Channel Temperature Determination in High-Power AlGaN/GaN HFETs Using Electrical Methods and Raman Spectroscopy

Self-heating in AlGaN/GaN HFETs was investigated using electrical analysis and micro-Raman thermography. Two typically employed electrical methods were assessed to provide a simple means of extracting average channel temperatures in devices. To quantify the accuracy of these electrical temperature measurements, micro-Raman thermography was used to provide submicron resolution temperature information in the source-drain opening of the devices. We find that electrical methods significantly underestimate peak channel temperatures, due to the fact that electrical techniques measure an average temperature over the entire active device area. These results show that, although electrical techniques can be used to provide qualitative comparisons between different devices, they have challenges for the accurate estimation of peak channel temperatures. This needs to be taken into account for lifetime testing and reliability studies based on electrical temperature measurements.     

New aspects and mechanism of kink effect in InAlAs/InGaAs/InPinverted HFETs

The kink effect in InAlAs/InGaAs/InP composite channel heterojunction field effect transistors (HFETs) was investigated as a function of temperature and optical excitation. Drain source and gate current measurements show that above 325 K the kink effect disappears while the impact ionization process is still present. The kink at low temperatures is suppressed by illumination with photons of energy above 1 eV. These results prove that this parasitic effect is mainly related to the presence of traps in the top layers     

超晶格光伏效应的温度特性

在18～300K 度范围内测量了 GaAs/AlGaAs 超晶格和 Ge_xSi_(1-x)/Si 应变层超晶格在不同温度下的光伏谱。在200K 以下,在 GaAs/AlGaAs 超晶格中观测到6个子带间光跃迁激子峰;在100K 以下,GaAs/AlGaAs 的光伏谱反映了超晶格台阶状态密度分布。在 Ge_xSi_1-x/Si 应变层超晶格中,观测到子带和连续带间的光跃迁。并对两类超晶格的光伏特性进行了比较分析。     

On the low-temperature static and dynamic properties ofhigh-performance silicon bipolar transistors

In a study performed over the temperature range of 400 to 77 K, Si bipolar transistors were found to have near-ideal characteristics at low temperatures with β as high as 80 at 77 K. Detailed calculations indicate that the conventional theory of the temperature dependence of β does not match the data. The discrepancy can be removed if it is assumed that a phenomenological thermal barrier to hole injection is present. Emitter-coupled logic (ECL) ring oscillators are functional at 85 K with no degradation in speed until about 165 K when compared to 358 K (85°C). Calculations using a delay figure of merit indicate that f_T, R_b, and C_c are the delay components most affected by low-temperature operation. The feasibility of reduced logic swing operation of bipolar circuits at low temperatures is examined. It is found that successful ECL circuit operation at reduced logic swings is possible provided emitter resistance is kept small and can be used to enhance low-temperature power-delay performance. These data suggest that conventionally designed high-performance bipolar devices are suitable for the low-temperature environment     

Improved device linearity of AlGaAs/InGaAs HFETs by a second mesaetching

The conventional mesa isolation process in AlGaAs/InGaAs heterostructure FETs results in the gate contacting the exposed highly doped region at the mesa sidewalls, forming a parasitic gate leakage path. In this work, we suppress the gate leakage from the mesa-sidewall and enhance microwave power performance by performing an additional second mesa etching. The device gate leakage characteristics under high-input power swing are particularly investigated to reveal an improvement in device linearity, which is sensitive to the sidewall gate leakage. This modified device (M-HFETs) provides not only a higher linear RF output power but also a lower IM3 product than those characteristics in conventional HFETs     

High-temperature performance of AlGaN/GaN HFETs and MOSHFETs

Performance of AlGaN/GaN HFETs and Al₂O₃/AlGaN/GaN MOSHFETs at the elevated temperatures up to 425 °C was investigated. Static output and transfer characteristics were measured and the saturation drain current, the peak transconductance and the series conductance as a function of temperature were evaluated. All these characteristic features of HFETs and MOSHFETs decreased with increased temperature. At 425 °C the devices exhibited ∼30% of their saturation drain current, peak transconductance and series conductance evaluated at room temperature. The device performance at elevated temperatures follows exactly the T^x dependence with a power x = −1.5. This indicates that the temperature dependence of the mobility of channel electrons due to phonon scattering is the predominant effect describing high-temperature performance of AlGaN/GaN HFETs and MOSHFETs.     

Low-temperature mobility behaviour in submicron MOSFETs and related determination of channel length and series resistance

The electron mobility behaviour in submicron MOSFETs is studied in the temperature range of 77–300 K. As the effective channel length is reduced, the effective mobility as well as the field-effect mobility are found to decrease and to become less temperature dependent. These experimental results are explained by the influence of series resistance and effective channel length, which are both temperature dependent. The possibility of accurate determination of series resistance and “pure” mobility is demonstrated. A new method is proposed to determine submicron MOSFET channel length at low temperatures.     

High-performance quantum cascade lasers with electric-field-freeundoped superlattice

An optimized design of quantum cascade lasers with electric field free undoped superlattice active regions is presented. In these structures the superlattice is engineered so that: (1) the first two extended states of the upper miniband are separated by an optical phonon to avoid phonon bottleneck effects and concentrate the injected electron density in the lower state and (2) the oscillator strength of the laser transition is maximized. The injectors' doping profile is also optimized by concentrating the doping in a single quantum well to reduce the electron density in the active material. These design changes result in major improvements of the pulse/continuous-wave performance such as a weak temperature dependence of threshold (T₀=167 K), high peak powers (100-200 mW at 300 K) and higher CW operating temperatures for devices emitting around at λ~8.5 μm     

Integration of a superlattice limiter into the HBT emitter forimproved operational reliability

A common problem limiting the output power of multiple finger heterojunction bipolar transistors (HBT's) is nonuniform current flow in the fingers, resulting from an underlying nonuniform temperature distribution. We have fabricated HBT devices containing an integrated superlattice region to help overcome this problem. We demonstrate that the superlattice functions as a temperature-dependent resistive current limiter in a single finger device at dc and RF. Furthermore, the RF performance of the HBT was not compromised by the inclusion of the superlattice structure     

SiC and GaN transistors - is there one winner for microwave power applications?

Wide bandgap semiconductors show promise for high-power microwave electronic devices. Primarily due to low breakdown voltage, it has not been possible to design and fabricate solid-state transistors that can yield radio-frequency (RF) output power on the order of hundreds to thousands of watts. This has severely limited their use in power applications. Recent improvements in the growth of wide bandgap semiconductor materials, such as SiC and the GaN-based alloys, provide the opportunity to now design and fabricate microwave transistors that demonstrate performance previously available only from microwave tubes. The most promising electronic devices for fabrication in wide bandgap semiconductors for these applications are metal-semiconductor field-effect transistors (MESFETs) fabricated from the 4H-SiC polytype and heterojunction field-effect transistors (HFETs) fabricated using the AlGaN/GaN heterojunction. These devices can provide RF output power on the order of 5-6 W/mm and 10-12 W/mm of gate periphery, respectively. 4H-SiC MESFETs should produce useful performance at least through X band and AlGaN/GaN HFETs should produce useful performance well into the millimeter-wave region, and potentially as high as 100 GHz.     

Dynamic current-voltage characteristics of III-N HFETs

A comparative study of the dynamic current-voltage (DI-V) characteristics of III-N heterojunction and double heterojunction field-effect transistors (HFETs and DHFETs) reveals that the current and RF power collapse in HFETs arise from modulation of device series resistances under large input signal. A model based on space-charge limited current through the depletion regions formed at the gate edges due to the charge trapping explains the DI-V behavior and other observations related to the RF current collapse in III-N HFETs.     

Low-power high-speed operation of submicron InP-InGaAs SHBTs at 1 mA

Scaling of submicron InP-InGaAs HBTs is investigated for low-power high-speed applications in mixed signal circuits. Device performance for transistors fabricated with a 0.5-/spl mu/m emitter width and varying emitter lengths are studied. The 0.5 /spl mu/m/spl times/2 /spl mu/m devices yielded excellent low-current RF performance, with an f/sub T/=173 GHz and an f/sub MAX/=187 GHz at 1 mA, the highest values reported for InP-based devices to date.     

Impact of twin-gate concept on silicon-on-insulator inverters

The low-temperature DC transfer characteristics of inverters, fabricated in a 1 μm silicon-on-insulator (SOI) CMOS technology, are investigated. As will be shown, the operation parameters, such as the noise margin, are degraded by the floating body effects, typical for a partially depleted technology and by low-temperature artefacts. However, by using the so-called twin-gate concept, considerable improvement in the inverter performance can be obtained, both at room temperature and at 4.2 K     

Mechanism of current collapse removal in field-plated nitride HFETs

An experimental study of the mechanism of RF current collapse removal in high-power nitride-based HFETs is presented. The results show that the conductivity of the dielectric material under the field plate plays a crucial role in the current collapse removal. Identical geometry field plated HFETs differing only in the FP dielectric conductivity show varying degree of current collapse removal. Devices with semiconducting dielectric layers exhibit perfectly linear RF power - drain bias dependence with the output powers of 20 W/mm at 55 V drain bias with essentially no current collapse. A trapped charge discharging model is presented to explain the removal of current collapse in FPd devices.     

Impact of well coupling on the spontaneous emission properties ofGaAs/AlGaAs multiple-quantum-well structures

The impact of well coupling on the emission spectra of multiquantum-well structures is discussed. Luminescence experiments are performed in the temperature range between 1.5 K and room temperature and at various excitation densities. High-excitation room temperature results are used for the calculation of gain profiles. With increasing coupling strength a transition from two-dimensional to three-dimensional behavior of the charge carriers is observed. In particular the two-dimensional gap is lowered, the light-hole-heavy-hole splitting is reduced, the influence of interface roughness on the line shapes is reduced, excitons cease to dominate the room-temperature luminescence, and the low-temperature recombination process switches from a non-k -conserving to a k-conserving one. Some of the fundamental advantages of quantum-well lasers, such as the improved TE/TM mode selection, the small spontaneous-to-stimulated emission ratio, and the tendency towards single-longitudinal-mode operation, are gradually lost. A detailed theory of electronic states in superlattices and of superlattice emission line shapes quantitatively explains these results     

The Kink Effect at Cryogenic Temperatures in Deep Submicron AlGaN/GaN HEMTs

The kink effect has been studied in deep submicron AlGaN/GaN high-electron mobility transistors by measuring their DC, RF and pulsed performance at cryogenic temperatures. In these devices, the kink effect is mainly due to traps: it appears at $T ≪ hbox{260} hbox{K}$ and can be removed either by applying UV light or biasing the gate with short pulses. Its appearance is related to the fluorine-based treatment ($hbox{CF}_{4}/hbox{O}_{2}$ plasma) used for etching the passivant, treatment which creates traps below and around the gate. This link between the kink and the etching treatment has also been confirmed in optically defined gate devices with different fluorine plasma exposure times.      

Gate current model for the hot-electron regime of operation inheterostructure field effect transistors

A model to describe the dependence of the gate current with source-to-drain voltage was developed and used to predict the performance of AlGaAs/InGaAs/GaAs HFETs. Our model describes the charge injection transistor (CHINT) regime of operation and account for real-space electron transport. In this model, the saturation of the hot-electron gate current is explained by the rapid drop in the energy relaxation time caused by the real-space transfer of electrons. Good correlation between the experimental and theoretical data was found for temperatures ranging from 198 to 398 K. Our experimental and theoretical results should be accounted for in the design of HFET devices and integrated circuits     

Thermal modeling and measurement of GaN-based HFET devices

In this letter, we present our thermal study results of GaN-based heterojunction field effect transistors (HFETs). In thermal computation, PAMICE code was used to calculate temperatures in a three-dimension (3-D) model. In the thermal measurement, nematic liquid crystal thermography was employed to determine the peak temperature on the surface of the device chip. The calculated and directly measured temperatures agree well. These methods are valuable in predicting the thermal performance of GaN-based HFET devices, in particular the power devices.     

Source resistance reduction of AlGaN-GaN HFETs with novel superlattice cap layer

We have developed a novel AlGaN-GaN heterojunction field effect transistor (HFET) with an ultralow source resistance by employing the novel superlattice (SL) cap structure. The particular advantage of the SL cap, i.e., the existence of multiple layers of the polarization-induced two-dimensional electron gas (2DEG) with high mobility and high concentration at each AlGaN-GaN interface, is fully exploited for lowering the lateral resistance and the potential barrier at the interface of the SL cap and the HFET barrier layer. By designing the AlGaN-GaN thickness ratio, we have established a method to obtain the optimized SL structure and have achieved an extremely low source resistance of 0.4 /spl Omega//spl middot/mm which is lower not only than HFETs with the conventional structure but also than those with the n-GaN cap structure. The SL cap HFET fabricated on a sapphire substrate exhibited excellent dc and RF performance, i.e., maximum transconductance of over 400 mS/mm, maximum drain current of 1.2 A/mm, a cutoff frequency of 60 GHz, a maximum frequency of oscillation of 140 GHz, and a very low noise figure minimum of 0.7 dB at 12 GHz.     

Flicker noise of GaN-based heterostructure field-effect transistors with Si-doped AlGaN carrier injection layer

Aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure field effect transistors (HFETs) with and without Si-doped AlGaN layer were fabricated and investigated. HFETs with the Si-doped AlGaN carrier-injection layer show better DC performance, and the transconductance is 150 mS/mm. However, the HFETs with Si-doped AlGaN layer present the deviation from the 1/f noise at low frequency. The Lorentz shape was observed in the noise spectrum. It suggests that traps might be more pronounced in this kind of structure. Therefore, the DC characteristics of HFETs can be improved by the insertion of Si-doped AlGaN layer, but it can result in more low-frequency noise with the carrier-injection layer.