Philip Mumford

Dynamic work function shift in cold cathode emitters using current carrying thin films

We analyze a new cold cathode emitter which consists of a thin wide band gap semiconductor material sandwiched between a metallic material, and a low work function semimetallic thin film. We show that under forward bias operation the electrons captured in the low work function material are responsible for an effective reduction of the semimetallic film work function, together with a substantial increase of the cathode emitted current. The dynamic work function shift is shown to increase with the amount of injected current. Potential material candidates are suggested to achieve low‐voltage (<20 V), room‐temperature cold cathode operation with emission currents approaching several hundred A/cm2 and large efficiencies.

Space-charge effects and current self-quenching in a metal/CdS/LaS cold cathode

We analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS(cadmium sulfide)/LaS(lanthanum sulfide) cold cathode. Our approach is based on an ensemble Monte Carlo description of electron transport assuming ballistic injection across the CdS and LaS layers. Under this approximation, the energy spectrum of the injected beam entering the air gap can be determined exactly as a function of the applied bias across the CdS layer. The effects of shot noise in the injected current are taken into account. For some of the biasing conditions considered here, space-charge effects are quite drastic and lead to dynamical effects which are responsible for the onset of current self-quenching similar to the Child–Langmuir regime of operation of thermionic cathodes. The limiting anode current density is found to be much larger than the Child–Langmuir limit. In the presence of strong space-charge effects, large oscillations in the minimum of the electrostatic potentia...

Current crowding effects in a CdS/LaS cold cathode

We analyze the importance of current crowding in a new cold cathode emitter that consists of a thin wide band-gap semiconductor material sandwiched between a metallic contact and a low work function semimetallic thin film. Potential material candidates are suggested to achieve low-voltage (<10 V), room-temperature cold cathode operation with emission currents of several tens of A/cm2. We calculate the lateral potential drop that occurs across the emission window of cold cathodes with rectangular geometry and describe its effects on the emitted current density profile. The power dissipation in the cold cathode is calculated as a function of a dimensionless parameter characterizing the importance of current crowding. We determine the range of dc bias over which cold cathodes of different width must be operated to minimize current crowding effects.

Onset of current self-quenching in a metal/CdS/LaS cold cathode in the presence of inelastic scattering in the CdS layer

We use an ensemble Monte-Carlo approach to analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS/LaS cold cathode and its influence on the energy distribution of the electrons collected at the anode. Our analysis is based on a mean-free path approach to include the effects of inelastic scattering in the CdS layer following Fowler–Nordheim injection across the metal/CdS interface. We investigate in detail the regime of current self-quenching and show the importance of the effects of space charge in narrowing the energy of the electron beam collected at the anode. We compare our results to an analytical treatment of the current self-quenching regime based on a hydrodynamic description of electron transport through the gap.

Maximum stability data gathering trees for mobile sensor networks

We describe a benchmarking algorithm Max.Stability-DG algorithm to determine the sequence of data gathering trees for maximum stability in mobile sensor networks such that the number of tree discoveries is the theoretical global minimum. The Max.Stability-DG algorithm assumes the availability of the complete knowledge of future topology changes and works based on a simple greedy principle: whenever a new data gathering tree is required at time instant t, we determine a spanning tree that exists for the longest time since t, transform the spanning tree to a rooted data gathering tree and use it until it exists. The whole procedure is repeated until the end of the session to obtain a sequence of longest-living stable data gathering trees. We prove the correctness of the Max.Stability-DG algorithm and also evaluate the performance of the Max.Stability-DG trees compared to minimum-distance spanning tree-based data gathering trees through extensive simulations under diverse operating conditions.

A Benchmarking Algorithm to Determine Maximum Stability Data Gathering Trees for Wireless Mobile Sensor Networks

The high-level contribution of this paper is the design of a benchmarking algorithm to determine a sequence of the longest-living stable data gathering trees for wireless mobile sensor networks (MSNs) such that the number of tree discoveries is the theoretical global minimum. Referred to as the Max. Stability-DG algorithm, the algorithm assumes the availability of the complete knowledge of future topology changes, and operates according to a greedy strategy: Whenever a new data gathering tree is needed at time instant t, determine a spanning tree that will exist for the longest time since t and derive a data gathering tree by conducting a Breadth First Search on the spanning tree. We prove the correctness of the Max. Stability-DG algorithm that it indeed determines the sequence of longest living stable data gathering trees. Since the Max. Stability-DG trees are based on spanning trees covering the entire network of live sensor nodes, the average lifetime and the number of tree discoveries incurred for the Max. Stability-DG trees will serve respectively as the upper bound and lower bound for any network-wide communication topology determined using any other algorithm for mobile sensor networks.

Node Failure Time and Coverage Loss Time Analysis for Maximum Stability Vs Minimum Distance Spanning Tree Based Data Gathering in Mobile Sensor Networks

A mobile sensor network is a wireless network of sensor nodes that move arbitrarily. In this paper, we explore the use of a maximum stability spanning tree -based data gathering (Max.Stability -DG) algorithm and a minimum-distance spanning tree-based data gathering (MST -DG) algorithm for mobile sensor networks. We analyze the impact of these two algorithms onthe node failure times and the resulting coverage loss due to node failures. Both the Max.Stability -DG and MST-DG algorithms are based on a greedy strategy of determining a data gathering tree when one is needed and using that tree as long as it exists. The Max.Stability-DG algorithm assumes the availability of the complete knowledge of future topology changes and determines a data gathering tree whose corresponding spanning tree would exist for the longest time since the current time instant; whereas, theMST-DG algorithm determines a data gathering tree whose corresponding spanning tree is the minimum distance tree at the current time instant. We observe the Max.Stability-DG trees to incur a longer network lifetime (time of disconnection of the network of live sensor nodes due to node failures), a larger coverage loss time for a particular fraction of loss of coverage as well as a lower fraction of coverage loss at any time. The tradeoff is that the Max.Stability-DG trees incur a lower node lifetime (the t ime of first node failure) due to repeated use of a data gathering tree for a longer time .

DSP Based Implementation of Direction of Arrival for Wideband Sources

There is a need for the computation of direction of arrival (DOA) for wideband sources for number of applications. There are number of algorithms available in the literature for wideband case however we focus on Coherent Signal Subspace method proposed by Wang and Kaveh. Most algorithms available in the literature follow some variation of CSS algorithm thereby increasing computational complexity in an effort to get more accurate, statistically stable and unbiased DOA. We are interested in computing DOA for wideband sources in real time. We chose CSS algorithm and investigated possibility of implementing it using commercially available digital signal processor (DSP) in an effort to achieve real time capability. DSPs offer flexibility, ease of development of embedded system, reduces design cost and offers use of high-level programming language such as C. In this work, we propose a DSP based architecture for detecting and estimating the DOA of wideband sources. It is known that DOA algorithms require computation of eigenvalues and eigenvectors. It would be best to find computational friendly algorithm for the computation of eigenvalues and eigenvectors. In this work eigenvalues and eigenvectors are computed using well known Householder and QR algorithms. CSS algorithm is then implemented in C and executed on DIOPSIStrade 740 by Atmel. DIOPSIStrade 740 (D740) is a high performance dual-core processing platform for real time applications. The CSS algorithm was then parallelized and a parallel architecture was then developed. This paper presents parallel architecture using DIOPSIStrade 740 (D740) and computes performance parameters.

Cold cathodes are heating up

One potential application of cold cathodes is flat panel displays. At present, the market for emissive displays is based on cathode ray tubes (CRTs) and liquid crystal displays (LCDs). However, these technologies do not offer the combination of bright, sharp images in a low power, lightweight and flat package. Cold cathodes should offer these advantages in the not-too-distant future. Since the international market for displays by the year 2000 is predicted to reach over

A Benchmarking Algorithm to Determine the Sequence of Stable Data Gathering Trees for Wireless Mobile Sensor Networks

20 billion (US), an increasing number of innovative cold cathode concepts should be expected.