DETECTORS

The present invention relates to a pyroelectric detector and to a method of improving the sensitivity of the pyroelectric material in the pyroelectric detector.

A typical known pyroelectric detector is illustrated schematically in Figure 1, in which a temperature change in a pyroelectric material P will generate a voltage change on the gate of a junction field effect transistor J that can be measured at its output 2 with suitable circuitry. Line 3 is the supply voltage line to the transistor J and line 1 is at ground. A resistor R arranged in parallel with the pyroelectric material P has the dual function of controlling the electrical time constant of the device and of correctly biasing the gate of the transistor J.

An objective of the present invention is to provide an improved pyroelectric detector to that illustrated in Figure 1.

According to a first aspect of the present invention there is provided a method of improving the sensitivity of a pyroelectric material in a pyroelectric detector, the method comprising applying a bias across the pyroelectric material to enhance the sensitivity of the pyroelectric material above that obtained when no bias is applied across the pyroelectric material.

According to a second aspect of the present invention there is provided a pyroelectric detector comprising a circuit arrangement incorporating a pyroelectric material, the arrangement being such that when in operation the pyroelectric material is subjected to an applied bias potential, the output of the pyroelectric detector for a

temperature change in the pyroelectric material being dependent upon the strength of the bias applied across the pyroelectric material.

In one embodiment the pyroelectric material is electrically coupled to the gate of a junction field effect transistor whereby a temperature change in the pyroelectric material generates a voltage change at the gate of the junction field effect transistor. In other embodiments the junction field effect transistor can be omitted or replaced by an alternative component as for example a MOSFET, a bipolar transistor or a. resistor if a sufficiently high radiation level were being measured.

In another embodiment a resistor is arranged in series with the pyroelectric material the resistor being provided between the pyroelectric material and a low or zero voltage line.

The invention will be described further, by way of example, with reference to the accompanying drawings in which:-

Figure 1 is a schematic illustration of a known pyroelectric detector; and,

Figure 2 is a schematic illustration of a pyroelectric detector according to an embodiment of the present invention.

An improved radiation detector is shown in Figure 2. A non- signal side (lead 4) of the pyroelectric element P is taken to an external high voltage source (not shown) so that the pyroelectric material is operated under a bias field. The pyroelectric material used should be one having properties which are enhanced under these conditions and greater sensitivity is obtained.

A detector structure may be fabricated, for example, in which the pyroelectric material P is a modified lead zirconate ceramic, J is a

field effect transistor and R is 10 ¹⁰ ohms. The device area can be approximately 1mm ² and its thickness approximately 50μm. When in the configuration of Figure 1 and placed in front of a radiation source modulated at 200 HZ a signal level of 50 mV is observed after amplification. When reconfigured to match Figure 2 with the line 4 at + 150 volts, the output level rises to 75 mV under the same conditions of radiation input and amplification.

Although the invention has been described above in relation to the embodiment shown in Figure 2 it will be understood that other embodiments which fall within the scope of the invention would be understood to a person skilled in the art.

For example, the pyroelectric material element P may be only one of a multiplicity of similar elements arranged as a one or two dimensional array.

It will also be appreciated that the transistor J serves as a convenient impedance converter. It could be omitted or replaced by an alternative component eg. a MOSFET, a bipolar transistor or even a resistor if sufficiently high radiation level are to be measured.

The resistor R may not be required or may be built into the material of P depending on the bias requirements of the transistor J.