Modulation-doped Field-effect Transistors Research Articles

Polarization dependent analysis of AlGaN/GaN HEMT for high power applications

Polarization dependent analysis for AlGaN/GaN HEMT has been done. The capacitance–voltage characteristics of lattice mismatched AlGaN/GaN modulation doped field effect transistor are obtained using charge controlled analysis for its microwave performance. The model includes the spontaneous and piezoelectric polarization effects and device transconductance and cutoff frequency are calculated. The results show excellent agreement when compared with experimental data thereby proving the validity of the model.

OPTICAL AND ELECTRICAL INVESTIGATION OF LOW DIMENSIONAL SELF-ASSEMBLEDInAsQUANTUM DOT FIELD EFFECT TRANSISTORS

Self-assembled InAs QD dot-in-a-well (DWELL) structures were grown on GaAs substrate by MBE system, and heterojunction modulation-doped field effect transistor (MODFET) was fabricated. The optical properties of the samples show that the photoluminescence of InAs/GaAs self-assembled quantum dot (SAQD) is at 1.265 μm at 300 K. The temperature-dependence of the abnormal redshift of InAs SAQD wavelength with the increasing temperature was observed, which is closely related with the inhomogeneous size distribution of the InAs quantum dot. According to the electrical measurement, high electric field current–voltage characteristic of the MODFET device were obtained. The embedded InAs QD of the samples can be regard as scattering centers to the vicinity of the channel electrons. The transport property of the electrons in GaAs channel will be modulated by the QD due to the Coulomb interaction. It has been proposed that a MODFET embedded with InAs QDs presents a novel type of field effect photon detector.

High-temperature electrical transport in AlxGa1−xN∕GaN modulation doped field-effect transistors

High-temperature electrical transports in AlGaN∕GaN modulation doped field-effect transistors (MODFETs) grown by metal organic chemical vapor deposition method have been studied experimentally. The MODFETs have a channel length of 1.5μm and a gate width of 40μm. The influence of pinchoff voltage, gate voltage, and source-drain voltage on the electrical transports has been addressed. The role of polarization and piezoelectric effect and the fundamental physics underlying leakage have also been elucidated. The maximum drain-source current is 525mA∕mm at a gate bias of 2V, source-drain bias of 4V, and temperature of 300K. The highest room temperature extrinsic transconductance is 155mS∕mm. The breakdown voltage is about 100V at 28 and 78°C.

RF Modeling of Large Gate-Width Multi-Finger Si/SiGe Power MODFETs

N-type large gate-width modulation-doped field-effect transistors (MODFETs) are fabricated on Si/SiGe modulation-doped heterostructure with fT and fmax of around 8 GHz and 20 GHz, respectively. These power devices demonstrate 15 to 16 dB power gain at 2 GHz, which make them suitable for low- and medium-power amplifiers for RF applications. We report here the RF modeling using small-signal equivalent circuit of these power devices. The extracted parameters are compared with prior work having similar gate length and the differences between this work and the previous one are discussed.

An analysis for AlGaN/GaN modulation doped field effect transistor using accurate velocity-field dependence for high power microwave frequency applications

An analytical model of Al0.15Ga0.85N/GaN modulation doped field effect transistor (MODFET), which uses an accurate velocity field relationship and incorporates the dominant effect of piezoelectric polarization induced charge at the AlGaN/GaN interface is presented. The effect of traps has also been taken into account. The calculated DC characteristics are in excellent agreement with the measured data. The model is extended to predict the microwave performance of the device. High current levels (>500mA/mm), large transconductance (160.83mS/mm) and a high cutoff frequency (9.6GHz) have been achieved analytically and are in close agreement with the experimental data.

Process for 20nm T gate on Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epilayer using two-step lithography and zigzag foot

After metallization, a 20nm T gate with a straight foot is not mechanically stable because the support given by the foot is too weak. We have proposed a zigzag gate foot to enhance mechanical support and developed a process using two-step electron beam lithography and zigzag foot shape to fabricate 20nm T gates for high performance Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs modulation-doped field-effect transistors. Two-step lithography reduces electron forward scattering by defining the foot on a thin (40nm) bottom layer of polymethyl methacrylate at the second step, the T-gate head having been developed at the first step. Adopting a low temperature development technique for the second step reduces the detrimental effect of head exposure on foot definition. With this process, stand-alone 20nm zigzag T gates have been successfully fabricated on an Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epitaxial wafer using a 20keV electron beam. With a higher-voltage electron beam, this process can be used to fabricate sub-20-nm T gates.

Fabrication and current-voltage characterization of a ferroelectric lead zirconate titanate/AlGaN∕GaN field effect transistor

We demonstrated ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics in a modulation-doped field effect transistors (MODFET) AlGaN∕GaN platform with ferroelectric Pb(Zr,Ti)O3 between a GaN channel and a gate metal. The pinch-off voltage was about 6–7V comparable to that of conventional Schottky gate MODFET. Counterclockwise hysteresis appeared in the transfer characteristics with a drain current shift of ∼5mA for zero gate-to-source voltage. This direction is opposite and much more pronounced than the defect induced clockwise hysteresis in conventional devices, which suggests that the key factor contributing to the counterclockwise hysteresis of the FFET is the ferroelectric switching effect of the lead zirconate titanate gate.

High-gain multi-finger power n -MODFET on Si substrate

A multi-finger n-type Si/SiGe modulation-doped field-effect transistor fabricated on Si substrate for RF power amplification is demonstrated for the first time. Load-pull measurements performed at 2 GHz on a ten-finger device with a gate length of 0.3 µm and a gate width of 750 µm show a maximum output power of 14 dBm and power gain of 16 dB at 1 dB compression point with power added efficiency of 15%.

Anomalous effects of temperature and UV illumination on the operation of AlGaN/GaN MODFET

The impact of high temperature rapid thermal annealing (RTA) on the mode of operation of AlGaN/GaN modulation doped field effect transistors (MODFETs) is reported. It is observed that annealing at high temperatures is capable of turning the normally depletion-mode (D-mode) characteristics of an AlGaN/GaN MODFET, towards that of an enhancement-mode (E-mode). This change is shown to be partly reversible through UV illumination. These results support the arguments on the extensive role of deep surface states on the operation of AlGaN/GaN MODFETs. According to this variation of characteristics, fabrication and characterization of close to E-mode AlGaN/GaN MODFETs are reported, using MBE grown material on sapphire. The devices demonstrate maximum extrinsic gate transconductance of 180 mS/mm. Unity current gain cutoff frequency ( f T) of 5 GHz and maximum oscillation frequency ( f max) of 14 GHz were measured.

Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (IDmax), the threshold voltage (Vth), the peak dc trans-conductance (gm), and unity current gain cut-off frequency (fT). The calculated values of fT (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.

Effects of RF and DC stress on AlGaN/GaN MODFETs: a low-frequency noise-based investigation

The impact of radio frequency (RF) and DC stress on passivated and unpassivated AlGaN/GaN modulation-doped field effect transistors (MODFETs) is investigated by means of DC and low-frequency noise (LFN) measurements. Unpassivated devices endure significant changes in the output resistance, gate, and drain noise current level after RF and DC stress. RF and DC stress of unpassivated devices leads to different degradation time constant and gate noise current. Besides, a positive shift in the pinch-off voltage is found to take place only after RF stress. In contrast to unpassivated devices, passivated devices do not show any considerable variation in the output resistance and gate, and drain noise current characteristics upon RF or DC stress. However, a positive shift in the pinch-off voltage upon RF stress is observed for both types of devices.

Transport Phenomena in Two-Dimensional Structures with Quantum Dots

A model that explains the unusual characteristics of the AlGaAs/GaAs modulation-doped field-effect transistor (MODFET) with InAs quantum dots incorporated in the GaAs channel is presented. It is shown that the negative charge of electrons confined in quantum dots decreases the threshold gate–drain voltage at which the channel is fully depleted. This provides an impact ionization of quantum dots at a low drain voltage. Because of the quantum dot ionization, the quantum dot MODFET transconductance becomes large and negative. The increased transconductance, due to the additional doping of the GaAs and InAs channels by impurities, exceeds 10 mS/mm. It is shown that the insertion of InAs quantum well with quantum dots into the GaAs quantum well increases the electron maximum drift velocity up to 10 cm/s, and consequently, quantum dot MODFET current gain cut-off frequency up to few hundred gigahertz.

High performances of InGaP∕InGaAs∕GaAs pseudomorphic modulation-doped field effect transistors using camel-gate structure

In this article, high-performance InGaP∕InGaAs∕GaAs δ-doped pseudomorphic modulation-doped field effect transistors (pMODFETs) employing n+-GaAs∕p+-InGaP∕n-InGaP camel-gate structure are characterized. Because of the depleted p-n junction in the gate region and the presence of the large conduction-band discontinuity of the InGaP∕InGaAs heterostructure, a large gate turn-on voltage is obtained. The dc and microwave characteristics of single- and double-δ-doped pMODFETs are demonstrated. The double-δ-doped pMODFET exhibits a higher drain saturation current, a larger transconductance, a broader gate voltage swing, and better high-frequency responses than the single-δ-doped device. The excellent performance of the studied devices is promising for linear and large signal amplifiers and high-frequency circuit applications.

A closed-form expression to analyze electronic properties in delta-doped heterostructures

We develop a closed-form model to describe the electronic properties for delta modulation doped heterostructures, particularly the 2DEG sheet charge density and the electric field distribution in the direction of growth. The model includes the effects of real-space charge transfer and carrier degeneracy. The electron transfer and quasi-equilibrium condition in growth direction have been used in order to express the 2DEG sheet charge density that is only a function of material parameters and constants. An empirical constant, corresponding to quantized energy states, has been employed to further simplify this description and to arrive at a closed-form expression. Results from the analytical expressions are shown to agree well with numerical simulations based on a self-consistent solution of a modified Schrödinger and Poisson equations. In addition to their use in modeling of current conduction in modulation doped field effect transistors (MODFETs), we expect that these expressions would serve as versatile tools in the modeling of optical and spectral effects that occur in the AlGaAs/GaAs heterostructures.

Low-energy plasma-enhanced chemical vapor deposition for strained Si and Ge heterostructures and devices

We review the potential of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) for the fabrication of strained Si and Ge heterostructures and devices. The technique is shown to be equally applicable to the formation of relaxed SiGe buffer layers, and to entire heterostructures including strained modulation doped channels. Pure Ge channels on Ge-rich linearly graded buffers are shown to exhibit low-temperature hole mobilities up to 120,000 cm 2 V −1 s −1, limited by remote impurity and background impurity scattering rather than interface roughness scattering. Strained-Si modulation-doped field-effect transistors (n-MODFETs) with excellent frequency response have been fabricated by combining LEPECVD and MBE for buffer layer and active layer growth, respectively. Maximum oscillation frequencies of n-MODFETs above 140 GHz have been achieved for active layer stacks both on buffers linearly graded to a Ge fraction of 40% at a rate of 10% per micron, and on constant composition buffers which are 10 times thinner. The use of a thin buffer results in significantly less device self-heating.

Characterization of extrinsic resistances in temperature behaviour modelling of InGaAs MODFETs

This work analyses the dc response of InGaAs channel modulation-doped field-effect transistors, when varying temperature from 300 to 400 K. An analytical model for the intrinsic drain current is derived from previous work, carried out for a similar AlGaAs channel device, in order to show explicitly the temperature dependence. The extrinsic resistances are numerically evaluated and added in a straightforward form to the model. Experimental output characteristics at different temperatures of an InGaAs MODFET, in static operation, are compared with those offered by the resulting extrinsic model and numerical simulations. Computed relative errors are around 10%.

Silicon Germanium Strained Layers and Heterostructures

The integration of strained-Si1–xGex into Si technology has enhanced the performance and extended the functionality of Si based circuits. The improvement of device performance is observed in both AC as well as DC characteristics of these devices. The category of such devices includes field effect as well as bipolar families. Speed performance in some based circuits has reached limits previously dominated by III-V heterostructures based devices. In addition, for some optoelectronics applications including photodetectors it is now possible to easily integrate strained-Si1–xGex based optical devices into standard Silicon technology. The impact of integrating strained and relaxed Si1–xGex alloys into Si technology is important. It has lead to stimulate Si research as well as offers easy options for performances that requires very complicated and costly process if pure Si has to be used. In this paper we start by discussing the strain and stability of Si1–xGex alloys. The origin and the process responsible for transient enhanced diffusion (TED) in highly doped Si containing layers will be mentioned. Due to the importance of TED for thin highly doped Boron strained-Si1–xGex layers and its degrading consequences, possible suppression design methods will be presented. Quantum well pchannel MOSFETs (QW-PMOSFETs) based on thin buried QW are solution to the low speed and weak current derivability. Different aspects of designing these devices for a better performance are briefly reviewed. Other FETs based on tensile strained Si on relaxed Si1–xGex for n-channel and modulation doped field effect transistors (MODFETs) showed excellent performance. Record AC performance well above 200GHz for fmax is already observed and this record is expected to increase in the coming years. Heterojunction bipolar transistors (HPTs) with thin strained-Si1–xGex highly doped base have lead to optimize the performance of the bipolar technology for many applications easily. The strategies of design and the most important designs of HBTs for optimum AC as well as DC are discussed in details. This technology is now mature enough and that is manifested in the appearance in the market nowadays. Si1–xGex based FETs circuits compatible with standard Si CMOS processes are soon expected to appear in the market. Finally, we briefly discuss the recent advances in Si1–xGex based infrared photodetectors.

High-Performance SiGe MODFET Technology

ABSTRACTAn overview of SiGe modulation-doped field-effect transistor (MODFET) technology is provided. The layer structures and mobility enhancements for both p- and n-channel modulation-doped quantum wells are described and compared to mobilities in Si/SiO2 inversion layers. Next, previous results on high-performance n- and p-MODFETs fabricated at IBM and elsewhere are reviewed, followed by recent results on laterally-scaled Si/SiGe n-MODFETs with gate lengths as small as 70 nm. We conclude with a discussion of the materials issues for the future vertical and lateral scaling of SiGe MODFETs.

High Mobility SiGe/Si n-MODFET Structures and Devices on Sapphire Substrates

ABSTRACTSiGe/Si n-type modulation doped field effect structures and transistors (n-MODFETs) have been fabricated on r-plane sapphire substrates. Mobilities as high as 1380 cm2/Vs were measured at room temperature. Excellent carrier confinement was shown by Shubnikov-de Haas measurements. Atomic force microscopy indicated smooth surfaces, with rms roughness less than 4 nm, similar to the quality of SiGe/Si n-MODFET structures made on Si substrates. Transistors with 2 μm gate lengths and 200 μm gate widths were fabricated and tested. An IDS of 9 mA was obtained by operating the transistor in an enhancement mode (positive VGS) and the maximum transconductance (gm) was 37 mS/mm at a VDS of 2.5 V. The transducer gain (Gt) measured with a loadpull system was 6.4 dB at 1 GHz for a VDS of 2.5 V and VGS=-0.4 V.

80 nm gate-length Si/Si 0.7 Ge 0.3 n -MODFET with 194 GHz f max

DC and RF performance of scaled n-channel Si/SiGe modulation-doped field effect transistors (n-MODFETs) grown by ultra-high vacuum chemical vapour deposition is reported. Devices with source-to-drain spacing of 300 nm, and gate length of 80 nm (70 nm) displayed fmax=194 GHz (175 GHz) and fT=70 GHz (79 GHz).