Micro-structure is like using micro-machining to make (most of time) Wheatstone bridge type circuit in a micro scale. Of course, band gap type device could be made in the form of a micro-structure. But in our case, we are trying to separate the sensors from their base materials. The sensitive materials in a micro-structure type IR sensor usually are PZT, PVDF, etc. They are usually pyroelectric and ferroelectric materials. Although some of the materials could be semi-conductive or even piezo-electric materials but people are using them based upon their pyroelectric and ferroelectric characteristics.
I searched some sites to give you more background. it is all depends on listener’s background, and how much they want to understand. Even Mike and I could give different definitions. In some of the cases, it is marketing that determine the name. Like Honeywell micro-structure mass flow sensor, the basic theory is the same as thermal mass flow sensor. But it sounds hi-tech calling "micro-structure".