Roderick A. B. Devine

Calculations of Radiation Dose-Rate Sensitivity of Bipolar Transistors

Mechanisms for dose-rate dependent effects of ionizing radiation are described. Bimolecular mechanisms are shown to produce reduced effects at high dose rates. Calculations using such mechanisms are shown to produce good agreement with data from devices affected by enhanced low dose-rate sensitivity (ELDRS).

Interface design to improve stability of polymer solar cells for potential space applications

Polymer solar cells may potentially fill the gap for a solar technology which satisfies the requirements of high specific power for space photovoltaics. Here, we address the issue of radiation hardness. Mathematical and analytical explanation for the observed degradation in Voc and guidance for designing more stable photocells have been provided.

Modeling of the X-irradiation Response of the Carrier Relaxation Time in P3HT:PCBM Organic-Based Photocells

Initial experimental work has demonstrated that x-ray bombardment of organic-based photocells (specifically P3HT:PCBM-based) leads to a reduction in the open-circuit voltage (Voc) without apparent change in the carrier relaxation time. The variation of Voc was suggested to be due to the injection and trapping of holes near the anode, which resulted in a decrease in the built-in potential. We have extended the experimental measurements to higher total dose (~1300 krad(SiO2)). Using standard inorganic modeling tools, a device model of the organic cell has been developed and predictions made. These predictions have been compared to the results of the previous and new experimental measurements and they demonstrate reasonable agreement between the two, thereby supporting the initial charge buildup hypothesis. Questions about the origin and behavior of the photo-carrier relaxation arise.

Insight into the multicomponent nature of negative bias temperature instability

A novel measurement technique is used to extract two physically distinct “permanent” (long lived on our experimental time scale, ≤12 000 s) and one recoverable charge components of the negative bias temperature instability in p-channel metal–oxide–semiconductor field effect transistors under inversion and n-channel devices under accumulation. The results suggest that the permanent components are present in both cases, while there is little, if any, recoverable charge present in the case of the n-channel device. A physical explanation is provided involving the band energy diagram to explain these observations.

Transient measurements of carrier relaxation time and density in the P3HT:PCBM organic photovoltaic cell

The authors have used transient photovoltage measurements to evaluate carrier relaxation times (τ) in P3HT:PCBM based photocells over a wide range of open circuit voltages. Satisfactory agreement is found with data obtained by low frequency impedance measurements. The authors find the differential capacitance measurements yield data consistent with the theoretical value expected based on Langevin recombination. The Langevin coefficient is three orders of magnitude smaller than the theoretical one. For the low light levels, the relaxation time variation is determined by the RC time constant behavior of the photodiode.

On the nature of “permanent” degradation in NBTI

This paper reports new high temperature measurements of Negative Bias Temperature Instability induced interface states in both NMOS and PMOS devices. Evidence of annealing of the interface states, previously thought to be “permanent”, is presented for measurements including a methodology which allows the direct measurement of the time dependent growth/recovery of the interface state component.

Tunneling discharge of positive trapped oxide charge in p-channel field effect transistors

Negative bias temperature instability has been studied in 130 nm and 90 nm channel length, SiO2 gate insulator field effect transistors at room temperature. Pulsed voltage stressing and subsequent recovery using times starting in the tens of microsecond regime were employed together with a single point data acquisition time ∼4 μs. Threshold voltage shifts were characteristic of oxide charge trapping as opposed to interface state generation. Recovery of the threshold voltage was modeled assuming quantum mechanical tunneling of trapped charges from the dielectric to the Si substrate.

Origin of Radiation Induced Damage in Organic P3HT:PCBM Based Photocells

Organic semiconductor photocells based upon P3HT:PCBM (1:1 by weight) have been subjected to X irradiation. The carrier lifetime has been determined using a pulsed optical method with continuous light to generate an open circuit voltage bias. No effect is observed on the carrier lifetime up to 300 krad (SiO2). However, changes in the open circuit voltage are observed and we argue that these result from compensation of the internal field which results from the built-in potential. The origin of the field modifications must lay in the presence of radiation induced trapped charge near the organic semiconductor/anode and/or cathode interfaces.

Ionizing radiation induced parametric variations in P3HT:PCBM organic photovoltaic cells

The authors have examined ionizing radiation effects in the organic photovoltaic material poly(3-hexylthiophene):([6, 6]-phenyl C61 butyric acid methyl ester) for total accumulated doses up to 300 krad(SiO2). The authors find that the open circuit voltage varies with the accumulation of irradiation; however, the other parameters such as relaxation time, short circuit current, and charge carrier density remain to first order constant. This behavior is inconsistent with observations on preirradiation devices in which all depend directly on the open circuit voltage.

Negative bias temperature instability threshold voltage shift turnaround in SiGe channel MOSFETs

Negative bias temperature instability (NBTI) has been measured at various temperatures in high-k gate insulator MOSFETs with buried Si0.65Ge0.35 channels and with regular Si surface channels. Previous studies on both surface channel Si devices and Si1−xGex buried channel devices provide evidence for net positive charge trapping as a function of electrical stressing time albeit reduced for the case of Si1−xGex compared to the Si case. In our case, for buried Si0.65Ge0.35, the authors find initial negative charge trapping followed by positive charge trapping at longer times, typically >10−1 s. The effect is accentuated at higher temperatures and yields a turnaround in the measured threshold voltage shift as a function of stress time. Closer examination of NBTI in high-k gate, Si surface devices stressed at room temperature and 90 °C suggests both electron and hole trapping may be present there although the majority effect is hole trapping.