K. Čičo

Gate insulation and drain current saturation mechanism in InAlN/GaN metal-oxide-semiconductor high-electron-mobility transistors

The authors investigate 2μm gate-length InAlN∕GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS HEMTs) with 12nm thick Al2O3 gate insulation. Compared to the Schottky barrier (SB) HEMT with similar design, the MOS HEMT exhibits a gate leakage reduction by six to ten orders of magnitude. A maximal drain current density (IDS=0.9A∕mm) and an extrinsic transconductance (gme=115mS∕mm) of the MOS HEMT also show improvements despite the threshold voltage shift. An analytical modeling shows that a higher mobility of electrons in the channel of the MOS HEMT and consequently a higher number of electrons attaining the velocity saturation may explain the observed increase in gme after the gate insulation.

Improved transport properties of Al2O3∕AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistor

The authors report on improved transport properties of Al2O3∕AlGaN∕GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). It is found that the drift mobility in the MOSHFET structures with 4nm thick Al2O3 gate oxide is significantly higher than that in HFETs. The zero-bias mobilities are 1950 and 1630cm2∕Vs for the MOSHFET and HFET, respectively. An ∼40% increase of the saturation drain current in the MOSHFETs compared to the HFETs seems to be larger than expected from the passivation effects. The MOSHFET devices show a higher transconductance (with peak values of ∼115mS∕mm) than the HFETs (∼70mS∕mm). Analysis of the device performance indicates a decrease of the parasitic series resistance together with an enhancement of the effective velocity of the channel electrons in the MOSHFET devices.

AlGaN/GaN metal?oxide?semiconductor heterostructure field-effect transistors with 4 nm thick Al2O3 gate oxide

AlGaN/GaN metal?oxide?semiconductor heterostructure field-effect transistors (MOSHFETs) with 4 nm thick Al2O3 gate oxide were prepared and their performance was compared with that of AlGaN/GaN HFETs. The MOSHFETs yielded ~40% increase of the saturation drain current compared with the HFETs, which is larger than expected due to the gate oxide passivation. Despite a larger gate-channel separation in the MOSHFETs, a higher extrinsic transconductance than that of the HFETs was measured. The drift mobility of the MOSHFETs, evaluated on large-gate FET structures, was significantly higher than that of the HFETs. The zero-bias mobility for MOSHFETs and HFETs was 1950 cm2 V?1 s?1 and 1630 cm2 V?1 s?1, respectively. These features indicate an increase of the drift velocity and/or a decrease of the parasitic series resistance in the MOSHFETs. The current collapse, evaluated from pulsed I?V measurements, was highly suppressed in the MOSHFETs with 4 nm thick Al2O3 gate oxide. This result, together with the suppressed frequency dispersion of the capacitance, indicates that the density of traps in the Al2O3/AlGaN/GaN MOSHFETs was significantly reduced.

Ultrathin InAlN/AlN Barrier HEMT With High Performance in Normally Off Operation

We present GaN-based high electron mobility transistors (HEMTs) with a 2-nm-thin InAlN/AlN barrier capped with highly doped n++ GaN. Selective etching of the cap layer results in a well-controllable ultrathin barrier enhancement-mode device with a threshold voltage of +0.7 V. The n++ GaN layer provides a 290-Omega/\square sheet resistance in the HEMT access region and eliminates current dispersion measured by pulsed IV without requiring additional surface passivation. Devices with a gate length of 0.5-mum exhibit maximum drain current of 800 mA/mm, maximum transconductance of 400 mS/mm, and current cutoff frequency fT of 33.7 GHz. In addition, we demonstrate depletion-mode devices on the same wafer, opening up perspectives for reproducible high-performance InAlN-based digital integrated circuits.

Thermally induced voltage shift in capacitance–voltage characteristics and its relation to oxide/semiconductor interface states in Ni/Al2O3/InAlN/GaN heterostructures

The Ni/Al2O3/InAlN/AlN/GaN metal-oxide-semiconductor heterostructure (MOS-H) is investigated using capacitance–voltage and capacitance–time characteristics in the temperature range of 25–300 °C. An anomalous positive voltage shift of the capacitance–voltage curve with increasing temperature was observed and attributed to the hole emission from the oxide/semiconductor interface states. Distribution of the interface states density, Dit(E), at the Al2O3/InAlN interface was evaluated using a modification of the constant-capacitance deep-level transient spectroscopy. The MOS-H capacitor threshold voltage shift under negative bias was repetitively recorded as a function of time at elevated temperatures. Dit in the range of 0.1–3 × 1013 eV−1 cm−2 was determined.

Schottky-barrier normally off GaN/InAlN/AlN/GaN HEMT with selectively etched access region

A Schottky-barrier normally off InAlN-based high-electron-mobility transistor (HEMT) with selectively etched access regions, high off-state breakdown, and low gate leakage is presented. Metal-organic chemical vapor deposition-grown 1-nm InAlN/1-nm AlN barrier stack is capped with a 2-nm-thick undoped GaN creating a negative polarization charge at a GaN/InAlN heterojunction. Consequently, the gate effective barrier height is increased, and the gate leakage as well as the equilibrium carrier concentration in the channel is decreased. After removal of the GaN cap at access regions by using a highly selective dry process, the extrinsic channel becomes populated by carriers. Normally off HEMTs with 8-μm source-to-drain distance and 1.8-μm -long symmetrically placed gate showed a source drain current of about 140 mA/mm. The HEMT gate leakage at a drain voltage of 200 V and grounded gate is below 10-7 A/mm with a three-terminal device breakdown of 255 V. The passivated InAlN surface potential has been calculated to be 1.45 V; significant drain current increase is predicted for even lower potential.

RF Performance of InAlN/GaN HFETs and MOSHFETs With

The RF performance of lattice-matched InAlN/GaN heterostructure field-effect transistors (HFETs) and Al₂O₃/InAlN/GaN metal-oxide-semiconductor HFETs (MOSHFETs) with varied gate length was evaluated. The current gain cutoff frequency <i>fT</i> and the maximum oscillation frequency <i>f</i> _max for the HFETs with 0.3-μm gate length were 54 and 58 GHz, respectively. An increase of <i>fT</i> to 61 GHz and of<i>f</i> _max to 70 GHz was obtained for the MOSHFETs. The HFETs and MOSHFETs with different gate length yielded an <i>fT</i> × <i>LG</i> product of 18 and 21 GHz · μm, respectively. These are higher values than reported yet on InAlN/GaN devices and similar to those known for AlGaN/GaN HFETs.

f_{T} \times L_{G}

Design considerations and performance of n++ GaN/InAlN/AlN/GaN normally off high-electron mobility transistors (HEMTs) are analyzed. Selective and damage-free dry etching of the gate recess through the GaN cap down to a 1-nm-thick InAlN barrier secures positive threshold voltage, while the thickness and the doping of the GaN cap influence the HEMT direct current and microwave performance. The cap doping density was suggested to be 2 × 1020 cm-3. To screen the channel from the surface traps, the needed cap thickness was estimated to be only 6 nm. Design is proved by an experiment showing a constant value of the HEMT dynamical access resistance, while a single-pulse experiment indicated almost collapse-free performance. On the other hand, it is found that the n++ GaN cap does not contribute to the HEMT drain current conduction, nor does it provide a path for the off-state breakdown. HEMTs with a gate length of 0.25 μm and a 4-μm source-to-drain distance show a drain-to-source current of 0.8 A/mm, a transconductance of 440 mS/mm, a threshold voltage of ~0.4 V, and a cutoff frequency of 50 GHz. A thin and highly doped GaN cap is also found to be suitable for the processing of normally on HEMTs by adopting the nonrecessed gate separated from the cap by insulation.

up to 21

The authors report on preparation and electrical characterization of InAlN/AlN/GaN metal-oxide-semiconductor (MOS) high electron mobility transistors (HEMTs) with Al2O3, ZrO2, and GdScO3 gate dielectrics. About 10 nm thick high-κ dielectrics were deposited by metal organic chemical vapor deposition after the Ohmic contact processing. Application of the gate dielectrics for 2 μm gate length MOS HEMTs leads to gate leakage current reduction from four to six orders of magnitude compared with Schottky barrier HEMTs. Among others, MOS HEMTs with an Al2O3 gate dielectric shows the highest transconductance (∼150 mS/mm) and maximum drain current (∼0.77 A/mm) and the lowest sheet resistance of ∼260 Ω/◻. MOS HEMTs with GdScO3 shows the highest breakdown electric field of about 7.0 MV/cm. A deep level transient spectroscopy (DLTS) based analysis revealed the maximum interface state density Dit up to 4×1012, 9×1012, and 3×1013 eV−1 cm−2 for Al2O3, ZrO2, and GdScO3/InAlN interface, respectively.

\hbox{GHz}\cdot \mu\hbox{m}

Application of GaAs-based metal-oxide-semiconductor (MOS) structures, as a “high carrier mobility” alternative to conventional Si MOS transistors, is still hindered due to difficulties in their preparation with low surface/interface defect states. Here, aluminum oxide as a passivation and gate insulator was formed by room temperature oxidation of a thin Al layer prepared in situ by metal-organic chemical vapor deposition. The GaAs-based MOS structures yielded two-times higher sheet charge density and saturation drain current, i.e., up to 4 × 1012 cm−2 and 480 mA/mm, respectively, than the counterparts without an oxide surface layer. The highest electron mobility in transistor channel was found to be 6050 cm2/V s. Capacitance measurements, performed in the range from 1 kHz to 1 MHz, showed their negligible frequency dispersion. All these results indicate an efficient suppression of the defect states by in situ preparation of the semiconductor structure and aluminum oxide used as a passivation and gate insu...