Abstract:
A pyroelectric demodulating detector (also termed a pyroelectric demodulator) is disclosed which utilizes an electrical resistor stacked upon a pyroelectric element to demodulate an rf or microwave electrical input signal which is amplitude-modulated (AM). The pyroelectric demodulator, which can be formed as a hybrid or a monolithic device, has applications for use in AM radio receivers. Demodulation is performed by feeding the AM input signal into the resistor and converting the AM input signal into an AM heat signal which is conducted through the pyroelectric element and used to generate an electrical output signal containing AM information from the AM input signal.

Description:
GOVERNMENT RIGHTS 
   This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 

   FIELD OF THE INVENTION 
   The present invention relates in general to pyroelectric devices, and in particular to a pyroelectric demodulating detector (also referred to herein as a pyroelectric demodulator) for demodulating an amplitude-modulated electrical signal. 
   BACKGROUND OF THE INVENTION 
   Amplitude demodulation of high-frequency radio signals received by an antenna in a radio receiver is conventionally performed using a mixer or diode demodulator which operates in a nonlinear regime. The mixer or diode demodulator has an electrical output response Φ out  that can be generally characterized in terms of its input signal Φ in  by a Taylor series expansion: 
             Φ   out     =       a   0     +       a   1     ⁢     Φ   in       +       a   2     ⁢     Φ   in   2       +       a   3     ⁢     Φ   in   3       +       a   4     ⁢     Φ   in   4       +   …           
where the input signal Φ in  can be amplitude modulated and characterized as a sinusoidal carrier having a time varying multiplying factor which contains information to be transmitted:
 Φ in   =b   0 (1 +m   0   ·m ( t ))cos(ω c   t ) 
where ω c  is the carrier frequency expressed in radians per second and is related to the carrier frequency f c  expressed in Hertz by ω c =2πf c .
 
   When this amplitude-modulated (AM) input signal Φ in  is passed through a conventional diode demodulator (also termed a “square-law” detector), the Taylor series expansion above is invoked, but only the squared term is useful to provide a demodulated output signal for recovery of the AM information. The squared term appears as: 
             Φ   out     =       a   2     ⁢           b   0   2     2     ⁡     [     1   +     cos   ⁡     (     2   ⁢     ω   c     ⁢   t     )         ]       ·     [     1   +     2   ⁢     m   ⁡     (   t   )         +       m   2     ⁡     (   t   )         ]               
In the above equation, the cos(2ω c t) term is filtered out by an RC filter formed by the “square-law” detector&#39;s output resistance and video capacitance, or by the use of an external filter. The resulting “square-law” detector output is then given by:
 
   
     
       
         
           
             Φ 
             out 
           
           = 
           
             
               a 
               2 
             
             ⁢ 
             
               
                 
                   b 
                   0 
                   2 
                 
                 2 
               
               ⁡ 
               
                 [ 
                 
                   1 
                   + 
                   
                     2 
                     ⁢ 
                     
                       m 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                   + 
                   
                     
                       m 
                       2 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                 
                 ] 
               
             
           
         
       
     
   
   The efficiencies of this “square-law” detection process are twofold. First, the coefficient a 2  reduces the available power in the output signal Φ out  from the “square-law” detector to a small fraction of the power in the input signal Φ in . Second, filtering out the cos(2ω c t) term becomes increasingly difficult as the modulation bandwidth of the amplitude-modulated m(t) signal containing the information being transmitted becomes a significant fraction of the carrier frequency ω c . For ultra-wide-band (UWB) signals the modulation bandwidth must be at least 25% of the carrier frequency ω c  to satisfy Federal Communications Commission (FCC) requirements. 
   The above analysis shows that conventional diode demodulators based on “square-law” detection are inefficient. In a conventional passive diode demodulator and filter combination, the insertion loss can be about −40 dB, or even lower. For a powered diode demodulator, the insertion loss can be reduced; but the overall power required including that for powering the diode demodulator can result in a ratio of receiver output power to total power which is on the order of −50 dB. 
   Similar losses can occur when a mixer is used for demodulation. In this case, the input signal Φ in  is multiplied by a local oscillator signal cos(ω LO t) in the mixer to provide an output signal Φ out  given by:
 
Φ out   =b   0 [1 +m ( t )cos(ω c   t )]·cos(ω LO   t )
 
The mixer generates up-converted and down-converted signal components in the output signal Φ out  as follows:
 
   
     
       
         
           
             Φ 
             out 
           
           = 
           
             
               b 
               0 
             
             ⁡ 
             
               [ 
               
                 
                   cos 
                   ⁢ 
                   
                     ( 
                     
                       
                         ω 
                         LO 
                       
                       ⁢ 
                       t 
                     
                     ) 
                   
                 
                 + 
                 
                   
                     
                       m 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     2 
                   
                   ⁢ 
                   
                     { 
                     
                       
                         cos 
                         ⁡ 
                         
                           [ 
                           
                             
                               ( 
                               
                                 
                                   ω 
                                   LO 
                                 
                                 - 
                                 
                                   ω 
                                   c 
                                 
                               
                               ) 
                             
                             ⁢ 
                             t 
                           
                           ] 
                         
                       
                       + 
                       
                         cos 
                         ⁡ 
                         
                           [ 
                           
                             
                               ( 
                               
                                 
                                   ω 
                                   LO 
                                 
                                 + 
                                 
                                   ω 
                                   c 
                                 
                               
                               ) 
                             
                             ⁢ 
                             t 
                           
                           ] 
                         
                       
                     
                     } 
                   
                 
               
               ] 
             
           
         
       
     
   
   When the mixer frequency ω LO  is equal to the carrier frequency ω c , the output signal Φ out  is demodulated as: 
             Φ   out     =         b   0     2     ⁢     m   ⁡     (   t   )               
The output signal Φ out  is reduced by at least 6 dB in this process, assuming perfect filtering. If the modulation bandwidth approaches the bandwidth of the input signal Φ in , the demodulated output signal Φ out  will be attenuated even more. Additionally, most receivers use multiple mixer and filter stages which can also reduce the output signal Φ out . Furthermore, the above analysis assumes perfect multiplication; whereas, in practice, the multiplication is accomplished by means of a nonlinear interaction induced by using the local oscillator to drive an active device into a nonlinear regime. This produces the same Taylor series expansion described above for the diode demodulator based on “square-law” detection so that the efficiency of mixing can be comparable to that of the diode demodulator particularly for low-level input signals.
 
   Other conventional demodulation schemes are essentially more complicated than those described above. A phase-locked loop demodulator behaves like a mixer in terms of its efficiency. Radio-frequency (rf) power detectors are generally diodes or linear conversion sensors and provide an insertion loss which is similar to the diode demodulator. 
   In conventional demodulators, the desired output signal containing information which has been impressed upon a carrier signal using amplitude modulation is only a small fraction of the receiver input signal so that the signal detection process is inefficient. Additionally, the problem of separating an AM information signal m(t) from the carrier frequency ω c  becomes very difficult and very inefficient as the frequency of the information signal m(t) approaches the carrier frequency ω c . For a UWB signal with a 25% modulation bandwidth, the detection problem is profoundly inefficient. What is needed is a more efficient type of demodulator which can be used to demodulate signals over a wide frequency range up to several GigaHertz (GHz) or more. 
   The present invention is a pyroelectric demodulator which operates passively to demodulate an AM electrical input signal to remove modulation at the carrier frequency ω c  and generate an electrical output signal containing the AM information. 
   These and other advantages of the present invention will become evident to those skilled in the art. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a pyroelectric demodulator for demodulating an amplitude-modulated (AM) electrical input signal to remove modulation at a carrier frequency and recover AM information. The pyroelectric demodulator comprises an electrical resistor to receive the AM electrical signal and to generate therefrom an AM heating signal in the electrical resistor which is substantially free from any signal at the carrier frequency; and a pyroelectric element in thermal communication with the electrical resistor to receive the AM heating signal and to generate therefrom an electrical output signal containing the AM information. 
   The pyroelectric demodulator can further comprise a substrate, with the pyroelectric element being in thermal communication with the substrate. The substrate can comprise, for example, silicon, silicon carbide, diamond, sapphire, ceramic or metal. 
   One side of the pyroelectric element can be attached to the substrate which acts as a heatsink; and the electrical resistor can be attached to another side of the pyroelectric element to act as a heat source. A pair of electrical input connections for providing the AM electrical signal to the electrical resistor can be suspended, at least in part, above the substrate to prevent a heat signal generated by the electrical resistor from bypassing the pyroelectric element. One or more electrical output connections from the pyroelectric element can also be suspended, at least in part, above the substrate to prevent the heat signal generated by the electrical resistor from being conducted through the electrical output connections and thereby bypassing the pyroelectric element. 
   The electrical resistor can comprise a layer of a resistive material such as tantalum nitride, nickel chromium (also termed nichrome), gold, copper, aluminum, tungsten or doped polycrystalline silicon. The layer of the resistive material can be located on a major surface of the pyroelectric element and electrically insulated therefrom by an intervening electrically-insulating layer (e.g. comprising a thin layer of silicon nitride, silicon dioxide or an adhesive such as epoxy). The electrical resistor can have a resistance substantially equal to 50 Ohms. 
   The pyroelectric element can comprise a pyroelectric material such as gallium nitride, lithium tantalate, lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), polyvinylidene fluoride (PVDF), triglycine sulfate (TGS), or sodium nitrite (NaNO 2 ). The pyroelectric element can comprise a pyroelectric material with a thickness of ≦2 microns when used to demodulate an rf or microwave electrical signal. In certain embodiments of the present invention, the pyroelectric element can be directly supported on the substrate. In other embodiments of the present invention, the pyroelectric element can be supported on a cantilever. 
   The AM information can have a carrier frequency up to about 10 GHz depending upon the size of the pyroelectric element. For UWB communications, the modulation bandwidth can be at least twenty-five percent of the carrier frequency. 
   The present invention also relates to a pyroelectric demodulator for demodulating an AM rf or microwave electrical signal to recover AM information therefrom. The pyroelectric demodulator comprises a pyroelectric element having a pair of major surfaces; a heat sink in thermal communication with one of the pair of major surfaces of the pyroelectric element; and an electrical resistor in thermal communication with the other of the pair of major surfaces of the pyroelectric element to receive the AM rf or microwave electrical signal and to generate therefrom a heat signal containing the AM information, and with the heat signal being conducted into the pyroelectric element to generate an electrical output signal containing the AM information. An electrically-insulating layer can be provided between the electrical resistor and the pyroelectric element for electrical isolation. 
   The pyroelectric element can comprise, for example, gallium nitride, lithium tantalate, PZT, PLZT, PVDF, TGS, or sodium nitrite. The exact thickness of a pyroelectric material within the pyroelectric element will, in general, depend upon a modulation bandwidth of the AM rf or microwave electrical signal and can be, for example, ≦2 microns. The electrical resistor can comprise a layer which is generally ≦1 micron thick, and can be formed from a resistive material such as tantalum nitride, nichrome, gold, copper, aluminum, tungsten or polycrystalline silicon which has been doped for electrical conductivity. The electrical resistor can have a resistance substantially equal to 50 Ohms. The heat sink can comprise a substrate material having a relatively high thermal conductivity such as silicon, silicon carbide, diamond, sapphire, ceramic, or metal. 
   Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
       FIG. 1A  shows a schematic plan view of a first example of the pyroelectric demodulator of the present invention. 
       FIG. 1B  shows a schematic cross-section view of the device of  FIG. 1A  along the section line  1 - 1  in  FIG. 1A . 
       FIGS. 2A and 2B  show schematic plan and cross-section views, respectively, of a second example of the pyroelectric demodulator of the present invention. 
       FIG. 3A  shows a schematic plan view of a third example of the pyroelectric demodulator of the present invention. 
       FIG. 3B  shows a schematic cross-section view of the device of  FIG. 3A  along the section line  2 - 2  in  FIG. 3A . 
       FIG. 4A  shows a schematic plan view of a fourth example of the pyroelectric demodulator of the present invention. 
       FIG. 4B  shows a schematic cross-section view of the device of  FIG. 4A  along the section line  3 - 3  in  FIG. 4A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1A and 1B , there is schematically illustrated in plan view and in cross-section view, respectively, a first example of the pyroelectric demodulator  10  of the present invention. The pyroelectric demodulator  10  comprises a pyroelectric element  12  which has a pair of major surfaces  14  and  16 , each of which has been metallized. In thermal communication with one major surface  14  is an electrical resistor  18 ; and in thermal communication with the other major surface  16  is a heatsink  20  (also referred to herein as a substrate) which can include a metallized surface  22  as shown in  FIGS. 1A and 1B , or can be formed entirely of metal (e.g. aluminum, copper, gold, etc.). 
   The pyroelectric demodulator  10  of  FIGS. 1A and 1B  can be formed as a hybrid device with the pyroelectric element  12  being attached to the heatsink  20  (e.g. with solder or an electrically-conductive epoxy), and with the electrical resistor  18  being attached to the pyroelectric element  12 . An electrically-insulating layer  24  (e.g. an adhesive such as epoxy, silicon dioxide or silicon nitride) can be used to provide electrical isolation between the resistor  18  and the pyroelectric element  12 . When the electrically-insulating layer  24  comprises an adhesive, the adhesive can be used to directly attach the resistor  18  to the pyroelectric element  12 . When silicon dioxide or silicon nitride is used as the electrically-insulating layer  24 , these materials can be deposited over the pyroelectric element  12  and then the electrical resistor  18  can be deposited directly onto the electrically-insulating layer  24 , or attached thereto with an adhesive. Alternately, the device  10  of  FIGS. 1A and 1B  can be formed entirely as a monolithic device by depositing and patterning a plurality of material layers one upon the other to build up the device  10  layer by layer as will be described in detail hereinafter. 
   In the pyroelectric demodulator  10  of  FIGS. 1A and 1B , an AM electrical signal to be detected (i.e. demodulated) can be provided to the electrical resistor  18 . This can be done using a pair of electrical input connections  26  (i.e. wires or deposited metal traces) which are suspended, at least in part, above the heatsink  20  to minimize heat transfer from the resistor  18  to the heatsink  20  through the electrical input connections  26 . Each electrical input connection  26  can be connected between a terminal  28  on the electrical resistor  18  and a contact pad  30  on the heatsink  20 . The AM electrical signal which is input to the electrical resistor  18  comprises a carrier frequency ω c  which has information impressed thereon by amplitude modulation. 
   The AM electrical signal is dissipated within the electrical resistor  18  and is entirely converted into heat to generate an AM heating signal which follows the AM information contained within the AM electrical signal, but which is not responsive to the higher carrier frequency ω c . This high responsivity of the heating signal to the AM information and relatively low responsivity to the carrier frequency ω c  can be tailored by an appropriate sizing of the electrical resistor  18  and its material and thermal characteristics. The electrical resistor  18  can comprise a thin (e.g. ≦1 μm thick) resistive layer  32  of a metal such as tantalum nitride, nickel chromium (also termed nichrome), gold, copper, aluminum, or tungsten. Alternately, the electrical resistor  18  can comprise a semiconductor such as polycrystalline silicon (also termed polysilicon) which is doped to provide a predetermined level of electrical resistivity (e.g. 50Ω) with an impurity dopant such as boron or phosphorous. Capacitive coupling of the AM electrical signal into the pyroelectric element  12  can also be minimized by making the resistive layer  32  and or the electrically-insulating layer  24  several times thicker than a “skin depth” at the carrier frequency ω c . The term “skin depth” refers to a depth which an rf signal at a particular frequency will penetrate into an electrically-conductive material. 
   Since the electrical resistor  18  is effectively thermally isolated in all directions except in a downward direction through the pyroelectric element  12 , the AM heating signal is forced to travel down through the pyroelectric element  12  and into the heatsink  20 . In traveling from the electrical resistor  18  down into the pyroelectric element  12 , the AM heating signal will create a changing temperature difference ΔT across a pyroelectric material between the two metallized surfaces  14  and  16  within the pyroelectric element  12 . This changing temperature difference ΔT will, in turn, generate an electrical output signal in the pyroelectric element  12  between the two metallized surfaces  14  and  16  which will contain the AM information from the electrical signal applied across the resistor  18 . The electrical output signal can be provided to additional contact pads  30  on the heatsink  20  by a pair of electrical output connections  34 . 
   In the example of  FIGS. 1A and 1B , the pyroelectric element  12  performs the demodulation of the AM electrical signal which is inputted into the resistor  18  by converting a temperature difference ΔT between the resistor  18  and the heatsink  20  into an electrical output voltage as constituent atoms within the pyroelectric material move back and forth in response to changes in temperature produced by the AM heating signal. 
   The thickness of the pyroelectric material in the pyroelectric element  12  between the metallized surfaces  14  and  16  can be selected so that conduction of the AM heating signal through the pyroelectric element  12  to the underlying heatsink  20  occurs at a rate which is sufficient for a modulation bandwidth of the information to be recovered from the AM electrical signal inputted into the resistor  18 . The rate at which heat conduction through the pyroelectric element  12  occurs can be selected, for example, to be resonant with the modulation bandwidth of the information to be recovered. This can be done by selecting the thickness of the pyroelectric material within the element  12 . In general, a higher modulation bandwidth will require a thinner pyroelectric element  12 . 
   When the thickness of the pyroelectric material within the pyroelectric element  12  is very small (e.g. ≦2 μm) to provide a modulation bandwidth in the GHz range for operation with a carrier frequency up to about 10 GHz, the heat flux q across the pyroelectric material is governed by an equation of phonon radiative transfer given by: 
           q   =       σ   ⁡     (       T   1   4     -     T   0   4       )             3   4     ⁢     (     L   l     )       +   1             
where σ is the Stefan-Boltzmann constant of radiative heat transfer, T 1  is the temperature on one side of the pyroelectric material, T 0  is the temperature on the other side of the pyroelectric material, L is the thickness of the pyroelectric material, and l is the phonon mean free path within the pyroelectric material. Further information on the equation of phonon radiative transfer (EPRT) can be found in an article by A. Majumdar, “Microscale Heat Conduction in Dielectric Thin Films,”  Journal of Heat Transfer , vol. 115, pp. 7-16, 1993, which is incorporated herein by reference.
 
   For a thickness L of the pyroelectric material which is about equal to the phonon mean free path l or less, heating incident at one side of the pyroelectric material will travel through the pyroelectric material at a speed which is about 57% of the acoustic velocity v a  in the pyroelectric material. Additionally, there will be a time lag of approximately one picosecond in converting a temperature difference ΔT across the pyroelectric material into a voltage difference between the metallized surfaces  14  and  16  of the pyroelectric element  12 . As an example, consider a pyroelectric demodulator  10  comprising a pyroelectric element  12  formed with a 1.5 μm thick layer of gallium nitride (GaN) for which v a =5800 m-s −1 . In such a device  10 , the thermal transport is governed by the equation of phonon radiative transfer with a thermal time constant in the GaN equal to 450 picoseconds [i.e. L/(0.57v a )] so that a demodulation bandwidth of this device  10 , which is inversely related to the thermal time constant, would be equal to about 2.2 GHz. 
   As another example, a pyroelectric demodulator  10  suitable for demodulating a received UWB AM electrical signal having a 25% bandwidth (i.e. 1.6 GHz bandwidth) centered at a carrier frequency of 6.5 GHz in the 3.1-10.6 GHz UWB frequency band allocated by the FCC for commercial use can be formed with a pyroelectric element  12  having a pyroelectric material thickness of approximately 2 μm when GaN is used as the pyroelectric material. For other pyroelectric materials such as lithium tantalate, PZT, PLZT, PVDF, TGS, sodium nitrite, etc., the exact thickness L of the pyroelectric material needed for demodulating an AM electrical signal with a predetermined signal bandwidth Δf can be determined from the acoustic velocity v a  for these materials using the equation: 
           L   =       0.57   ⁢     v   a         Δ   ⁢           ⁢   f             
when the thickness L of the pyroelectric material is about equal to the mean free path of phonons l or less.
 
   For cases where the thickness L of the pyroelectric material is appreciably larger than the mean free path of phonons l, the heat transport through the pyroelectric material can be determined from the general three-dimensional heat flow equation for the temperature T given by: 
                 ∇   2     ⁢   T     +       τ   T     ·       ∂     ∂   t       ⁡     [       ∇   2     ⁢   T     ]         +       1   k     ·     [     Q   +       τ   q     ·       ∂   Q       ∂   t           ]         =         1   α     ·       ∂   T       ∂   t         +         τ   q     α     ·         ∂   2     ⁢   T       ∂     t   2                   
where Q is the heat source, in W-m 3  (i.e. the AM heating signal produced by the electrical resistor  18 ), α is the thermal diffusivity of the pyroelectric material in the element  12 , τ T  is the inverse of the phase lag of the temperature gradient, τ q  is the inverse of the phase lag of heat flux, and k is the thermal conductivity of the pyroelectric material. Additional parameters for determining the heat flow through a given pyroelectric material include the density ρ, heat capacity C p , phonon mean free path l, and acoustic velocity v a . Further details of the thermal transport through materials governed by the general three-dimensional heat flow equation above can be found in a book by D. Y. Tzou entitled  Macro - to Microscale Heat Transfer: The Lagging Behavior  (Taylor and Francis Publishing Co., 1997, pp. 1-34), which is incorporated herein by reference.
 
   In the case of GaN, the heat flux time delay τ q  is about 4 picoseconds at room temperature, and only increases slightly at an elevated temperature of 150° C. The temperature gradient time delay τ T  is also comparable to q. Since the time delays τq and τ T  are both much smaller than the 450 picosecond thermal time constant of GaN, the general three-dimensional heat flow equation for the temperature T above for thicker (L           l) pyroelectric materials reduces to:
                 ∇   2     ⁢   T     +       1   k     ⁢   Q       =       1   α     ·       ∂   T       ∂   t               
where the heat Q provided by the electrical resistor  18  serves as an excitation source for heat transfer to the pyroelectric material, and the resulting temperature profile across the pyroelectric material follows the well-known Fourier diffusion model. Using this equation, a GaN pyroelectric demodulator  10  can be formed which is suitable for demodulating AM electrical input signals having a modulation bandwidth in the range of about 10 MHz-1 GHz.
 
   The response time of the pyroelectric demodulator  10  of the present invention is not limited by the pyroelectric material, which responds to a change in temperature and rearranges its crystal lattice on a time scale on the order of 1 picosecond, but instead is limited by electrical parameters such as capacitance and inductance, and by the heating response time of the resistor  18 . In addition to having a response time which can be tailored by the size and thickness of the pyroelectric material and the overlying resistor  18 , the pyroelectric demodulator  10  can provide a relatively large electrical output signal, thereby overcoming limitations of conventional demodulators based on “square-law” detection. 
   As an example, the pyroelectric coefficient P V  of GaN along the c-axis is reported to be 7×10 5  Volts-meter −1  per degree Kelvin of temperature difference ΔT across the GaN pyroelectric material. For a pyroelectric element  12  having a 2-μm-thick layer of GaN pyroelectric material, the above value of P V  would be expected to provide a theoretical output voltage of 1.4 Volts if the temperature difference across the GaN pyroelectric material were 1° C. Such a temperature difference could theoretically be produced by a 0.5 μm thick polysilicon 50Ω resistor  18  with a 10 μm square size and 116 picogram mass based on the heat capacity of silicon which is about 0.7 J-gm −1 ° C. −1  and using an electrical input power of about 80 picoJoules which is equivalent to an AM electrical input signal with 90 microVolts of peak-to-peak voltage or 64 microVolts root-mean-square (rms) voltage. This simplified calculation assumes that the resistor  18  is thermally isolated so that all of the heat produced in the resistor  18  flows through the pyroelectric element  12 . Even if there are inefficiencies and thermal losses not taken into account in the above simplified calculation, the pyroelectric demodulator  10  of the present invention should still be able to provide a net voltage gain in demodulating an AM electrical input signal, thereby providing an advantage over conventional demodulators. 
   Returning to the example of the pyroelectric demodulator  10  in  FIGS. 1A and 1B , this device  10  can be fabricated on many different types of substrates  20 , which preferably have a relatively high heat conductivity, including substrates comprising silicon, silicon carbide, diamond, sapphire, ceramic, and metal. The primary characteristic required for the substrate  20  is a relatively high thermal conductivity to efficiently and quickly remove heat from the pyroelectric element  12 . Those skilled in the art will understand that other types of substrates than those specifically mentioned above can be used for practice of the present invention. Additionally, although a single device  10  is shown and described herein, those skilled in the art will understand that a plurality of pyroelectric demodulators  10  can be formed on a common substrate  20 . This can be done to demodulate a plurality of channels of rf or microwave signals (e.g. in a multi-channel receiver). This also allows fabrication costs to be reduced by using batch fabrication processes to fabricate a plurality of individual devices  10  on the common substrate  20  and subsequently to separate out the individual devices  10  (e.g. by sawing or dicing) for individual packaging thereof. 
   When the pyroelectric demodulator  10  is formed as a hybrid device, the size of the pyroelectric element  12  can range from a few millimeters lateral dimensions and about 0.1 mm thickness down to a few tens of microns lateral dimensions and a few hundred nanometers thickness. Such devices  10  are expected to be suitable for use with carrier frequencies in the 100 kHz to about 1 GHz range, and with modulation bandwidths up to several MHz or more. 
   Smaller devices  10 , which are suitable for operation with modulation bandwidths and carrier frequencies in the GHz range (i.e. up to a few GHz modulation bandwidth and 10 GHz carrier frequency) can be monolithically fabricated on the substrate  20 . This can be done, for example, by depositing one or more metal layers over the substrate  20  and patterning the metal layers by reactive ion etching to form a lower electrical contact for the pyroelectric element  12 . The metal layers can comprise, for example, a layer of titanium up to a few tens of nanometers thick followed by a layer of platinum up to a few hundred nanometers thick. The titanium and platinum layers can be sputter deposited over the substrate  20 . 
   The pyroelectric material can then be deposited over the metal layers on the substrate  20 . This can be done, for example, by spin coating a commercially available polymerizable sol-gel precursor solution containing metal cations of lead, zirconium and titanium (e.g. available from Mitsubishi Materials Corp.) to provide a layer about 40-50 nm thick which can then be dried and pyrolyzed on a hot plate in air at about 400° C. for one minute to form a PZT layer. A seed layer of lead titanate (PT) can be optionally spun onto the substrate  20  and pyrolyzed at 400° C. prior to forming the PZT layer to enhance perovskite formation in the PZT layer, thereby improving the quality of the PZT layer and all subsequently-deposited PZT layers. 
   Subsequent PZT layers can be similarly spin coated over the substrate  20  and pyrolyzed to build up the thickness of the pyroelectric material. After deposition and pyrolysis of a few layers of the PZT, a rapid thermal annealing step can be performed at a temperature of about 700° C. in an oxygen ambient. This process of spin coating, pyrolysis and annealing can be repeated, as needed, to build up the pyroelectric material to a predetermined overall thickness ≦2 μm depending upon the modulation bandwidth desired for the pyroelectric demodulator  10 . 
   Further details of sol-gel deposition of PZT can be found in an article by T. G. Cooney et al., “Processing of Sol-Gel Derived PZT Coatings on Non-Planar Substrates,” Journal of Micromechanics and Microengineering , vol. 6, pp. 291-300, 1996; and in another article by A. A. Talin et al., “Epitaxial PbZr 0.52 Ti 0.48 O 3  films on SrTiO 3 /(001)Si Substrates Deposited by Sol-Gel Method,”  Applied Physics Letters , vol. 81, pp. 1062-1064, 5 Aug. 2002, both of which are incorporated herein by reference. Those skilled in the art will understand that PLZT and lithium tantalate can also be formed by sol-gel deposition in a manner similar to that described above, and as described in more detail in the references cited above. Additionally, those skilled in the art will understand that there are other methods well-known to the art for depositing pyroelectric materials such as PZT, PLZT, lithium tantalate, PVDF, TGS and sodium nitrite including sputter deposition, pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MOCVD), and electro-spray deposition (ESD). If the pyroelectric material is not adequately poled during deposition, poling can be performed after fabrication of the pyroelectric demodulator  10  is completed. Poling aligns domains within the pyroelectric material along a preferred polar axis by applying a voltage of generally less than 20 Volts across the pyroelectric element  12 . This can be done while the pyroelectric element  12  is heated up near to or above the Curie temperature of the pyroelectric material. 
   After the pyroelectric material has been formed, an upper contact metallization comprising platinum or chromium and platinum up to a few hundred nanometers thick can be sputter deposited over the pyroelectric material. Reactive ion etching can be used to pattern the pyroelectric material and the upper contact metallization. 
   A layer  24  of an electrically-insulating material such as silicon dioxide or silicon nitride can then be deposited over the upper contact metallization to electrically insulate the upper contact metallization of the pyroelectric element  12  from the electrical resistor  18  which will be formed on top of the pyroelectric element  12 . The electrically-insulating layer  24  can be up to a few hundred nanometers thick. 
   The electrical resistor  18  can then be formed over the electrically-insulating layer  24  by depositing and patterning the resistive layer  32  comprising tantalum nitride, nichrome, gold, copper, aluminum, tungsten, or doped polysilicon. These materials forming the resistive layer  32  can be deposited by sputtering or chemical vapor deposition. The terminals  28  for the electrical resistor  18  can be formed, for example, from sputter-deposited layers of nichrome, palladium and gold. After deposition the resistance of the electrical resistor  18  can be stabilized by heating of the resistor  18  to an elevated temperature of a few hundred ° C. Additionally, the resistive film  32  can be laser trimmed, if needed, to provide a resistance of about 50 Ohms. 
   When the lateral dimensions and thickness of the pyroelectric element  12  and the electrical resistor  18  become very small (e.g. less than about 50 μm on a side) then deposited and patterned metallizations (also termed traces) can be used for the electrical input and output connections  26  and  34 . The use of patterned metallizations substantially reduces the cross-sectional area of the connections  26  and  34  since the metallizations can be a fraction of a micron thick (e.g. 0.2-0.3 μm), and can have a width of, for example, 1-2 μm. Additionally, the connections  26  and  34  can be formed as air bridges suspended above the substrate  20  to further reduce the heat loss through the connections  26 . This is schematically illustrated in a second example of the pyroelectric demodulator  10  of the present invention in  FIGS. 2A and 2B . 
   The second example of the pyroelectric demodulator  10 , which is shown schematically in plan view in  FIG. 2A  and in cross-section view in  FIG. 2B , can be formed on a substrate  20  which can comprise, for example, silicon, silicon carbide, diamond, sapphire, ceramic, or metal. The pyroelectric element  12  can be formed on the substrate  20 , or alternately on a pedestal  36  as schematically illustrated in  FIG. 2B . The pedestal  36 , which can be formed from the substrate  20  (e.g. by etching away a portion of the substrate  20 ) or deposited thereon (e.g. as a layer of polycrystalline silicon, silicon nitride, silicon carbide, diamond or a diamond-like material, etc.) can be used to electrically isolate the pyroelectric element  12  from the substrate  20  and to ensure a substantially vertical flow of heat from the electrical resistor  18  through the pyroelectric element  12  and to the underlying substrate  20 . 
   In the example of  FIGS. 2A and 2B , the pyroelectric demodulator  10  can be built up on the substrate layer by layer, with a sacrificial material (e.g. silicon dioxide or a silicate glass, or alternately polysilicon) being provided underneath the input and output connections  26  and  34  at locations where the air bridges are to be formed, and with spacers  38  being provided to support a terminal end of each input and output connection  26  or  34 . The spacers  38  can be formed, for example, from silicon nitride when the sacrificial material comprises silicon dioxide or a silicate glass such as TEOS which is deposited from the decomposition of tetraethylortho silicate using low-pressure chemical vapor deposition (LPCVD). When the sacrificial material comprises polysilicon, the spacers  38  can be formed from silicon dioxide or a silicate glass (e.g. TEOS). The sacrificial material and spacers  38  can be deposited by MOCVD, LPCVD, or plasma-enhanced chemical vapor deposition (PECVD), and patterned using reactive ion etching. 
   In the example of  FIGS. 2A and 2B , external electrical connections can be made to the terminal end of the input and output connections  26  and  34 , or the connections  26  and  34  can be extended to form electrical connections to other components located on the substrate  20 . As an example, the demodulated electrical output signal from the pyroelectric element  12  can be connected to an amplifier or a signal processor located on the substrate  20  via the output connections  34 . The amplifier or signal processor can be formed, for example, using well-known complementary metal-oxide semiconductor (CMOS) technology. 
     FIGS. 3A and 3B  show another example of the pyroelectric demodulator  10  of the present invention. In  FIGS. 3A and 3B , the pyroelectric element  12  is formed on a cantilever  40  extending outward from a pedestal  36 , with the pyroelectric material in the element  12  being poled in a direction substantially parallel to the substrate  20 . This arrangement allows one side of the pyroelectric element  12  to be heat-sinked to the pedestal  36  and substrate  20  while the other side of the pyroelectric element  12  is thermally isolated on the cantilever  40 . The pyroelectric demodulator  10  of  FIGS. 3A and 3B  operates by sensing a temperature difference ΔT produced across the width of the pyroelectric element  12  due to heating produced by the AM electrical signal input into the electrical resistor  18 . 
   In  FIGS. 3A and 3B , the cantilever  40  can comprise a material such as silicon or silicon nitride. As an example, a silicon-on-insulator (SOI) substrate  20  can be used, with the cantilever  40  being formed from a monocrystalline silicon layer, and with the pedestal  36  being formed from an oxide layer (e.g. silicon dioxide) in the SOI substrate  20  which separates the monocrystalline silicon layer from a monocrystalline silicon body. As another example, the pedestal  36  can be built up from a deposited layer of polysilicon or silicon nitride, with the cantilever  40  being formed from another deposited layer of polysilicon or silicon nitride. A sacrificial material such as silicon dioxide or a silicate glass can be provided underneath the cantilever  40  and later removed so that the cantilever  40  is free standing. The exact thickness of the cantilever  40  will, in general, depend upon the size and thickness of the pyroelectric element  12  and can be, for example, up to a few microns. 
   After the pyroelectric element  12  and the electrical resistor  18  have been built up as previously described, a portion of the pedestal  36  can be undercut with a selective wet etchant (e.g. comprising hydrofluoric acid when the pedestal  36  comprises silicon dioxide). Similarly, when a sacrificial material is provided beneath the cantilever  40 , the sacrificial material can be removed with the selective wet etchant. After fabrication is completed, the pyroelectric material in the pyroelectric demodulator  10  can be horizontally poled by applying a voltage across the output electrical connections  34  with the device  10  heated near to or above the Curie temperature of the pyroelectric material. 
   A fourth example of the pyroelectric demodulator  10  of the present invention is schematically illustrated in  FIGS. 4A and 4B . In this device, the pyroelectric element  12  is epitaxially grown upon the substrate  20 . The pyroelectric element  12  can comprise, for example, a semiconductor pyroelectric material such as gallium nitride (GaN) which can be epitaxially grown on a silicon carbide or sapphire substrate. A silicon carbide substrate  20  can be doped for electrical conductivity to facilitate making a electrical connection to a lower surface of the GaN pyroelectric material which can be doped (e.g. n-type doped) during epitaxial growth. Epitaxial growth of GaN is well-known in the art (see e.g. U.S. Pat. No. 6,599,362; an article by C. I. H. Ashby et al., “Low-Dislocation-Density GaN from a Single Growth on a Textured Substrate,”  Applied Physics Letters , vol. 77, pp. 3233-3235, 13 Nov. 2000; and a review article by O. Ambacher, “Growth and Applications of Group III-Nitrides,”  Journal of Physics D: Applied Physics , vol. 31, pp. 2653-2710, 1998; all of which are incorporated herein by reference). 
   After epitaxial growth and patterning of the GaN pyroelectric element  12 , the remainder of the pyroelectric device  10  can be built up as previously described using a series of deposition and patterning steps to form the input and output electrical connections  26  and  34 , the electrical resistor  18 , and an electrically-insulating layer  24  separating the resistor  18  from an upper metallized surface  14  of the pyroelectric element  12 . 
   The device  10  of  FIGS. 4A and 4B  provides suspended air-bridge strip-line electrical input connections  26  to allow operation at GHz carrier frequencies while minimizing the heat loss through these connections  26 . The upper metallized surface  14  of the pyroelectric element  12  is also connected to an air-bridge output electrical connection  34 , while a lower surface of the pyroelectric element  12  can be connected through the substrate  20  to a contact pad  30  formed on the substrate. 
   The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.