Modulation-doped FETs Research Articles

Impact of device geometry and doping strategy on linearity and RF performance in Si/SiGe MODFETs

Based on careful calibration in respect of 70 nm n-type strained Si channel Si/SiGe modulation doped FETs (MODFETs) fabricated by Daimler Chrysler, numerical simulations have been used to study the impact of the device geometry and various doping strategies on device performance and linearity. Both the lateral and vertical layer structures are crucial to achieve high RF performance or high linearity. The simulations suggest that gate length scaling helps to achieve higher RF performance, but degrades the linearity. Doped channel devices are found to be promising for high linearity applications. Trade-off design strategies are required for reconciling the demands of high device performance and high linearity simultaneously.

Photoresponse of microwave transistors to high-frequency modulated lightwave carrier signal

Described in this paper are the photoresponse characteristics of microwave transistors, both unipolar [metal-semiconductor FETs (MESFETs) and modulation-doped FETs (MODFETs)] and bipolar [heterojunction bipolar transistors (HBTs)]. Investigation includes time- and frequency-domain measurements. For unipolar device FETs, the two dominant photodetection mechanisms, photoconductive and photovoltaic, are clearly identified within the same device for the first time. It is shown that even high-speed FETs are limited to a photonic bandwidth of a few megahertz, if photodetection and amplification are to be achieved simultaneously. In contrast, bipolar HBTs can provide optical gain up to the millimeter-wave range. It is shown that their bandwidth to a modulated optical input is closely related to the microwave bandwidth, and that parameters such as base-access resistance and base-emitter capacitance are critical to photoresponse optimization.

Undoped SiGe heterostructure field effect transistors

Summary form only given. Undoped SiGe heterostructure MISFETs which are an analog of III-V HIGFETs and use a biaxially strained Si channel with a SiGe gate insulator are reported. The devices were grown on a buffer layer of relaxed SiGe, as were previously reported SiGe modulation-doped heterostructure FETs, but here no doping is used; instead, a gate field is used to form the electron channel. Undoped Si/SiGe heterostructure MISFETs show good FET characteristics with low gate leakage at 77 K. FETs with 1.5- mu m-gate-length and SiGe insulator thickness of 100 nm have a maximum transconductance greater than 100 mS/mm at 77 K. By reducing the insulator thickness to 50 nm, a maximum transconductance greater than 150 mS/mm was obtained for a 2. 5- mu m-gate-length device. A field effect mobility at 77 K of 16500 cm/sup 2//V-s, extracted from long gate-length device data, is comparable to that of modulation-doped high-mobility samples. >

Modeling of the source resistance in modulation-doped FETs

A distributed multiple layer transmission line (MTL) model has been developed for the planar source ohmic contacts as encountered in modulation-doped FETs. Detailed current and voltage distributions under the MTL system have been obtained by numerical solutions of the coupled differential equations. The results clearly demonstrate that source resistance R S cannot be estimated simply from knowledge of the behavior of series resistance R NG in non-gated TLM test structures. The effects of the heavily-doped cap layer and the variation of the gate-source spacing on R NG and R S have been studied. Differences between our results based on the MTL model and that obtained from the lumped element contact end-resistance (LECR) model are explained satisfactorily. Our results show that the LECR method could be used to calculate the series resistance of the MODFET structures provided appropriate end elements are first determined by the distributed MTL model. Our results clearly indicate that attempts to reduce R S by increasing the cap layer thickness or decreasing the gate-source spacing would have only limited effectiveness. However, reduction of the interfacial contact resistivities is more effective in the reduction of R S , as demonstrated in the case of pseudomorphic structures. Graphical results are presented that clearly show the dependence of R S on the structural parameters of representative conventional and pseudomorphic test MODFETs.

Accurate modeling of the source resistance in modulation-doped FETs

A distributed multiple-layer transmission line (MTL) model has been developed for the source and drain ohmic contacts of modulation-doped FETs. Results based on the authors' model clearly demonstrate that the source resistance in MODFETs cannot be estimated simply from a knowledge of the behavior of the series resistance in nongated TLM test structures. Differences between the results based on the MTL model and those obtained from the previously used lump-element contact end-resistance (LECR) model are explained. >

MODFETs soar to 400 GHz

The operation of MODFETs (modulation-doped FETs) is explained. These devices now have switching delays under 5 ps, current-gain cutoff frequencies of about 250 GHz, and maximum oscillation frequencies in excess of 400 GHz. They also perform frequency division up to 26 GHz. Experimental ring oscillators, frequency dividers, static RAMs, and gate arrays are described. The microwave performance of MODFETs is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Selectively regrown contacts to field-effect transistors with two-dimensional electron-gas channels

The authors have fabricated for the first time heterostructure field-effect transistors where the two-dimensional electron gas (2-DEG) channel is directly contacted by selectively regrown epitaxial GaAs contacts. Both modulation-doped FETs (MODFETs) and semiconductor-insulator-semiconductor FETs (SISFETs) were fabricated. Contact resistances were low, as evidenced by high transconductances and improvements to the transconductance at low temperatures. The low resistance and shallow nature of the regrown contacts should permit scaling of these structures to very small dimensions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An analytic model for MODFET capacitance-voltage characteristics

An analytic model for the capacitance-voltage (C-V) characteristics of n-channel modulation doped FETs (MODFETs) is derived. Gauss law is used to relate the net areal gate charge density in an AlGaAs/GaAs MODFET to the electric field intensity at the metal-AlGaAs interface. An analytic expression for the electric field intensity which accounts for the neutralization of donors and the generation of free electrons is derived. The gate capacitance is derived as a closed-form analytic function of the gate voltage. The expression derived is easily computable and affords physical insight. The results, when compared with numerical calculations and experimental data, yield good agreement over a wide range of gate voltages. >

Electron velocity saturation in heterostructure field-effect transistors

Results on gate-length scaling of the performance of enhancement-mode heterostructure field-effect transistors (HFETs) for gate lengths of between 0.4 and 10 mu m are reported. The devices studied were fabricated by a self-aligned gate process. Transconductances as large as 534 mS/mm were achieved with 0.4- mu m devices. Two types of pseudomorphic AlGaAs/InGaAs/GaAs heterostructure are compared. One of them is used for modulation-doped FETs and the other for doped-channel FETs. It is found that the effects of electron velocity saturation are different for the two types of device due to the dominance of charge transfer and gate leakage in the conventional modulation-doped device. The experimental results are explained in the framework of a simple charge control model. >

High-gain, V-band, low-noise MMIC amplifiers using pseudomorphic MODFETs

State-of-the-art, 60-GHz, low-noise MMICs based on pseudomorphic modulation-doped FETs, with 0.25- mu m*60- mu m gates offset 0.3 mu m from the source ohmic, are discussed. Single-state low-noise amplifiers (LNAs) exhibited minimum noise figures of 2.90 dB with 4.1 dB of associated gain at 59.25 GHz. Dual-state MMICs had minimum noise figures of 3.5 dB and 10.8 dB of associated gain at 58.50 GHz. Cascaded four-stage LNAs (two dual-stage MMICs) had minimum noise figures of 3.7 dB and over 20.7 dB of associated gain at 58.0 GHz. Finally, when biased for maximum gain, the four-stage amplifier exhibited over 30.4 dB of gain at 60.0 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

60-GHz pseudomorphic-MODFET low-noise MMIC amplifiers

The development of V-band low-noise monolithic microwave integrated circuits (MMICs) based on pseudomorphic modulation-doped FETs (P-MODFETs) is presented. These dual-stage MMICs incorporate P-MODFETs, with 0.35- mu m*60- mu m gates, as the active elements, electron-beam-written tuning elements, and DC-blocking and bias networks. The dual-stage chips exhibited a maximum gain of 10.2 dB at 59.5 GHz and a minimum noise figure of 5.3 dB, with an associated gain of 8.2 dB at 58.2 GHz. A cascaded four-stage amplifier using two MMIC modules exhibited 5.8-dB minimum noise figure with an associated gain of 18.3 dB at 58 GHz and up to 21.1 dB of maximum gain. >

Determination of source and drain series resistances of ultra-short gate-length MODFETs

A practical method to evaluate the parasitic source and drain series resistances of modulation-doped FETs is presented. The method is based on a modified gate-probe technique that uses the gate current crowding phenomenon. It is suitable for complex heterostructure devices since the carrier transport mechanism and physical structure properties around the gate region need not conform to ideal models. >

Static random-access memory based on self-aligned-gate (Al,Ga)As/ n + -GaAs superlattice modulation-doped FETs

A 64 bit, fully decoded static random-access memory (SRAM) has been fabricated utilising self-aligned-gate (Al,Ga)As/n+-GaAs superlattice modulation-doped FETs (MODFETs) for the first time. Read access times of 1.1 ns at 270 μW/bit and minimum write-enable pulse widths less than 2 ns were demonstrated at room temperature. Typical room-temperature extrinsic transconductances and output conductances of 240 mS/mm and 7 mS/mm, respectively, were observed for the superlattice MODFET devices.

Modulation-doped GaAs/(Al,Ga)As heterojunction field-effect transistors: MODFETs

Recently developed modulation-doped field-effect transistors (MODFETs) now hold the record for high-speed logic. In this device structure only the larger bandgap (Al,Ga)As layer is doped with donors and the GaAs layer is left undoped. Electrons minimize their energy by diffusing out of the (Al,Ga)As into the lower potential GaAs where they form a two-dimensional electron gas near the heterointerface. Since the electrons and donors are spatially separated, ionized impurity scattering is avoided making it possible to obtain extremely high electron mobilities. Even at electron densities of 1019cm-3or ∼ 1 × 1012cm-2per interface, transport properties of these heterostructures are comparable to pure bulk GaAs. Modulation-doped FETs using this heterojunction system offer many advantages among which are a small gate to conducting channel separation (∼ 300 A) leading to extremely high transconductances, a large current-carrying capability (∼ 500 mA/mm per interface), a small source resistance, and a small saturation voltage. The benefits improve quite substantially at 77 K where the transconductance increases to ∼ 500 mS/mm. Improved device performance has been obtained in both high-speed and digital applications. Modulation-doped FETs used as low-noise amplifiers exhibit noise figures as low as 0.4 dB with 14-dB gain at 10 GHz at 77 K and a noise temperature of 3.5 K at 3.3 GHz with the MODFET cooled to 15 K. The rate of increase in noise temperature is about 1 K/GHz. Quarter micrometer devices have exhibited a current-gain cutoff frequency of 70 GHz. When used as inverters in logic circuits, propagation delays of 12 ps and under 10 ps have been obtained at 300 and 77 K, respectively. Delay times as low as 5 ps are quite possible at 77 K. Static RAMs with 4-kbit complexity exhibited access times of about 2 ns. These results are far superior to any other three-terminal device which is the primary reason why numerous university, industrial, and government laboratories in the U.S., Europe, and Japan have sizable MODFET programs.

Strained-quantum-well, modulation-doped, field-effect transistor

A new modulation-doped field-effect transistor structure which employs a thin (8 nm) strained layer of In 0.25 Ga 0.75 As as the channel is described. The prototype device with a 2.9 μm gate length had a peak, room-temperature, unilluminated extrinsic transconductance of 91 mS/mm (VDS = 1 V). If device performance scaled linearly with gate length to 1 μm, as do (Al, Ga)As/GaAs, single-interface, modulation-doped FETs, this is the best room-temperature performance reported to date for an (In, Ga)As channel, modulation-doped transistor. Furthermore, the transconductance of the strained-quantum-well transistor proved to be almost constant with gate voltage, varying less than 13% over a 0.5 V range at 77 K. This will allow the fabrication of extremely linear microwave amplifiers and supports the assertion that the current conduction path is the (In, Ga)As channel.

Temperature dependence of the I–V characteristics of modulation-doped FETs

Both normally on and normally off modulation-doped FETs have been fabricated from MBE grown AlGaAs/GaAs heterojunction structures. The static I–V characteristics of recessed-gate FETs were measured over the temperature range 77–400 K. A large change in the threshold voltage Vt with temperature T was observed. The observed Vt(T) data are explained, in part, in terms of a simple model based on the temperature-dependent occupation of deep donor traps in the AlGaAs layer. The traps were characterized by different energies for emission and capture. The transient capacitance of large area modulation-doped Schottky diodes was also measured. The capacitance data is explained by the time-dependent occupation of the same donor traps. The value of the activation energy for carrier emission was estimated to be 450 mV while the donor level was found to be about 42 mV below the conduction band.