B.K. Choi

The energy dependence of proton-induced degradation in AlGaN/GaN high electron mobility transistors

The effects of proton irradiation at various energies are reported for AlGaN/GaN high electron mobility transistors (HEMTs). The devices exhibit little degradation when irradiated with 15-, 40-, and 105-MeV protons at fluences up to 10/sup 13/ cm/sup -2/, and the damage completely recovers after annealing at room temperature. For 1.8-MeV proton irradiation, the drain saturation current decreases 10.6% and the maximum transconductance decreases 6.1% at a fluence of 10/sup 12/ cm/sup -2/. The greater degradation measured at the lowest proton energy considered here is caused by the much larger nonionizing energy loss of the 1.8-MeV protons.

Heavy-ion-induced breakdown in ultra-thin gate oxides and high-k dielectrics

Presents experimental results on single-event-induced breakdown in sub-5-nm plasma-enhanced SiO/sub 2/, nitrided SiO/sub 2/, Al/sub 2/O/sub 3/, HfO/sub 2/, and Zr/sub 0.4/Si/sub 1.6/O/sub 4/ dielectrics typical of current and future-generation commercial gate oxides. These advanced oxides are found to be quite resistant to ion-induced breakdown. Radiation-induced soft breakdown was observed in some films with 342 MeV Au (LET=80 MeV/mg/cm/sup 2/) but not 340 MeV I (LET=60 MeV/mg/cm/sup 2/). The critical voltage to hard breakdown was found to scale with the square root of the physical oxide thickness, not with the energy stored on the gate capacitance. Alternative dielectrics with equivalent oxide thickness substantially below their physical thickness were found to exhibit significantly higher voltage to hard breakdown than SiO/sub 2/ counterparts. All of the samples reached ion-induced hard breakdown at applied voltages well above typical operating power-supply voltages; these findings bode well for the use of advanced commercial integrated circuits in space systems.

Long-term reliability degradation of ultrathin dielectric films due to heavy-ion irradiation

High-energy ion-irradiated 3.3-nm oxynitride film and 2.2-nm SiO/sub 2/-film MOS capacitors show premature breakdown during subsequent electrical stress. This degradation in breakdown increases with increasing ion linear energy transfer (LET), increasing ion fluence, and decreasing oxide thickness. The reliability degradation due to high-energy ion-induced latent defects is explained by a simple percolation model of conduction through SiO/sub 2/ layers with irradiation and/or electrical stress-induced defects. Monitoring the gate-leakage current reveals the presence of latent defects in the dielectric films. These results may be significant to future single-event effects and single-event gate rupture tests for MOS devices and ICs with ultrathin gate oxides.

Process Dependence of Proton-Induced Degradation in GaN HEMTs

The 1.8-MeV proton radiation responses are compared for AlGaN/GaN HEMTs grown under Ga-rich, N-rich, and NH3-rich conditions. The NH3-rich devices are more susceptible to proton irradiation than the Ga-rich and N-rich devices. The 1/ f noise of the devices increases with increasing fluence. Density functional theory calculations show that N vacancies and Ga-N divacancies lead to enhanced noise in these devices.

Total-dose and single-event effects in switching DC/DC power converters

Total-dose and single-event effects in discrete switching DC/DC power converters are examined using a combination of circuit measurements and simulations. The total-dose experiments focus on the response of the power MOSFET used as the switching element for the converters. The efficiencies of two different types of converters (boost and buck) degrade with increasing total dose, leading to eventual functional failure. The single-event transient response of the converters is determined by the response of the feedback control circuitry. Radiation response is studied using both electrical measurements and simulation techniques, and issues affecting circuit failure are identified.

Characterization of 1.8-MeV proton-irradiated AlGaN/GaN field-effect transistor structures by nanoscale depth-resolved luminescence spectroscopy

We have used depth-resolved cathodoluminescence spectroscopy to examine AlGaN/GaN modulation-doped field-effect transistors that display degraded source-drain current characteristics after 1.8-MeV proton irradiation, along with bulk heterojunction field-effect transistor material after similar proton irradiation. For both cases, we have observed distinct changes in spectral emission features due to decreased internal electric-field strength and new point defects within different layers of the device structure with nanometer-scale depth resolution. These changes can account for the degraded electrical characteristics.

Nanodiamond Lateral VFEM Technology for Harsh Environments

This paper reports the first total dose tests on a nanocrystalline diamond lateral vacuum field emission microelectronics (VFEM) technology. This technology operates efficiently at both low and high temperatures (200degC) and is inherently ldquohardrdquo to radiation. No measurable change in device response is observed after 15 Mrad(SiO2) total dose exposure, signifying an emerging electronics for extreme environment.

Temperature dependence and irradiation response of 1/f-noise in MOSFETs

Measured the 1/f-noise of 3 /spl mu/m/spl times/16 /spl mu/m nMOS transistors with gate-oxide thickness of 48 nm as a function of frequency (f), gate voltage (V/sub g/), and temperature (T). For a temperature range of 85 K/spl les/T/spl les/320 K, noise measurements were performed at frequencies of 0.3 Hz/spl les/f/spl les/1 kHz with V/sub g/-V/sub th/=2 V, where V/sub th/ is the threshold voltage. Devices were operated in strong inversion in their linear regimes. A detailed comparison of the temperature and frequency dependences of the 1/f-noise of MOS transistors shows the importance of thermally activated charge exchange between the Si channel and defects in the oxide. After X-ray irradiation, the noise power increases after positive-bias irradiation and decreases after postirradiation annealing, in agreement with previous work. For these devices and experimental conditions, detailed comparisons of the temperature dependencies of the noise magnitude and frequency dependence show that the 1/f-noise of nMOS transistors is very well described by the model of Dutta and Horn. As a result, we are able to extract the energy distributions before and after irradiation for the near-interfacial oxide (border) traps that cause the majority of the noise.

Uniformity conditioning of diamond field emitter arrays

The authors present recent advances in the uniformity conditioning of diamond field emitter arrays (DFEAs). Postfabrication conditioning procedures consisting of thermal annealing and high field/current operation have been examined. Nonuniformity due to varying contamination states of the emitters can be mitigated by moderate temperature (∼150–300°C) operation. Operating the emitters at elevated current levels was found to enhance the spatial uniformity in a self-limiting manner. The conditioning mechanism is most likely thermal-assisted field evaporation of the diamond nanotips, however, the nature of the dc tests does not definitively exclude back bombardment as a possible contributor. Pulsed testing is underway to remove this ambiguity, provide conditioning for high-density arrays, and demonstrate the operational current density limits of DFEAs.

Aging and baking effects on the radiation hardness of MOS capacitors

A decrease in the oxide-charge trapping efficiency of Al- and TaSi-Al-gate MOS capacitors with oxide thicknesses ranging from 33 to 100 nm was observed after more than 14 years of room-temperature storage. The decrease in trapping efficiency can be reduced or even eliminated in Al-gate (and to a lesser degree TaSi-Al) devices by baking them at temperatures up to /spl sim/200 /spl deg/C. This change in aged-device radiation response after baking is largely independent of baking time, at least for 1-18 h bakes. Poly-Si gate capacitors processed and stored under similar conditions show no significant change in radiation hardness due to aging or baking. Mechanisms responsible for this behavior and implications of the results for hardness assurance are discussed.