R. Stoklas

Investigation of trapping effects in AlGaN/GaN/Si field-effect transistors by frequency dependent capacitance and conductance analysis

We report on frequency dependent capacitance and conductance analysis of the AlGaN/GaN/Si heterostructure field-effect transistors (HFETs) and Al2O3/AlGaN/GaN/Si metal-oxide-semiconductor HFETs (MOSHFETs) in order to investigate the trap effects in these devices. The capacitance of the HFETs exhibits significantly higher frequency dispersion than that of the MOSHFETs. Two different types of traps were found from the conductance analysis on both types of devices, fast with the time constant τ≅(0.1–1) μs and slow with τ=8 ms. The density of trap states evaluated on the HFETs was DT≅2.5×1012 and up to 1013 cm−2 eV−1 for the fast and slow traps, respectively. Analysis of the MOSHFETs yielded only slightly lower DT of the fast traps (≅1.5×1012 cm−2 eV−1), but nearly two orders of magnitude lower density of slow traps (≤4×1011 cm−2 eV−1) than those of the HFETs. This indicates an effective passivation of slow surface related traps but less influence on fast (probably bulk related) trapping states by applying an...

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.

Characterization of AlGaN/GaN metal-oxide-semiconductor field-effect transistors by frequency dependent conductance analysis

We report on the frequency dependent conductance measurements of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). The properties of the devices with as-deposited and annealed 9-nm-thick Al2O3 gate oxide were investigated. The trap density in the range of 1011 cm−2 eV−1 was evaluated for the nonannealed devices. However, the conductance versus frequency peaks were significantly broader than those expected from theory, which indicates a surface potential fluctuation due to nonuniformities in the oxide charge and interface traps. Additionally, the dependence of the trap state time constant on gate voltage showed a deviation from the expected exponential function. However, the annealed devices (680 °C, 5 min) yielded a slightly lower (∼75%) trap density. Moreover, the conductance versus frequency data and the time constant versus gate voltage dependence of the annealed devices were in full agreement with the theoretical ones. The results show that the frequency dependen...

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.

Trapping effects in Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistor investigated by temperature dependent conductance measurements

Frequency dependent conductance measurements at varied temperature between 25 and 260 °C were performed to analyze trapping effects in the Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors. The trap states with a time constant τT,f≅(0.1–1) μs (fast) and τT,s=10 ms (slow) were identified. The conductance measurements at increased temperatures made it possible to evaluate the fast trap states in about a four times broader energy range than that from room temperature measurement. The density of the fast traps decreased from 1.4×1012 cm−2 eV−1 at an energy of 0.27 eV to about 3×1011 cm−2 eV−1 at ET=0.6 eV. The density of the slow traps was significantly higher than that of the fast traps, and it increased with increased temperature from about 3×1012 cm−2 eV−1 at 25–35 °C to 8×1013 cm−2 eV−1 at 260 °C.

Aluminum oxide as passivation and gate insulator in GaAs-based field-effect transistors prepared in situ by metal-organic vapor deposition

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...

InAlN/GaN/Si heterostructures and field-effect transistors with lattice matched and tensely or compressively strained InAlN

Properties of InAlN/GaN heterostructures and field-effect transistors with nearly lattice matched (InN=18%) and tensely (13%) or compressively (21%) strained InAlN barrier layer were evaluated. The sheet charge density increased from 1.1×1013 to 2.2×1013 cm−2 with decreased InN mole fraction. The saturation drain current as well as the peak extrinsic transconductance increased inversely proportional to the InN mole fraction—from 1 A/mm to 1.4 A/mm (VG=2 V) and from 190 to 230 mS/mm. On the other hand, the threshold voltage shifted to higher values with increased InN mole fraction. The pulsed current-voltage measurements (1 μs pulse width) yielded relatively low and nearly identical gate lag for all devices investigated. These results show that InAlN/GaN heterostructures with tensely strained InAlN can be useful for high-frequency and high-power devices and with compressively strained InAlN might be useful for the preparation of enhancement mode GaN-based devices.

InGaAs/GaAs metal-oxide-semiconductor heterostructure field-effect transistors with oxygen-plasma oxide and Al2O3 double-layer insulator

Surface condition before an insulator deposition is the key issue for the preparation of reliable GaAs-based metal-oxide-semiconductor (MOS) devices. This study presents the preparation and properties of InGaAs/GaAs MOS structures with a double-layer insulator consisting of an oxygen-plasma oxide covered by Al2O3. The structures were oxidized during 75 s and 150 s. Static measurements yielded a saturation drain current of ∼250 mA/mm at VG = 1 V. Capacitance measurements showed improved performance in the depletion region compared with the structures without the double-layer insulator. Trapping effects were investigated by conductance vs. frequency measurements. The trap state density was in order of 1011 cm−2·eV−1 with a continuous decrease with increased trap energy. The carrier mobility evaluation showed peak values of 3950 cm2/V·s for 75 s and 4570 cm2/V·s for 150 s oxidation times with the sheet charge density ≅2 × 1012 cm−2. The results demonstrate great potential of the procedure that was used to pr...

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.

Defect states characterization of non-annealed and annealed ZrO2/InAlN/GaN structures by capacitance measurements

InAlN/GaN metal-oxide-semiconductor structures with non-annealed and annealed ZrO2 gate insulators were characterized by capacitance-voltage (C−V) measurements. A significant capacitance hysteresis in both channel depletion and barrier accumulation regions was observed on the non-annealed structures. Fixed positive charge in the gate insulator was identified from the negative shift of the C−V curves. The C−V hysteresis was negligible and the threshold voltage decreased with a corresponding increase of the sheet charge density by 6 × 1012 cm−2 after annealing. The C−V slope in the accumulation region increased and the flat-band voltage decreased with decreased frequency. This confirms a decrease of the oxide/barrier interface trap state density with increased their activation energy. Capacitance saturation in the accumulation region occurs at lower values than it is in the insulator capacitance. Measurements at increased temperature up to 150 °C show a shift of the flat-band voltage to lower values. Both f...