Recessed-gate AlGaAs-InGaAs-GaAs pseudomorphic HEMT withSi-planar-doped etch stop layer

An improved slot etch technique based on an Si planar doped layer has been applied to gate recessing in the fabrication of AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs). The devices exhibited comparable g_m with much better breakdown and leakage behaviour than conventional pseudomorphic HEMT devices     

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     

Fabrication and operation of a velocity modulation transistor

The velocity modulation transistor (VMT) has two channels with differing velocities. Small vertical distances between these channels can be achieved using epitaxial growth, opening the opportunity for higher speed than the high electron mobility transistor (HEMT). Experimental results from a VMT realized using the AlGaAs/GaAs system are given. The VMT channel carrier population as a function of input gate voltage is calculated for HEMTs and VMTs using a one-dimensional (1-D) numerical model. This supports a proposed equivalent circuit model for the VMT, which is used to compare VMT performance to that of HEMTs. A noise model for the VMT is developed, and this model suggests that HEMT-like noise is achievable with good carrier confinement. The dual gate, dual-channel VMT, while more complex than the HEMT, may be useful in applications such as analog-to-digital converters (ADCs) and microwave amplifiers     

Se-doped AlGaAs/GaAs HEMTs for stable low-temperature operation

The fabrication of Se-doped AlGaAs/GaAs high electron mobility transistors (HEMTs) is discussed. Because the DX center concentration in Se-doped AlGaAs layers is lower than in Si-doped layers, the drain-current collapses much less at 77 K. The Se-doped HEMTs are therefore suitable for application in low-temperature LSI     

Current-gain cutoff frequency comparison of InGaAs HEMTs

The current-gain cutoff frequency performance of pseudomorphic InGaAs/AlGaAs (20% InAs composition) high-electron-mobility transistors (HEMTs) on GaAs is compared to that of lattice-matched InGaAs/InAlAs HEMTs on InP. The current-gain cutoff frequency (f_t) characteristics as a function of gate length (L_g) indicate that the f_t-L_g product of ~26 GHz-μm in InGaAs/InAlAs HEMTs is 23% higher than that of ~21 GHz-μm in InGaAs/AlGaAs HEMTs. The performance of InGaAs/AlGaAs HEMTs is also 46% higher than that of conventional GaAs/AlGaAs HEMTs (~18 GHz-μm). These data are very useful in improving the device performance of HEMTs for a given gate length     

V-band high-efficiency high-power AlInAs/GaInAs/InP HEMT's

The authors report on the state-of-the-art power performance of InP-based HEMTs (high electron mobility transistors) at 59 GHz. Using a 448-μm-wide HEMT with a gate length of 0.15 μm, an output power of 155 mW with a 4.9-dB gain and a power-added efficiency of 30.1% were obtained. By power-combining two of these HEMTs, an output power of 288 mW with 3.6-dB gain and a power-added efficiency of 20.4% were achieved. This is the highest output power reported with such a high efficiency for InP-based HEMTs, and is comparable to the best results reported for AlGaAs/InGaAs on GaAs pseudomorphic HEMTs at this frequency     

Degradation and recovery of AlGaAs/GaAs p-HEMT irradiated by high-energy particle

Results of an extensive study on the irradiation damage and its recovery behavior resulting from thermal annealing in AlGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs) subjected to a 220-MeV carbon, 1-MeV electrons and 1-MeV fast neutrons are presented. The drain current and effective mobility decrease after irradiation, while the threshold voltage increases in positive direction. The decrease of the drain current and mobility is thought to be due to the scattering of channel electrons with the induced lattice defects and also to the decrease of the electron density in the two dimensional electron gas region. Isochronal thermal annealing shows that the device performance degraded by the irradiation recovers. The decreased drain current for output characteristics recovers by 75% of pre-rad value after 300°C thermal annealing for AlGaAs HEMTs irradiated by carbon particles with a fluence of 1×10¹² cm⁻². The influence of the materials and radiation source on the degradation is also discussed with respect to the nonionizing energy loss. Those are mainly attributed to the difference of particle mass and the probability of nuclear collision for the formation of lattice defect in Si-doped AlGaAs donor layer. A comparison is also made with results obtained on irradiated InGaP/InGaAs p-HEMTs in order to investigate the effect of the constituent atom. The damage coefficient of AlGaAs HEMTs is also about one order greater than that of InGaP HEMTs for the same radiation source. The materials and radiation source dependence of performance degradation is mainly thought to be attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects in Si-doped donor layer.     

Comparison of 80-200 nm gate lengthAl0.25GaAs/GaAs/(GaAs:AlAs), Al0.3GaAs/In0.15GaAs/GaAs, and In0.52AlAs/In0.65GaAs/InPHEMTs

In this paper we present a comparative study of the high frequency performance of 80-200 mm gate length Al_0.25GaAs/GaAs/(GaAs:AlAs) superlattice buffer quantum well (QW) HEMTs, Al_0.3GaAs/In_0.15GaAs/GaAs pseudomorphic HEMTs and In_0.52AlAs/In_0.65GaAs/InP pseudomorphic HEMTs. From an experimental determination of the delays associated with transiting both the intrinsic and parasitic regions of the devices, effective electron velocities in the intrinsic channel region under the gate of the HEMT's were extracted. This analysis showed no evidence of any systematic increase in the effective channel velocity with reducing gate length in any of the devices. The effective electron velocity in the channel of the pseudomorphic In_0.65GaAs/InP HEMTs, determined to be at least 2.5×10⁵, was was around twice that of either the Al_0.25GaAs/GaAs quantum well or pseudomorphic In_0.15GaAs/GaAs HEMTs, resulting in 80 nm gate length devices with f_T's of up to 275 GHz. We also show that device output conductance is strongly material dependent. A comparison of the different buffer layers showed that the (GaAs:AlAs) superlattice buffer was most effective in confining electrons to the channel of the Al_0.25GaAs/GaAs HEMTs, even for 80 nm gate length devices. We propose this may be partly due to the presence of minigaps in the superlattice which provide a barrier to electrons with energies of up to 0.6 eV. The output conductance of pseudomorphic In_0.65GaAs/InP HEMTs was found to be inferior to the GaAs based devices as carriers in the channel have greater energy due to their higher effective velocity and so are more difficult to confine to the 2DEG     

Theoretical analysis of HEMT breakdown dependence on device designparameters

A two-dimensional numerical analysis is presented to investigate the breakdown characteristics of single- and double-channel AlGaAs/GaAs HEMTs. The influence of the doped layer thickness and the thickness of an undoped i-layer under the gate is analyzed. Impact ionization is considered to be the dominant breakdown mechanism. All simulations reveal the existence of a high electric field region near the gate contact. Breakdown occurs in the gate-drain region and the (breakdown) path which maximizes the ionization integral is entirely in the AlGaAs layer. For increased donor layer thickness, single-channel devices biased near pinchoff have gate-drain breakdown voltages varying from 8 to 14 V with corresponding peak electric field values in the range of 8.2×10⁵ to 2.4×10⁶ V/cm. The breakdown voltage increases with increasing gate bias |V _gs| due to a screening effect of transverse from longitudinal electric field. Double-channel HEMTs have slightly higher breakdown than single-channel, especially near pinchoff and for thin donor layers     

Analysis of delta-doped and uniformly doped AlGaAs/GaAs HEMT's byensemble Monte Carlo simulations

Transport properties and device performance of delta-doped and uniformly doped AlGaAs/GaAs high electron mobility transistors (HEMTs) with identical threshold voltages and gate capacitors are investigated using two-dimensional self-consistent ensemble Monte Carlo simulations. The model includes the effects of real-space transfer and carrier degeneracy, as well as the influence of DX centers and surface states. A one-to-one comparison of simulation results for the two devices demonstrates superior performance for the delta-doped HEMT and provides a physical basis for the observed improvements. In particular, the delta-doped HEMT maintains its superior device performance as gate bias is increased. Reasons for these improvements are reported     

High microwave and ultra-low noise performance of fullyion-implanted GaAs MESFETs with Au/WSiN T-shaped gate

Fully ion-implanted n⁺ self-aligned GaAs MESFETs with high microwave and ultra-low-noise performance have been fabricated. T-shaped gate structures composed of Au/WSiN are employed to reduce gate resistance effectively. A very thin and high-quality channel with high carrier concentration can be formed by adopting the optimum annealing temperature for the channel, and the channel surface suffers almost no damage by using ECR plasma RIE for gate formation. GaAs MESFETs with a gate length as short as 0.35 μm demonstrated a maximum oscillation frequency of 76 GHz. At an operating frequency of 18 GHz, a minimum noise figure of 0.81 dB with an associated gain of 7.7 dB is obtained. A K_f factor of 1.4 estimated by Fukui's noise figure equation, which is comparable to those of AlGaAs/GaAs HEMTs with the same geometry, reveals that the quality of the channel is very high     

0.25-μm gate millimeter-wave ion-implanted GaAs MESFETs

Quarter-micrometer gated ion-implanted GaAs MESFETs which demonstrate device performance comparable to AlGaAs/InGaAs pseudomorphic HEMTs (high-electron mobility transistors) have been successfully fabricated on 3-in-diameter GaAs substrates. The MESFETs show a peak extrinsic transconductance of 480 mS/mm with a high channel current of 720 mA/mm. From S-parameter measurements, the MESFETs show a peak current-gain cutoff frequency f_t of 68 GHz with an average f_t of 62 GHz across the wafer. The 0.25-μm gate MESFETs also exhibit a maximum-available-gain cutoff frequency f_t greater than 100 GHz. These results are the first demonstration of potential volume production of high-performance ion-implanted MESFETs for millimeter-wave application     

Super-low-noise performance of direct-ion-implanted 0.25-μm-gateGaAs MESFET's

The authors report on advanced ion implantation GaAs MESFET technology using a 0.25-μm `T' gate for super-low-noise microwave and millimeter-wave IC applications. The 0.25×200-μm-gate GaAs MESFETs achieved 0.56-dB noise figure with 13.1-dB associated gain at 50% I_DSS and 0.6 dB noise figure with 16.5-dB associated gain at 100% I_DSS at a measured frequency of 10 GHz. The measured noise figure is comparable to the best noise performance of AlGaAs/GaAs HEMTs and AlGaAs/InGaAs/GaAs pseudomorphic HEMTs     

Determination of AlGaAs/GaAs HEMT parasitic resistances

An improved technique has been developed to measure source and drain parasitic resistances of AlGaAs/GaAs HEMTs. Similar to the measurement technique typically used for MESFETs, a positive d.c. gate crowding current is applied. Because of the structure of the HEMT, this gate current must be kept very small in order to prevent significant leakage into the AlGaAs layer, which would result in current paths not present in normal operation of the device. The small d.c. gate current necessary to limit the current in this leakage path did not yield a usable signal-to-noise ratio of the measured gate-source, gate-drain and drain-source voltages needed to calculate the parasitic resistances. To overcome this problem, modulation of the drain current with a low-frequency a.c. signal coupled with lock-in techniques to measure the desired voltages was implemented. The resulting improvement in signal-to-noise ratio has made the gate crowding technique suitable for measuring the parasitic resistances of AlGaAs/GaAs HEMTs.     

22 nm In0.75Ga0.25As channel-based HEMTs on InP/GaAs substrates for future THz applications

In this work, the performance of L_g=22 nm In_0.75Ga_0.25As channel-based high electron mobility transistor (HEMT) on InP substrate is compared with metamorphic high electron mobility transistor (MHEMT) on GaAs substrate. The devices features heavily doped In_0.6Ga_0.4As source/drain (S/D) regions, Si double δ-doping planar sheets on either side of the In_0.75Ga_0.25As channel layer to enhance the transconductance, and buried Pt metal gate technology for reducing short channel effects. The TCAD simulation results show that the InP HEMT performance is superior to GaAs MHEMT in terms of f_T, f_max and transconductance (g_{m_max}). The 22 nm InP HEMT shows an f_T of 733 GHz and an f_max of 1340 GHz where as in GaAs MHEMT it is 644 GHz and 924 GHz, respectively. InGaAs channel-based HEMTs on InP/GaAs substrates are suitable for future sub-millimeter and millimeter wave applications.     

Ultra-submicrometer-gate AlGaAs/GaAs HEMTs

Ultra-submicrometer-gate AlGaAs/GaAs high-electron-mobility transistors (HEMTs) that have gate lengths ranging from 25 to 85 nm and were fabricated using electron-beam lithographic techniques on epitaxial wafers grown by molecular beam epitaxy are discussed. These devices show that velocity overshoot and short-gate geometry effects play an important role for the gate lengths less than 100 nm. The maximum intrinsic transconductance is 215 mS/mm, and the effective saturated electron velocity reaches 3×10⁷ cm/s for a 30-nm HEMT     

An extraction technique for small signal intrinsic parameters of HEMTs based on artificial neural networks

This paper presents a fast and accurate procedure for extraction of small signal intrinsic parameters of AlGaAs/GaAs high electron mobility transistors (HEMTs) using artificial neural network (ANN) techniques. The extraction procedure has been done in a wide range of frequencies and biases at various temperatures. Intrinsic parameters of HEMT are acquired using its values of common-source S-parameters. Two different ANN structures have been constructed in this work to extract the parameters, multi layer perceptron (MLP) and radial basis function (RBF) neural networks. These two kinds of ANNs are compared to each other in terms of accuracy, speed and memory usage. To validate the capability of the proposed method in small signal modeling of GaAs HEMTs, data and modeled values of S-parameters of a 200 μm gate width 0.25 μm GaAs HEMT are compared to each other and very good agreement between them is achieved up to 30 GHz. The effect of bias, temperature and frequency conditions on the extracted parameters of HEMT has been investigated, and the obtained results match the theoretical expectations. The proposed model can be inserted to computer-aided design (CAD) tools in order to have an accurate and fast design, simulation and optimization of microwave circuits including GaAs HEMTs.     

Simulation on the effect of non-uniform strain from the passivation layer on AlGaN/GaN HEMT

High electron mobility transistors (HEMTs) based on the III-nitride material system have attracted interest for high-frequency electronic components operating at high-power levels. Nitride based HEMTs can achieve power, bandwidth and efficiency levels that exceed the performance of Si, GaAs or SiC based devices. At present, a major limitation of nitride HEMTs is their failure to achieve reliability on par with Si-LDMOS or GaAs pHEMT devices. The development of SiN_x passivation layers have largely mitigated the gate lag effect, however, this passivation layer introduces an additional strain that forms a non-uniform polarization induced charge. Furthermore, this excess strain can locally relax the film eliminating the piezoelectric induced charge in addition to forming defects that act as electron traps.     

Experimental and theoretical noise analysis of microwave HEMTs

Low-frequency noise power and high-frequency noise figures in HEMTs (high electron mobility transistors) were measured and compared with calculations based on a one-dimensional noise model to characterize their low-noise properties. It was found that the drain noise current parameter Q in HEMTS is lower than that in GaAs MESFETs. The strong correlation between drain- and induced-gate-noise currents in HEMTs is due to the asymmetric distribution of noise generation along a channel, and the drain noise current is nearly canceled by those induced-gate-noise current. The intrinsic thermal noise from source and gate resistances is about 25% of the total output noise in the 0.25-μm gate-length HEMT considered     

0.25-μm pseudomorphic HEMTs processed with damage-free dry-etchgate-recess technology

Damage-free, dry-etched 0.25-μm T-shape gate pseudomorphic InGaAs channel HEMTs have been demonstrated. A Freon-12-based discharge was used in either electron cyclotron resonance (ECR) or reactive ion etching (RIE) systems to perform the gate recess process. Etching selectivity of more than 200 was obtained between the GaAs cap layer and the underlying AlGaAs donor layer. Self-bias voltages of -30 to -50 V were used in the etching process to minimize the damage. Pre- and post-etch clean steps were utilized to achieve uniform etch and removal of any dry-etch-related residues. Schottky diodes fabricated on n-GaAs subjected to either dry or wet etching showed no differences of barrier height, zero-bias depletion depth, and ideality factor. By using the dry etch for gate recess, very tight threshold voltage uniformity was obtained. The devices showed I-V characteristics comparable to that of devices fabricated with a wet chemical process