the following detailed description of the invention refers to the accompanying drawings . although the description includes exemplary embodiments , other embodiments are possible , and changes may be made to the embodiments described without departing form the spirit and scope of the invention . instead , the scope of the invention is defined by summary of the invention and appended claims , if any . wherever possible , the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts . [ 0054 ] fig1 illustrates a bolometer including a thin film metal control electrode 1 , which has the dual function of acting as an electrical contact and optical reflector , bottom insulator layer 2 , semiconductor layer 3 , top insulator 4 , and top metal film 5 . electrical contact to the semiconductor layer 3 is made for the purpose of bias and signal readout by contacts 6 and 7 . heavily doped semiconductor layers 8 and 9 may advantageously be placed between the semiconductor layer 3 and contacts 6 and 7 to obtain low - contact resistance . the detector structure may be generally supported on pillars 10 and 11 above a single crystal silicon wafer substrate 12 . the pillars may be advantageously solid metal structures prepared by such processes as copper or aluminium damascene technology , or formed by thin film processes such as image reversal lithography and lift - off . generally , it is preferred that the silicon wafer will have formed in its surface a microelectronic circuit for signal conditioning and readout . [ 0056 ] fig2 illustrates one possible arrangement for thermal isolation . in this simplified plan view , the metal film 5 is shown on top of the semiconductor layer 3 . the bias contact electrodes 6 and 7 make electrical contact to each end and on top of the semiconductor layer and extend to legs 13 and 14 . the legs have the dual purpose of electrical interconnection and thermal isolation , and are supported at the ends by pillars 10 and 11 . the control electrode 1 extends to legs 15 and 16 , which function in similar manner as legs 13 and 14 . it will be understood that legs 15 and 16 extend from the bottom of the semiconductor layer . because of the inherent strength and mechanical stability of the four - leg structure , each leg can be formed with small cross - sectional area to attain a low thermal conductance . however , other constructions may equally well be applicable . for example , in a further configuration four pillars of low thermal conductance may be positioned at the periphery of the detector structure . the detector will generally be formed on top of a ‘ sacrificial layer ’ such as a polyimide layer ( not shown ), which is coated on the silicon wafer 12 . vias are opened in the polyimide layer by conventional lithography and reactive ion etching ( rie ), and pillars 10 and 11 extend through the vias to the underlying silicon wafer . the following description provides information as to one possible order of manufacture of the detector components . metal film 1 is first deposited using conventional lithography and patterning by either lift - off or sputter etching . the preferred metals are nickel - chrome alloy or titanium . the thickness of this layer must be sufficient to act as an infrared reflector in the waveband 8 to 13 μm , and this requirement is met if the sheet resistance of the film exceeds 10 ohms per square . dielectric layer 2 is in preference silicon nitride , but may be another suitable dielectric material such as silicon carbide , and is patterned using rie . the semiconductor material forming layer 3 will be an amorphous silicon or silicon alloy , including one of : a . phosphorous or boron gas - phase doped amorphous silicon hydrogen alloy , a - si : h , prepared by plasma enhanced chemical vapour deposition ( pecvd ) or physical deposition ( sputter or thermal evaporation ); c . an alloy of silicon with nitrogen , boron , phosphorous , carbon , antimony or tin , prepared by co - deposition of silicon and the alloying material by physical vapour deposition the ability to vary the electrical resistivity and activation energy of silicon by gas phase doping or alloying with germanium is well known . the use of silicon alloyed with nitrogen , boron , phosphorous , carbon , antimony or tin for resistance bolometers provides a novel alternative of which only nitrogen has been previously been known in the present application . varying the alloying metal content significantly alters the electrical properties of the material to meet specific performance requirements . these alloys may or may not have hydrogen within the material . one objective of this material embodiment is to reduce the amount of hydrogen or eliminate it completely for the purpose of achieving low values of excess noise . microcrystalline silicon and polycrystalline silicon are cited as optional to amorphous silicon . following deposition and patterning of the silicon layer , layers 8 and 9 may be deposited and patterned . a low contact resistance is achieved by conventional methods such as gas - phase doping or ion implantation , or by using low resistivity alloys formed of the materials cited above for layer 3 . layer 4 is in preference silicon nitride , but other suitable dielectric such as silicon carbide may be employed . metal layer 5 is in preference nickel - chrome alloy or titanium , but other metals including platinum may be suitable alternatives . this layer is semi - transmitting to infrared radiation in the waveband 8 to 13 μm , and has a nominal sheet resistance of 377 ohm per square . the detector ‘ stack ’ comprising layers 1 , 2 , 3 , 4 and 5 comprises an integral optical interference filter ( also known as an optical cavity ) for absorption of infrared radiation in the desired waveband . this is a desirable , although not an essential feature of the invention . it may be noted that the top metal 5 is optional , in particular if layer 4 has the appropriate absorption index to meet the requirement for infrared absorption . the final operation is to remove the sacrificial layer by plasma etching , causing the detector to be thermally isolated from the underlying substrate . in an alternative arrangement , shown in fig3 layers 2 , 3 4 and 5 may be deposited in sequence and patterned using the same photo mask , the metal film 5 being sputter etched and the other films patterned using rie . it is to be understood that the technology described herein can be applied to either single detector elements or two - dimensional arrays of detectors . the latter may be in the form of sparse or close - packed arrays . a sparse array enables some versatility in the means for thermal isolation . for example , the folded legs shown in fig3 must be used for close - packed arrays , but for sparse arrays the legs can extend outwards to any convenient location . a large number of detectors or detector arrays can be fabricated by step - and - repeat lithographic methods , and then diced into separate chips for mounting , bonding and packaging . in other embodiments more than one control electrode may be employed to enable dual control functions in the same detector element . it will be understood by those skilled in the art that a detector prepared by the methods described will also function in passive mode , so that this specification is intended to include new methods for fabrication of micro - bolometers in general . advantageously , the detector or detector array will be integrated on the same silicon wafer as the associated roic , however , one or more of the signal conditioning and readout circuits may be located off - chip . furthermore signal - processing circuits may also be located on or off the detector chip . the detector may be operated with either a steady state ( dc ) or modulated bias current applied to the bias electrodes . a voltage applied to the control electrodes will alter the conductivity of the semiconductor , by virtue of the well - known theory of thin film filed effect transistors , thereby changing the bias current and signal voltage . this characteristic is particularly advantageous for an array of detectors where , with the aid of associated signal processing , the bias may be adjusted on a detector - to - detector basis to remove fixed pattern noise arising form small variations in the detective performance of individual detectors at a given operational temperature . furthermore , it is possible to adjust for larger signals that occur when the detector temperature changes , thus removing the requirement for temperature stabilisation . the ability to control bias current also suggests that applying a modulated control voltage could modulate the bias current . this alternative embodiment might be advantageous in that the signal would also be modulated and detectable by techniques such as phase sensitive detection . in some applications it may be advantageous to change the activation energy for electrical conduction of the semiconductor layer . this may be useful to adjust for variations that occur in semiconductor preparation , where the properties are difficult to control , or where the applied control voltage shifts the properties into a region of higher activation energy , hence sensitivity . those skilled in the art will appreciate that the present invention provides for significant improvements over existing technology , including , but not limited , to the following : a . the ability to tune the electrical resistance of the detector to match optimum input impedance of the associated roic ; b . the activation energy may be tuned to give a desired temperature coefficient of resistance ; thus when applied to a focal plane detector array the uniformity of response across the array can be enhanced ; c . by integrating the detector with an appropriately designed roic it will be possible to utilise the ability to control the resistance and activation energy in order to self - bias the circuit , taking the form of a feedback circuit to compensate for changes in resistance and activation energy due to ambient temperature drift ; and further advantages and improvements may very well be made to the present invention without deviating from its scope . although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment , it is recognized that departures may be made therefrom within the scope and spirit of the invention , which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus .