high structural and optical quality was established for the AlInGaN MQWs. Incorporating the PALE grown quaternary MQWs as the active layer of light emitting diode (LED) on sapphire or SiC substrates we also demonstrated room temperature deep ultraviolet electroluminescence under dc and pulsed electrical pumping. The peak emission wavelength can be tuned from 305 nm to 340 nm with spectrum FWHM of about 20 nm by varying the alloy compositions of the quaternary AlInGaN active layers using PALE. Comparative study of LEDs over sapphire and SiC substrates was also done in order to determine the influence of epilayer design on the performance parameters and the role of the substrate absorption.