Gate breakdown in MESFETs and HEMTs

A new model for gate breakdown in MESFETs and HEMTs is presented. The model is based upon a combination of thermally assisted tunneling and avalanche breakdown. When thermal effects are considered it is demonstrated that the model predicts increasing drain-source breakdown as the gate electrode is biased towards pinch-off, in agreement with experimental data. The model also predicts the gate current versus bias behavior observed in experimental data. The model is consistent with various reports of breakdown and light emission phenomena reported in the literature     

High-performance InAlAs/InGaAs HEMTs and MESFETs

The fabrication and microwave performance of heterostructure InAlAs/InGaAs HEMTs (high-electron-mobility transistors) and MESFETs are described. Maximum stable gains of 14.3 dB for a HEMT and 12 dB for a MESFET at 26.5 GHz have been achieved. These are believed to be record gains for FETs having gates as long as 0.7 μm     

Improved noise model for MESFETs and HEMTs in lower gigahertz frequency range

A noise model based on an equivalent circuit is applied to an HEMT. Besides the frequency dependence of the most important noise parameters (F/sub min/, R/sub opt/, X/sub opt/, R/sub N/) two apparent experimental facts are explained: the limited R/sub opt/, X/sub opt/ and the increase of R/sub N/ with decreasing frequency.<>     

Cryogenic noise performance of HEMTs and MESFETs between 300 and700 MHz

The authors present the noise performance of amplifiers using HEMTs and MESFETs at room temperature and cryogenic temperatures, in the frequency range 300-700 MHz. Results demonstrate that these microwave devices can be applied at frequencies down to at least 300 MHz, giving amplifier noise temperatures below 2 K at 20 K ambient temperature     

DC and pulsed measurements of on-state breakdown voltage in GaAs MESFETs and InP-based HEMTs

The BV_DS–I_D breakdown characteristics of MESFET and HEMT devices measured at constant gate current are correlated with conventional measurements of gate current due to impact-ionization. The influence of thermal effects on breakdown DC measurements is demonstrated. By adopting pulsed measurements, we confirm that on-state breakdown voltage of InP HEMTs decreases by increasing the temperature, while the opposite occurs in GaAs based MESFETs and HEMTs. We show that DC measurements are not suitable for evaluating on-state breakdown of power MESFETs and HEMTs, and we propose pulsed measurements as a viable alternative.     

Light emission in AlGaAs/GaAs HEMTs and GaAs MESFETs induced by hotcarriers

Light emission in submicrometer gate AlGaAs/GaAs HEMTs and GaAs MESFETs has been observed at high drain bias values (>4.0 V). The spectral distribution of the emitted photons in the 1.7-2.9-eV range does not correspond to a simple Maxwellian distribution function of the electron energies in the channel. Light emission is observed in correspondence with nonnegligible gate and substrate hole currents, due to the collection of holes generated by impact ionization     

Pulsed measurements and circuit modeling of weak and strong avalanche effects in GaAs MESFETs and HEMTs

An extensive characterization of the on-state breakdown characteristics of GaAs based MESFETs and HEMTs has been carried out by means of DC and pulsed measurements and of circuit simulations. A computer-controlled, three-terminal Transmission Line Pulse (TLP) system with 50-100 ns pulse width and sub-ns risetime has been developed, which allows automated pulsed measurements of device I-V characteristics. The TLP system has been adopted for nondestructive measurements of the on-state breakdown characteristics of GaAs MESFETs and HEMTs up to unprecedented values of gate current density (I/sub G//W=30 mA/mm has been reached), in strong avalanche conditions. The device behavior in strong avalanche conditions is dominated by a parasitic bipolar effect (PBE) similarly to SOI and bulk Si MOSFETs. By taking into account this and other parasitic effects, an equivalent circuit model, suitable for SPICE simulations has been developed. The proposed model is capable of predicting the exact behavior of the gate and drain currents in both weak and strong avalanche conditions.     

Q and V band power InGaAs MESFETs

Excellent power performance at 44 and 60 GHz demonstrated by 0.25 mu m gate InGaAs MESFETs is reported. From small-signal S-parameter measurements, these devices typically have extrinsic f/sub t/ greater than 100 GHz. At 1 dB compression point, a 150 mu m wide MESFET shows a power added efficiency of 33% with an output power of 60.8 mW (0.43 W/mm) at 44 GHz. For 200 mu m wide devices, the power added efficiency was measured to be 19% with an output power of 82 mW (0.41 W/mm) at 60 GHz. These results indicate that InGaAs MESFETs are viable millimetre-wave power devices.<>     

Symptoms of stress-induced gain degradation in power MESFETs

GaAs metal-semiconductor field effect transistors configured as microwave power amplifiers have been observed to degrade under normal device operations at high gate-to-drain fields. The nature of this degradation is an increase in the gate current, with a subsequent decrease in the gain. We present evidence that crystallographic defects in the active region are responsible for this “power slump” and that these defects originate during device operation due to the high strain fields which exist as the result of passivation layer processing. Strain data and x-ray topographic images support our assertion that passivation layer processing induces high strain in and around the gate-to-drain region of the device. Topographic images show that an increase in dislocation density occurs in the highly stressed regions after power slump. By varying deposition parameters, we can produce passivation films, which induce less stress in the active region, resulting in less dislocation generation and a less severe power slump.     

High-temperature characteristics of 2-D MESFETs

Experimental data of 2-D MESFETs, which utilize sidewall Schottky contacts to degenerate two-dimensional electron gas, indicate a much weaker temperature dependence of the drain current compared to conventional MESFETs in the temperature range from 25-150°C. Measured drain current characteristics show that the 2-D MESFET structure exhibits negligible threshold voltage shift with temperature in this temperature range. The negligible threshold voltage shift can be explained in terms of a nearly temperature independent built-in voltage related to the degeneracy of the two-dimensional electron gas. Furthermore, the low-field mobility extracted from the measured transconductance exhibits a smaller degradation with increasing temperature compared to conventional MESFETs. For our devices, the mobility drops by approximately 25% over the temperature range 25-125°C, compared to 40-50% for conventional MESFETs. The smaller temperature variations of the low-field mobility are linked to a more effective screening of impurity scattering by the two-dimensional electron gas     

Frequency-dependent electrical characteristics of GaAs MESFETs

Output resistance and transconductance of GaAs MESFETs have been observed to change significantly at very low frequencies. Extensive measurements of these characteristics as a function of device bias are reported. Direct measurements of the dispersive behavior between DC and 100 kHz and over a broad temperature range have been made on ion-implanted monolithic microwave IC (MMIC) devices. Conductance deep level transient spectroscopy (DLTS) and microwave S-parameter measurements have also been made to investigate this behavior. These measurements reveal that surface or channel-substrate interface traps in the material are most likely to be responsible for the observed behavior. A new equivalent-circuit model which accounts for many of the observed characteristics is developed. Unlike previously proposed equivalent circuits, the model does not rely on physically unrealistic circuit element values in order to obtain accurate performance predictions. The bias dependence of circuit element values is computed for one device. Effects not described by the model are also discussed     

Degradation mechanisms induced by temperature in power MESFETs

Two independent failure mechanisms, mainly induced by temperature, were observed in unpassivated, Au-metallised, commercially available power MESFETs. They result in a high gate/source (drain) leakage and in an increase in the open channel resistance, and may explain the `burn-out? of the devices in practical applications.     

Optically-controlled switching characteristics of silicon MESFETs

The switching time of a silicon MESFET device can be controlled by photons incident on the transparent or semitransparent gate, which may be treated as the virtual gate in addition to usual gate. Studies have been made on the optically-controlled switching characteristics of the Silicon MESFET which show that the internal gate-source capacitance increases with increasing radiation flux density under normally OFF conditions and decreases under normally ON conditions. Further, the drain-to-source resistance is found to be reduced with increasing radiation flux density at a particular value of absorption coefficient (or wavelength of radiation). The effect of radiation becomes predominant over the impurity concentration at flux-density ≥ 10¹⁸/m². Also it is observed that RC time constant decreases initially with increasing radiation up to 10¹⁸/m². At a radiation intensity equal to 10¹⁹/m², the RC time constant gradually decreases with increased doping level.     

Gold-based gate-sinking enhanced by inhomogeneities in power MESFETs

Gate contact degradation of power MESFETs with gold-based gate metallisations is enhanced by local inhomogeneities, such as thermal gradients and border effects.     

Thermally induced parasitic ungated FET in power MESFETs

The onset of a parasitic FET between the gate and the source (drain) has been observed by monitoring the reverse gate diode I−V characteristics of power GaAs MESFETs, subjected to accelerated aging tests. Such anomalous characteristics are attributed to localized microreactions at the gate metal-GaAs interface, which can be preferential sites for the occurrence of burn-out phenomena.     

35 GHz GaAs power MESFETs and monolithic amplifiers

GaAs MESFETs optimized for power operation at 35 GHz are described. Various doping levels and potential barrier layers at the interface between the buffer and the active layers were studied. The best power performance was obtained from an FET on a very heavily doped active layer. A device on an AlGaAs heterobuffer had further improved output power. The best devices delivered output power densities of 0.8 W/mm with 23% efficiency, 0.71 W/mm with 34% efficiency, and 0.61 W/mm with 41% efficiency. Monolithic power amplifiers with a 400-μm FET generated 200 mW of output power. These amplifiers were monolithically power combined, resulting in 600 mW of output power at 34 GHz     

Modelling DC characteristics of HEMTs

A simple empirical equation is presented to relate the drain current to drain-source and gate-source voltages of the high electron mobility transistors (HEMTs). This equation covers the entire ID/VDS characteristics. The accuracy of this equation is better than the theoretically developed separate equations.     

Monolithic integration of pseudomorphic power and low-noise HEMTs

Monolithic integration of pseudomorphic power and low-noise high electron mobility transistors (HEMTs) on a GaAs substrate has been demonstrated using specially designed multiple epitaxial layers. MBE-grown layers with three heterojunctions were selectively recessed to provide optimum structures for low-noise and power operations. At 18 GHz, a record power-added efficiency of 59% with 8 dB gain and 0.4 W/mm power density was obtained for the power HEMT. The low-noise device from the same slice achieved a noise figure of 1 dB with 9 dB associated gain at the same frequency.<>     

On-state breakdown in power HEMTs: measurements and modeling

We have carried out a systematic study of on-state breakdown in a sample set of InAlAs/InGaAs HEMT's using a new gate current extraction technique in conjunction with sidegate and temperature-dependent measurements. We find that as the device is turned on, the breakdown voltage limiting mechanism changes from a TFE-dominated process to a multiplication-dominated process. This physical understanding allows the creation of a phenomenological physical model for breakdown which agrees well with all our experimental results, and explains the relationship between BV_on and the sheet carrier concentration. Our results suggest that depending on device design, either on-state or off-state breakdown can limit maximum power     

Optically controlled current-voltage characteristics of ion-implanted MESFETs

Optically controlled MESFETs are useful as optical devices for optical communications, and as photodetectors. In this paper, a theoretical model for the I–V characteristics of these MESFETs is presented. The model considers the nonuniform Gaussian doping for ion-implanted channels. It takes both the photogenerated carriers as well as the doping generated residual carriers into account. It is noted that the density of photogenerated carriers in the channel due to diffusion is much less than that due to drift. Treatment both under gradual channel approximation and saturation velocity approximation has been presented. The gradual channel and the velocity saturation approximations are applied to study the I–V characteristics of long-channel and short-channel MESFETs, respectively. Results for both long-channel and short-channel MESFETs indicate that drain saturation current and transconductance can be improved by properly fixing the optical flux, and the absorption coefficient of the material.