Gate-drain Spacing Research Articles

Improvement of the off-state breakdown voltage with field plate and low-density drain in AlGaN/GaN high-electron mobility transistors**Project supported by the Key Program of the National Natural Science Foundation of China (Grant No. 61334002) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant Nos. 61404100 and 61106106).

We present an AlGaN/GaN high-electron mobility transistor (HEMT) device with both field plate (FP) and low-density drain (LDD). The LDD is realized by the injection of negatively charged fluorine (F–) ions under low power in the space between the gate and the drain electrodes. With a small-size FP and a LDD length equal to only 31% of the gate-drain spacing, the device effectively modifies the electric field distribution and achieves a breakdown voltage enhancement up to two times when compared with a device with only FP.

Effect of OFF-state stress induced electric field on trapping in AlGaN/GaN high electron mobility transistors on Si (111)

The influence of electric field (EF) on the dynamic ON-resistance (dyn-RDS[ON]) and threshold-voltage shift (ΔVth) of AlGaN/GaN high electron mobility transistors on Si has been investigated using pulsed current-voltage (IDS-VDS) and drain current (ID) transients. Different EF was realized with devices of different gate-drain spacing (Lgd) under the same OFF-state stress. Under high-EF (Lgd = 2 μm), the devices exhibited higher dyn-RDS[ON] degradation but a small ΔVth (∼120 mV). However, at low-EF (Lgd = 5 μm), smaller dyn-RDS[ON] degradation but a larger ΔVth (∼380 mV) was observed. Our analysis shows that under OFF-state stress, the gate electrons are injected and trapped in the AlGaN barrier by tunnelling-assisted Poole-Frenkel conduction mechanism. Under high-EF, trapping spreads towards the gate-drain access region of the AlGaN barrier causing dyn-RDS[ON] degradation, whereas under low-EF, trapping is mostly confined under the gate causing ΔVth. A trap with activation energy 0.33 eV was identified in the AlGaN barrier by ID-transient measurements. The influence of EF on trapping was also verified by Silvaco TCAD simulations.

Effect of SiNx gate insulator thickness on electrical properties of SiNx/In0.17Al0.83N/AlN/GaN MIS–HEMTs

The effect of SiNx thickness on device characteristics such as threshold voltage, carrier density, and carrier mobility have been determined for a metal–organic chemical-vapor-deposition grown In0.17Al0.83N/AlN/GaN structure with an ultra-thin In0.17Al0.83N/AlN (2.3/1nm) barrier layer. The SiNx gate dielectric was deposited ex situ in an RF plasma assisted molecular beam epitaxy system. The threshold voltage shifts negatively and the carrier density increases as the SiNx thickness is increased from 1 to 6nm due to the presence of a positive charge at the SiNx/In0.17Al0.83N interface. An interfacial charge of +3.84×1013cm−2 was extracted through the dependence of threshold voltage on insulator thickness. While remote charge scattering from the interfacial charge is shown to limit the carrier mobility, values as high as 1550cm2/Vs were achieved. Low gate and off-state drain leakage currents of less than 500nA/mm, a drain current ON/OFF ratio of approximately 107, and a normalized three terminal breakdown voltage of approximately 60–80V/μm gate–drain spacing were achieved on these ultra-thin In0.17Al0.83N/AlN barrier devices by implementing a thin SiNx gate insulator. The ability to maintain a short gate-to-channel distance while utilizing a gate insulator for reduced leakage current and improved breakdown can provide a pathway for higher power millimeter-wavelength amplifier performance.

Characterization of InAlN/GaN high-electron-mobility transistors grown on Si substrate using graded layer and strain-layer superlattice

We report on In0.25Al0.75N/GaN heterostructures grown on Si using a graded buffer and a thick strained-layer superlattice. These heterostructures showed a smooth surface with a root mean square value of 0.2 nm, a high sheet carrier density of 1.7 × 1013 cm−2, and a Hall mobility of 1100 cm2 V−1 s−1. The In0.25Al0.75N/GaN high-electron-mobility transistors (HEMTs) showed a high drain current density of 755 mA/mm with a low specific on-resistance of 0.39 mΩ·cm2. Furthermore, In0.25Al0.75N/GaN HEMT with the gate–drain spacing of 20 µm showed a high power device figure-of-merit of 1.62 × 108 V2 Ω−1 cm−2.

Effect of neutron irradiation on the electrical properties of AlGaN/GaN high electron mobility transistors

SiN-passivated AlGaN/GaN high electron mobility transistors (HEMTs) are exposed to 1 MeV neutron at fluences up to 1015 cm-2. The device shows a negligible degradation at neutron fluences below 1014 cm-2, while the gate leakage current (Ig) slightly changes (the forward IF increases, the reverse IR decreases.) at low fluencies and the IR degrades dramatically at fluences higher than 1014 cm-2. Moreover, near the knee voltage, the transconductance decreases at fluences up to 1015 cm-2, but the Schottky characteristicis become degraded after neutron irradiation. And the 20-hour annealing results do not show any significant annealing recovery effect at room temperature, while the parameters also continues to degrade a little. Therefore, the drain current (near the knee voltage) and the IF degradation of SiN-passivated AlGaN/GaN HEMT can be attributed to the irradiation induced defects in SiN passivation layers, demonstrating that the effectiveness of the SiN layer in passivating surface state in the source-gate spacer and gate-drain spacer is undiminished by neutron irradiation. And the Ohmic contact is so relatively robust to neutron, but the Schottky characteristics degrade obviously. The annealing results prove that the damage induced by neutron may be recovered more difficultly. SiN-passivated AlGaN/GaN HEMT appear to be an attractive candidate for space and terrestrial applications where resistance to displacement damage is required.

High‐voltage RESURF AlGaN/GaN high electron mobility transistor with back electrode

A novel reduced surface field (RESURF) AlGaN/GaN high electron mobility transistor (HEMT) with back electrode is proposed. The back electrode is electrically grounded and attached to the AlN nucleation layer after the substrate is removed. The back electrode, which attracts the electric field lines at the AlGaN/GaN interface by inducing negative charges, leads to a more uniform horizontal electric field along the channel and, hence, a significant improvement in breakdown voltage. Meanwhile, there is negligible negative impact on the ON-state resistance. Numerical simulation demonstrates a breakdown voltage of 1701 V and an ON-state resistance of 10.45 Ω mm with a gate-drain spacing of 6 μm for the proposed device, compared to a breakdown voltage of 1118 V and an ON-state resistance of 10.34 Ω mm for conventional RESURF AlGaN/GaN HEMT.

Influence of a drain field plate on the forward blocking characteristics of an AlGaN/GaN high electron mobility transistor

In this paper, the influence of a drain field plate (FP) on the forward blocking characteristics of an AlGaN/GaN high electron mobility transistor (HEMT) is investigated. The HEMT with only a gate FP is optimized, and breakdown voltage VBR is saturated at 1085 V for gate—drain spacing LGD ≥ 8 μm. On the basis of the HEMT with a gate FP, a drain FP is added with LGD = 10 μm. For the length of the drain FP LDF ≤ 2 μm, VBR is almost kept at 1085 V, showing no degradation. When LDF exceeds 2 μm, VBR decreases obviously as LDF increases. Moreover, the larger the LDF, the larger the decrease of VBR. It is concluded that the distance between the gate edge and the drain FP edge should be larger than a certain value to prevent the drain FP from affecting the forward blocking voltage and the value should be equal to the LGD at which VBR begins to saturate in the first structure. The electric field and potential distribution are simulated and analyzed to account for the decrease of VBR.

Direct-current and radio-frequency characteristics of passivated AlGaN/GaN/Si high electron mobility transistors

AlGaN/GaN/Si HEMTs grown by molecular beam epitaxy have been investigated using spectroscopy capacitance, direct and pulse current–voltage and small-signal microwave measurements. Passivation of the HEMT devices by SiO2/SiN with NH3 and N2O pretreatments is made in order to reduce the trapping effects. As has been found from DLTS data, some of electron traps are eliminated after passivation. This has led to an improvement in the drain current. To describe the electron transport, we have developed a charge-control model by including the deep traps observed from DLTS experiments. The thermal and trapping effects have been, on the other hand, studied from a comparison between direct-current and pulsed conditions. As a result, a gate-lag and a drain-lag were revealed indicating the presence of deep lying centers in the gate-drain spacing. Finally, small-signal microwave results have shown that the radio-frequency parameters of the AlGaN/GaN/Si transistors are improved by SiO2/SiN passivation and more increasingly with N2O pretreatment.

Influence of Physical Parameters on Microwave Noise Characteristics of Al0.3Ga0.7N/Al0.05Ga0.95N/GaN Composite-Channel HEMTs

THE Al0.3Ga0.7N/Al0.05Ga0.95N/GaN CC-HEMT structure is shown in [12]. It consists of 2 µm sapphire substrates, a 2.5 µm GaN undoped minor channel layer, a 6 nm Al0.05Ga0.95N undoped major channel layer, a 3 nm Al0.3Ga0.7N undoped spacer layer, a 21 nm doped(1e18) carrier supplier layer and a 2nm undoped cap layer. The devices have a source-gate spacing of Lsg=0.5 µm, gate-drain spacing of Lgd=1 µm, a 1 µm -gate-length and a gate width of 1 µm [12]. The band gap (Eg), affinity and electron mobility of layers of this device are shown in Table I. The polarization charge density at the interface of Al0.3Ga0.7N/Al0.05Ga0.95N and Al0.05Ga0.95N/GaN is calculated. The polarization charge density is 1.12e13 cm -2

The effects of 60Co γ-ray irradiation on the DC characteristics of enhancement-mode AlGaN/GaN high-electron-mobility transistors

The effects of 60Co γ-ray irradiation on the DC characteristics of AlGaN/GaN enhancement-mode high-electron-mobility transistors (E-mode HEMTs) are investigated. The results show that having been irradiated by 60Co γ-rays at a dose of 3 Mrad (Si), the E-mode HEMT reduces its saturation drain current and maximal transconductance by 6% and 5%, respectively, and significantly increases both forward and reverse gate currents, while its threshold voltage is affected only slightly. The obvious performance degradation of E-mode AlGaN/GaN HEMTs is consistent with the creation of electronegative surface state charges in the source-gate spacer and gate-drain spacer after being irradiated.

N-Polar GaN MIS-HEMTs With a 12.1-W/mm Continuous-Wave Output Power Density at 4 GHz on Sapphire Substrate

This letter presents a high-performance N-polar AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor grown by metal-organic chemical vapor deposition on sapphire substrate. The devices were passivated with Si <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> deposited by plasma-enhanced CVD and consisted of a gate structure recessed through the Si <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> passivation, with integrated slant field plates, to prevent dc to RF dispersion and improve the breakdown voltage. Devices with a drawn gate length of 0.7 μm and a gate drain spacing of 0.8 μm showed a breakdown voltage of 170 V, corresponding to a three-terminal leakage current of 1 mA/mm. Due to the high breakdown voltage of the devices, continuous-wave RF power measurements at 4 GHz could be measured at a drain bias of 50 V, yielding an output power density of 12.1 W/mm. To the best of our knowledge, this is the highest power density reported so far for an N-polar device and also matches the highest power density reported for a Ga-polar HEMT on the sapphire substrate.

Polarization Engineering in GaN Power Devices

This paper reviews novel design of epitaxial structure for GaN-based power devices taking advantages of the material's unique polarization. This can be called as polarization engineering which enables low on-state resistances and high breakdown voltages. AlGaN/GaN superlattice and polarization-matched InAlGaN quaternary alloy capping layers effectively reduce the series resistance of AlGaN/GaN heterojunction field effect transistors (HFETs) by reducing the potential barriers above the heterojunction. Natural super junction (NSJ) model is proposed, which well explains the limitless increase of the breakdown voltages of GaN transistor by the extension of the gate-drain spacing. This model is applied to diodes with multi channels of AlGaN/GaN resulting in low on-state resistances and high breakdown voltages. The presented polarization engineering is very promising for future GaN power devices with the improved performances.

High vertical breakdown strength in with low specific on‐resistance AlGaN/AlN/GaN HEMTs on silicon

AbstractOff‐state and vertical breakdown characteristics of AlGaN/AlN/GaN high‐electron‐mobility transistors (HEMTs) on high‐resistivity (HR)‐Si substrate were investigated and analysed. Three‐terminal off‐state breakdown (BVgd) was measured as a function of gate–drain spacing (Lgd). The saturation of BVgd with Lgd is because of increased gate leakage current. HEMTs with Lgd = 6 µm exhibited a specific on‐resistance RDS[ON] of 0.45 mΩ cm2. The figure of merit (FOM = BVgd2/RDS[ON]) is as high as 2.0 × 108 V2 Ω–1 cm–2, the highest among the reported values for GaN HEMTs on Si substrate. A vertical breakdown of ∼1000 V was observed on 1.2 µm thick buffer GaN/AlN grown on Si substrate. The occurrence of high breakdown voltage is due to the high quality of GaN/AlN buffer layers on Si substrate with reduced threading dislocations which has been confirmed by transmission electron microscopy (TEM). This indicates that the AlGaN/AlN/GaN HEMT with 1.2 µm thick GaN/AlN buffer on Si substrate is promising candidate for high‐power and high‐speed switching device applications. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Dynamic model of AlGaN/GaN HFET for high voltage switching

We analyze the channel and surface charge dynamics of a high voltage AlGaN/GaN Heterostructure Field Effect Transistor (HFET) operating as a power switch. We demonstrate that operation in the voltage range exceeding 100V without field plates involves alternating of the surface compensating charge in the gate–drain spacing. Developed model predicts the switching speed limitation of power HFET determined by the surface recharge rate and provides new, voltage-scalable design approach of AlGaN/GaN power devices.

Influence of the device geometry on the Schottky gate characteristics of AlGaN/GaN HEMTs

In this work, we investigate the relevance of device geometry to the Schottky gate characteristics of AlGaN/GaN high electron mobility transistors. Changes of three-terminal gate turn-on voltage and gate leakage current on the gate—drain spacing, source—gate spacing and recess depth have been observed. Further examinations comparing device simulations and measurements suggest that gate turn-on voltage is influenced by the distribution of electric potential under the gate region which is related to the geometry. By proper design of the device, high gate turn-on voltage can be obtained for both depletion-mode and recessed enhancement-mode devices.

Excellent Stability of GaN-on-Si High Electron Mobility Transistors with 5 µm Gate–Drain Spacing Tested in Off-State at a Record Drain Voltage of 200 V and 200 °C

AlGaN/GaN high electron mobility transistors (HEMTs) were electrically stressed in pinch-off condition at a drain voltage up to 200 V for 200 h at an ambient temperature of 200 °C. The tested transistors which were grown and processed on 4-in. silicon substrate showed negligible degradation. This proves that a combination of a high quality AlGaN/GaN/AlGaN double heterostructure, the in-situ Si3N4 deposition technique and an accurately optimized gate technology result in excellent device stability under harsh conditions.

Electrothermal Monte Carlo Simulation of GaN HEMTs Including Electron–Electron Interactions

A Monte Carlo device simulator was developed to investigate the electronic transport properties in AlGaN/GaN high-electron mobility transistors (HEMTs). Electron-electron interactions were included using a particle-particle-particle-mesh coupling scheme. Quantum corrections were applied to the heterointerface using the effective potential approach due to Ferry. Thermal effects were also included by coupling the particle-based device simulator self-consistently with an energy balance solver for the acoustic and optical phonons. The electrothermal device simulator was used to observe the temperature profiles across the device. Hot spots or regions of higher temperatures were found along the channel in the gate-drain spacing. Results from electrothermal simulations show self-heating degradation of performance at high source-drain bias. More importantly, the observed nonequilibrium phonon effects may play an important role in determining the thermal distribution in these HEMTs, resulting in reliability issues such as current collapse.

Effect of Gate–Drain Spacing for In0.52Al0.48As/In0.53Ga0.47As High Electron Mobility Transistors Studied by Monte Carlo Simulations

We performed two-dimensional Monte Carlo (MC) simulations of 60-nm-gate InP-based lattice-matched In0.52Al0.48As/In0.53Ga0.47As high electron mobility transistors (HEMTs) to clarify the effect of the gate–drain spacing Lgd on device performance. The calculated maximum transconductance gm and cutoff frequency fT increase with decreasing Lgd down to 50 nm, which agrees with experimental values. To explain the increase in gm and fT, we obtained electron velocity profiles in the InGaAs channel layer. Electron velocity overshoot under the gate is enhanced with decreasing Lgd. The resulting average electron velocity under the gate increases with decreasing Lgd. The enhancement of the electron velocity overshoot can be explained by using potential profiles in the HEMT. Potential profile under the gate in the InGaAs channel becomes steeper with decreasing Lgd.

Low Specific On-Resistance AlGaN/AlN/GaN High Electron Mobility Transistors on High Resistivity Silicon Substrate

High performance undoped AlGaN/AlN/GaN high electron mobility transistors (HEMTs) on high resistivity Si were achieved by implementing an ohmic-recess contact by inductively coupled plasma etching using four layers of Ti/Al/Ni/Au metal. About a 17.5% increase in drain current density and a 12.5% increase in extrinsic transconductance were observed on ohmic-recess-etched HEMTs. This is due to the reduction in the parasitic resistance such as contact resistance, drain resistance, and source resistance of the device. The ohmic-recess-etched AlGaN/AlN/GaN HEMT with a gate–drain spacing exhibited the lowest specific on-resistance of with an off-state breakdown voltage of 194 V. The highest ever reported figure-of-merit has been achieved in the ohmic-recess-etched AlGaN/AlN/GaN HEMTs on silicon. Ohmic recess is useful for undoped AlGaN/AlN/GaN HEMTs to achieve low with high figure-of-merit.

Effect of GaN Buffer Layer Growth Pressure on the Device Characteristics of AlGaN/GaN High-Electron-Mobility Transistors on Si

AlGaN/GaN High-electron-mobility transistor (HEMT) structures on silicon were grown by Metalorganic chemical vapor deposition (MOCVD) with various growth pressures during the growth of 1.5 µm thick GaN. The grown samples were characterized by X-ray diffraction, secondary ion mass spectroscopy, and photoluminescence analysis which revealed increased dislocation density, high C impurity compensation of free carriers and yellow luminescence for low pressure samples. For GaN grown at 200 Torr pressure, the formation of highly resistive buffer with C concentration as high as 3.8 ×1017 cm-3 leads to reduced buffer leakage over one order of magnitude and an enhanced breakdown voltage of 425 V for a HEMT with gate–drain spacing of 4 µm. However, unlike atmospheric pressure grown samples, the presence of unintentional C in the semi-insulating GaN degraded the channel conduction and resulted in severe current collapse.