Abstract:
An improved resonant tunneling device (RTD) oscillator is provided by supplying electrical power to the RTD device  23  using a photocell  21  and a light source  25  such that essentially no spurious resonances are possible.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to resonant tunneling devices (RTDs) and more particularly to methods for supplying electrical power to resonant tunneling devices. 
     2. Background of Invention 
     Sub-millimeter wavelength microwave oscillators are essential microwave components. They are used as local oscillators within millimeter wavelength radio receivers and as the basic frequency-determining function within millimeter wavelength radio transmitters. Since the possible bandwidth of information impressed on a frequency carrier is proportional to the carrier frequency, oscillators that operate above 1000 GHz (1 Terahertz) would allow unprecedented modulation bandwidth relative to current technology. Typically, field effect transistors, bipolar transistors, impact ionization diodes, and Gunn-effect diodes are used in microwave oscillators, but these devices have inherent frequency limitations due to their size which limits speed due to transit time effects. Resonant tunneling diodes (RTDs) are semiconductor devices with very high intrinsic gain and switching speed and would allow microwave oscillators to be constructed for frequency bands well above 1000 GHz. However, resonant tunneling diode oscillator circuits are very difficult to electrically bias so that the bias circuitry does not resonate with the RTD and produce unwanted modes of oscillation. Reddy et. al. provided one method for properly biasing RTDs in microwave oscillators based on the use of transmission lines and Schottky diodes. REF: M. Reddy, et. al. “Bias Stabilization for Resonant Tunnel Diode Oscillators,” IEEE Microwave and Guided Wave Letters, Vol. 5, No.7, July 1995. However, this method still allows the parasitic resonances to occur in the power supply circuitry. Therefore there remains the general problem of providing RTD-based circuits that oscillate at ultra high frequency while suppressing lower frequency oscillations. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention the problem of providing a low inductance, high capacitance means to supply power is to use a photocell to power and bias the RTD such that essentially no spurious resonances are possible. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a power supply with leads to an RTD oscillator according to prior art; 
     FIG. 2 illustrates a circuit diagram illustrating an embodiment of the present invention; 
     FIG. 3 illustrates the structure of one embodiment of the present invention; 
     FIG. 4 the structure of a second embodiment of the present invention; 
     FIG. 5 illustrates I-V plot of an RTD; 
     FIG. 6 is a plot of differential conductance versus voltage of an RTD; 
     FIG. 7 illustrates an equivalent electrical model of a p-n junction photocell; 
     FIG. 8 illustrates an I-V plot for p-n junction photocell; 
     FIG. 9 illustrates the I-V plot for RTD and p-n junction photocell; 
     FIG. 10 illustrates a circuit diagram of an RTD oscillator according to an embodiment of the present invention; 
     FIG. 11 illustrates an RTD oscillator structure according to an embodiment of the present invention; 
     FIG. 12 illustrates another embodiment using a traveling wave antenna; 
     FIG. 13 illustrates another embodiment using a dipole antenna; and 
     FIG. 14 illustrates an entire array pumped with light. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Resonant tunneling diodes have intrinsic electrical gain which allows them to be used in amplifiers and oscillators up to at least several hundreds of GHz. Referring to FIG. 1 a typical RTD-based oscillator  10  may contain RTD  11  with a power supply  12  and resonant circuit  13 . Resonant circuit  13  may consist of an inductor  14  and capacitor  15  tuned to the desired radio frequency. The power supply  12  may consist of a RF bypass capacitor  16 , a voltage source such as battery  17 , and wires consisting of inductors  18  and  19  that connect the voltage source to the bypass capacitor, RTD, and tuned circuit. The RTD has electrical gain from zero frequency (dc) to an upper limit Fmax. Because the gain of the RTD tends to decrease with frequency, the lowest characteristic resonant frequency of the circuitry of oscillator  10  will tend to set the frequency of RTD oscillation. This is a special problem at high microwave frequencies where the RTD gain tends to roll off with increasing frequency. Due to the inherently greater gain at low frequencies, a circuit containing an RTD may tend to oscillate at a low frequency set by the relatively large values of inductance in the power supply wiring rather than at the desired microwave frequency. This problem has been noted by M. Reedy. et. el. (See prior citation in text) where special precautions were required to properly bias and decouple the power supply circuit from a 6.9 GHz RTD oscillator. In the approach of Reddy, a tuned transmission line and bypass capacitor were used to isolate the bias voltage source form the RTD and microwave resonator. However, this approach will not be preferable at frequencies of 500 GHz or greater where the RTD gain is rolling off very rapidly with increasing frequency. Even well-decoupled power wiring may still cause undesired oscillation effects. 
     Referring to FIG. 2 there is illustrated a circuit diagram of one embodiment of the present invention where a photocell is used to supply electrical power an RTD. In this embodiment a photocell in  21  is in series with the RTD device  23 . The photocell  21  absorbs optical radiation  25  and converts this radiation into electrical power for energizing the RTD. 
     Referring to FIG. 3 there is illustrated a cross-sectional view of an epitaxial structure of the series structure of FIG. 2 according to one embodiment of the present invention. The structure comprises the multilayer RTD  31  sitting upon a multilayer p-n junction photocell  35 . The photocell  35  sits upon a substrate  36 . The terminals are  37  and  38  across which a further circuit may be connected. Optical illumination  32  and  33  are provided to empower the circuit. 
     Referring to FIG. 4 there is illustrated a second structure comprised of a multilayer RTD  41  sitting on a common substrate  43  and a multilayer photocell  45  sitting a common substrate. Layer  46  is common to both the RTD  41  and photocell  45 . Optical illumination  42  and  44  are provided to empower the circuit. The terminals  48  and  49  are connected respectively to the RTD and photocell and may be further connected to an external circuit. 
     As is very well known in the art, a resonant tunneling diode, also known as the quantum well diode, utilizes the tunneling of electrons through a pair of potential energy barriers to produce negative differential conductance effects. A RTD typically consists of a multi-layer stack (heterostructure) of epitaxially grown semiconductor material so formed to produce a single quantum energy well surrounded by twin energy barriers. Electrons incident upon this structure will be reflected from the structure except for certain well defined energies. These energies correspond to resonant energy levels within the quantum well, and conductance through the structure for these energies is very efficient. A typical design for a very high frequency RTD is given in Table 1. These layers are grown using molecular beam epitaxy or chemical vapor deposition methods using compounds of Indium (In), Arsenic (As), Gallium (Ga), Aluminum (Al), and Phosphorous (P). RTDs based on InGaAs quantum wells and AlAs tunnel barriers are easily grown on standard InP substrates. Various dopants such as silicon and boron can be used to alter the conductivity of each layer. Such diodes can also be fabricated using AlGaAs compounds on GaAs substrates. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical Resonant Tunneling Diode Heterostructure. 
               
             
          
           
               
                   
                   
                   
                 Doping 
                 Thickness in Angstroms or 
               
               
                   
                 Layer Name 
                 Material 
                 Density 
                 Monolayers 
               
               
                   
                   
               
               
                   
                 Top Contact 
                 InGaAs 
                 2E19/cc 
                  500 Angstroms 
               
               
                   
                 Spacer 
                 InGaAs 
                 undoped 
                  50 Angstroms 
               
               
                   
                 Aluminum 
                 AlAs 
                 undoped 
                   6 monolayers 
               
               
                   
                 Quantum 
                 InGaAs 
                 undoped 
                  10 Angstroms 
               
               
                   
                 Well 
               
               
                   
                 Quantum 
                 InAs 
                 undoped 
                  20 Angstroms 
               
               
                   
                 Well 
               
               
                   
                 Quantum 
                 InGaAs 
                 undoped 
                  10 Angstroms 
               
               
                   
                 Well 
               
               
                   
                 Aluminum 
                 AlAs 
                 undoped 
                   6 monolayers 
               
               
                   
                 Spacer 
                 InGaAs 
                 undoped 
                  70 Angstroms 
               
               
                   
                 Bottom 
                 InGaAs 
                 2E19/cc 
                 2000 Angstroms 
               
               
                   
                 Contact 
               
               
                   
                 Substrate 
                 InP 
                 undoped 
                 0.018 inches 
               
               
                   
                   
               
             
          
         
       
     
     Under electrical bias between the top contact and the bottom contact, the electrical current in the diode will first rise with increasing bias, then precipitously drop with further bias, then rise again with even further bias as shown in FIG.  5 . FIG. 5 shows the current versus voltage response of the RTD of Table 1 that exhibits 100,000 amperes/cm 2  of current at the first resonance. The response is for a device of 1 square micrometer of cross-sectional area. The initial rise and peak in the diode current, at about 0.28 volts of bias, is due to quantum mechanical tunneling of electrons through the lowest energy conduction band resonance established within the heterostructure. The second rise in current after 0.5 volts of bias is due to the onset of tunneling through the next higher energy resonance. The falloff in current, at bias voltages between 0.28 and 0.5 volts in the Figure has a negative slope. The negative differential conductance (or conversely the negative differential resistance) associated with this bias regime for this device is shown in FIG.  6 . Within this bias range the RTD can be used as a gain element for amplification or oscillation. The maximum negative differential conductance (−6.96E-3 ohms −1 ) occurs at about 0.35 volts in this exemplary device, which is also the bias point of maximum available gain. Connecting the RTD to a resonant electrical circuit can produce amplification and oscillation if the magnitude of the RTD gain exceeds the losses within the circuit. This ability to support amplification and oscillation within an electronic circuit is characteristic of any electrical device that exhibits negative differential conductance. REF: “Basic Theory And Application of Tunnel Diodes,” S. P. Gentile, Van Nostrand, Princeton N.J. (1962) At the present state of the art, the resonant tunneling diode is the fastest semiconductor with intrinsic electronic gain. RTDs have been shown to oscillate above 700 GHz. (REF:Brown et. al. “High Frequency Resonant Tunneling Oscillators,” Microwave and Optical Technology Letters, Vol. 4, No.1, January 1991, pp-19-23) Therefore, the fastest oscillators based on semiconductor gain elements must currently employ resonant tunneling diodes. 
     An established equivalent electrical model of a p-n junction photocell is shown in FIG.  7 . (Reference: “Physics of Semiconductor Devices, S. M. Sze, John Wiley and Sons, Chapter 14, (1981) [ISBN 0-471-05661-8]” The photocell  100  is represented by a controlled current source  102 , a shunt diode  104 , a junction capacitance  106  and a series output resistance  108 . The capacitor  106  is generally a direct function of the area of the diode. The series resistor  108  is dependent upon the types and thicknesses of the materials used to fabricate the diode. The amplitude I L  of the current source  102  is directly proportional to the level Ip of optical illumination  110 . The output voltage across the terminals of the photodiode are determined by the external load resistance, the photocurrent I L , the current versus voltage response of the internal diode  104 , and the voltage drop across the series resistor  105 . FIG. 8 shows the calculated dc output current versus output voltage response of a typical p-n junction photocell. A series resistance of 10 ohms was used in the calculation. As the level of optical illumination increases from one, three, and 6 times a reference value, the short-circuit output current form the diode increases respectively to 0.5, 1.5, and 3 milliamperes. For the same range of illumination, the open-circuit voltage across the diode falls in the range of 0.34 and 0.39 volts. The dc resistance of the diode changes with illumination. The lowest value of equivalent resistance of the photodiode occurs in the region of zero current. At the highest level of illumination shown in FIG. 8, the dc resistance of the photocell is about 34 ohms near the open-circuit condition. 
     A photodiode based on metal-semiconductor junctions, called the Schottky photocell, is an alternative to the p-n junction diode. Many different types of p-n and Schottky photocell are possible, and many semiconductor materials may be used to build photocells. 
     FIG. 9 shows the I-V curve  58  of the RTD of FIG.  5  and the I-V characteristic of a typical p-n or Schottky diode photocell under several levels of optical illumination. At low levels of optical illumination, the I-V characteristic of a photocell may be described by curve  52 . At moderate levels of optical illumination, the I-V characteristic of a photocell may be described by curve  54 . At high levels of optical illumination, the I-V characteristic of a photocell may be described by curve  56 . To properly bias the RTD for operation as an amplifier or oscillator, the optical illumination of the photocell must be adjusted such that the I-V characteristic of the photocell intercepts the I-V of the RTD at its point of maximum negative differential resistance  59  of the RTD. This condition is illustrated best by curve  54 . 
     To employ the RTD as a stable amplifier or oscillator it is necessary to electrically bias the RTD with a dc voltage that maintains the average voltage across the RTD within its negative resistance region. 
     The electrical properties of photocell  21  should meet several requirements to serve as a useful power source for RTD  23 . First, it should present a very low impedance at the desired RF operating frequency so that the photocell does not impeded RF currents within the circuit. The requirement of a low impedance can be met by using a photocell of sufficient area that its junction capacitance provides a sufficiently low reactive impedance to the RTD. In one embodiment, the magnitude of the reactance of this junction capacitance may be made less than 1 ohm at the operating frequency. At an operating frequency of 500 GHz, the junction capacitance of the photocell  21  may be set greater than or equal to ½πF={fraction (1/6.28)}*500E9=0.3 Picofarads. Typical p-n and Schottky photocells can achieve this junction capacitance with an area of about 100 square micrometers. The second requirement of the photocell is that it present a sufficiently low dc resistance to the RTD circuit to ensure proper bias stability. The dc resistance presented by the photocell to the RTD must be less than the magnitude of the peak negative differential resistance of the RTD. This ensures that the intercept of the I-V load line established by the photocell, as in curve  54  of FIG. 9, intercepts the RTD I-V at only one bias point. In equation form, proper operation of an amplifier or oscillator containing a series-connected RTD and photocell requires that 
     
       
         −R RTD (dc)&gt;R Photocell (dc)  Equation 1 
       
     
     where R RTD (dc) is the differential dc resistance of the RTD and R Photocell (dc) is the equivalent differential dc conductance of the photocell connected in series with the RTD. As an example, the peak negative differential resistance of the RTD of FIG. 9 is about ({fraction (1/−6.46)}E-3)=−154 ohms. For this RTD to be correctly used in an amplifier or oscillator the effective dc resistance placed across the RTD must be less than 154 ohms. To provide design margin, R Photocell  is should be limited to a value no greater than ½ of (−R RTD ). If the rest of the circuit also has non-zero dc resistance, the dc resistance of the photocell must be reduced further by a similar amount. The differential dc resistance indicated by the simulated I-V trace  54  is about 40 ohms at bias point  59 . Therefore, a photocell that provides an I-V similar to curve  54  would have a suitable differential dc resistance. 
     Referring to FIG. 10 an RTD-based oscillator  120  may contain photocell  121 , RTD  123  and resonant circuit  124 . Resonant circuit  124  may consist of an inductor  126  and capacitor  128  tuned to the desired oscillation frequency. FIG. 11 shows the cross-section of a monolithic oscillator  130  consisting of a photocell circuit  131 , an RTD  133  and an inductive strap  136 . Inductive strap  136  has an equivalent inductance and parasitic capacitance so as to provide the function of resonator  124  in oscillator  120 . The N+ layer  138  serves to interconnect the end of the inductor  136  to photocell  131 . In this embodiment, the material used for N+ layer  138  and the substrate  140  may be selected such that they are optically transparent to the spectral region absorbed by photocell  131 . This allows optical illumination  142  of photocell  131  to be coupled via the substrate side of the oscillator. Generally, optical illumination  143  may be used to empower the circuit. 
     Referring to FIG. 12 there is illustrated an embodiment of an optically power oscillator using a traveling wave antenna  71  which may be covered with a conductive gold layer has a RTD and solar cell series set  77  placed across the walls  73  and  74  at a point from wall  78  that resonates the system at a desired frequency. The serially connected RTD and photocell will have a particular value of impedance at any given frequency. The impedance measured across the slot formed by wall  73  and  74  will vary as a function of the distance away from the shorting wall  78 . For example, very near the wall  78 , the impedance of the slot will measure out as a very low inductance. For points slightly further away than one quarter of a electrical wavelength, the slot will measure out as slightly capacitive. To operate the circuit of FIG. 12 as an oscillator at a frequency F, the set  77  should be located along the slot such that the impedance of the slot is the complex conjugate of the impedance of the set  77 . In this manner, the effective reactance of the set  77  will be cancelled by the conjugate reactance of the slot. This is the preferred point of location of set  77  to serve as an oscillator at frequency F. This circuit may be used as a transmitter or local oscillator in a high frequency receiver circuit. The circuit may be fabricated on the same substrate that carries the set  77 , or the antenna  71  may be deposited onto a substrate such as quartz or Mylar plastic and the set  77  physically and electrically bonded to the antenna using standard flip-chip or other well known techniques. 
     Referring to FIG. 13 there is illustrated a serially connected RTD and photocell set  81  placed across the dipole  83  so that the combination can work as either a transmitter or free-space local oscillator for a receiver circuit. The impedance measured across the dipole  83  will vary as a function of the distance away from the center  84 . For example, very near the center  84 , the impedance of the dipole will measure out as a very low and inductive. The impedance of points slightly further away than center will increase. To operate the circuit of FIG. 13 as an oscillator at a frequency F, the set  81  should be electrically attached to the dipole such that the impedance of the dipole is the complex conjugate of the impedance of the set  81 . In this manner, the effective reactance of the set  81  will be cancelled by the conjugate reactance of the dipole. This is the preferred point of location of set  81  to serve as an oscillator at frequency F. This circuit may be used as a transmitter or local oscillator in a high frequency receiver circuit. The circuit may be fabricated on the same substrate that carries the set  81 , or the antenna  83  may be deposited onto a substrate such as quartz or Mylar plastic and the set  81  physically and electrically bonded to the antenna using standard flip-chip or other well known techniques. 
     Referring to FIG. 14 is an illustration of an entire array  61  of resonant oscillators on a common substrate  62 . Each oscillator is comprised of a dipole antenna and a RTD and photocell. The RTD and photocell are connected in series to form a set. The end terminals of each set are connected to each dipole such that the set is impedance-matched to the dipole. Each dipole oscillator may be made an electrical half wavelength long to permit the entire array to oscillate in a coherent, that is phase locked, manner. The array  61  of dipole oscillators is pumped by light from source  63 . The intensity and uniformity of the light source are adjusted so that each dipole oscillator is equally powered. 
     In some uses of the invention, it may be desirable to fabricate the RTD and photocell on different substrates or by incompatible epitaxial growth methods. In one embodiment, the RTD may be fabricated on a separate substrate, mechanically or chemically diced into individual units, and then mechanically and electrically bonded to one of the photocell terminals using well know flip-chip or multi-chip module assembly.