p-channel modulation-doped field-effect transistors based on AlSb0.9As0.1/GaSb

Operation of the first AlSbAs/GaSb p-channel modulation-doped field-effect transistor (MODFET) is reported. Devices with 1-μm gate length exhibit transconductance of 30 and 110 mS/mm at room temperature and 80 K, with respective maximum drain current densities of 25 and 80 mA/mm. The low field Hall mobility and sheet carrier density of this modulation doped structure were 260 cm²/V-s and 1.8×10 ¹² cm^-2 at room temperature and 1700 cm²/V-s and 1.4×10¹² cm^-2 at 77 K. Calculations based on these results indicate that room-temperature transconductances of 200 mS/mm or greater could be achieved. This device can be integrated with an InAs n-channel HFET for complementary circuit applications     

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/Al0.15Ga0.85As modulation-doped field effect transistors (MODFET's) exhibiting extremely good dc characteristics have been successfully fabricated, dc transconductance in these strained-layer structures of 270 mS/mm were measured for 1-µm gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.     

Characterization of InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic InyGa1-yAs/Al0.15- Ga0.85As (0.05 le y le 0.2) MODFET's grown by MBE have been characterized at dc (300 and 77 K) and RF frequencies. Transconductances as high as 310 and 380 mS/mm and drain currents as high as 290 and 310 mA/mm were obtained at 300 and 77 K, respectively, for 1-µm gate lengths and 3-µm source-drain spacing devices. Lack of persistent trapping effects,I-Vcollapse, and threshold voltage shifts observed with these devices are attributed to the use of low mole fraction AlxGa1-xAs while still maintaining 2DEG concentrations of about 1.3 × 10¹²cm^-2. Detailed microwave S-parameter measurements indicate a current gain cut-off frequency Of 24.5 GHz Wheny = 0.20, which is as much as 100 percent better than similar GaAs/AlGaAs MODFET structures, and a maximum frequency of oscillation of 40 GHz. These superior results are in part due to the higher electron velocity of InGaAs as compared with GaAs. Velocity field measurement performed up to 3 kV/cm using the magnetoresistance method indicates an electron saturation velocity of greater than 1.7 × 10⁷cm/s at 77 K fory = 0.15, which is 20 percent higher than GaAs/AlGaAs MODFET's of similar structure.     

Low-frequency noise characteristics of AlInAs/GaInAs modulation-doped field-effect transistors

The output noise voltage of AlInAs/GaInAs MODFETs grown by both MOCVD and MBE was measured at frequencies from 1 MHz to 1.5 GHz under different bias conditions for the first time. For frequencies below 500 MHz the noise voltage showed a 1/f dependence with a corner frequency around 200 MHz. The low-frequency noise was larger at the bias conditions giving higher transconductance.<>     

High speed p-type SiGe modulation-doped field-effect transistors

We report on the fabrication and characterization of high-speed p-type modulation-doped field-effect transistors (MODFETs) with 0.7-μm and 1-μm gate-lengths having unity current-gain cut-off frequencies (f_T) of 9.5 GHz and 5.3 GHz, respectively. The devices were fabricated on a high hole mobility SiGe heterostructure grown by ultra-high-vacuum chemical vapor deposition (UHV-CVD). The dc maximum extrinsic transconductance (g_m) is 105 mS/mm (205 mS/mm) at room temperature (77 K) for the 0.7-μm gate length devices. The fabricated devices show good pinch-off characteristics and have a very low gate leakage current of a few μA/mm at room temperature and a few nA/mm at 77 K     

Cryogenic temperature performance of modulation-doped field-effect transistors

Reports S-parameter measurements of AlInAs/GaInAs/InP modulation-doped field-effect transistors (MODFETs) at cryogenic temperatures. The current gain at 80 K is 3 dB higher than at 300 K, and the current gain cutoff frequency f/sub T/ increases from 32 GHz at 300 K, to 42 GHz at 80 K, which is the first observation of higher f/sub T/ by direct measurement.<>     

Ultra-high speed modulation-doped field-effect transistors: atutorial review

A tutorial review on the modulation-doped field-effect transistor (MODFET) and its application to ultra-low-noise, medium-power, and ultra-wide-band traveling-wave amplifiers as well as ultra-high-speed digital logic circuits is presented. It is believed that with further advances in material growth and device scaling significant improvements in cutoff frequencies, switching speed, noise, and power will be achieved in the near future     

Inverted GaAs/AlGaAs modulation-doped field-effect transistors with extremely high transconductances

Inverted GaAs/AsGaAs MODFET's with transconductances as high as 1810 mS/mm at 77 K and 1180 mS/mm at 300 K are fabricated using a self-aligned process. The devices have the gate-heterojunction interface spacing of only 100 Å, and the observed values of the transconductance are limited primarily by the source series resistance and by the gate current. The MODFET characteristics are interpreted using the charge control velocity saturation model which takes into account the gate current. The obtained results show a great potential of inverted MODFET's for ultrahigh-speed applications.     

Self-aligned modulation-doped (Al,Ga)As/GaAs field-effect transistors

The first modulation-doped (Al,Ga)As/GaAs field-effect transistors (MODFET's) have been fabricated using a self-aligned ion-implantation process. Measured extrinsic transconductances of 190 mS/mm were achieved at 300 K with source resistances of 1 Ω.mm. The highest currents yet reported for such device structures, in excess of 350 mA/mm, were obtained. A value of the maximum two-dimensional electron gas concentration of nearly 1.2 × 10¹²cm^-2was obtained from an analysis of the FET drain current-voltage characteristics using the charge-control model. These results hold promise for the practical fabrication of very high speed integrated circuits based on MODFET's, using a completely planar self-aligned ion-implantation process.     

Langmuir-Blodgett deposited cadmium gate inverted InP-GaInAs modulation-doped field-effect transistors

We report on the Langmuir-Blodgett film deposition and plasma etching of cadmium distearate on n-Gao.47Ino.53As to form a high-barrier-height Schottky barrier. Using this technique to form the gate electrode, we fabricated a 1?m-gate-length inverted InP-GalnAs modulation-doped field-effect transistor (MODFET) with an extrinsic transconductance of 170mS/mm and a cutoff frequencyfT of 19 GHz.     

On the low-temperature degradation of (AlGa)As/GaAs modulation-doped field-effect transistors

A comprehensive study of the anomalous low-temperature behavior of modulation,doped (Al, Ga)As/GaAs field-effect transistors is reported, Experiments on the effect of bias stress on the current-voltage characteristics in lateral resistors, 1-µm gate FET's, and a novel dual-gate tester are presented along with the results of freeze-out, optical spectroscopy, and trapping kinetics experiments. Both modulation-doped (Al, Ga)As/GaAs heterostructures and isolated (Al, Ga)As layers are examined. The results delineate the conditions under which threshold shift andI-Vcollapse occur. Based on these results a detailed physical model which explains these effects is proposed. One important conclusion of this work is that the collapse is not related to hot electron injection from the high mobility channel, as suggested earlier, but is a property of a highly doped AlGaAs layer under bias.     

Dual-gate AlGaN/GaN modulation-doped field-effect transistors with cut-off frequencies f/sub T/>60 GHz

We demonstrate dual-gate AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with gate-lengths of 0.16 /spl mu/m and 0.35 /spl mu/m for the first and second gates, respectively. The dual-gate device exhibits a current-gain cut-off frequency f/sub T/>60 GHz, and can simultaneously achieve a high breakdown voltage of >+100 V. In comparison to single-gate devices with the same gate length 0.16 /spl mu/m, dual-gate FETs can significantly increase breakdown voltages, largely increasing the maximum allowable drain bias for high power application. The continuous wave (CW) output power is in excess of 3.5 W/mm at 8.2 GHz. The corresponding large-signal gain is 12 dB and the power added efficiency is 45%. The dual-gate device with different gate lengths shows the capability of providing simultaneous high cut-off frequencies, and high breakdown voltages for broadband power amplifiers.     

Investigation of the influence of the well and the barrierthicknesses in GaSb/AlSb/GaSb/AlSb/InAs double-barrier interbandtunneling structures

The tunneling currents of GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling (DBIT) structures were studied experimentally by varying the thickness of the well and the barrier layers systematically. The optimal thicknesses for the GaSb well and the AlSb barriers were found to be 6.5 and 1.0 nm, respectively, to obtain a high peak current density (19 kA/cm²), with a large peak-to-valley ratio of 4. The high peak current in the DBIT structure shows the strong effect of the resonant coherence of the wave function across the double barrier. For the case of a small GaSb well width (3 nm), a drastic reduction of the peak current was observed, an effect suggesting that the electron-wave function in the InAs couples primarily to the quantized light hole state in the GaSb well     

Model for the temperature dependence of the threshold voltage of modulation-doped field-effect transistors

A model for the temperature dependence Of the threshold voltage of modulation-doped FET's caused by traps in doped AlGaAs is presented. The model takes into account the charge distribution in the depletion region determined by the temperature and time-dependent occupation of traps. The theory shows excellent agreement with experiment in the temperature range 77 to 400 K.     

The n-channel SiGe/Si modulation-doped field-effect transistor

At the heterointerface of Si1-xGex/Si the existence of two-dimensional carrier gas has recently been demonstrated. The electrons are confined inside the large-gap material Si. We report the first fabrication of n-channel modulation-doped SiGe/Si hetero field-effect transistors by use of molecular-beam epitaxial growth. Though neither layer sequence nor parasitic resistances were optimized, these first transistors exhibit an extrinsic transconductance of 40 mS/mm for a gate length of 1.6 µm. This value is higher than that of conventional Si MESFET's of comparable carrier concentration. Technological processing steps and device evaluation are described.     

Population inversion in two-dimensional InAs/GaSb/AlSb systems

We analyse the potentiality of InAs/GaSb/AlSb tunnel structures for creation of the population inversion and stimulated radiation both in the presence and in the absence of magnetic field.     

Terahertz electroluminescence from GaSb/AlSb quantum cascade laser

A phonon depopulation GaSb/AlSb quantum cascade laser was fabricated for the first time. The low longitudinal-optical phonon energy and small effective electron mass of GaSb enables a low electric field at the lasing threshold and a wide design range. The frequency of oscillation was designed to be 2.6 THz. With an increase of injection current, an abrupt increase of luminescence was observed.     

Investigation of negative differential resistance phenomena inGaSb/AlSb/InAs/GaSb/AlSb/InAs structures

The negative differential resistance (NDR) phenomena were observed in GaSb/AlSb/InAs/-GaSb/AlSb/InAs resonant interband tunnel structures. Electrons have resonantly achieved interband tunneling through the InAs/GaSb broken-gap quantum well. The InAs well width causes significant variations of the peak current density and NDR behaviors. The peak current density varies exponentially with the AlSb barrier thickness. The multiple NDR behavior was observed with appropriate InAs well and AlSb barrier thicknesses, e.g., 30 Å thick AlSb barrier and 240 Å wide InAs well. Only single negative resistance has, otherwise, been seen. The three-band model was used to interpret the effect of the InAs well and AlSb barrier on the current-voltage characteristics of GaSb/AlSb/InAs/GaSb/AlSb/InAs structures     

High-performance Si/SiGe n-type modulation-doped transistors

Enhancement-mode Si/SiGe n-type modulation-doped transistors with a 0.5-μm-length T-gate have been fabricated. Peak transconductances of 390 mS/mm at room temperature and 520 mS/mm at 77 K have been achieved. These high values are attributable to a combination of the high quality of the material used, having a room temperature mobility of 2600 cm²/V-s at an electron sheet concentration of 1.5×10¹² cm², and an optimized layer design that minimizes the parasitic series resistance and the gate-to-channel distance     

Current—Voltage and capacitance—Voltage characteristics of modulation-doped field-effect transistors

A model describingI-VandC-Vcharacteristics of modulation doped FET's is developed and used to predict the performance of AlxGa1-xAs/GaAs FET's in good agreement with our experimental results. It is shown that the change in the Fermi energy with the gate voltage changes the effective separation between the gate and the two-dimensional electron gas by about 80 Å. Current-voltage characteristics were calculated using a two piece as well as a three piece linear approximation for the electron velocity and compared with experimental results. At 300 K, the two piece model overestimates the current predicted by the three piece model only by approximately 10-20 percent. At 77 K, however, the three piece linear approximation for the velocity field characteristic should be used since the electron mobility decreases very abruptly at about 200 V/cm. The effect of the nonlinear source resistance is also discussed along with the gate-to-source and gate-to-drain capacitances, parameters of paramount importance in determining device performance. These capacitances are calculated as functions of gate-to-source and drain-to-source voltages below saturation.