Non-stationary and transient transport effects are expected to play crucial roles in determining the performance of ultra-small electronic devices. This is especially true for structures fabricated of III-V compound and alloy materials. The nature of carrier transport in such structures demands the use of sophisticated modeling and simulation techniques in order to describe and predict experimental observations and device performance. We will address these modeling challenges by developing a moment-equation simulation tool as well as making enhancements in our existing Monte Carlo code. A prime objective and contribution of this research will be the development of an efficient coupling technique for the two simulation approaches. Details of the nature of suitable and compatible boundary conditions will be explored. We will also make a detailed assessment of the validity and utility of the moment-equation approach for ultra-small device simulation. Our target material systems will be In(x)Ga(1-x)As, In(x)Al(l-x)As, and InP, chosen because of the increased technological importance of these materials in opto-electronic communication systems. In order to fully exercise these simulation tools, we will focus our attention on a device characterized by both small geometries and large, rapidly varying fields, the heterojunction bipolar transistor or HBT. Our work will lead to an improved understanding of HBT device physics and new device designs exhibiting higher cut-off frequencies. These designs will exploit the transport phenomena studied with the new simulation tools. The coordinated implementation of the Monte Carlo and moment-equation tools and the assessment of the moment-equation approach in a device content will hasten the development of useful and efficient simulation tools suitable for device design. This simulation effort will also provide the necessary background for our future experimental investigations of carrier transport in HBT-related structures.