Transconductance Dispersion Research Articles

A nonlinear model for frequency dispersion and DC intrinsic parameter extraction for GaN-based HEMT

We propose in this study a practical nonlinear model for the AlGaN/GaN high electron mobility transistors (HEMTs) to extract DC intrinsic transconductance (gmDC), output conductance (gdsDC), and electron mobility from the intrinsic parameter set measured at high frequencies. An excellent agreement in I–V characteristics of the model with a fitting error of 0.11% enables us successfully extract the gmDC, gdsDC, and the total transconductance dispersion. For this model, we also present a reliable analysis scheme wherein the frequency dispersion effect due regional surface states in AlGaN/GaN HEMTs is taken into account under various bias conditions.

Total Ionizing Dose (TID) Effects in GaAs MOSFETs With La-Based Epitaxial Gate Dielectrics

Epitaxially grown La-based oxide has shown promise as a gate dielectric for GaAs substrate materials with a low interface trap density in the mid to low $10^{11}$ cm $^{\mathrm {-2}}$ eV $^{\mathrm {-1}}$ range. Total ionizing dose (TID) effects have been studied on GaAs MOSFETs with Al2O3/La2O3 and Al2O3/La1.8Y0.2O3 gate oxides. Charge trapping mechanisms in GaAs MOSFETs are studied by the AC transconductance dispersion method. Al2O3/La2O3 gated devices show a combination of electron and hole trapping, whereas Al2O3/La1.8Y0.2O3 gated devices show primarily hole trapping.

101-GHz InAlN/GaN HEMTs on Silicon With High Johnson’s Figure-of-Merit

In this brief, the InAlN/GaN high-electron mobility transistors (HEMTs) on silicon substrate with high Johnson’s figure-of-merit (J-FOM) are presented. A trilayer photoresist of polymethylmethacrylate (PMMA)/copolymer/PMMA associated with a T-shaped gate is used to reduce the parasitic resistance while maintaining high current gain cutoff frequency. The small dc-to-RF transconductance dispersion of only 1.1% suggests a good quality SiN x passivation layer, and the $f_{\rm MAX}$ of 101 GHz and $f_{T}$ of 60 GHz can be simultaneously obtained with a 0.11- $\mu \text{m}$ foot length and 1.5- $\mu \text{m}$ source-drain distance. In addition, the three-terminal OFF-state breakdown measurements reveal a source-drain breakdown voltage (BV $_{\rm DS})$ of 21 V ( $V_{\rm DG}=31$ V). The results lead to a high J-FOM of 1.3 THz $\,\cdot \,\text{V}$ , which has not been reported for the InAlN/GaN HEMTs on silicon substrate.

A complete and accurate surface-potential based large-signal model for compound semiconductor HEMTs

A complete and accurate surface potential based large-signal model for compound semiconductor HEMTs is presented. A surface potential equation resembling the one used in conventional MOSFET models is achieved. The analytic solutions from the traditional surface potential theory that developed in MOSFET models are inherited. For core model derivation, a novel method is used to realize a direct application of the standard surface potential model of MOSFETs for HEMT modeling, without breaking the mathematic structure. The high-order derivatives of I—V/C—V remain continuous, making the model suitable for RF large-signal applications. Furthermore, the self-heating effects and the transconductance dispersion are also modelled. The model has been verified through comparison with measured DC IV, Gummel symmetry test, CV, minimum noise figure, small-signal S - parameters up to 66 GHz and single-tone input power sweep at 29 GHz for a 4 × 75 μm × 0.1 μm InGaAs/GaAs power pHEMT, fabricated at a commercial foundry.

Border Traps in Ge/III–V Channel Devices: Analysis and Reliability Aspects

The aim of this review paper is to describe the impact of so-called border traps (BTs) in high- k gate oxides on the operation and reliability of high-mobility channel transistors. First, a brief summary of the physics of BTs will be given, describing the charge trapping and release in terms of the elastic tunneling model. It will be also pointed out how information on the BT properties can be extracted from popular measurement techniques such as low-frequency (1/f) noise and variable-frequency charge pumping. In the next two parts, the impact of BTs on metal-oxide-semiconductor structures fabricated on Ge or III-V channel materials is outlined, with particular emphasis on the development of novel or adapted measurement techniques such as AC transconductance dispersion or trap spectroscopy by charge injection and sensing. Finally, the effect of BTs on the operation and reliability of high-mobility channel MOSFETs is discussed. It is also shown that the density of BTs is closely linked to the quality or defectivity of the high- k gate stack, indicating room for improvement by optimization of processing or by implementation of a suitable bulk-oxide defect passivation step.

Iron-induced deep-level acceptor center in GaN/AlGaN high electron mobility transistors: Energy level and cross section

Dynamic transconductance dispersion measurements coupled with device physics simulations were used to study the deep level acceptor center in iron-doped AlGaN/GaN high electron mobility transistors (HEMT). From the extracted frequency dependent trap-conductance, an energy level 0.7 eV below the conduction band and a capture cross section of 10−13 cm2 were obtained. The approach presented in this work avoids the non-equilibrium electrical or optical techniques that have been used to date and extracts the device relevant trap characteristics in short channel AlGaN/GaN HEMTs. Quantitative prediction of the trap induced transconductance dispersion in HEMTs is demonstrated.

AC Transconductance Dispersion (ACGD): A Method to Profile Oxide Traps in MOSFETs Without Body Contact

We introduce an ac transconductance dispersion method (ACGD) to profile the oxide traps in an MOSFET without needing a body contact. The method extracts the spatial distribution of oxide traps from the frequency dependence of transconductance, which is attributed to charge trapping as modulated by an ac gate voltage. The results from this method have been verified by the use of the multifrequency charge pumping (MFCP) technique. In fact, this method complements the MFCP technique in terms of the trap depth that each method is capable of probing. We will demonstrate the method with InP passivated InGaAs substrates, along with electrically stressed Si N-MOSFETs.

The origin of low-frequency negative transconductance dispersion in a pseudomorphic HEMT

Measurements of low-frequency transconductance dispersion at different temperatures and conductance deep level transient spectroscopic (CDLTS) studies of an AlGaAs/InGaAs pseudomorphic high electron mobility transistor (p-HEMT) were carried out. The experimental results show the presence of defect states at the AlGaAs/InGaAs hetero-interface. A mobility degradation model was developed to explain the low-frequency negative transconductance dispersion as well as the apparent ‘hole’ like peaks observed in the CDLTS spectra. This model incorporates a time-dependent change in the two-dimensional electron gas mobility due to ionized impurity scattering by the remaining charge states at the adjoining AlGaAs/InGaAs hetero-interface.

Characterization of the frequency dispersion of transconductance and drain conductance of GaAs MESFET

Frequency dispersions of the transconductance and the drain conductance of ion-implanted gallium arsenide (GaAs) metal-semiconductor field-effect transistors (MESFETs) are measured and analyzed. In the linear region of the MESFET (low drain voltage), a positive transconductance dispersion is observed, which is caused by the deep-level traps at the surface between the source and the gate. In the saturation region (high drain voltage), however, a negative transconductance dispersion becomes dominant. The drain conductance does not show a dispersion in the linear region, while a distinct positive dispersion is observed in the saturation region with the same activation energy as the negative transconductance dispersion. The difference of the dispersion activation energy of the MESFET with and without the p-buried layer beneath the channel indicates that the negative transconductance and the drain conductance dispersion are caused by the deep-level traps at the channel-substrate interface. Because there exists the high electric field at the drain edge of the gate and an electron accumulation layer is formed, the potential in the channel becomes lower when the drain current is larger with high gate voltage. The emission of electrons from electron traps with lower potential is the cause of the negative frequency dispersion.

Transconductance frequency dispersion measurements on fully implanted 4H-SiC MESFETs

Transconductance frequency dispersion measurements were performed on fully implanted MESFETs made on bulk semi-insulating 4H-SiC to observe bulk, as well as surface, traps in the channel region. A 30% reduction in the transconductance was observed by increasing the frequencies from 0.1 Hz to 10 MHz. Five traps were recorded in this study. Bulk traps observed at 0.52 and 0.65 eV in the gated channel region, which were also seen in the capacitance-DLTS study, are believed to be due to the nitrogen implant-defect complex and vanadium deep donor dopant, respectively. Surface traps of the ungated channel region between the source and gate and gate and drain were observed at 0.18, 0.3 and 0.4 eV. The trap at 0.4 eV has shown an upward transconductance dispersion behavior with increasing frequency, whereas all the remaining four traps have shown a downward frequency dispersion behavior. The origins of the surface traps are not known at this time.

Low frequency effects of surface states on 4H-SiC metal-semiconductor field effect transistor

The process-related surface state effect is investigated of the fabrication of SiC devices and a nonlinear model for 4H-SiC power metal-semiconductor field effect transistor (MESFET) is proposed, which takes into account the surface-related parameters. The frequency- and temperature- dependent transconductance dispersion is readily demonstrated in terms of the improved model. Simulation results show that larger dispersion and higher transition frequency occur in 4H-SiC MESFET than in GaAs MESFET. The advantage of this analytical model over the two-dimensional numerical simulation is the simplicity of calculations, therefore it is suitable for the processing improvement of SiC devices.

Effects of hot-electron stress on electrical performances in AlGaAs/InGaAs pseudomorphic high electron transistors

The effects of hot-electron stress on electrical properties in AlGaAs/InGaAs pseudomorphic high electron mobility transistors were investigated using current-voltage (I–V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), and transconductance dispersion measurements. After hot-electron stress, the three-terminal gate-drain breakdown characteristics were improved and the gate-drain capacitance was decreased even though no difference was found in both DLTS and transconductance dispersion results. These results suggest that hot electrons were trapped at the interface of the passivation layer, Si3N4, with AlGaAs, locating between gate and source/drain electrodes, leading to the increase of the depletion region under the ungated region. On the other hand, the two-terminal gate-drain breakdown characteristics were deteriorated by hot-electron stress. This was due to the reduction of the Schottky barrier height.

Interpretation of transconductance dispersion in GaAs MESFET using deep level transient spectroscopy

The negative transconductance dispersion in a GaAs metal-semiconductor field-effect transistor (MESFET) was interpreted using both surface leakage current and capacitance deep level transient spectroscopy (DLTS) measurements. The transconductance of the device was reduced by 10% in the frequency range of 10 Hz /spl sim/1 kHz. The transition frequency shifted to higher frequency region with the increase of device temperature. The activation energy for the change of the transition frequency was determined to be 0.66/spl plusmn/0.02 eV. It was found that the activation energy for the conductance of electrons on the surface of GaAs was 0.63/spl plusmn/0.01 eV. In the DLTS spectra, two types of hole-like signals with activation energies, 0.65/spl plusmn/0.07 eV (H1) and 0.88/spl plusmn/0.04 eV (H2), were observed. The activation energy of H1 trap agrees well with those obtained from the transconductance dispersion and surface leakage current measurements. This demonstrates that surface state H1 causes the generation of surface leakage current, leading to the transconductance dispersion in the MESFET. Using the experimental results, a model for the evolution of hole-like signal by surface states in the capacitance DLTS is proposed.

Effects of deep levels on transconductance dispersion in AlGaAs/InGaAs pseudomorphic high electron mobility transistor

The effects of deep levels on the transconductance dispersion in an AlGaAs/InGaAs pseudomorphic high electron mobility transistor was interpreted using capacitance deep level transient spectroscopy (DLTS). Transconductance was decreased by 10% in the frequency range of 10 Hz–10 kHz at the negative gate bias, but it was increased at the positive one. In the DLTS spectra, two hole trap-like signals corresponding to surface states were only observed at the negative pulse bias, whereas the DX-center with the activation energy of 0.42±0.01 eV were observed at the positive one. The activation energy agrees well with that obtained from the temperature dependence of the positive transconductance dispersion, 0.39±0.03 eV. These provide evidence that the positive and negative transconductance dispersions are due to the DX center and surface states, respectively.

Correlation between fluctuating conductivity of the channel, transconductance dispersion and noise of a MESFET

Theoretical models of the noise spectra of MESFETs have been researched based on experimental data with the aim of obtaining a correlation with the fluctuating conductivity and transconductance dispersion in terms of frequency, bias, temperature and surface state. The results obtained correspond well with the experimental data. Moreover, we show the influence of the transconductance dispersion on the surface state and some sources of 1/f noise.

Experimental evidence of surface conduction contributing to transconductance dispersion in GaAs MESFETs

Low-frequency transconductance dispersion in GaAs metal semiconductor field effect transistors (MESFET's) is commonly attributed to the presence of a high density of surface states under the passivated source-gate and gate-drain separation regions. In this paper, we have found that they also contribute to a temperature dependent surface reverse leakage current with a thermal activation energy of 0.43 eV and a surface electron concentration (2.5/spl times/10/sup 12/ cm/sup 2/). Conductance deep-level transient spectroscopy (DLTS) spectra of these MESFETs have shown apparent hole-like peaks with emission energy of 0.48 eV and capture activation energy of 0.05 eV. These data were used for a model-based simulation and the results were compared with those obtained from experimental transconductance versus temperature measurements performed at various frequencies. A close agreement between these provides conclusive proof that the surface conduction at the GaAs-passivant interface is the major cause of the low-frequency transconductance dispersion observed. Finally, a possible explanation for the characteristic activation energy of 0.43 eV for Si/sub 3/N/sub 4/ passivant film on GaAs has been presented.

Low-frequency transconductance dispersion in InAlAs/InGaAs/InP HEMT's with single- and double-recessed gate structures

The effects of trapping mechanisms on the transconductance of single- and double-recessed InAlAs/InGaAs/InP HEMT's are examined. Measurements at room temperature indicate transconductance dispersion occurring primarily between 100 Hz and 1 MHz. A detailed examination of the dispersion yields two mechanisms with different activation energies which were determined by measuring the transition frequencies as functions of temperature. One mechanism, causing negative dispersion, has an activation energy of 0.17 eV and was found only in the double-recessed structure. The other mechanism, causing positive dispersion and common to both structures, has a dominant transition with an activation energy of 0.51 eV at low fields. The first mechanism appears to be associated with surface states, while the second is caused by electron traps in the InAlAs or its interface with the InGaAs channel. Transient response measurements were also used to examine the location of the traps and to study the field dependence of the characteristic times.

Evidence of interface trap creation by hot-electrons in AlGaAs/GaAs high electron mobility transistors

We report on the hot-electrons induced degradation in AlGaAs/GaAs high electron mobility transistors (HEMTs), consisting of a decrease in the drain current and an increase in the parasitic drain resistance. The amount of the degradation is proportional to the impact-ionization rate which is related to the electron energy. Transconductance dispersion measurements and drain current deep level transient spectroscopy (DLTS) have been used to identify interface traps which are located at the AlGaAs/GaAs interface in the gate-drain access region and are the causes of the observed degradation.

Numerical analysis of frequency dispersion of transconductance in GaAs MESFETs

A fully two-dimensional numerical model for the transconductance dispersion in GaAs MESFETs is presented. According to simulated results, the dominating surface traps belong to the hole trap type in order to obtain consistent results with reported measurements. The AC frequency-dependent modulation of negative surface charge can explain this anomalous phenomenon. The holes injecting from and emitting out of the gate metal electrode interact with the surface hole traps, and result in the change of the gate-to-source and the gate-to-drain resistances, which in turn cause the change in transconductance. The gate voltage and the gate length effects on the dispersion are also considered. Good agreement with reported results is obtained.

Analysis of Gate Lag in GaAs Metal-Semiconductor Field-Effect Transistor Using Light Illumination

We have developed a novel method for analyzing the gate lag effect in gallium arsenide (GaAs) metal-semiconductor field-effect transistor (MESFET) using light illumination. It is estimated that the density of trapped electrons at the surface of an active channel layer is above 6×1011 cm-2 from the dependence on photon flux density. Photon energy dependence shows that the electrons are mainly trapped at the GaAs surface. Angle-resolved analysis indicates that the trapped electrons at the active channel layer between the gate and the drain mostly account for the gate lag effect. Temperature dependence of the transconductance (g m) dispersion shows that the activation energy of this trap is 0.33 eV. Two-dimensional device simulation demonstrates the similar transient characteristics of the drain current, which originates from the electrons trapped at the GaAs surface.