It might reasonably be expected that semiconductor devices would inevitably displace thermal devices in all aspects of gas sensing. However, thermal sources and detectors are inexpensive and suffice for a very wide range of gas sensing applications. The advent of thin film sources provided the necessary extended wavelength coverage beyond 4.5 µm compared with glass-envelope filament lamps, and the emitted power is generally higher for thermal sources. Because of the complexity of fabricating semiconductor devices with the materials used, there is a significant economic barrier to them entering the marketplace. They do offer the advantages of being robust and having a long lifetime and, when used as both source and detector, much higher modulation frequencies are available. The temperature dependence of semiconductor devices is higher than thermal devices, which requires temperature stabilisation, or compensation. Incorporating a modest cooling with the temperature stabilisation in semiconductor devices would be advantageous but at the expense of electrical power and some complexity. It is, perhaps, where reliability is paramount that semiconductor systems are likely to be introduced. © 2006 Springer-Verlag London Limited.