Abstract:
An apparatus includes a distributed resonant tunneling section with a plurality of inductive portions that are coupled in series with each other between first and second nodes, such that a respective further node is present between each adjacent pair of the inductive portions. The distributed resonant tunneling section also has a plurality of resonant tunneling device portions which are each coupled between a third node and a respective one of the further nodes.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates in general to high-frequency circuits and, more particularly, to techniques for effecting high-frequency amplification or oscillation.  
       BACKGROUND OF THE INVENTION  
       [0002]     A variety of amplifiers and oscillators exist for applications with operational frequencies below approximately 100 GHz. These include solid-state amplifiers and oscillators which are based on Gunn-effect diodes, impact avalanche transit time diodes, field effect transistors, and/or bipolar transistors. Other known approaches include vacuum sources such as klystrons, traveling wave tubes, and gyrotrons.  
         [0003]     However, there are other types of systems in which there is a need for amplifiers and/or oscillators capable of operating at higher frequencies. For example, microwave systems need high-frequency amplifiers to improve the reception of signals, need high-frequency oscillators to serve as local oscillators in receiver circuits, and need high-frequency oscillators to serve as power oscillators in transmitter circuits. High-frequency amplifiers and oscillators for these applications have traditionally been implemented with large vacuum-tube devices, such as gyratrons, or with inefficient frequency-multiplied solid-state sources and parametric amplifiers. In this regard, frequency-multiplied solid-state sources translate an input signal at one frequency into a higher harmonic frequency, but at poor power conversion efficiency. Parametric amplifiers use driven, non-linear reactive elements to achieve power gain at high frequencies. While these existing approaches have been generally adequate for their intended purposes, they have not been satisfactory in all respects.  
       SUMMARY OF THE INVENTION  
       [0004]     One form of the present invention relates to forming a distributed resonant tunneling section, and includes: coupling a plurality of inductive portions in series with each other between first and second nodes in a manner so that a respective further node is present between each adjacent pair of the inductive portions; and coupling each of a plurality of resonant tunneling device portions between a third node and a respective one of the further nodes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which:  
         [0006]      FIG. 1  is a diagrammatic perspective view of part of an apparatus which is an integrated circuit, and which embodies aspects of the present invention;  
         [0007]      FIG. 2  is a diagrammatic fragmentary side view of the structure shown in  FIG. 1 ;  
         [0008]      FIG. 3  is a graph depicting a curve showing how an electrical current within a resonant tunneling diode structure in the embodiment of  FIG. 1  will vary in response to variation of a voltage applied across that structure;  
         [0009]      FIG. 4  is a circuit schematic showing an apparatus which is an alternative embodiment of the apparatus of  FIG. 1 ;  
         [0010]      FIG. 5  is a diagrammatic view of a circuit in which a distributed resonant tunneling diode structure from the embodiment of  FIG. 1  is used to effect amplification;  
         [0011]      FIG. 6  is a diagrammatic view of a circuit in which the distributed resonant tunneling diode structure from the embodiment of  FIG. 1  is used to effect oscillation;  
         [0012]      FIG. 7  is a schematic diagram of a circuit  251 , which is an equivalent circuit for the distributed resonant tunneling diode structure from the embodiment of  FIG. 1 ;  
         [0013]      FIG. 8  is graph showing the result of a computer simulation of the operation of the circuit shown in  FIG. 6 ; and  
         [0014]      FIG. 9  is a diagrammatic fragmentary perspective view showing an apparatus in the form of an integrated circuit, which is an alternative embodiment of the integrated circuit of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  is a diagrammatic perspective view of part of an apparatus which is an integrated circuit  10 , and which embodies aspects of the present invention.  FIG. 2  is a diagrammatic fragmentary side view of the structure shown in  FIG. 1 . The integrated circuit  10  includes a substrate  12  which, in the disclosed embodiment, is made of indium phosphide (InP). It should be understood that the specific materials discussed herein for various parts of the integrated circuit  10  are exemplary, and the integrated circuit  10  could be implemented using other materials and/or other semiconductor technologies.  
         [0016]     An elongate structure  14  is formed on top of the substrate  12  and, as shown in  FIG. 2 , has ends  16  and  17  which are at spaced locations. The distance between the ends  16  and  17  is the electrical length L of the structure  14 . The structure  14  is referred to herein as a distributed resonant tunneling diode (DRTD) structure.  
         [0017]     The DRTD structure  14  includes an electrically conductive layer  21 , which is provided on the top surface of the substrate  12 , and which extends from the end  16  to the end  17 . In the disclosed embodiment, the conductive layer  21  is a doped semiconductor material, and in particular is indium gallium arsenide (InGaAs), which is doped to make it an n+ type semiconductor material.  
         [0018]     The DRTD structure  14  also includes, on top of the layer  21 , a stack of five further layers  22 - 26  which each extend from the end  16  to the end  17 . In a transverse direction, the layers  22 - 26  are each substantially narrower than the layer  21 , and are provided approximately in the center of the layer  21 .  
         [0019]     The layer  25  is an electrically conductive layer that is similar in thickness and composition to the layer  21 . In particular, it is a doped semiconductor material. In the disclosed embodiment, it is indium gallium arsenide (InGaAs), which is highly doped in order to make it an n+ type semiconductor material. The center layer  23  is also made of InGaAs, but is not doped, or is only lightly doped. The layers  22  and  24  are each made of aluminum arsenide (AlAs), and are thus electrically insulating layers. In a vertical direction, the five layers  21 - 25  collectively define a resonant tunneling diode (RTD) structure.  
         [0020]     The layer  26  is an electrical contact. The DRTD structure  14  includes two further electrical contacts  28  and  29 , which are provided on top of the layer  21 , and which each extend from the end  16  to the end  17  of the structure  14 . The contacts  28  and  29  are provided on opposite sides of the stack that includes the layers  22 - 26 , and are each spaced from this stack. In the disclosed embodiment, the contacts  26  and  28 - 29  are all made of gold. However, these contacts could alternatively made of any other suitable material which is electrically conductive. The contact  26  and the layer  25  effectively correspond to one conductor of a transmission line, and the contacts  28 - 29  and the layer  21  effectively correspond to the other conductor of the transmission line, with the RTD structure of the layers  21 - 25  disposed between these two conductors along the length thereof.  
         [0021]     With reference to  FIG. 2 , broken lines are used to diagrammatically show how a terminal or node  41  of a circuit can be electrically coupled to the contact  26  of the DRTD structure  14  at the end  16  thereof, and to show how another terminal or node  42  of the circuit can be electrically coupled to each of the other contacts  28  and  29  at the end  16 .  FIG. 2  also shows how an additional terminal or node  43  can be coupled to the contact  26  at the end  17 , and how a terminal or node  44  can be coupled to each of the contacts  28  and  29  at the end  17 .  
         [0022]     As indicated by broken lines in  FIG. 1 , the DRTD structure  14  of  FIGS. 1 and 2  can be conceptually subdivided into a plurality of identical sections, several of which are identified by reference numerals  51 - 54 . These sections are discussed later. The DRTD structure  14  is shown in  FIG. 1  with an elongate configuration, in order to facilitate an understanding of the present invention. However, it would alternatively be possible for the DRTD structure to have other shapes.  
         [0023]     As mentioned above, the layers  21 - 25  collectively form an RTD structure in a vertical direction.  FIG. 3  is a graph depicting a curve that shows how an electrical current through this RTD structure will vary in response to variation of a voltage applied across the RTD structure, or in other words a voltage applied between the contact  26  and one or both of the contacts  28  and  29 . It will be noted that the curve has a region  71  where the slope is negative. In effect, this represents a negative resistance characteristic of the RTD structure. As is known in the art, a positive resistance will absorb power, and thereby attenuate electrical signals. Conversely, a negative resistance such as that shown at  71  represents the opposite of attenuation, or in other words gain. A device with gain can be used to implement a circuit such as an oscillator or an amplifier.  
         [0024]      FIG. 4  is a circuit schematic showing an apparatus  110  which is an alternative embodiment of the apparatus  10  of  FIG. 1 . Equivalent parts are identified by the same reference numerals. The apparatus  110  includes a plurality of discrete inductors coupled in series with each other between two terminals  41  and  43 , four of which are shown at  121 - 124 . This circuit also includes a plurality of discrete RTDs, four of which are shown at  126 - 129 . Each of these RTDs has one end coupled to the right end of a respective inductor, and the other end coupled to a common conductive run which extends between two terminals  42  and  44 . The inductors and the RTDs collectively form a distributed resonant tunneling diode (DRTD) structure  130 , which is functionally comparable to the DRTD structure  14  in  FIG. 1 . The electrical path between the terminals  41  and  43  (including the inductors  121 - 124 ) effectively corresponds to one conductor of a transmission line, and the electrical path between the terminals  42  and  44  effectively corresponds to the other conductor of the transmission line. The RTDs, including those at  126 - 129 , effectively extend between these transmission line conductors at spaced locations therealong.  
         [0025]     The DRTD structure  130  in  FIG. 4  can be conceptually divided into a plurality of identical sections, several of which are identified by reference numerals  131 - 134 . These identical sections each include an inductor and an RTD. With reference to  FIG. 1 , the sections  51 - 54  of the DRTD structure  14  correspond conceptually to the sections  131 - 134  of the DRTD structure  130  in  FIG. 4 . In a sense, the circuitry within any one of the sections  131 - 134  in  FIG. 4  represents a simplified equivalent circuit for the physical structure within any one of the sections  51 - 54  in  FIG. 1 .  
         [0026]     In the DRTD structure  130  of  FIG. 4 , the inductors (including those at  121 - 124 ) can each be viewed as having an incremental coupling inductance ΔL, and the RTDs (including those at  126 - 129 ) can each be viewed as having an incremental shunt capacitance ΔC. With this in mind, the effective impedance Z EFF  of the DRTD structure  130  will be roughly Z EFF ={square root}(ΔL/ΔC).  
         [0027]      FIG. 5  is a diagrammatic view of a circuit in which the DRTD structure  14  of  FIG. 1  is used to effect amplification. It would alternatively be possible to substitute the DRTD structure  130  of  FIG. 4  for the DRTD structure  14  in the circuit of  FIG. 5 .  FIG. 5  shows a direct current (DC) source  201  and an alternating current (AC) source  202 , which are coupled in series with each other between the terminals  41  and  42 . The DC source  201  is a low-impedance source such as a battery, which applies across the terminals  41 - 42  a DC bias voltage selected so that the RTD structure within the DRTD structure  14  is biased to operate in its negative resistance region ( 71  in  FIG. 3 ). The AC source  202  is a low-impedance circuit which applies an AC input signal between the input terminals  41 - 42 . As this AC signal travels from the input terminals  41 - 42  to the output terminals  43 - 44 , it is amplified by the DRTD structure  14 .  
         [0028]     A load  206  is coupled between the output terminals  43 - 44 . The load  206  has an impedance Z LOAD  which is matched to the effective impedance Z EFF  exhibited by the DRTD structure  14  when viewed from the output terminals  43  and  44 . For the purpose of effecting amplification, the electrical length L ( FIG. 2 ) of the DRTD structure  14  can be selected to be any convenient length, so long as no reflections occur that could produce oscillatory feedback. The circuitry coupled to the terminals  41 - 44  which is external to the DRTD structure  14  could be an integral part of the integrated circuit  10  ( FIG. 1 ) Alternatively, it could be implemented with discrete components that are external to the integrated circuit.  
         [0029]      FIG. 6  is a diagrammatic view of a circuit in which the DRTD structure  14  is used to effect oscillation. The DRTD structure  130  of  FIG. 4  could be substituted for the DRTD structure  14  in the circuit of  FIG. 6 . In  FIG. 6 , a DC source  201  is coupled in series with a switch  231  between the terminals  41  and  42 . The switch  231  is an electronic switch of a known type. The switch  231  is closed in order to start operation of the oscillator circuit, and then remains continuously closed.  
         [0030]     In  FIG. 6 , the DRTD structure  14  has an electrical length L, which is the physical length of the structure  14  times the apparent dielectric constant of the composite structure  14 . For example, if the apparent dielectric constant of the structure  14  as seen by electromagnetic waves traveling through the structure  14  is  3 . 3 , then the physical length of the structure  14  is (L/3.3). The electrical length of the DRTD structure  14  is selected to be an integer multiple of one-quarter wavelength of the selected frequency at which oscillation is to occur. This permits a standing wave to develop and to be maintained within the DRTD structure  14 . In the embodiment disclosed in  FIG. 6 , the electrical length L is selected to be one-half of a wavelength of the frequency of interest, in order to optimize boundary conditions and prevent oscillation at lower frequencies.  
         [0031]     In more detail, in order to support oscillation, the external circuitry attached to each end of the structure  14  needs to have an impedance which is different from the apparent terminal impedance of the structure  14 . These impedance discontinuities at the ends of the structure  14  cause reflections of traveling electromagnetic waves within the structure  14 , and the standing wave created by these reflections is amplified within the structure  14  so as to overcome losses and sustain oscillation. The relation of the impedance of the structure  14  to these end impedances determines the selected length of the structure  14 .  
         [0032]     In particular, if the circuits at each end of the structure  14  have impedances which are both less than or both greater than the impedance of the structure  14 , then the electrical length of the structure  14  is selected to be an integer number of one-half wavelengths of the selected frequency. In contrast, if the circuit at one end of the structure  14  has an impedance which is less than the impedance of structure  14 , and the circuit at the other end of the structure  14  has an impedance which is greater than the impedance of the structure  14 , then the electrical length of the structure  14  is selected to be an integer number of quarter wavelengths of the selected frequency.  
         [0033]     In  FIG. 6 , the load  206  has an impedance Z LOAD  which is selected to create a termination mismatch with respect to the effective impedance Z EFF  exhibited by the DRTD structure  14  at the terminals  43  and  44 . The mismatch may be reactive or resistive, or a combination of both. As discussed above, this termination mismatch is needed in order to provide reflections at the load  206  which are suitable for sustaining standing wave oscillation within the DRTD structure  14 .  
         [0034]      FIG. 7  is a schematic diagram of a circuit  251 , which is an equivalent circuit for the DRTD structure  14  shown in  FIG. 1 . The circuit  251  has a plurality of identical sections which are coupled in series with each other, and four of these sections are identified by reference numerals  51 - 54 . These sections  51 - 54  of the circuit  251  are each an equivalent circuit for the respective corresponding section  51 - 54  in the DRTD structure  14  in  FIG. 1 . Since the sections of the circuit  251  are identical, only the circuitry within the section  51  is described below in detail.  
         [0035]     More specifically, the section  51  includes an inductor  261  and a resistor  262 , which are coupled in series with each other, and a circuit node  263  is present between them. An inductor  266  and a resistor  267  are coupled in series with each other between the node  263  and a further node  268 . A capacitor  271  and a resistor  272  are coupled in parallel with each other between the node  268  and a common line  273 . The section  51  has a portion  276 , which includes the inductor  266 , the resistor  267 , the capacitor  271  and the resistor  272 . The portion  276  corresponds to the RTD structure in the section  51  of the structure  14  in  FIGS. 1-2 , or in other words the layers  21 - 25 . The inductor  261  and the resistor  262  represent inductive and resistive components of transmission line characteristics that are inherent to the section  51  of the structure  14  in  FIG. 1 .  
         [0036]     A computer simulation was carried out for the oscillator circuit of  FIG. 6 , using the equivalent circuit  251  of  FIG. 7  to model the DRTD structure  14 . The frequency of oscillation for the simulation was selected to be 580 GHz, and thus the electrical length L of the DRTD structure  14  was selected to be one-half of the wavelength of a 580 GHz signal. The equivalent circuit was configured so that the RTD portion  276  in each of the sections  51 - 54  was representative of a 120 kA/cm 2  RTD. The speed index of such an RTD relates the large-signal switching of the RTD to its internal characteristics, and is about 240 GHz. But in the negative resistance region, the gain-bandwidth product of the RTD can be significantly greater than its speed index. The simulation was configured so that the output of the oscillator would be 54 microwatts into a purely resistive load of 20 ohms. For the simulation, the equivalent circuit  251  was configured to give the RTD  14  an effective impedance Z EFF  of about 50 ohms. The DC source  201  of  FIG. 6  was configured to have an impedance of approximately zero ohms for the simulation. In the simulation, the switch  231  ( FIG. 6 ) was closed at a time T=0, and  FIG. 8  is graph showing the result of the simulation over time at ten different points A-J which were distributed uniformly along the electrical length L of the DRTD structure  14 .  
         [0037]      FIG. 9  is a diagrammatic fragmentary perspective view showing an apparatus in the form of an integrated circuit  310 , which is an alternative embodiment of the integrated circuit  10  of  FIG. 1 . Equivalent parts are identified by the same reference numerals, and the following discussion focuses on the differences.  
         [0038]     In particular, the only significant difference between the integrated circuits  10  and  310  is that the layer  25  in the integrated circuit  10  of  FIG. 1  has been replaced with a different layer  325  in the integrated circuit  310  of  FIG. 9 . The layer  325  is substantially thicker than the layer  25 , and is not heavily doped. Instead, the layer  325  is a lightly doped layer of indium gallium arsenide (InGaAs) which, in the disclosed embodiment, has a level of doping that is about the same as that used for the layer  23 . The increased thickness of the layer  25  serves to increase the effective distance between the electrically conductive contact  26  and the electrically conductive layer  21 .  
         [0039]     To the extent that the contact  26  and the layer  21  are comparable to the conductors of a transmission line, the increased thickness of the layer  325  increases the gap between them, which in turn reduces the effective capacitance between them. This allows the structure shown in  FIG. 9  to be used at lower operational frequencies than the structure of  FIG. 1 , and with lower transmission losses. In addition, by reducing the capacitance of the amplifying medium, the losses and bandwidth of the circuit at high operating frequencies will improve. The reduced capacitance also raises the impedance of the multi-layer structure of  FIG. 9 , which makes it easier to match the impedance of this structure to external circuits or loads, such as an antenna.  
         [0040]     Due to the fact that the layer  325  is not heavily doped, the embodiment of  FIG. 9  does not have ohmic contact between the contact  26  and the layer  325 . Instead, a Schottky diode structure is effectively formed between the contact  26  and the layer  325 . One consideration resulting from this Schottky diode structure is that polarity becomes a factor, for example when coupling a DC source such as a battery to the structure of  FIG. 9 . In contrast, the structure shown in  FIG. 1  is electrically symmetric, and does not present an issue of polarity.  
         [0041]     The present invention provides a number of advantages. One such advantage results from the provision of structure which can be used to implement circuits such amplifiers or oscillators that operate at very high frequencies, for example up to about 1,000 GHz. Further, by combining several RTD devices, or by using an elongate RTD structure, increased power-handling capability can be obtained, and can be tailored to meet the needs of a particular application. Examples of applications include generation of coherent signals for receiver down-conversion, and power sources for transmitters. In addition, properly terminated, the disclosed structure can provide low-noise amplification for use in the front end of a receiver circuit.  
         [0042]     Although selected embodiments have been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.