Metal-insulator-semiconductor Heterostructure Field-effect Transistors Research Articles

Depletion- and Enhancement-Mode p-Channel MISHFET Based on GaN/AlGaN Single Heterostructures on Sapphire Substrates

We report on p-channel metal-insulator-semiconductor heterostructure field-effect transistors (MISHFET) based on p-GaN/uid-GaN/Al0.29Ga0.71N single heterostructures on sapphire substrates, grown by metalorganic vapor phase epitaxy (MOVPE). The impact of p-GaN layer removal and channel layer thickness adjustment by dry-etching on the characteristics of the MISHFET are investigated. Depending on the remaining GaN thickness (tGaN), the fabricated MISHFET show either depletion-mode (d-mode) operation with a threshold voltage Vth of 3.8 V and an on-current |ID,on| of 9.5 mA/mm (tGaN = 21 nm) or enhancement-mode (e-mode) operation with Vth of -2.3 V and |ID,on| of 1.5 mA/mm (tGaN = 12 nm). Independent of the etching depth, all devices exhibit a very low off-state drain current |ID,off| 10-8 mA/mm and a steep subthreshold swing (SS) between 80 and 89 mV/dec. Similar to n-channel devices, a Vth instability caused by charge trapping at the dielectric/semiconductor interface is found, emphasizing that careful interface engineering is required for good device performance.

Impact of Al x Ga1−x barrier thickness and Al composition on electrical properties of ferroelectric HfZrO/Al2O3/AlGaN/GaN MFSHEMTs

Ferroelectric (FE) HfZrO/Al2O3 gate stack AlGaN/GaN metal-FE-semiconductor heterostructure field-effect transistors (MFSHEMTs) with varying Al x Ga1−x N barrier thickness and Al composition are investigated and compared by TCAD simulation with non-FE HfO2/Al2O3 gate stack metal–insulator–semiconductor heterostructure field-effect transistors (MISHEMTs). Results show that the decrease of the two-dimensional electron gas (2DEG) density with decreasing AlGaN barrier thickness is more effectively suppressed in MFSHEMTs than that in MISHEMTs due to the enhanced FE polarization switching efficiency. The electrical characteristics of MFSHEMTs, including transconductance, subthreshold swing, and on-state current, effectively improve with decreasing AlGaN thickness in MFSHEMTs. High Al composition in AlGaN barrier layers that are under 3-nm thickness plays a great role in enhancing the 2DEG density and FE polarization in MFSHEMTs, improving the transconductance and the on-state current. The subthreshold swing and threshold voltage can be reduced by decreasing the AlGaN thickness and Al composition in MFSHEMTs, affording favorable conditions for further enhancing the device.

Suppression of current dispersion in AlGaN/GaN MISHFETs with in-situ AlN passivation layer

We demonstrate effective reduction of interface states and current collapse in AlGaN/GaN metal insulator semiconductor heterostructure field effect transistor (MISHFETs) with Al2O3 and in-situ AlN passivation layer. Here, first, a 3 nm-thick in-situ AlN layer is grown on AlGaN/GaN structure by using MOCVD at 1070 °C, and second a 7 nm-thick atomic layer deposition (ALD) Al2O3 layer is immediately deposited on the AlN layer. The X-ray photoelectron spectroscopy (XPS) analyses for the Ga 3d core level and multi-frequency capacitance–voltage (C–V) measurement show that the density of interface states originated from the Ga-O bonds, which is dramatically reduced with the in-situ AlN passivation layer. The AlGaN/GaN-based MISHFET with the Al2O3/in-situ AlN layers exhibits the low gate-lag effect (8%), remarkably less than that of the device with the typical GaN capping layer (21.5%). These results indicate that the Al2O3/in-situ AlN double passivation layer is very effective in suppressing the current collapse, and hence, it is found to be the promising method for the fabrication of AlGaN/GaN MISHFETs with enhanced performance.

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

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.

Gate leakage current induced trapping in AlGaN/GaN Schottky-gate HFETs and MISHFETs

This study examined the correlation between the off-state leakage current and dynamic on-resistance (RON) transients in AlGaN/GaN heterostructure field-effect transistors (HFETs) with and without a gate insulator under various stress conditions. The RON transients in a Schottky-gate HFET (SGHFET) and metal-insulator-semiconductor HFET (MISHFET) were observed after applying various amounts of drain-source bias stress. The gate insulator in the MISHFET effectively reduced the electron injection from the gate, thereby mitigating the degradation in dynamic switching performance. However, at relaxation times exceeding 10 ms, additional detrapping occurred in both the SGHFET and MISHFET when the applied stress exceeded a critical voltage level, 50 V for the SGHFET and 60 V for MISHFET, resulting in resistive leakage current build-up and the formation of hot carriers. These high-energy carriers acted as ionized traps in the channel or buffer layers, which subsequently caused additional trapping and detrapping to occur in both HFETs during the dynamic switching test conducted.

High-κ insulating materials for AlGaN/GaN metal insulator semiconductor heterojunction field effect transistors

High-κ insulating materials (HfO2, HfO2/Al2O3, HfAlOx, and HfSiOx) were deposited by atomic layer deposition (ALD) on AlGaN/GaN to form Metal–Insulator–Semiconductor Heterostructure Field Effect Transistors (MISHFETs) and were electrically and structurally characterized. The objective of this study is to characterize the interface quality and correlate the results with electrical phenomena for each insulating material. Although there are many studies using HfO2 and Al2O3 on AlGaN, there is limited experimental data using ternary compounds such as HfAlOx or HfSiOx, compared to their binary counterparts. In this work, interface trap density, Dit, was extracted by the conductance method using on-chip metal–insulator–semiconductor heterostructure capacitors (MISHCAPs). HfO2 was measured to have the lowest trap density at low energies on the order of 1012cm−2eV−1 and quickly reduced about one order of magnitude less than the others at higher trap energies. HfO2/Al2O3, HfAlOx, and HfSiOx all had similar trap densities on the order of 1012cm−2eV−1. Ultra-low gate leakage levels were achieved, especially for HfAlOx on the orders of 10−12A/mm. Our studies indicate that HfAlOx provides the best electrical characteristics such as lowest gate leakage current, largest channel carrier density and resistance to self-heating effects without the vulnerability to low crystallization temperatures.

Influence of acceptor-like traps in the buffer on current collapse and leakage of E-mode AlGaN/GaN MISHFETs

Current collapse has been observed in transient simulations even with highly efficient surface passivation by HfO2. In this work, current collapse and buffer-leakage mechanisms in E-mode (normally off) AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MISHFETs) have been investigated by physics-based simulations. The influences on both current collapse and off-state leakage from the energy levels of the acceptor-like traps in the buffer and also the density of the traps have been systematically studied. It is shown that traps with higher energy levels will induce less current collapse and less buffer leakage. It is demonstrated that current collapse is closely related with the change of the densities of ionized acceptor-like traps before and after the stress. Results further indicate that with increasing buffer trap density, the off-state leakage significantly decreases while current collapse increases. Finally, a non-uniform distribution of the acceptor-like traps in the buffer has been proposed to balance the trade-off between current collapse and buffer leakage.

Analysis of AlN/AlGaN/GaN metal-insulator-semiconductor structure by using capacitance-frequency-temperature mapping

AlN/AlGaN/GaN metal-insulator-semiconductor (MIS) structure is analyzed by using capacitance-frequency-temperature (C-f-T) mapping. Applying sputtering-deposited AlN, we attained AlN/AlGaN/GaN MIS heterostructure field-effect transistors with much suppressed gate leakage currents, but exhibiting frequency dispersion in C-V characteristics owing to high-density AlN/AlGaN interface states. In order to investigate the interface states deteriorating the device performance, we measured temperature-dependent frequency dispersion in the C-V characteristics. As a result, we obtained C-f-T mapping, whose analysis gives the activation energies of electron trapping, namely the interface state energy levels, for a wide range of the gate biases. This analysis method is auxiliary to the conventional conductance method, serving as a valuable tool for characterization of wide-bandgap devices with deep interface states. From the analysis, we can directly evaluate the gate-control efficiency of the devices.

Effect of AlN growth temperature on trap densities of in-situ metal-organic chemical vapor deposition grown AlN/AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors

The trapping properties of in-situ metal-organic chemical vapor deposition (MOCVD) grown AlN/AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) with AlN layers grown at 600 and 700 °C has been quantitatively analyzed by frequency dependent parallel conductance technique. Both the devices exhibited two kinds of traps densities, due to AlN (DT-AlN) and AlGaN layers (DT-AlGaN) respectively. The MIS-HFET grown at 600 °C showed a minimum DT-AlN and DT-AlGaN of 1.1 x 1011 and 1.2 x 1010 cm-2eV-1 at energy levels (ET) -0.47 and -0.36 eV. Further, the gate-lag measurements on these devices revealed less degradation ∼ ≤ 5% in drain current density (Ids-max). Meanwhile, MIS-HFET grown at 700 °C had more degradation in Ids-max ∼26 %, due to high DT-AlN and DT-AlGaN of 3.4 x 1012 and 5 x 1011 cm-2eV-1 positioned around similar ET. The results shows MIS-HFET grown at 600 °C had better device characteristics with trap densities one order of magnitude lower than MIS-HFET grown at 700 °C.

Investigation of plasma-oxidized aluminium as a gate dielectric for AlGaN/GaN MISHFETs

Nitride-based metal insulator semiconductor heterostructure field effect transistors (MISHFETs) have shown great potential, recently. An elegant technological approach is presented to fabricate MISHFETs with the gate dielectric formed only in the gate region. Aluminium is deposited in the gate region, oxidized by an inductively coupled oxygen plasma, subsequently covered by the evaporated gate contact metal and finally lifted off, all using a single patterning step. The electrical properties of the dielectric fabricated in this manner can be enhanced by annealing. Leakage currents comparable to those of other dielectric deposition methods are achieved. A subthreshold swing below 80 mV dec−1 indicates a high-quality interface.

Application of Sputtering-Deposited AlN Films to Gate Dielectric for AlGaN/GaN Metal–Insulator–Semiconductor Heterojunction Field-Effect Transistor

AlN films deposited by RF magnetron sputtering are applied to AlGaN/GaN metal–insulator–semiconductor heterostructure field-effect transistors (MIS-HFETs) as a gate dielectric. X-ray photoelectron spectroscopy (XPS) was used to characterize the AlN films, showing their chemical bonds and the bandgap by N 1s electron energy loss spectroscopy. The AlGaN/GaN MIS-HFET with a gate length of 150 nm was successfully fabricated, exhibiting low gate leakage currents for both reverse and forward biases, which are at least four orders of magnitude lower than those of reference Schottky-HFETs. Although these results support the possibility of sputtering-deposited AlN as a gate insulator, there are AlN/AlGaN interface states unfavorable for device performance, which were investigated by the frequency dispersion in the capacitance–voltage (C–V) characteristics.

Structure and electrical characteristics of AlGaN/GaN MISHFET with Al2O3 thin film as both surface passivation and gate dielectric

An AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistor (MISHFET) with about 40 nm Al2O3 for both surface passivation and gate dielectric has been investigated and compared with the regular metal-semiconductor heterostructure field-effect transistor (MESHFET). The output characteristic measurements have shown that the MISHFET yielded 34% increase of the saturation drain current compared to the MESHFET. The Hall effect measurements of AlGaN/GaN two-dimensional electron gas (2DEG) coated with Al2O3 thin films indicated an increase of mobility and density of 2DEG, and thus a decrease of the parasitic series resistance. The XRD analysis of the AlGaN/GaN heterostructure showed that strain was introduced into the AlGaN barrier layer with Al2O3 coating. The energy band calculations showed that the biaxial tensile stress should possibly be the main mechanism for the performance improvement of the MISHFET.

Performance of AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors with AlN gate insulator prepared by reactive magnetron sputtering

The reactive magnetron sputtering technique at room temperature was used to prepare AlN dielectric layer for application in AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs). Two different gas ambiences (mixture of nitrogen and argon N2/Ar=0.35 and N2 alone, respectively) were applied. An increase in the saturation drain current (∼50%) and transconductance (∼45%) of the MIS-HFETs compared with HFET counterparts (IDS=363 mA/mm and gm=114 mS/mm) was observed. AlN/AlGaN/GaN MIS-HFET, where only the nitrogen gas ambience was used to deposit an AlN dielectric layer, reduced the gate leakage current up to four orders of magnitude (∼10−8 A/mm at −5 V) in comparison with the HFET. The improvements of the dynamic characteristics of the MIS-HFETs (reduction of the current collapse and density of the slow traps) were achieved. The N2 MIS-HFET exhibits better improvements of the static and dynamic parameters than the ArN2 MIS-HFET, and therefore the deposition of the AlN layer in the N2 alone is more effective.

Analytical charge control model for AlGaN/GaN MIS-HFETs including an undepleted barrier layer

An analytical charge control model considering the insulator/AlGaN interface charge and undepleted Al-GaN barrier layer is presented for AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) over the entire operation range of gate voltage. The whole process of charge control is analyzed in detail and partitioned into four regions: I—full depletion, II—partial depletion, III—neutral region and IV—electron accumulation at the insulator/AlGaN interface. The results show that two-dimensional electron gas (2DEG) saturates at the boundary of region II/III and the gate voltage should not exceed the 2DEG saturation voltage in order to keep the channel in control. In addition, the span of region II accounts for about 50% of the range of gate voltage before 2DEG saturates. The good agreement of the calculated transfer characteristic with the measured data confirms the validity of the proposed model.

Impact of gate dielectric thickness on the electrical properties of AlGaN/GaN MISHFETs on Si(111) substrate

AbstractWe report on the fabrication and characterization of AlGaN/GaN metal–insulator–semiconductor heterostructure field effect transistors (MISHFETs) using HfO2 and Al2O3 as gate dielectric, deposited by atomic layer deposition (ALD). Improved device performance has been observed for all MISHFET devices as compared to the reference HFET. The additional dielectric layer underneath the gate metallization leads to a significant decrease of gate leakage currents and thus prevents device degradation and losses. Furthermore, the 2DEG sheet carrier concentration increases depending on the k‐value of the oxide as well as the oxide thickness. The thin (3–9 nm) dielectric layer is serving simultaneously as surface passivation, mitigating dc‐to‐rf dispersion. However, less has been reported about the impact of gate insulator thickness on the electrical performance, in particular in terms of dc‐to‐rf dispersion. In this paper, full recovery of dc drain current is demonstrated with a MISHFET with 9 nm Al2O3 oxide thickness.

Improved Electrical Performance and Thermal Stability of HfO2 / Al2O3 Bilayer over HfO2 Gate Dielectric AlGaN/GaN MIS–HFETs

AlGaN/GaN metal–insulator–semiconductor heterostructure field-effect transistors (MIS–HFETs) with a bilayer gate dielectric have been fabricated, characterized, and compared with gate dielectric MIS–HFETs. Physical and electrical characterizations have revealed the enhanced properties of the bilayer dielectric over that of the single layer. As a result, the fabricated MIS–HFETs exhibit better electrical performance and thermal stability than the transistors. The maximum drain current of the MIS–HFETs has increased by , while the off-state drain current has reduced by nearly 1 order of magnitude than that of the MIS–HFETs. After the thermal stress at elevated temperatures (400 and ) for a prolonged duration up to 500 min, the irreversible device performance degradation, including drain current, peak transconductance, threshold voltage, and gate leakage current for the MIS–HFETs, is substantially less than that for the MIS–HFETs. A longer lifetime of at has also been estimated for the former, compared to that of the latter of .

Physical and electrical characteristics of hafnium oxide films on AlGaN/GaN heterostructure grown by pulsed laser deposition

Physical and electrical characteristics of hafnium oxide (HfO 2) films grown by pulsed laser deposition (PLD) technique and their application in AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) have been investigated. The PLD-grown amorphous HfO 2 films exhibit good constituent uniformity and stoichiometry. The conduction band offset for HfO 2/GaN heterostructure is evaluated to be 1.7 eV. The dielectric constant of HfO 2 is estimated as ∼ 20 and the effective oxide charge density is ∼ 8.9 × 10 11 cm − 2 . The fabricated PLD-grown HfO 2 MIS-HFETs show a much better electrical performance than the conventional Schottky gate-HFETs, including a larger maximum drain current (31.5%), larger gate voltage swing (8.5%), smaller gate leakage current (two orders of magnitude), and smaller degradation rate at an elevated temperature operation.

Characterization of AlGaN/GaN MISHFETs on a Si substrate by static and high-frequency measurements

AlGaN/GaN/Si metal–insulator–semiconductor heterostructure field-effect transistors (MISHFETs) with SiN and Al2O3 gate insulators are characterized by static and high-frequency measurements, and their performance is compared with nonpassivated and SiN-passivated heterostructure field-effect transistors (HFETs). The saturation drain current increased from ∼500 mA mm−1 for the HFETs to ∼770 mA mm−1 for the MISHFETs. The peak extrinsic transconductance of the MISHFETs (147 mS mm−1 for 8 nm SiN and 220 mS mm−1 for 4 nm Al2O3) is higher than expected due to the increased gate-to-channel separation. Similarly, small signal microwave characterization yielded an increase in the current gain cut-off frequency (from 3.2 to 7.2 GHz) and the maximum frequency of oscillation (from 12.3 to 20.4 GHz) for the MISHFETs with 2 µm gate length compared to the HFET counterparts. Finally, the density of trap states, evaluated from the frequency-dependent conductance measurements, was ≅3 × 1012 cm−2 eV−1 for the HFETs but only ≅1.8 × 1012 cm−2 eV−1 for the MISHFETs. All of these demonstrate the capability of AlGaN/GaN MISHFETs of preparing high-performance and cost-effective devices for high-power microwave applications on a Si substrate.

Improvement of device characteristics in MIS AlGaN/GaN heterostructure field‐effect transistors by designing insulator/AlGaN structures

AbstractTo obtain a guideline for improving the device performance of GaN‐based Metal‐Insulator‐Semiconductor (MIS) heterostructure field‐effect transistors (HFETs), both HFET and MIS HFETs were fabricated and compared, whose gate capacitances are designed to be equal. With an increased insulator thickness and decreased AlGaN thickness, a high transconductance was successfully obtained that was equivalent to that of HFET, in addition to the reduced gate leakage current in MIS HFETs. Moreover, Al2O3‐based MIS HFETs were revealed to exhibit superior DC characteristics over Si3N4 MIS HFETs. Since deposition of insulators changes the electrical properties of heterostructures, considering the insulator/AlGaN structures as the total barrier layers is effective and indispensable in designing the MIS AlGaN/GaN HFETs. On the basis of the device design, MIS HFETs can be fabricated that exhibit superior device characteristics over those in conventional HFETs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gate dielectric engineering of quarter sub micron AlGaN/GaN MISHFET: A new device architecture for improved transconductance and high cut-off frequency

In this paper analytical modeling for a novel three region gate dielectric engineered AlGaN/GaN Metal Insulator Semiconductor heterostructure field effect transistor (MISHFET) device architecture is presented which shows high transconductance and enhanced cut-off frequency at quarter micron gate lengths. Using a three region analysis along the horizontal direction in the gate dielectric region the expressions for transconductance and cut-off frequency of the device are obtained. It has been observed that using these gate dielectric schemes, improvements on device performance are observed over conventional MISHFET structures. Relative comparison of T and Γ-gate shaped structures is done with uniform gate dielectric profile and enhancement in microwave performance is observed. The proposed model is capable of modeling electrical characteristics like drain current, output conductance and threshold voltage of various other existent structures like uniform gate dielectric MISHFETs, HFETs and T-gate HFETs. The present model is based on closed form expression and does not involve any fitting parameter. The results obtained are compared with experimental data and show excellent agreement, thereby proving the validity of the model.