A study of requirements elicitation and validation within an industrial environment is reported The key features in this part of the requirements process are: scenarios, (1.-; the prime means of elicitation; identification of domain objects, to capture the language of the domain and Fagan inspections for scenario validation by stakeholders. The process has been evaluated from both the requirements engineer s perspective and the viewpoint of the various stakeholders. The findings highlight a number of issues, both positive and negative, which are discuss ed. The deficiencies identified have stimulated our research. In particular, it is our contention, that requirements documentation needs to break away from the fixation with purely textual documents to ones that are media rich. Examples of this research, such as our hypermedia Scenario Manager, will be described 1: Introduction Increasing stakeholder involvement in the exploration and generation of system requirements is a recurring theme through much of the requirements engineering literature. Proponents of such a view often present this as an axiom for improving the quality of requirement". However, in reality this is an area where there is more theory than reported practice. An opportunity to redress this balance presented itself early in 1993. At this time one of Philips' professional organisations was beginning the development of a new system and, as part of this, was pioneering the use of an object oriented method, specifically, OOSE (Object Oriented Software Engineering) [Jacobson 92]. At the time OOSE was one of the few tool supported 00 methods using scenarios (which are referred to as use-cases by Jacobson) and their inclusion was one of its prime attractions. We note that other 00 methods e.g. [Booch 93], [Rumbaugh 94] have 0-8186-7017-7/95 $04.00 © 1995 IEEE 10 recently adopted use-cases. In addition to the use of scenarios the requirements generation process also included elicitation of domain objects and the validation of scenarios by stakeholders using Fagan inspections (Fagan 76]. We were therefore afforded an excellent opportunity to observe this process, how it evolved over 18 months and to capture the views of stakeholders and requirements engineers regarding their participation. This evaluation forms the focus of the first part of this paper.In the second part of the paper we briefly describe ideas and aspects of our research that have been stimulated by the case study. In particular, we consider the use of hypermedia for requirements documentation and its use in representing scenarios. 2: Case studyThe new system being developed is the software for a range of radiotherapy machines. These are commercial products aimed at the professional medical market. This software is to perform system control, system monitoring and provide an interface to operators and service engineers. A number of case-studies examining the use of scenarios have been performed, some of the more recent are: [Dardenne 93], [potts 94], [McDaniels 94] & [Kyng 94]. This paper g...
The switching speed of the Lateral Insulated Gate Transistor (LIGT) is slow compared to that of similar LDMOS power devices.The LIGT described in this paper, however, is designed to have both fast switching and high current conduction. The speed improvement is achieved by using a modified LIGT structure, where an additional n+ region is added to the p+ injector. The turn-off time of this modified LIGT is less than 450 nanoseconds, while turn-on is under 100 nanoseconds. Computer simulation is used t o understand the role of the shorted anode in improving the switching speed of the LIGT. A comparison with fabricated devices is shown to be in good qualitative agreement.
TESSA is a new program to simulate the coupled electrothermal behaviour of semiconductor devices in 2 & 3 dimensions. This paper describes the equations used to capture the electrothermal physics and the algorithms used to solve them. Simulations are reported for a multi-fingered bipolar power transistor. These show, for the first time, that the characteristic form of the temperature distribution, within the transistor, is quite different for the saturated and unsaturated cases. The effect of temperature on the partitioning of current between emitter fiigers is investigated. In addition, the components of heat generation e.g. recombination heat and their relative importance are calculated.1 .O Introduction.The numerical simulation of power devices supports many aspects of device design. In particular, prediction of breakdown voltages [l], current gains and certain areas of switching behaviour [2], [3] now form an important part of the design methodology. However, there still remain a number of areas of device performance that are not adequately supported by simulation. One of the most important of these is coupled thermal and electrical behaviour.The locally elevated temperatures produced by self heating of power devices can strongly effect the DC and transient performance of a device, even leading to device failure (e.g. thermal runaway in bipolar devices). Such interactions are, at present, not well understood from a device designer's perspective, despite their importance in defining the limits of device operation. To provide the capability to predict and understand the behaviour of devices in which thermal effects play a role a new 2/3D device simulator TESSA (Thermal and Electronic Semiconductor SimulAtor) has been developed. This paper describes the TESSA program, the equations solved and algorithms used. To illustrate the use of the program a number of transistor simulations are described. Device Equations.The physics of a semiconductor device, sufficient to describe the coupled electrical and thermal behaviour, can be captured in a number of partial differential equations. Those used in TESSA are: (1) V.(EVW) = q(n -p + N) 1 + an -V.J,-R at (3) (4) aT c p -= V.(KVT) + H at Here equation (1) is Poisson's equation, equations (2) and (3) are the electron and hole current continuity equations and (4) is the heat conduction equation. The dependent variables are potential w , electron and hole concentrations n and p and the lattice temperature T. The other physical quantities are dielectric constant E, net impurity concentration N , net recombination-generation R, specific heat capacity c, density p, thermal conductivity K and net heat generation H. These main equations have a number of subsidiary equations, for (2) and (3) we have the electron and hole transport equations: 3 J, = q p,,n E,, + q D,vn + q n DZVT, -7 q D,nV In(%) (5) and for holes:The drift fields E, , (n or p may be substituted for V) are assumed to be given by the gradients of the respective band edges and contain effects due to bandgap narrowin...
SUMMARYA novel algorithm has been implemented within our new semiconductor device simulation program TESSA that allows large signal periodic time-domain simulation to be performed by integrating over just one period of the excitation. Conventionally the need to integrate out initial transients, over many cycles, had effectively made such simulations impracticable. The algorithm can also be implemented in the field of circuit simulation so a combined large signal circuitidevice simulation package is now a practical possibility.
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