D.C. Radulescu

Characterization of ultra-high-speed pseudomorphic AlGaAs/InGaAs (on GaAs) MODFETs

The authors report a detailed characterization of ultrahigh-speed pseudomorphic AlGaAs/InGaAs (on GaAs) modulation-doped field-effect transistors (MODFETs) with emphasis on the device switching characteristics. The nominal 0.1- mu m gate-length device exhibit a current gain cutoff frequency (f/sub t/) as high as 152 GHz. This value of f/sub t/ corresponds to a total delay of approximately 1.0 ps and is attributed to the optimization of layer structure, device layout, and fabrication process. It is shown that the electron transit time in these very short gate-length devices still accounts for approximately 60% of the total delay, and, as a result, significant improvements in switching speed are possible with further reductions of gate length. The results reported clearly demonstrate the potential of the pseudomorphic AlGaAs/InGaAs MODFET as an ultrahigh-speed device. Its excellent switching characteristics are attributed to the high saturation velocity ( approximately 2*10/sup 7/ cm/s), 2DEG sheet density (2.5*10/sup 12/ cm/sup -2/), and current drive capability (>200 mA/mm at the peak transconductance). >

Influence of quantum-well width on device performance of Al/sub 0.30/Ga/sub 0.70/As/In/sub 0.25/Ga/sub 0.75/As (on GaAs) MODFETs

An experimental study in which the quantum well width (W) is varied from 45 to 200 AA is discussed. Optimum device performance was observed at a well width of 120 AA. The 0.2- mu m*130- mu m devices with 120-AA quantum-well width typically exhibit a maximum channel current density of 550 mA/mm, peak transconductance of 550 mS/mm, and peak current gain cutoff frequency (f/sub T/) of 122 GHz. These results have been further improved in subsequent fabrications employing a trilevel-resist mushroom-gate process. The 0.2- mu m*50- mu m devices with mushroom gate exhibit a peak transconductance of 640 mS/mm, peak f/sub T/ of 100 GHz, and best power gains cutoff frequency in excess of 200 GHz. These results are among the best ever reported for GaAs-based FETs and are attributed to the high two-dimensional electron gas (2DEG) sheet density, good low-field mobility, low ohmic contact, and the optimized mushroom gate process. >

0.2- mu m gate-length atomic-planar doped pseudomorphic Al/sub 0.3/Ga/sub 0.7/As/In/sub 0.25/Ga/sub 0.75/As MODFETs with f/sub T/ over 120 GHz

The authors report the DC and RF performance of nominally 0.2- mu m-gate length atomic-planar doped pseudomorphic Al/sub 0.3/Ga/sub 0.7/As/In/sub 0.25/Ga/sub 0.75/As modulation-doped field-effect transistors (MODFETs) with f/sub T/ over 120 GHz. The devices exhibit a maximum two-dimensional electron gas (2 DEG) sheet density of 2.4*10/sup 12/ cm/sup -2/, peak transconductance g/sub m/ of 530-570 mS/mm. maximum current density of 500-550 mA/mm, and peak current-gain cutoff frequency f/sub T/ of 110-122 GHz. These results are claimed to be among the best ever reported for pseudomorphic AlGaAs/InGaAs MODFETs and are attributed to the high 2 DEG sheet density, rather than an enhanced saturation velocity, in the In/sub 0.25/Ga/sub 0.75/As channel.<<ETX>>

A high-current pseudomorphic AlGaAs/InGaAs double quantum-well MODFET

Double quantum-well modulation-doped field-effect transistors (MODFETs) with planar-doped lattice-strained AlGaAs/InGaAs structure have been fabricated and characterized at DC and microwave frequencies. At 300 K the 0.3- mu m gate devices show a full channel current of 1100 mA/mm with a constant extrinsic transconductance of 350 mS/mm over a broad gate voltage range of 1.6 V. Excellent microwave performance is also achieved with a maximum available gain cutoff frequency f/sub mag/ of 110 GHz and a current gain cutoff frequency f/sub r/ of 52 GHz. A maximum output power of 0.7 W/mm with 30% efficiency is obtained at 18 GHz.<<ETX>>

Influence of substrate misorientation on defect and impurity incorporation in GaAs/AlGaAs heterostructures grown by molecular‐beam epitaxy

GaAs/AlGaAs heterostructures have been grown by molecular‐beam epitaxy on GaAs substrates intentionally oriented (tilted) a few degrees (0–6.5) off the (001) plane towards either (111)A, (111)B, or (011). We observe that the 4‐K photoluminescence and low‐field electron transport properties of these structures may be functions of the substrate tilt angle and tilt direction depending on the concentration of impurities incorporated during growth. A substrate tilt during molecular‐beam epitaxy is observed to have the largest effect on these properties when the background impurity concentration in the molecular‐beam epitaxial machine is high. This supports our contention that the observed changes in material characteristics are due to differences in the incorporation of defects and impurities. The incorporation of defects and impurities are reduced by using substrates tilted toward (111)A in comparison to nominally flat (001) substrates or substrates tilted toward (111)B.

Effects of substrate misorientation and background impurities on electron transport in molecular-beam-epitaxial grown GaAs/AlGaAs modulation-doped quantum-well structures

Single GaAs quantum wells, clad with Al0.3Ga0.7As, and modulation doped with silicon introduced in the Al0.3Ga0.7As after the quantum wells are grown have been grown by molecular‐beam epitaxy on GaAs substrates tilted a few degrees from the nominal (001) plane towards either of the (111) planes. The low‐field two‐dimensional electron gas mobility is observed to be a function of the tilt angle (0°, 2°, 4°, 6.5°) and of the direction of tilt [towards (111)A or (111)B]. The two‐dimensional electron gas mobilities in quantum‐well structures grown on substrates tilted towards (111)A are larger than those in structures grown on nominally flat (001) substrates. The improvement in two‐dimensional electron gas transport is attributed to an improvement in the quality of the inverted interface (i.e., GaAs grown on AlGaAs). Quantum wells grown on substrates tilted toward (111)A also exhibit larger two‐dimensional electron gas mobilities than quantum wells grown on substrates tilted toward (111)B for a given angle of ...

Variable angle spectroscopic ellipsometry: Application to GaAs‐AlGaAs multilayer homogeneity characterization

Variable angle spectroscopic ellipsometry has been applied to a GaAs‐AlGaAs multilayer structure to obtain a three‐dimensional characterization using repetitive measurements at several spots on the same sample. The reproducibility of the layer thickness measurements is of order 10 A, while the lateral dimension is limited by beam diameter, presently of order 1 mm. Thus, the three‐dimensional result mainly gives the sample homogeneity. In the present case we used three spots to scan the homogeneity over 1 in. of a wafer, which had molecular‐beam epitaxially grown layers. The thickness of the AlGaAs, GaAs, and oxide layers and the Al concentration x varied by 1% or less from edge to edge. This result was confirmed by two methods of data analysis. No evidence of an interfacial layer was observed on top of the AlGaAs.

Observation of high-frequency high-field instability in GaAs/InGaAs/AlGaAs DH-MODFETs at K band

The authors report the observation of high-field instability at room temperature with oscillation frequency as high as 24 GHz in GaAs/InGaAs/AlGaAs double-heterojunction-MODFETs (DH-MODFETs) of 1.2 mu m gate length. Negative drain differential resistance was also observed in these devices under various forward gate biases. The nature of this instability is believed to be caused by the efficient removal of the real-space transferred hot two-dimensional electrons in the AlGaAs layer through the forward-biased Schottky gate. A tuned oscillator, with a fundamental oscillation frequency as high as 19.68 GHz, has also been demonstrated at a gate bias of 1.3 V.<<ETX>>

dc characterization of the AlGaAs/GaAs tunneling emitter bipolar transistor

AlGaAs/GaAs tunneling emitter bipolar transistors (TEBT’s), grown by molecular beam epitaxy, have been fabricated. Device structures with two different tunneling barrier Al mole fractions and each for two different barrier thicknesses were characterized at room temperature. A differential current gain of 410 was achieved using a single 50 A AlAs tunneling barrier. Devices with either thinner barriers (20 A) or Al0.38Ga0.62As barriers had lower gains. Al0.24Ga0.76As/GaAs heterojunction bipolar transistors and GaAs homojunction bipolar transistors without tunneling barriers were also fabricated and characterized, for comparison. The performance of the homojunction devices was improved in all cases by inserting the tunneling barrier. The variations of the base and collector currents were measured for all devices and, for the TEBT’s they showed a functional dependence on the interfacial barrier Al mole fraction and thickness, which was attributed to carrier tunneling through the barrier. Furthermore, the abov...

Comparisons of microwave performance between single-gate and dual-gate MODFETs

Both a 1.2- mu m and a 0.3- mu m gate length, n/sup +/-GaAs/InGa/n/sup +/-AlGaAs double-heterojunction MODFET have been fabricated with single-gate and dual-gate control electrodes. Extrinsic DC transconductance of 500 mS/mm has been achieved from a 0.3- mu m single-gate MODFET. The device also has a current gain cutoff frequency f/sub T/ of 43 GHz and 14-dB maximum stable gain at 26 GHz with the stability factor k as low as 0.6 from the microwave S-parameter measurements. At low-frequency dual-gate MODFETs demonstrate higher gain than the single-gate MODFETs. However, the k of dual-gate MODFETs approaches unity at a faster rate. Power gain roll-off slopes of 3-, 6-, and 12-dB/octave have been observed for the dual-gate MODFETs.<<ETX>>