Ultrahigh-frequency performance of submicrometer-gate ion-implantedGaAs MESFETs

A study of the high-frequency performance of short-gate ion-implanted GaAs MESFETs with gate lengths of 0.3 and 0.5 μm is discussed. Excellent DC and microwave performance have been achieved with an emphasis on the reduction of effective gate length during device fabrication. From f_t of 83 and 48 GHz for 0.3-0.5-μm gate devices, respectively, an electron velocity of 1.5×10⁷ cm/s is estimated. An f_t of 240 GHz is also projected for a 0.1-μm-gate GaAs MESFET. These experimental results are believed to be comparable to those of the best HEMTs (high-electron-mobility transistors) reported and higher than those generally accepted for MESFETs     

Microwave performance of ion-implanted InP JFETs

These devices have a planar structure with the channel and gate regions formed by the selective implantation of silicon and beryllium into an Fe-doped semi-insulating InP substrate. The nominal gate length is 2 μm with a channel doping of 10¹⁷ cm^-3 and thickness of 0.2 μm. The measured values of f_T and f_max are 10 and 23 GHz, respectively. Examination of the equivalent circuit parameters and their variation with bias led to the following conclusions: (a) a relatively gradual channel profile results in lower than desired transconductance, but also lower gate-to-channel capacitance; (b) although for the present devices, the gate length and transconductance are the primary performance-limiting parameters, the gate contact resistance also reduces the power gain significantly; (c) the output resistance appears lower than that of an equivalent GaAs MESFET, and requires a larger V_DS to reach its maximum value; and (d) a dipole layer forms and decouples the gate from the drain with a strength that falls between that of previously reported GaAs MESFETs and InP MESFETs     

Improved breakdown voltage in GaAs MESFETs utilizing surface layersof GaAs grown at a low temperature by MBE

A metal-semiconductor field-effect transistor (MESFET) utilizing surface layers of GaAs grown at a low temperature by MBE (LT GaAs) under the gate electrode has been fabricated. The high trap density of LT GaAs reduces the surface fields of the FET, suppresses gate leakage, and increases the gate-drain breakdown voltage without sacrificing current drive capability. An undoped AlAs layer is incorporated between the LT GaAs layer and the channel as a barrier to the diffusion of excess As from the LT GaAs layer to the channel. A 74-μm-gate-width device demonstrated an improved breakdown voltage of 34.85 V with a g _m of 144 mS/mm and an I_dss of 248 mA/mm     

Low-temperature DC characteristics of pseudomorphic Ga0.18In0.82P/InP/Ga0.47In0.53As HEMT

A high-gap strained GaInP material chosen to increase Schottky barrier height on InP is discussed. This material has been used for the first time in high electron mobility transistor (HEMT) fabrication on InP. For these devices the best g_m of a 1.3-μm gate HEMT is 300 mS/mm. Transistors of 3-μm gate length are studied at low temperature (100 to 293 K). Their DC electrical characteristics improve upon cooling. The best improvement is measured at the lowest temperature (+54% for g_m at 105 K). The structure is stable and does not present any g_m or I_ds collapse at low temperature, unlike AlGaAs/GaAs heterostructures     

InGaAs metal-semiconductor field-effect transistor withLangmuir-Blodgett deposited gate structure

A high-transconductance n-channel, depletion-mode InGaAs metal-semiconductor field-effect transistor (MESFET) with a Langmuir-Blodgett deposited gate fabricated on organometallic chemical vapor deposition (OMCVD)-grown InGaAs lattice matched to InP is reported. The fabrication process is similar to epitaxial GaAs FET technology and is suitable for making optoelectronic integrated circuits (OEICs) for long-wavelength fiber-optic communications systems. Devices with 1-μm gate and 6×10¹⁶ channel doping achieved 162-mS/mm extrinsic transconductance and -1.8-V pinch-off voltage. The effective saturation velocity of electrons in the channel was measured to be between 3.5 and 3.9×10⁷ cm/s. The drain current ( I_dss), 300 mA/mm at V_ds=2.5 V, is the highest current capability reported for depletion-mode InGaAs MESFET devices with low pinch-off voltages     

Quarter-micrometer gate ion-implanted GaAs MESFET's with an f1 of 126 GHz

The fabrication and characterization of a 0.25-μm-gate, ion-implanted GaAs MESFET with a maximum current-gain cutoff frequency f_t of 126 GHz is reported. Extrapolation of current gains from bias-dependent S-parameters at 70-100% of I _dss yields f₁'s of 108-126 GHz. It is projected that an f₁ of 320 GHz is achievable with 0.1-μm-gate GaAs MESFETs. This demonstration of f₁'s over 100 GHz with practical 0.25-μm gate length substantially advances the high-frequency operation limits of short-gate GaAs MESFETs     

A recessed-gate InAlAs/n+-InP HFET with an InP etch-stoplayer

The authors have exploited both the attractive transport properties and the etch selectivity of InP in a novel InAlAs/n⁺ -InP metal-insulator-doped-channel heterostructure FET (MIDFET). In several other material systems, the MIDFET has been shown to be well-suited to high-power telecommunications applications. The device employs InP both as the channel layer and as an etch-stop layer in a selective-etch recessed-gate process. L_g=1.8-μm devices achieve g_m and I_D,max values of 224 mS/mm and 408 mA/mm, respectively, the highest recorded values for an InP channel HFET with L_g⩾0.8-μm, including MODFETs. These figures combine with a breakdown voltage of 10 V and peak values of f _T and f_max of 10.5 and 28 GHz, respectively. The selective-etch recessed-gate process contributes to excellent device performance while maintaining a tight 60-mV threshold voltage distribution (13 mV between adjacent devices)     

Characteristics including electron velocity overshoot for0.1-μm-gate-length GaAs SAINT MESFET's

Short-channel effects, substrate leakage current, and average electron velocity are investigated for 0.1-μm-gate-length GaAs MESFETs fabricated using the SAINT (self-aligned implantation for n⁺-layer technology) process. The threshold-voltage shift was scaled by the aspect ratio of the channel thickness to the gate length ( a/L_g). The substrate leakage current in a sub-quarter-micrometer MESFET is completely suppressed by the buried p layers and shallow n⁺-layers. The average electron velocity for 0.1- to 0.2-μm-gate-length FETs is estimated to be 3×10⁶ cm/s from the analysis of intrinsic FET parameters. This high value indicates electron velocity overshoot. Moreover, a very high f_T of 93.1 GHz has been attained by the 0.1-μm SAINT MESFET     

High-performance In0.08Ga0.92As MESFETs onGaAs(100) substrates

In_0.08Ga_0.92As MESFETs were grown in GaAs (100) substrates by molecular beam epitaxy (MBE). The structure comprised an undoped compositionally graded In_xGa_1-x As buffer layer, an In_0.08Ga_0.92As active layer, and an n⁺-In_0.08Ga_0.92As cap layer. FETs with 50-μm width and 0.4-μm gate length were fabricated using the standard processing technique. The best device showed a maximum current density of 700 mA/mm and a transconductance of 400 mS/mm. The transconductance is extremely high for the doping level used and is comparable to that of a 0.25-μm gate GaAs MESFET with an active layer doped to 10¹⁸ cm^-3. The current-gain cutoff frequency was 36 GHz and the power-gain cutoff frequency was 65 GHz. The current gain cutoff frequency is comparable to that of a 0.25-μm gate GaAs MESFET     

Ultralow-noise W-band pseudomorphic InGaAs HEMT's

Low-noise planar doped pseudomorphic (PM) InGaAs high-electron-mobility transistors (HEMTs) with a gate length of 0.1 μm for W-band operation are discussed. These devices feature a multiple-finger layout with air bridges interconnecting the sources to reduce gate resistance. The device exhibits a minimum noise figure of 2.5 dB with an associated gain of 4.7 dB at 92.5 GHz. This result demonstrates the feasibility of using PM InGaAs HEMTs for W-band low-noise receivers without the need for using lattice-matched InP HEMTs     

Delay time analysis for 0.4- to 5-μm-gate InAlAs-InGaAs HEMTs

InAlAs-InGaAs HEMTs with 0.4- to 5-μm gate lengths have been fabricated and a maximum f_T of 84 GHz has been obtained by a device with a 0.4-μm gate length. A simple analysis of their delay times was performed. It was found that gradual channel approximation with a field-dependent mobility model with E_c of 5 kV/cm holds for long-channel devices (L _g>2 μm), while a saturated velocity model with a saturated velocity of 2.7×10⁷ cm/s holds for short-channel devices (L_g<1 μm)     

Millimeter-wave ion-implanted gradedInxGa1-xAs MESFETs grown by MOCVD

The authors present the fabrication and characterization of ion-implanted graded In_xGa_1-xAs/GaAs MESFETs. The In_xGa_1-xAs layers are grown on GaAs substrates by MOCVD (metal-organic chemical vapor deposition) with InAs concentration graded from 15% at the substrate to 0% at the surface. 0.5-μm gate MESFETs are fabricated on these wafers using silicon ion implantation. In addition to improved Schottky contact, the graded In_xGa _1-xAs MESFET achieves maximum extrinsic transconductance of 460 mS/mm and a current-gain cutoff frequency f_t of 61 GHz, which is the highest ever reported for a 0.5-μm gate MESFET. In comparison, In_0.1Ga_0.9As MESFETs fabricated with the same processing technique show an f_t of 55 GHz     

GaAs power MESFET with 41-percent power-added efficiency at 35 GHz

A GaAs power MESFET has been optimized for Ka-Band operation. The device has an n⁺ ledge channel structure with a 0.25-μm gate on MBE-grown material. An output power density of 0.71 W/mm was achieved with 5.2-dB gain and 34% power-added efficiency. When tuned for maximum efficiency, a power-added efficiency of 41% was obtained with a power density of 0.61 W/mm and a gain of 5.6 dB     

Half-micrometer gate-length ion-implanted GaAs MESFET with 0.8-dBnoise figure at 16 GHz

Ion-implanted GaAs MESFETs with half-micrometer gate length have been fabricated on 3-in-diameter GaAs substrates. At 16 GHz, a minimum noise figure of 0.8 dB with an associated gain of 6.3 dB has been measured. This noise figure is believed to be the lowest ever reported for 0.5- and 0.25-μm ion-implanted MESFETs, and is comparable to that for 0.25-μm HEMTs at this frequency. By using the Fukui equation and the fitted equivalent circuit model, a K_f factor of 1.4 has been obtained. These results clearly demonstrate the potential of ion-implanted MESFET technology for K-band low-noise integrated circuit applications     

High-transconductance insulating-gate InP/InGaAs buried p-bufferDH-MODFETs grown by MOVPE

Buried p-buffer double heterostructure modulation-doped field-effect transistors (BP DH-MODFETs) with an InGaAs quantum-well channel were fabricated with high transconductance and good breakdown voltage, by placing the metal gate directly on Fe-doped InP insulating layer. Excellent extrinsic DC transconductance of 560 mS/mm and a high gate-to-drain diode breakdown voltage (greater than 20 V) were achieved at room temperature with FETs of 1.2-μm gate length. Unity currently gain cutoff frequency f_T of 24 GHz and maximum oscillation frequency f_max of 60 GHz were demonstrated for a drain to source voltage V_DS=4 V, which corresponds to an average electron velocity of 2.2×10⁷ cm/s in the quantum well     

High-performance millimeter-wave ion-implanted GaAs MESFETs

GaAs MESFETs (metal-epitaxial-semiconductor-field-effect transistors) with ion-implanted active channels have been fabricated on 3-in-diameter GaAs substrates which demonstrate device performance comparable with that of AlGaAs/InGaAs pseudomorphic HEMT (high-electron-mobility transistor) devices. Implanted MESFETs with 0.5-μm gate lengths exhibit an extrinsic transconductance of 350 mS/mm. From S-parameter measurements, a current-gain cutoff frequency f₁ of 48 GHz and a maximum-available-gain cutoff frequency f_max greater than 100 GHz are achieved. These results clearly demonstrate the suitability of ion-implanted MESFET technology for millimeter-wave discrete device, high-density digital, and monolithic microwave and millimeter-wave IC applications     

Self-aligned bottom-gate submicrometer-channel-length a-Si-:Hthin-film transistors

Fully self-aligned bottom-gate thin-film transistors (TFTs) fabricated by using a back substrate exposure technique combined with a metal lift-off process are discussed. Ohmic contact to the sources and drains is accomplished by a 40-nm-thick layer of phosphorous-doped microcrystalline silicon. Devices with channel lengths ranging from 0.4 to 12 μm are processed with overlap dimensions between the gate and the source and the gate and the drain ranging from 0.0 to 1.0 μm. Analysis of the conductance data in the linear voltage regime reveals a parasitic drain-to-channel and source-to-channel resistance that is 14% of the channel resistance for a 10-μm device and 140% for a 1-μm device. Thus, increase in the device speed caused by reducing the channel length does not follow expected behavior. A similar situation exists in the nonlinear regime. The on-current of the devices starts to saturate below channel lengths of 2 μm. Current on/off ratios taken at V_d=5 V and V_G=15 V and 0 V, respectively, are approximately 1×10⁶ for the 1- and 12-μm-long devices. The on/off ratio is reduced to 1×10⁵ for the 0.4-μm device     

5-60-GHz high-gain distributed amplifier utilizing InP cascodeHEMTs

A high-gain InP MMIC cascode distributed amplifier was developed which has 12 dB of gain from 5 to 60 GHz with over 20-dB gain control capability and a noise figure of 2.5-4 dB in the Ka band. Lattice-matched InAlAs/InGaAs cascode HEMTs on InP substrate with 0.25-μm gate length were the active devices. Microstrip was the transmission medium for this MMIC with an overall chip dimension of 2.3 mm×0.9 mm. The gain/noise figure advantages of the InP HEMT over the AlGaAs HEMT and the superior gain performance of the cascode HEMT over the common-source HEMT are demonstrated     

An experimental determination of electron drift velocity in0.5-μm gate-length ion-implanted GaAs MESFET's

A microwave technique was used to determine the electron drift velocity in an ion-implanted GaAs MESFET with a 0.5×100-μm gate. The characteristics of the velocity versus drain-to-source voltage for a GaAs MESFET exhibit a peak velocity of 4.0, 3.3, and 2.2×10 ⁷ cm/s at 100%, 65%, and 31% of I_dss, respectively. This work presents the first experimental determination of electron drift velocity at various gate biases and provides verification for velocity overshoot in ion-implanted GaAs MESFETs     

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