There are a number of reasons why POET was supported in the US by SBIR funding and why India is backing monolithic GaAs as their solution for space applications.

2. State of the Art 

In the imaging arts, next generation imaging systems must operate at very high frequencies and have a high resistance to radiation flux such that the imager is said to be "radiation hardened." State-of-the-art imagers are constructed as silicon integrated circuits in the form of CCDs or active pixel arrays. In the CCD, a linear array of pixels is clocked sequentially to a common output amplifier. In the active pixel array, the array is x-y addressable and each pixel is output to its own dedicated amplifier (the array is output on a row by row or column by column basis). 

Silicon technology is limited by the presence of the silicon oxide in both the active and passive regions of the integrated circuit in a number of ways. A main limitation is the sensitivity of the oxide to radiation flux. The radiation creates traps and other charged defects in the insulator which alter the internal voltage thresholds in both active and passive regions within the integrated circuit. After a certain cumulative exposure level, these threshold changes render the circuit inoperable. The gate oxide creates limitations in other ways as well. The silicon CCD couples one pixel to the other via overlapping gates. Each overlapping gate creates a small region of thicker oxide between pixels which inhibits charge transfer and therefore sets a speed limitation upon the CCD. These oxide barriers are fundamental to the silicon CCD and constitute a transfer speed limitation. Some approaches have been employed to eliminate these effects such as the virtual phase CCD. However, these structures are then faced with barriers created by implant misalignment and a lack of well capacity. In any event the transfer speed in the silicon CCD rarely exceeds a few MHz. 

A further limitation of the silicon CCD is its spectral sensitivity. The silicon CCD absorbs radiation across its energy gap and therefore is insensitive to radiation with a wavelength longer than about 1 um. It is also insensitive to ultraviolet (UV) radiation.