Separation of complex gaseous mixtures using gas chromatography (GC) is an important step in analytical systems for environmental monitoring, medical diagnosis, and forensic science. Due to its high resolving power, analysis speed, and small sample size, GC, has become the premier technique for separation and analysis of volatile and semi-volatile organic compounds. Miniaturization of analytical systems has become a major trend which is mainly driven by advancements in microfabrication techniques and a need for portable lab-on-a-chip systems for onsite monitoring. Microfabricated columns have been explored for applications in analytical processes like GC in several research studies. These microGC columns typically have open rectangular or open circular cross sections which is a result of the etching process utilized in the fabrication. This work reports the fabrication and performance of a new generation of silicon-on-glass micro-electro-mechanical systems (MEMS) based GC columns with microposts namely "semi-packed." These columns can be fabricated on a 2 cm2-die for a 1 m-long channel or a 1 cm2-die for a 25 cm-long channel. The semi-packed columns have a higher sample capacity as the overall surface area is larger than that of open rectangular columns of the same dimensions. The separation efficiency of these columns is also superior to that of open columns due to the presence of the microposts. As compared to conventional packed columns, the semi-packed columns show lower pressure drops and a more uniform flow profile, both of which contribute to, performance in terms of separation efficiency.