A thermopile arrangement is disclosed in WO 98/50763 in a gas flow sensor based on porous silicon. The thermopile generates an electric potential in response to a temperature change induced by a gas flow, and a hot resistor is also incorporated, heated with constant power.

It is now found that thermopile arrangements can be constructed using silicon technology which can be used to generate useful quantities of electric current.

The invention comprises a current generating thermopile arrangement having a plurality of thermocouples, characterised in that the device comprises: a thick porous silicon layer; "hot"thermocouple contacts on said thick porous silicon layer; a a bulk crystalline silicon member; and "cold"thermocouple contacts on said bulk, crystalline silicon layer.

The arrangement may be further characterised by having an encapsulation affording a ready heat path from an external contact such as a human finger to the hot thermocouple contacts.

The arrangement may be incorporated in a circuit with a device adapted to be powered by current from the arrangement when activated by heat contact. The device may comprise a light emitting diode. The circuit may be incorporated in a child's toy, which can light up on finger contact.

One embodiment of a current generating thermopile arrangement according to the invention will how be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a thermocouple arrangement; Figure 2 is a view of an arrangement like Figure 1 encapsulated; and Figure 3 is a view of a circuit including an arrangement like Figure 2 and light emitting nodes in a child's toy.

The drawings illustrate a current generating arrangement 11 having a plurality of thermocouples 12.

The arrangement comprises a crystalline silicon die 12, a thick porous silicon layer 13 and a passivation layer 14.

"Hot"thermocouple contacts 15 are on the thick porous silicon layer,"cold" thermocouple contacts 16 are on the bulk crystalline layer-the die 12.

The layers can be made by micromachining and/or deposition, the thermocouple elements and pads 17 for power take-off by deposition and/or etching in the usual way.

By approximately linking the thermocouples in series and/or parallel an arrangement having an area not much bigger than a child's fingerprint can be made to generate enough current at a suitable voltage to power a light emitting diode.

The arrangement may be encapsulated in any desired fashion save that, as seen in Figure 2, there is an area 21 above the"hot"thermocouple contacts 15 which affords a ready heat path thereto. Of course, the aim is to heat these contacts, while leaving the"cold"contacts at a lower temperature.

The arrangement 11 can be deployed in a child's toy, such as the toy cat 31 of Figure 3, the arrangement 11 being incorporated in a button 32 and being connected in a circuit 34 including two light emitting diodes, one in each eye 33. When a child presses on the button 32, the eyes 33 will light up.

Of course, the arrangement may be used also to power more serious devices.

For example, to provide for back illumination for liquid crystal displays of electronic watches, calculators and organisers is-by comparison with the rest of the device-a heavy drain on battery power. By providing a device 11 in a keyboard button, enough current can be generated by finger contact to provide for screen illumination without loading the battery. Indeed sufficient power may be generated for the entire operation of a calculator or organiser, as by having a finger contact area in or by the keyboard area.