Michael J. O'Loughlin

Radiation Effects in High Electron Mobility Transistors: Total Dose Gamma Irradiation

AlGaAs/GaAs high electron mobility transistors (HEMTs) have been exposed to 90 Mrad(GaAs) of Co60 gamma radiation. Despite the heavy exposure, the peak transconductance for selected transistors was only diminished by approximately 4%. In addition, threshold voltage shifts of up to 80 mV were observed for both enhancement and depletion mode devices.

High energy neutron irradiation effects in GaAs modulation-doped field effect transistors (MOSFETs): threshold voltage

The effects of high energy neutrons on MODFETs were studied for fluences approaching 1*10/sup 15/ cm/sup -2/. Neutron-induced threshold-voltage shifts are described by application of a finite-temperature, strong-inversion, depletion-layer, charge-control model. The model indicates that the shifts are a consequence of electron trapping in the GaAs layer near the AlGaAs/GaAs interface. This allows a convenient parameterization of the neutron degradation by accounting for these trapped electrons as effective acceptors, defining an effective acceptor introduction rate, and applying the charge-control model to relate this introduction rate to the threshold voltage. The analysis shows that neutron degradation in these heterostructures is dominated by the change in the depletion layer charge and the shift in the Fermi level with neutron fluence. These dominant mechanisms depend on GaAs material parameters only. >

Modified quantum-well infrared photodetector designs for high-temperature and long-wavelength operation

A principal concern with the performance of GaAs/AlGaAs multiple quantum well long- wavelength detectors is obtaining high detectivity at relatively high temperatures. We have fabricated GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) with graded barrier regions by varying the aluminum concentration during the barrier growth to improve high temperature performance. The effect of barrier grading has been characterized by the measurement of dark current and responsivity, leading to an evaluation of the temperature dependent detectivity. Detectivity of 2 X 109 cm(root)Hz/W at 100 K has been measured on graded barrier QWIPs. We also report on the investigation of QWIPs with cut- off wavelengths ((lambda) c) greater than 12 micrometers . A QWIP with (lambda) c approximately 13.8 micrometers was grown with peak responsivity (Rp) of 1.2 A/W at 10 K. Both Rp and (lambda) c were weakly bias dependent. The maximum detectivity of 6 X 1012 cm(root)Hz/W was at 10 K at 11.7 micrometers and (lambda) c equals 14.5 micrometers .

Long-wavelength GaAs quantum-well IR detectors: low-temperature performance characteristics

We have fabricated and characterized several GaAs/AlGaAs multiquantum well infrared detectors at temperatures ranging from 6 K to 77 K. The detectors were designed to have a single bound state in the quantum well and the first excited state in the continuum above the AIGaAs conduction band edge. The difference in energy between the two levels, as determined by the quantum well width and aluminum mole fraction in the barrier, was chosen such that absorption would occur in the 8-14 tm wavelength region. Each detector was characterized by FTIR absorption, dark current, responsivity, spectral noise density, and thermal activation energy measurements. The -maximum observed detectivity is 1 .8 x 1 012 cmIHz/W at ?= 8.3 jim and 6 K.

Advances in multiple-quantum-well IR detectors

The performance of GaAs/AlGaAs multiple quantum well long-wavelength infrared detectors is evaluated for potential applications in focal plane arrays. A number of GaAs/AlGaAs quantum well infrared detectors with absorption between 7 to 12 micrometers have been fabricated and characterized. In these samples, the quantum well width and barrier height were held approximately constant, while the AlGaAs barrier thickness was varied from 300 to 500 angstrom. These detectors were characterized by FTIR absorption, dark current, responsivity, spectral noise density, and thermal activation energy measurements at temperatures ranging from 6 to 80 K. A maximum detectivity of 4 X 1013 cm(root)Hz/W at 6 K is obtained at (lambda) equals 8.4 micrometers for the 500 angstrom barrier sample.

Neutron degradation of the I-V characteristics of AlGaAs/GaAs modulation-doped field-effect transistors

A triangular-well, one-subband depletion layer model has been developed which applies over the range of I-V characteristics from subthreshold to saturation, some nine orders of magnitude in source-drain current. The model has been extended to describe neutron degradation of source-drain current and transconductance.