Richard J. Krantz

Applied Field and Total Dose Dependence of Trapped Charge Buildup in MOS Devices

A rate equation for charge buildup which includes carrier sweep out, geminate recombination, hole/ electron trapping, and effects of internal fields is developed. The first moment of the resulting charge distribution is calculated to yield the midgap voltage shift as a function of irradiation time. The initial midgap voltage shift per dose and the maximum midgap voltage shift are derived. The field dependence of these quantities is shown to be a consequence of the field dependence of the hole/electron capture cross sections and geminate recombination escape probability The results of this formulation show that the E-1/2 decrease in the midgap shift per dose with field described in the literature is due to the decrease of the hole capture cross section with increasing applied field. The theory is validated by comparison with experimental results obtained on 225 A thermal oxide on p-type silicon test capacitors irradiated under bias at room temperature.

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. >

Total dose-induced charge buildup in nitrided-oxide MOS devices

Nitrided oxides and reoxidized nitrided oxides processed at various nitridation temperatures for various nitridation times have been irradiated. The total dose response of these nitrided oxides has been analyzed and compared to that of radiation-hard control oxides. To aid in the analysis, the charge-trapping model of R.J. Krantz et al. (1987) has been extended to include electron trapping and qualitatively applied to simulate the experimental results. Nitridation temperature was found to have a significant effect on the radiation response of thin ( approximately 150 AA) nitrided oxides and reoxidized nitrided oxides. The data show that oxides nitrided at 1050 degrees C and reoxidized accumulate less fixed charge (by a factor of approximately 2) than the control oxides. Oxides nitrided at 950 degrees C and reoxidized accumulate substantially more fixed charge (by a factor of approximately 5) than the controls or any of the nitrided samples. The analysis indicates that nitridation creates neutral hole traps as well as neutral electron traps, and that reoxidation can decrease the concentration of hole and electron traps. >

Subthreshold I-V characteristics of AlGaAs/GaAs MODFETs: the role of unintentional acceptors

A strong-inversion depletion-layer model of threshold has been extended to describe subthreshold I-V characteristics in MODFETs. The results of this calculation yield the MODFET equivalent of the MOSFET charge sheet subthreshold model. For a typical molecular-beam-epitaxy-grown structure, the subthreshold current may differ by two orders of magnitude for a given gate voltage V/sub g/ and drain-to-source voltage V/sub ds/ as the acceptor doping varies from 10/sup 13/ to 10/sup 15/ cm/sup -3/. For these acceptor doping densities, the V/sub g/, for a given V/sub ds/, needed to maintain a constant subthreshold current varies by only approximately 0.1 V. If the acceptor density is increased to 10/sup 17/ cm/sup -3/, a large increase of approximately 0.8 V in the gate voltage is required to maintain a constant subthreshold current. These changes in subthreshold current with acceptor concentration in the bulk GaAs are significant and need to be included in an accurate MODFET model. >

The role of acceptor density on the high channel carrier density I-V characteristics of AlGaAs/GaAs MODFETs

Abstract A triangular-well, one-subband, depletion layer model has been developed for the high density region of a MODFET. The high density operation is defined as when the channel carrier density, in the entire channel, is equal to or greater than m1kT/πh2. This high density model has been used to describe the effects of the depletion layer charge on the I–V characteristics. An approximation for the experimentally determined threshold voltage is derived. It is shown that for small acceptor densities, ∼1013 cm−3, the experimentally determined threshold voltage may differ from the strong inversion threshold voltage by ∼0.25 V. We show that this discrepancy is due to the effect of the depletion layer in the device capacitance and the AlGaAs layer capacitance. The effective layer thickness, Δd, is shown to decrease from ∼90 A at an acceptor density of 1013 cm−3 to 75 A at 1017 cm−3.

The Influence of Fermi-Level Pinning at the GaAs Substrate on HEMT threshold Voltage

A two-dimensional quantum well, strong-inversion model of threshold in AlGaAs/GaAs high-electron-mobility transistors (HEMTs) has been extended to include the effects of Fermi-level pinning at the semi-insulating boundary of the thin active GaAs layer. The pinning of the Fermi level at the semi-insulating boundary results from an abundance of mid-gap traps in the substrate and couples the carrier channel at the AlGaAs/GaAs interface to the substrate. This communication between the carrier channel and the substrate causes the threshold voltage characteristics of the pinned devices to be substantially different from those of a conventional, semi-infinite HEMT structure. The discrepancy in the threshold voltage approaches 250 mV for a 0.1-μm active-layer device having a typical acceptor doping of ∼1014 cm−3. The quantum-well HEMT threshold model has also been compared to a classical analysis of the threshold voltage. For low acceptor doping and thin GaAs layers (<0.5 μm), neglecting quantum effects can result in significant errors in the threshold voltage calculation. For state-of-the art AlGaAs/GaAs microcircuits, in which device dimensions are shrinking and unintentional acceptor densities are decreasing, analyses of the I-V characteristics of thin, fully depleted devices must be carried out in the electric quantum limit and include the effects of the semi-insulating substrate.

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.