GaN Heterostructure Field-effect Transistors Research Articles

Terahertz emission spectroscopy of GaN-based heterostructures

This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices.

Simulation of the RF Power Performance of a GaN HFET and Comparison to Experiment

The large-signal RF power performance of an AlGaN/GaN heterostructure field-effect transistor (HFET) is simulated by technology computer-aided design (TCAD) software, and compared to measurement. A clear procedure for extraction of the simulation physical parameters is described. Trapping effects are included, but temperature effects are not. Good agreement between simulation and measurement is demonstrated, paving the way for efficiency optimization of GaN HFETs using TCAD.

Synthesis of thermally stable HfOxNy as gate dielectric for AlGaN/GaN heterostructure field-effect transistors**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0403000).

In this paper, we adopted thermally stable HfOxNy as gate dielectric for TiN/HfOxNy/AlGaN/GaN heterostructure field-effect transistors (HFETs) application. It demonstrated that the surface morphologies, composition, and optical properties of the HfOxNy films were dependent on oxygen flow rate in the O2/N2/Ar mixture sputtering ambient. The obtained metal–oxide–semiconductor heterostructure field-effect transistors by depositing HfO2 and HfOxNy dielectric at different oxygen flow rates possessed a small hysteresis and a low leakage current. After post deposition annealing at 900 °C, the device using HfOxNy dielectric operated normally with good pinch-off characteristics, while obvious degradation are observed for the HfO2 gated one at 600 °C. This result shows that the HfOxNy dielectric is a promising candidate for the self-aligned gate process.

Corrections to “Density of Traps at the Insulator/III-N Interface of GaN Heterostructure Field-Effect Transistors Obtained by Gated Hall Measurements” [Sep 15 893-895

In the above paper, equation 5 should read (1) $dV_{B} ={d\left ({ {q\Delta n} }\right )} / {C_{B} }$ Equation 6 should read (2) $D_{it} \left ({ {E_{f} +{q\Delta n}/ {C_{B} }} }\right )\approx \frac {C_{B} C_{I} }{q^{2}}\left [{ {\left ({ {\frac {d\left ({ {q\Delta n} }\right )}{dV_{G} }} }\right )^{-1}-\frac {1}{C_{HF} }} }\right ]$

Density of Traps at the Insulator/III-N Interface of GaN Heterostructure Field-Effect Transistors Obtained by Gated Hall Measurements

We apply the gated Hall method to obtain the density of trap distribution, $\text{D}_{\mathrm {it}}$ (E), at the insulator/III-N interface of a heterostructure field effect transistor. It is shown that $\text{D}_{\mathrm {it}}$ is proportional to the difference between the steady-state and the high-frequency prediction of the gate-induced two-dimensional electron gas concentrations. The $\text{D}_{\mathrm {it}}$ profile at the SiN x /GaN interface in the energy range of 1.2–2.3 eV below the GaN conduction band is obtained as a demonstration of the method.

T-Shaped Gate GaN HFETs on Si With Improved Breakdown Voltage and <inline-formula> <tex-math notation="TeX">$f_{\rm MAX}$ </tex-math></inline-formula>

In this letter, we have fabricated GaN heterostructure field-effect transistors (HFETs) with submicrometer T-shaped gate on Si substrate to realize compact solid-state power amplifiers used in Ku-band and higher. An AlGaN buffer layer was incorporated under the GaN channel layer, which was expected to enhance the breakdown voltage without decreasing the gain. The fabricated GaN HFETs with a gate length (L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> ) of 0.16 μm showed a three-terminal breakdown voltage (BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) of 98 V and a peak maximum oscillation frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> ) of 226 GHz, with a simultaneous current gain cutoff frequency ( f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of 63 GHz at VDS = 15 V. In addition, a minimum noise figure (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fmin</sub> ) of 1.01 dB with an associated gain of 16 dB at 14 GHz was obtained at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 5 V. A 0.1-mm-wide GaN HFETs exhibited a saturated power density of 3.82 W/mm at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> = 30 V, indicating the successful reduction of current collapse. To the best of our knowledge, this is the first demonstration of a GaN HFETs on Si, which combines high BV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , high f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> , high power density, and low NF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> at the Ku-band.

Hot-electron fluctuations and transistor performance: hot-phonon effects

The frequency performance of a GaN heterostructure field-effect transistor is discussed in terms of plasmon-assisted dissipation of LO-mode heat accumulated by non-equilibrium longitudinal optical phonons (hot phonons). Hot phonons cause additional scattering of electrons and also facilitate defect formation. Plasmon–LO-phonon resonance is experimentally resolved in a wide range of experiments such as fluctuations, dissipation, hot-electron transport, transistor frequency performance, transistor reliability, and transistor phase noise.

Elimination of Gate Leakage in GaN FETs by Placing Oxide Spacers on the Mesa Sidewalls

This letter reports the fabrication of a GaN heterostructure field-effect transistor with oxide spacer placed on the mesa sidewalls. The presence of an oxide spacer effectively eliminates the gate leakage current that occurs at the channel edge, where the gate metal is in contact with the 2-D electron gas edge on the mesa sidewall. From the two-terminal gate leakage current measurements, the leakage current was found to be several nA at VG=-12 V and at VG=-450 V. The benefits of the proposed spacer scheme include the patterning of the metal electrodes by plasma etching and a lower manufacturing cost.

Electron density window for best frequency performance, lowest phase noise and slowest degradation of GaN heterostructure field-effect transistors

The problems in the realm of nitride heterostructure field-effect transistors (HFETs) are discussed in terms of a novel fluctuation–dissipation-based approach impelled by a recent demonstration of strong correlation of hot-electron fluctuations with frequency performance and degradation of the devices. The correlation has its genesis in the dissipation of the LO-mode heat accumulated by the non-equilibrium longitudinal optical phonons (hot phonons) confined in the channel that hosts the high-density hot-electron gas subjected to a high electric field. The LO-mode heat causes additional scattering of hot electrons and facilitates defect formation in a different manner than the conventional heat contained mainly in the acoustic phonon mode. We treat the heat dissipation problem in terms of the hot-phonon lifetime responsible for the conversion of the non-migrant hot phonons into migrant acoustic modes and other vibrations. The lifetime is measured over a wide range of electron density and supplied electric power. The optimal conditions for the dissipation of the LO-mode heat are associated with the plasmon-assisted disintegration of hot phonons. Signatures of plasmons are experimentally resolved in fluctuations, dissipation, hot-electron transport, transistor frequency performance, transistor phase noise and transistor reliability. In particular, a slower degradation and a faster operation of GaN-based HFETs take place inside the electron density window where the resonant plasmon-assisted ultrafast dissipation of the LO-mode heat comes into play. A novel heterostructure design for the possible improvement of HFET performance is proposed, implemented and tested.

Camelback channel for fast decay of LO phonons in GaN heterostructure field-effect transistor at high electron density

Fluctuation technique is used to measure hot-phonon lifetime in dual channel GaN-based configuration proposed to support high-power operation at high frequencies. The channel is formed of a composite Al0.1Ga0.9N/GaN structure situated in an Al0.82In0.18N/AlN/Al0.1Ga0.9N/GaN heterostructure. According to capacitance–voltage measurements and simultaneous treatment of Schrödinger–Poisson equations, the mobile electrons in this dual channel configuration form a camelback density profile at elevated hot-electron temperatures. The hot-phonon lifetime was found to depend on the shape of the electron profile rather than solely on its sheet density. The camelback channel with an electron sheet density of 1.8 × 1013 cm−2 demonstrates ultrafast decay of hot phonons at hot-electron temperatures above 600 K: the hot-phonon lifetime is below ∼60 fs in contrast to ∼600 fs at an electron sheet density of 1.2 × 1013 cm−2 obtained in a reference Al0.82In0.18N/AlN/GaN structure at 600 K. The results suggest a suitable method to increase the electron sheet density without the deleterious effect caused by inefficient hot-phonon decay observed in a standard design at similar electron densities.

GaN HFET MMICs with integrated Schottky-diode for highly efficient digital switch-mode power amplifiers at 2 GHz

This work describes the integration of Schottky diodes into fast GaN MMIC process technology suitable for the realization of switch-mode power amplifier core chips for class-S operation at 2 GHz. With the demonstration of this technology, the so-called third-quadrant issue, which reduces the efficiency in band pass-Δ-Σ class-S operation can be diminished on device level. Compared to a hybrid diode assembly, the broadband properties of the amplifier module with on-chip-integrated diode can be improved by the reduction of parasitic losses. The GaN heterostructure field effect transistors (HFETs) with integrated series diode show a cut-off frequency of 28 GHz with drain breakdown voltages exceeding −100 and +100 V and comparable large signal performance to conventional GaN HFETs at 10 GHz. MMIC core chips for class-D and class-S switch-mode power amplifier modules are demonstrated for the operation at mobile communication frequencies between 0.45 and 2 GHz and signal bit rates up to 8 Gbps. The circuits yield broadband output power levels between 4 and 9 W with efficiencies of up to 80%.

The lifetime of optical phonons in a single heterostructure

The lifetime of long-wavelength optical phonons in bulk semiconductors is known to be a function of electron density, decreasing with increasing density. An explanation of this has been given in terms of the effect of plasmon coupling on the anharmonic decay route. This explanation is limited to small wave vectors and is not applicable to the case of phonons in general because their interaction with electrons involves larger wave vectors for which the plasma coupling is Landau damped. Nevertheless, a similar decrease of lifetime with density is observed for hot phonons in the channel of a GaN Heterostructure Field Effect Transistor (HFET). We show that this dependence on electron density can be associated with the interaction with the barrier interface mode and the temperature dependence of the phonon lifetimes. It is pointed out that a complete account of the shortening of phonon lifetime in a working HFET requires an analysis of the specific thermal and electron transport properties of thesystem. Important differences arise between HFETs grown on sapphire and on SiC.

Compressively Strained InxAl1-xN/Al0.22Ga0.78N/GaN (x = 0.245–0.325) Heterostructure Field Effect Transistors with Regrown AlGaN Contact Layers

Compressively strained InxAl1-xN (15 nm)/Al0.22Ga0.78N (3 nm)/GaN heterostructure field effect transistors (FETs) with regrown AlGaN contact layers were fabricated. The increase in compressive strain in InAlN reduced the polarization charge, i.e., the density of two-dimensional electron gas decreased from 6.5 ×1012 to 1.3 ×1012 cm-2 as the In content of InAlN increased from 0.245 to 0.325. With the insertion of the AlGaN layer, electron mobility of as high as 1570 cm2 V-1 s-1 was achieved at the In content of 0.245. Selectively regrown AlGaN contact layers reduced the sheet resistance from 17,000 to 584 Ω/sq. at the access layer for In0.325Al0.675N/Al0.22Ga0.78N/GaN. We fabricated FETs with this structure. The FETs without the regrown AlGaN contact layer did not operate at all owing to the high resistance. In contrast, the devices with the regrown AlGaN did. The maximum transconductance is 60 mS/mm, and the drain current is 0.11 A/mm. The threshold voltage becomes shallower, changing from -3.2 to -0.2 V with the increase in In content from 0.245 to 0.325. These results indicate that it is possible to fabricate enhancement-mode FETs with InAlN/AlGaN/GaN heterostructures.

Effect of hot phonon lifetime on electron velocity in InAlN/AlN/GaN heterostructure field effect transistors on bulk GaN substrates

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.

Current collapse transient behavior and its mechanism in submicron-gate AlGaN∕GaN heterostructure transistors

The current collapse transient behavior of a practical submicron AlGaN∕GaN heterostructure field effect transistor (HFET) is investigated, and its mechanism is proposed. First, the steady-state and transient characteristics of the Schottky diode obtained by connecting the source and drain electrodes of the transistor have been investigated. The steady-state characteristics can be explained by the thin surface barrier model, indicating the presence of tunneling injection of electrons. Turn-on and turn-off transient characteristics of the reverse current of Schottky diode showed very slow nonexponential transients covering six orders of magnitude of time scale from milliseconds to thousands of seconds. They are very similar to those of a large planar Schottky diode studied recently by the authors. The HFET device showed a clear current collapse behavior after a gate stress beyond pinch off. Pulsed gate stress visualized drain current transients which again included very slow nonexponential transients covering six orders of magnitude of time scale. The whole experimental results are explained consistently by a model in which the current collapse is due to surface state charging near the source side and drain side of the gate edge where its rate limiting process is not the usual Shockley–Read–Hall capture-emission process but the dispersive electron transport through the surface states by time-continual hopping, which is triggered by the tunneling injection process at the gate edge.

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.

Ultrathin AlN∕GaN heterostructure field-effect transistors with deposition of Si atoms on AlN barrier surface

We deposited Si atoms on the AlN barrier surface of an ultrathin AlN∕GaN heterostructure field-effect transistor (HFET). This induced a remarkable change in the electrical properties of the two-dimensional electron gas. A 2-nm-thick Si layer reduced the sheet resistance of an AlN∕GaN HFET (AlN barrier, 2nm) from 60356to388Ω∕sq. The effect on the Ohmic contact was also significant: the presence of an undermost layer of Si atoms under Ohmic contacts produced a low specific contact resistance of 1.7×10–6Ωcm2. A 50-nm-gate AlN∕GaN HFET with a Si layer exhibited excellent device characteristics with a current-gain cutoff frequency of 106GHz.

NON-IDEAL CURRENT TRANSPORT IN HETEROSTRUCTURE FIELD EFFECT TRANSISTORS

We describe a new analytical model of a Heterostructure Field Effect Transistors (HFETs) that accounts for electron trapping in the gate-drain spacing of the device and for related non-ideal device behavior. Under conditions of a very strong trapping, the electron velocity saturates outside the gate, in the trapping region, and the negative trapped charge leads to relatively large differential output conductance at the drain voltages exceeding the knee voltage. Also under the conditions of severe trapping, the negative trapped charge leads to the positive offset of the output current-voltage (I-V) characteristic. The model describes quite well numerous experimental data for passivated and unpassivated AlGaN / GaN HFETs with and without field plates (FP) under different bias conditions.

Steady state and transient behavior of currents in AlGaN∕GaN planar Schottky diodes and mechanism of current collapse

To get information on the current collapse mechanism under gate bias stress in AlGaN∕GaN heterostructure field effect transistors, this article investigates the steady state and transient behavior of current in a planar Schottky metal contact formed on a standard AlGaN∕GaN high electron mobility transistor wafer. Schottky contacts with reduced reverse leakage currents were prepared by using an oxygen gettering process, and their electrical characteristics were studied by current-voltage, capacitance-voltage, and current-time measurements at various temperatures. When the voltage pulse was applied into the pinch-off region, an excess current flew, showing very slow nonexponential transients covering six orders of magnitude of time from millisecconds to thousand seconds. Turn-off transients were also slow and highly nonexponential. The current transients are shown to be due to charging and discharging of the virtual gate capacitor with the electrons in and out of surface trapping states where the rate limiting process is the hopping process of electrons. The process can be described by a dispersive hopping transport, leading to charging behavior with a stretched exponential dependence on time and discharging behavior with power law dependence on time. Theoretical formulas showed good agreements with experiments, and provide ample description of the transients processes directly related to the current collapse.

Development of High-Frequency GaN HFETs for Millimeter-Wave Applications

This paper describes the device fabrication process and characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) aimed for millimeter-wave applications. We developed three novel techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N/GaN HFETs with a gate length of 30nm had a maximum drain current density of 1.6A/mm and a maximum transconductance of 402mS/mm. The use of these techniques led to a current-gain cutoff frequency of 181GHz and a maximum oscillation frequency of 186GHz.