Transient stress characterization of AlGaN/GaN HEMTs due to electrical and thermal effects

Transient Thermal Characterization of AlGaN/GaN HEMTs Under Pulsed Biasing

Abstract: The development of steady-state thermal characterization techniques for AlGaN/GaN high-electron mobility transistors (HEMTs) has been used to measure the device’s peak temperature under DC conditions. Despite these methods enabling the accurate quantification of the device’s effective thermal resistance and power density dependence, transient thermometry techniques are necessary to understand the nanoscale thermal transport within the active GaN layer where the highly localized joule heating occurs. One technique that has shown the ability to achieve this is transient thermoreflectance imaging (TTI). The accuracy of TTI is based on using the correct thermoreflectance coefficient. In the past, alternative techniques have been used to adjust the thermoreflectance coefficient to match the correct temperature rise in the device. This paper provides a new method to accurately determine the thermoreflectance coefficient of a given surface and is validated via an electrical method: gate resistance thermometry (GRT). Close agreement is shown between the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by GRT. Overall, TTI can now be used to thermally map GaN HEMTs under pulsed conditions providing simultaneously a submicrosecond temporal resolution and a submicrometer spatial resolution.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

Abstract: In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

Development of a Versatile Physics-Based Finite-Element Model of an AlGaN/GaN HEMT Capable of Accommodating Process and Epitaxy Variations and Calibrated Using Multiple DC Parameters (Postprint)

Abstract: : We present a physics-based finite-element model of operation of an AlGaN/GaN HEMT with device geometry inputs taken from transmission electron microscope cross sections and calibrated by comparison with measured electrical data comprising standard field-effect transistor metrics and less well-known model parameters. A variety of electrical outputs from the model are compared to experiment, and the level of agreement is reported.

The Influence of Microwave Pulse Width on the Thermal Burnout Effect of an LNA Constructed by the SiGe HBT

Abstract: This paper presents the influence of the pulse width on the thermal burnout effect of a low-noise amplifier (LNA) constructed by a silicon–germanium heterojunction bipolar transistor (SiGe HBT) when it is injected by microwave pulses. Based on the characteristics of microwave pulses and the structure of the SiGe HBT, a theoretical model to predict the impact of the pulse width and power on the thermal burnout effect of the LNA is established by solving the heat conduction equation. The derivation of the theoretical model requires the pulse width less than a microsecond level. Using at least two groups of simulated or measured results to fit the coefficients, the proposed theoretical model can predict the other effect of the pulse width, which can greatly reduce the experimental costs and guide the rational selection of the pulse width in numerical simulations. At last, the theoretical model is verified by numerical simulations and experimental measurements. The results show that within the scope of pulse width (less than the microsecond level) the burnout power threshold can be effectively reduced by increasing the pulse width.

Study on the electrical degradation of AlGaN/GaN MIS-HEMTs induced by residual stress of SiN x passivation

Abstract: In this paper, we report a new phenomenon in C-V measurement of different gate length MIS-HEMTs, which can be associated with traps character of the AlGaN/GaN interface. The analysis of DC measurement, frequency dependent capacitance-voltage measurements and simulation show that the stress from passivation layer may induce a decrease of drain output current I ds , an increase of on-resistance, serious nonlinearity of transconductance g m , and a new peak of C-V curve. The value of the peak is reduced to zero while the gate length and measure frequency are increasing to 21 μm and 1 MHz, respectively. By using conductance method, the SiN x /GaN interface traps with energy level of E C −0.42 eV to E C −0.45 eV and density of 3.2 × 10 12 ∼ 5.0 × 10 12 eV −1 cm −2 is obtained after passivation. According to the experimental and simulation results, formation of the acceptor-like traps with concentration of 3 × 10 11 cm −2 and energy level of E C −0.37 eV under the gate on AlGaN barrier side of AlGaN/GaN interface is the main reason for the degradation after the passivation.

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Abstract: Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

Uniaxial-process-induced strained-Si: extending the CMOS roadmap

Abstract: This paper reviews the history of strained-silicon and the adoption of uniaxial-process-induced strain in nearly all high-performance 90-, 65-, and 45-nm logic technologies to date. A more complete data set of n- and p-channel MOSFET piezoresistance and strain-altered gate tunneling is presented along with new insight into the physical mechanisms responsible for hole mobility enhancement. Strained-Si hole mobility data are analyzed using six band k/spl middot/p calculations for stresses of technological importance: uniaxial longitudinal compressive and biaxial stress on [001] and [110] wafers. The calculations and experimental data show that low in-plane and large out-of-plane conductivity effective masses and a high density of states in the top band are all important for large hole mobility enhancement. This work suggests longitudinal compressive stress on [001] or [110] wafers and channel direction offers the most favorable band structure for holes. The maximum Si inversion-layer hole mobility enhancement is estimated to be /spl sim/ 4 times higher for uniaxial stress on (100) wafer and /spl sim/ 2 times higher for biaxial stress on (100) wafer and for uniaxial stress on a [110] wafer.

Elastic constants of gallium nitride

Abstract: The elastic constants of GaN have been determined using Brillouin scattering; in GPa they are: C11=390, C33=398, C44=105, C66=123, C12=145, and C13=106. Our values differ substantially from those quoted in the literature which were obtained from the determination of mean square displacement of atoms measured by x‐ray diffraction.

Reliability of GaN High-Electron-Mobility Transistors: State of the Art and Perspectives

Abstract: Failure modes and mechanisms of AlGaN/GaN high-electron-mobility transistors are reviewed. Data from three de-accelerated tests are presented, which demonstrate a close correlation between failure modes and bias point. Maximum degradation was found in "semi-on" conditions, close to the maximum of hot-electron generation which was detected with the aid of electroluminescence (EL) measurements. This suggests a contribution of hot-electron effects to device degradation, at least at moderate drain bias (VDS 30-50 V), new failure mechanisms are triggered, which induce an increase of gate leakage current. The latter is possibly related with the inverse piezoelectric effect leading to defect generation due to strain relaxation, and/or to localized permanent breakdown of the AlGaN barrier layer. Results are compared with literature data throughout the text.

GaN HEMT reliability

Abstract: This paper reviews the experimental evidence behind a new failure mechanism recently identified in GaN high-electron mobility transistors subject to electrical stress. Under high voltage, it has been found that electrically active defects are generated in the AlGaN barrier or at its surface in the vicinity of the gate edge. These defects reduce the drain current, increase the parasitic resistance and provide a path for excess gate current. There is mounting evidence for the role of the inverse piezoelectric effect in introducing mechanical stress in the AlGaN barrier layer and eventually producing these defects. The key signature of this mechanism is a sudden and non-reversible increase in the gate leakage current of several orders of magnitude. This degradation mechanism is voltage driven and characterized by a critical voltage below which degradation does not occur. This hypothesis suggests several paths to enhance the electrical reliability of GaN HEMTs which are borne out by experiments.