Channel High Electron Mobility Transistors Research Articles

2DEG properties in InGaN/InN/InGaN‐based double channel HEMTs

AbstractThis paper describes the 2DEG properties of InGaN/InN/InGaN double channel high electron mobility transistor (DHEMT). These include an in‐depth theoretical investigation of 2DEG sheet carrier concentration and mobility. The 2DEG mobility is calculated using ensemble Monte Carlo taking into account all relevant scatterings. The calculated sheet carrier concentration reaches as high as 1.77×1014 cm‐2 for DHEMT. While for single channel high electron mobility transistor (SHEMT) the calculated sheet carrier concentration was found to be 7.06×1013 cm‐2. The maximum total 2DEG mobility is found to be around 12×104 and 8×104 cm2V‐1s‐1, respectively, for DHEMT and SHEMT for a sheet carrier concentration of ns = 5×1012 cm‐2 at 77 K. Thus, the 2DEG sheet carrier concentration and mobility for dual channel is higher than conventional single channel HEMTs. The high values of 2DEG sheet carrier concentration and mobility strongly suggest that the InGaN/InN/InGaN based double channel HEMT is very promising for the fabrication of high performance high speed device (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of two‐dimensional electron gas in AlGaN channel HEMTs with high crystalline quality

AbstractEpitaxial structures of AlGaN channel high electron mobility transistors (HEMTs) were grown on sapphire and AlN substrates. Reduction in the full width at half maximum of X‐ray rocking curve for (10‐12) peak of the AlGaN channel layer owing to the reduction of threading dislocation densities resulted in a sharp decrease in the sheet resistance of 2‐dimensional electron gas (2DEG). In the case of AlGaN channel HEMTs, it was found that improvement of the crystalline quality of AlGaN channel layers is essential to the reduction of the sheet resistance of 2DEG. The use of AlN substrates resulted in improved crystalline quality of the AlGaN layer and lower 2DEG resistance, suggesting the high potential of AlN substrates for AlGaN channel HEMTs (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the nonlinear plasma resonance in gated two-dimensional semiconductor conduction channels

In the Dyakonov–Shur detector [IEEE Trans. Electron Devices 43, 380 (1996)] a short channel high electron mobility transistor is used for the detection of electromagnetic radiation in the low terahertz range. The quality of the plasma resonance utilized in such device is strongly temperature dependent. Starting with the Boltzmann equation, we derived the viscous hydrodynamic model with temperature dependent transport coefficients and obtained the temperature dependence of the quality factor of the resonance. We find that in high mobility channels the quality of the resonance is strongly limited by the viscosity of the electron fluid.

Analysis of degradation mechanisms in lattice-matched InAlN/GaN high-electron-mobility transistors

We address degradation aspects of lattice-matched unpassivated InAlN/GaN high-electron-mobility transistors (HEMTs). Stress conditions include an off-state stress, a semi-on stress (with a partially opened channel), and a negative gate bias stress (with source and drain contacts grounded). Degradation is analyzed by measuring the drain current, a threshold voltage, a Schottky contact barrier height, a gate leakage and an ideality factor, an access, and an intrinsic channel resistance, respectively. For the drain-gate bias <38 V parameters are only reversibly degraded due to charging of the pre-existing surface states. This is in a clear contrast to reported AlGaN/GaN HEMTs where an irreversible damage and a lattice relaxation have been found for similar conditions. For drain-gate biases over 38 V InAlN/GaN HEMTs show again only temporal changes for the negative gate bias stresses; however, irreversible damage was found for the off-state and for the semi-on stresses. Most severe changes, an increase in the intrinsic channel resistance by one order of magnitude and a decrease in the drain current by ∼70%, are found after the off-state ∼50 V drain-gate bias stresses. We conclude that in the off-state condition hot electrons may create defects or ionize deep states in the GaN buffer or at the InAlN/GaN interface. If an InAlN/GaN HEMT channel is opened during the stress, lack of the strain in the barrier layer is beneficial for enhancing the device stability.

Detection of Cl- Ions with AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) with Ag/AgCl gate are found to exhibit significant changes in channel conductance upon exposing the gate region to various concentrations of chorine ion solutions. Ag/AgCl gate electrode, prepared by potentiostatic anodization, changes the electrical potential when encounters chorine ions. This gate potential changes lead to a change of surface charge in the gate region on the HEMT, induced a higher positive charge on the AlGaN surface, and finally increases the pizeo-induced charge density in the HEMT channel. These anions create an image positive charge on the Ag gate metal for the required neutrality, thus increase the drain current of the HEMT. The HEMT source-drain current showed a clear dependence on the chorine concentration. The limit of detection (LOD) achieved was 1×10-8 M using a 20µm × 50µm gate sensing area.

Impact of SF 6 plasma treatment on performance of AlGaN/GaN HEMT

Selective plasma treatment of an AlGaN/GaN heterostructure in the RF discharge of the electronegative SF 6 gas was studied. Shallow recess-gate etching of AlGaN (∼5 nm) was performed in CCl 4 plasma through a photoresist mask. Subsequently, recess-gate etching followed in situ by SF 6 plasma. The plasma treatment provides the following advantages in the technology of AlGaN/GaN high-electron mobility transistors (HEMT): It (1) simplifies their technology; (2) ensures sufficient selectivity; and (3) enables the technologist to set the threshold voltage of the HEMTs controllably. At the same time, the treatment can (1) provide the AlGaN/GaN heterostructure with surface passivation; (2) modify the 2DEG in any area of a HEMT channel; and (3) make it possible to convert a HEMT operation from depletion mode to enhancement mode. The treatment also improved significantly the DC and RF parameters of HEMTs studied.

Comparison of characteristics of AlGaN channel HEMTs formed on SiC and sapphire substrates

The fundamental device characteristics of AlGaN channel high electron mobility transistors formed on 4H-SiC and sapphire substrates are compared, and it is found that the drivability is apparently better for the SiC substrate. Judging from the simultaneous enhancement in mobility and the carrier concentration of the two-dimensional electron gases for the SiC substrate, the advantages of the SiC substrate originate from the excellent crystal quality of the AlGaN channel layer including the interface with the AlGaN barrier layer, which should be due to less lattice mismatch with the substrate.

Analysis of Gate Delay Scaling in In0.7Ga0.3As-Channel High Electron Mobility Transistors

The intrinsic and the parasitic gate delay of In0.7Ga0.3As-channel high electron mobility transistors (HEMTs) are analyzed. In this paper, we extract the intrinsic parameters of sub-100-nm gate pseudomorphic InGaAs channel HEMTs and estimate the intrinsic and parasitic gate delay. We discuss how the intrinsic and parasitic gate delay are affected by scaling of the gate length and we estimate the parasitic gate capacitance from this gate delay analysis. We also compare the result of the gate delay analysis with lattice-matched InGaAs-channel HEMTs to examine the effect of use of the pseudomorphic InGaAs channel in the gate delay.

Changes of Electrical Characteristics for AlGaN/GaN HEMTs Under Uniaxial Tensile Strain

This letter investigates the characteristics of unpassivated AlGaN/GaN high-electron mobility transistors (HEMTs) under uniaxial tensile strain. Mechanical stress can produce additional charges that change the HEMT channel current. This phenomenon is dependent upon gate orientation and may be the result of the piezoelectric effect and changes in electron mobility due to the applied uniaxial stress. In addition, results show that tensile strain reduces the transient current, which is likely due to the additional donorlike surface states created through the piezoelectric effect.

Remarkable breakdown voltage enhancement in AlGaN channel high electron mobility transistors

The channel layer substitution of a wider bandgap AlGaN for a conventional GaN in high electron mobility transistors (HEMTs) is an effective method of enhancing the breakdown voltage. We demonstrated a remarkable breakdown voltage enhancement in these AlGaN channel HEMTs. The obtained maximum breakdown voltages were 463 and 1650V in the Al0.53Ga0.47N∕Al0.38Ga0.62N HEMT with the gate-drain distances of 3 and 10μm, respectively. This result is very promising for the further higher power operation of high-frequency HEMTs.

Oblique modes effect on terahertz plasma wave resonant detection in InGaAs∕InAlAs multichannel transistors

We report on the demonstration of narrow terahertz plasma wave resonant detection at low temperature in 200nm gate length InGaAs∕InAlAs multichannel high electron mobility transistors. We observe that the resonant detection linewidth is smaller than in full channel high electron mobility transistors. We interpret this shrinking by the effect of multichannel geometry that does not allow oblique plasma mode propagation along the channel.

Detection of chloride ions using an integrated Ag∕AgCl electrode with AlGaN∕GaN high electron mobility transistors

Al Ga N ∕ Ga N high electron mobility transistors (HEMTs) with an Ag∕AgCl gate exhibit significant changes in channel conductance upon exposing the gate region to various concentrations of chloride (Cl−) ion. The Ag∕AgCl gate electrode, prepared by potentiostatic anodization, changes electrical potential when it encounters Cl− ions. This gate potential changes lead to a change of surface charge in the gate region of the HEMT, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMT channel. These anions create an image positive charge on the Ag gate metal for the required neutrality, thus increasing the drain current of the HEMT. The HEMT source-drain current was highly dependent on Cl− ion concentration. The limit of detection achieved was 1×10−8M using a 20×50μm2 gate sensing area.

AlGaN Channel High Electron Mobility Transistors: Device Performance and Power-Switching Figure of Merit

In this paper, AlGaN channels for high electron mobility transistors (HEMTs) have been evaluated based on a power device figure of merit. AlGaN-channel HEMTs grown on SiC substrates by plasma-assisted molecular beam epitaxy (PAMBE) were fabricated. Maximum saturation current of 0.55 A/mm was obtained at VGS=1 V. Current-gain cutoff ( ft) and power-gain cutoff ( fmax) frequencies obtained from small signal measurements were ft=13.2 GHz and fmax=41 GHz. Pulsed current–voltage (I–V) measurements at 200 ns showed no dispersion in I–V curves. Large signal continuous wave (CW) measurement yielded an output power density of 4.5 W/mm with power added efficiency (PAE) of 59% at 4 GHz. This work demonstrates the potential of AlGaN channel HEMTs for high voltage switching and microwave power applications.

A comparative study on single and double channel AlGaN/GaN high electron mobility transistor

AbstractWe present a numerical investigation on the DC and AC characteristics of AlGaN/GaN single (SCHEMT) and double channel high electron mobility transistors (DCHEMT) using the APSYS simulation program. The simulation results indicate that the DCHEMT has a slightly wider range of transconductance and cut‐off frequency for device operation at high current levels than SCHEMT. Besides, we also examine the effect of barrier layer thickness between the two channels in DCHEMT on the transconductance profile. It is found that the operation range of transconductance can be tailored by adjusting the barrier layer thickness. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced Electron Saturated Drift Velocity in AlGaAs/GaAs/AlGaAs Heterostructures

A new approach for reduction of scattering rate of electrons by polar optical phonons in the double barrier heterojunction quantum well is proposed. This approach is based on the phonon localization in narrow phonon wells. The enhancement of the electron saturated drift velocity in the Al0:2Ga0:8As=GaAs=Al0:2Ga0:8As high electron mobility transistor channel is envisaged theoretically and observed experimentally. The drift velocity in the channel in high electric flelds (E > 10 kV/cm) exceeded the maximal drift velocity in bulk GaAs (vmax = 10 7 cm/s) and achieved the value of 4 £ 10 7 cm/s.

Low-frequency noise properties of double channel AlGaN/GaN HEMTs

Low-frequency noise in MOCVD-grown AlGaN/GaN/AlGaN/GaN double channel high electron mobility transistors (HEMTs) on sapphire substrate was investigated over a wide range of temperatures from 80 K to 300 K. Generation–recombination (g–r) noise was observed arising from the traps with activation energies 140 meV, 188 meV and 201 meV. Hooge parameter was estimated to be 1.6 × 10 −3 at room temperature. Our work demonstrates good dc and low-frequency noise properties for the device.

Fabrication of InAs composite channel high electron mobility transistors by utilizing Ne-based atomic layer etching

High electron mobility transistors (HEMTs) with InAs∕InGaAs composite channel were fabricated by employing low damage, highly selective Ne-based atomic layer etching (ALET) for the dry gate recess process. Ne-based ALET exhibited very high etch selectivity of InP over InAlAs which is suitable for dry gate recess process removing InP etch stop layer on top of InAlAs Schottky barrier layer. The plasma induced damage on the exposed InAlAs surface due to the ALET was much lower than that due to the conventional Ar-based reactive ion etching (RIE), which was verified by atomic force microscopy and Hall measurements. For further comparison, dc characteristics were compared for the two types of 0.3μm HEMTs fabricated by utilizing ALET and conventional RIE during the dry gate recess processes.

Advanced modelling of GaN/AlGaN HEMT devices with field plate

AbstractIn this paper we carry out detailed study and numerical modelling of the operation of the GaN/AlGaN High Electron Mobility Transistor (HEMT) with Field Plate (FP) optimisation. The FP has the effect of smoothing the potential gradient along the HEMT channel near the gate edge on the drain side, resulting in the enhancement of the breakdown voltage. We show that the main effect of the FP is a substantial decrease in the longitudinal component of the field EL, along with little variation of the normal component ET. In addition to the main peak of the EL field at the gate edge, there is a secondary peak under the FP edge which grows rapidly with drain bias and eventually surpasses the first. The appearance of the second peak should be taken into account in optimising of the HEMT design. The I‐V characteristics of the FP‐gated device remain practically the same as those of a device without a FP. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Research of breakdown characteristic of InP composite channel HEMT

Density gradient quantum model has been used to analyse the breakdown characteristic of InP-based composite channel HEMT(high electron mobility transistor). The collision ionization in composite channel and quantum effect has been taken into consideration. We payed great attention to the relationship of on-state breakdown voltage with respect to the thickness of In0.7Ga0.3As channel, and promoted a method to enhance the on-state breakdown voltage. A commercial 2D-device simulation program. Sentaurus has been used to simulate the on-state breakdown voltage of the device. A comparison has been made between the result of simulated and measured data. The result shows that with the reduction of the thickness of In0.7Ga0.3As channel, on-state breakdown voltage of the device will be enhanced greatly without reducing saturation current, which has a significant meaning for promoting the power performance of InP-based HEMT.

S F 6 ∕ O 2 plasma effects on silicon nitride passivation of AlGaN∕GaN high electron mobility transistors

The effects of various plasma and wet chemical surface pretreatments on the electrical characteristics of AlGaN∕GaN high electron mobility transistors (HEMTs) passivated with plasma-deposited silicon nitride were investigated. The results of pulsed IV measurements show that samples exposed to various SF6∕O2 plasma treatments have markedly better rf dispersion characteristics compared to samples that were either untreated or treated in wet buffered oxide etch prior to encapsulation. The improvement in these characteristics correlates with the reduction of carbon on the semiconductor surface as measured with x-ray photoelectron spectroscopy. HEMT channel sheet resistance was also affected by varying silicon nitride deposition parameters.