The inherent structural incompatibility between lasers and transistors is an obstacle in the monolithic integration of these two devices for optoelectronic integrated circuits (OEICs). One attractive solution could be in devising a common device layer structure which would support both functionalities. This would enable easy integration without selective area regrowth or reduce wire interconnects to link the laser and transistor. Among others, recently presented at Photonics West '97 was two independent attempts to exploit the idea of using a common epitaxial layer structure to fabricate both a laser and a transistor. These have been mostly empirical in nature. The common layer structure strategy imposes more constraints in the design and also on the experimental conditions required to achieve optimal performances in both laser and transistor functions than that for an individual function. However, in making heterojunction bipolar transistor (HBT) compatible laser structures, design issues reach beyond the study of layer compatibility to include issues involving contact configuration and doping profile re-engineering. A self-consistent theoretical investigation is called for to examine these new related issues and to further expand our comprehension of the device characteristics and performance possibilities. In this paper, we report on our findings of an in-depth theoretical investigation of the unique operational features of a new class of laser structures that are HBT compatible. They are, in fact, 'doping-converted' lasers fabricated from modified HBT structures. We highlight some of the new, largely unexpected, effects arising from the necessity of doping- conversion and of HBT compatible contact configuration. We describe quantitative results using the two example structures recently reported at Photonics West '97. These two examples, with similar QW insertion locations in the HBT structure, exhibit dramatically different lasing threshold and differential efficiency, and thus serves as a good demonstration of the underlying physics involved with the effects arising from contact configuration and doping profile re-engineering.