Dimitris Pavlidis

Evaluation of the factors determining HBT high-frequency performance by direct analysis of S-parameter data

A novel parameter extraction formalism for the evaluation of heterojunction bipolar transistor (HBT) device physics is presented. The technique uses analytically derived expressions for direct calculation of the HBT T-model equivalent circuit element values in terms of the measured S-parameters. All elements are directly calculated with the exception of the emitter leg of the T-model. This approach avoids errors due to uncertainty in fitting to large, overdetermined equivalent circuits, and does not require the use of test structures and extra measurement steps to evaluate parasitics. Detailed bias-dependent results for the directly calculated circuit elements are presented. An analysis of the short circuit current gain that separates the transit times and RC products and allows evaluation of their individual contribution to the measured f/sub T/ and significance in limiting the HBTs high-frequency performance is reported. >

Capacitance–voltage characterization of AlN/GaN metal–insulator–semiconductor structures grown on sapphire substrate by metalorganic chemical vapor deposition

Electrical characterization of AlN/GaN interfaces was carried out by the capacitance–voltage (C–V) technique in materials grown by metalorganic chemical vapor deposition. The high-frequency C–V characteristics showed clear deep-depletion behavior at room temperature, and the doping density derived from the slope of 1/C2 plots under the deep depletion condition agreed well with the growth design parameters. A low value of interface state density Dit of 1×1011 cm−2 eV−1 or less around the energy position of Ec−0.8 eV was demonstrated, in agreement with an average Dit value estimated from photoassisted C–V characteristics.

Ga/sub 0.51/In/sub 0.49/P/GaAs HEMT's exhibiting good electrical performance at cryogenic temperatures

The DC and microwave characteristics of Ga/sub 0.51/In/sub 0.49/P/GaAs HEMTs grown by metalorganic chemical vapor deposition (MOCVD) are presented. Devices with 1- mu m-long gates show transconductances of 163 and 213 mS/mm at 300 and 77 K, respectively. Their maximum cutoff frequency is 17.8 GHz. Deep traps in the doped layer are evaluated at low temperature by the threshold voltage shift and current collapse phenomena. GaInP/GaAs HEMTs show no current collapse and have almost zero threshold voltage shift compared to AlGaAs/GaAs and InAlAs/InGaAs where the corresponding values are 0.5 and 0.25 V, respectively. >

Large-signal microwave performance of GaN-based NDR diode oscillators

The GaN material parameters relevant to the negative diAerential resistance (NDR) devices are discussed, and their physical models based on the theoretical predictions and experimental device characteristics are introduced. Gunn diode design criteria were applied to design the GaN NDR diodes. A higher electrical strength of the GaN allowed operation with higher doping (10 17 cm ˇ3 ) and at a higher bias (90 V for a 3 lm thick diode). The transient hydrodynamic simulations were used to carry out the harmonic power analysis of the GaN NDR diode oscillators in order to evaluate their large-signal microwave characteristics. The GaAs Gunn diode oscillators were also simulated for a comparison and verification purposes. The dependence of the oscillation frequency and output power on the GaN NDR diode design and operating conditions are reported. It was found that, due to the higher electron velocities and reduced time constants, GaN NDR diodes oAered twice the frequency capability of the GaAs Gunn diodes (87 GHz vs. 40 GHz), while their output power density was 2 10 5 W/cm 2 compared with10 3 W/cm 2 for the GaAs devices. The reported improvements in the microwave performance are supported by the high value of the GaN Pf 2 Z figure of merit, which is 50‐100 times higher than the GaAs, indicating a strong potential of the GaN for the microwave signal generation. ” 2000 Elsevier Science Ltd. All rights reserved.

Improved quality GaN by growth on compliant silicon-on-insulator substrates using metalorganic chemical vapor deposition

The use of compliant silicon-on-insulator (SOI) substrates instead of Si substrates is shown to improve the quality of epitaxial GaN layers by releasing the strain and absorbing the generated threading dislocations in the thin Si overlay of the SOI substrate. GaN layers have been grown on SOI substrates by low-pressure metalorganic chemical vapor deposition and various growth conditions and compared with GaN layers grown on Si substrates. Crystal uniformity, surface morphology, and number of threading dislocations of GaN layers grown on SOI substrates are improved compared to layers grown directly on Si substrates as evidenced by x-ray diffraction spectroscopy (XRD) and transmission electron microscopy. Full width at half maximum XRD values improved from 672 to 378 arcsec by growth on SOI instead of Si substrates. The GaN layers grown directly on Si substrates are highly resistive while all as-grown GaN layers on SOI substrates are unintentionally n type. For a 1–2 μm thick GaN layer grown on SOI, the ele...

Low‐ and high‐field transport properties of pseudomorphic InxGa1−xAs/In0.52Al0.48As (0.53≤x≤0.65) modulation‐doped heterostructures

We have grown pseudomorphic InxGa1−xAs/In0.52Al0.48As modulation‐doped heterostructures by molecular‐beam epitaxy under carefully controlled growth conditions. Mobilities as high as 13 900, 74 000, and 134 000 cm2/V s are measured at 300, 77, and 4.2 K in a heterostructure with x=0.65. Shubnikov–de Haas measurements indicate that the change in the effective mass with increasing In is not significant and is not responsible for the enhancement in mobilities. We believe that the improvement results from reduced alloy scattering, reduced intersubband scattering, and reduced impurity scattering, all of which result from a higher conduction‐band offset and increased carrier confinement in the two‐dimensional electron gas. The high‐field electron velocities have been measured in these samples using pulsed current‐voltage and pulsed Hall measurements. A monotonic increase in velocities is observed both at 300 and 77 K with an increase of In content in the channel. Velocities of 1.55×107 and 1.87×107 cm/s are meas...

First Observation of Bias Oscillations in GaN Gunn Diodes on GaN Substrate

In this paper, we report on the bias oscillation of GaN-based Gunn diodes realized on a n+-GaN substrate. Different contact materials, ambient gases, and pulsewidths were used and compared with regard to device stability. A wide negative- differential-resistance (NDR) region was measured for electrical- field values E larger than a threshold field Eth of 150 kV/cm. Electrical fields much higher than the threshold value did not lead to any electromigration effects or discharging problems from the contacts. The drift velocity derived from the current-voltage characteristics, diode geometry, and doping concentration in the active layer was estimated to be 1.9 times 107 cm/s. Bias oscillations were obtained for the GaN Gunn diodes in the presence of a series inductance.

Design and experimental characteristics of strained In/sub 0.52/Al/sub 0.48/As/In/sub x/Ga/sub 1-x/As (x>0.53) HEMTs

Strained In/sub 0.52/Al/sub 0.48/ As/In/sub x/Ga/sub 1-x/As (x>0.53) HEMTs (high electron mobility transistors) are studied theoretically and experimentally. A device design procedure is reported that is based on band structure and charge control self-consistent calculations. It predicts the sheet carrier density and electron confinement as a function of doping and thickness of layers. The DC performance at 300 K is presented. Wafer statistics demonstrate improvement of device characteristics with excess indium in the channel (g/sub m,/ /sub intr/=500 and 700 mS/mm for x=0.60 and 0.65). Microwave characterization shows the f/sub T/ improvement (f/sub T/=40 and 45 GHz for x=0.60 and 0.65, respectively) and the R/sub ds/ limitations of the 1- mu m-long-gate HEMTs. >

Luminescence of GaN nanocolumns obtained by photon-assisted anodic etching

GaN nanocolumns with transverse dimensions of about 50 nm were obtained by illumination-assisted anodic etching of epilayers grown by metalorganic chemical vapor deposition on sapphire substrates. The photoluminescence spectroscopy characterization shows that the as-grown bulk GaN layers suffer from compressive biaxial strain of 0.5 GPa. The majority of nanocolumns are fully relaxed from strain, and the room-temperature luminescence is free excitonic. The high quality of the columnar nanostructures evidenced by the enhanced intensity of the exciton luminescence and by the decrease of the yellow luminescence is explained by the peculiarities of the anodic etching processing.

A D-band monolithic fundamental oscillator using InP-based HEMTs

The design and experimental characteristics of the first fundamental D-band monolithic high-electron-mobility transistor (HEMT) oscillator are reported. The circuit is based on a dual feedback topology and uses 0.1- mu m pseudomorphic double heterojunction InAIAs/In/sub 0.7/Ga/sub 0.3/As HEMTs. It includes on-chip bias circuitry and an integrated E-field probe for direct radiation into the waveguide. An oscillation frequency of 130.7 GHz was measured and the output power level was -7.0 dBm using HEMTs of small gate periphery (90 mu im). This represents the highest frequency of fundamental signal generation out of monolithic chips.<<ETX>>