Tunable-frequency Gunn diodes fabrication with focused ion beams
another object of the invention is to provide a tunable-frequency Gunn diode which has a doping concentration which varies along the path from cathode to anode. 
     Yet another object of the invention is to provide a frequency tunable Gunn diode which can be fabricated as a planar device, which can be integrated with other devices on the same substrate. 
     SUMMARY OF THE INVENTION 
     The foregoing and other objects and advantages are achieved in a tunable frequency Gunn diode which is fabricated using focused ion beam implantation to vary the doping profile of the diode along the drift path between cathode and anode. The use of focused ion beam technology allows fabrication of a planar structure which can be integrated with other devices directly into monolithic circuits. Further, diodes with different lateral doping profiles and frequency versus bias characteristics can be defined on a single wafer or even in the same circuit. 
     The device is small and eliminates the need for external resonant cavity tuning or for complex electronic tuning schemes. 
     A three-terminal gated device is also described. In this variation, a graded doping or tapered Gunn diode is combined with a Schottky barrier gate. At fixed ohmic contact bias, the frequency of oscillation of the device current can be controlled by varying the gate voltage. 
     The invention will be more fully understood from the detailed description which follows, which should be read in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, 
     FIG. 1A is a schematic top view of a Gunn diode according to the invention; 
     FIG. 1B is a diagram of a typical doping profile for the device of FIG. 1A; 
     FIG. 2 is a flow chart of the basic Gunn diode fabrication technique of the present invention; 
     FIG. 3 is an exemplary graph of the frequency dependence of transit-time mode oscillations in Gunn diodes fabricated in accordance with the teachings herein, as a function of applied voltage; 
     FIG. 4 is a diagram of a typical doping profile of a second embodiment of a Gunn diode according to the invention, illustrating also a shortening of the active region; 
     FIG. 5A is a schematic top view of a three terminal device according to the present invention, being the device of FIG. 1A to which a Schottky barrier gate has been added; 
     FIG. 5B is a diagram of a typical doping profile for the device of FIG. 5A; and 
     FIG. 6 is a flow chart illustrating the method of fabricating the device of FIG. 5A. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1A, a top view is shown of a Gunn diode 10 according to the present invention. The diode comprises a pair of ohmic contacts 12 and 14, respectively comprising the cathode and anode contacts, and a graded semiconductor area 16. Typical dimensions for the length, 1, of the active semiconductor is about 80 μm, and typical width, w, is about 10 μm. A typical doping profile 18 is shown in FIG. 1B. There, the dose is varied linearly from 10 13  to 3×10 13  cm -2  over the length of the active region 16. 
     The steps involved in fabricating such a device are illustrated in FIG. 2. First, in step 22 a semi-insulating GaAs substrate, or other suitable material, is implanted using a focused beam of Si ++   ions of about 140 keV energy. Conventional focused ion beam equipment is used for this process. The doping is graded between the contacts 12 and 14. The grading may be uniform and linear, as in the example of FIG. 1B, or it may be non-uniform or nonlinear. Of course, the doping profile will affect the characteristics of the device. Next, the implant is annealed at about 850° C. for about 20 minutes, in step 24. The annealing step may use a cap consisting of about 700 Angstroms of Si 3  N 4  and 3,000 angstroms of S i  0 2 . Then ohmic contacts of, for example, nickel/germanium/gold, may be defined using optical lithography and lift-off technigues and alloyed for about 30 seconds at about 450° C. 
     Focused ion implantation equipment and technigues are well-known and commercially available, though not previously used to make such devices. Accordingly, the reader may refer to any convenient text or source on the subject for any necessary details not covered herein 
     Experimentally, it has been found that a voltage on the order of 20-25 V is typically reguired to induce transit time mode Gunn oscillations in such devices of an 80 μm length. At lower applied voltages, the peak electric field in the device is everywhere below the threshold for domain nucleation, and oscillations do not occur. When the device has a uniform active region length of 80 μm (and a carrier concentration of about 10 13  /cm 3 ), it will oscillate at about 1.6 GHz at a bias of 34 V. The oscillation frequency shows a relatively weak dependence on bias, dropping to about 1.57 GHz when the bias is raised to 37 V. In contrast, a device with the lateral doping gradient of FIG. 1B has been seen to display a very strong dependence of frequency on bias. The transit time mode oscillation frequency has been observed in a prototype device to be smoothly tunable with DC bias over a 17 GHz range. This dependence is displayed in FIG. 3. As the bias across the device 10 is increased from 26 to 37 V, the frequency decreases from 23 to 6 GHz. The device does not oscillate for biases below 26 V, and the oscillation power decays rapidly to negligible levels for biases above 37 V. 
     Analytically, it appears that only the initial portion of the device contributes to the variable frequency effect and a shorter device could achieve the same performance with lower parasitic resistance, allowing operating bias levels four to five times lower. 
     Digressing briefly, a few remarks are in order regarding the fabrication of such Gunn diodes. The Gunn domains propagate in the sloped active region, nucleating at the concentration discontinuity, labelled &#34;A&#34; in FIG. 1B. The actual ohmic contacts (i.e., cathode and anode) are kept away from the active region so as not to interfere with domain propagation and nucleation. This is achieved by extending two uniformly doped segments 19 and 20 on either side of the active region, such that they extended under the ohmic contact pads. They are sufficiently long to allow for alignment error at the ohmic contact mask level (typically a few microns). They are doped sufficiently highly to make good ohmic contact. Thus, these two extensions free the active area from interference from the ohmic contacts Domain propagation depends only on the implanted profile (which can be accurately controlled with the focused ion beam) and not on the position of the ohmic contacts. If the cathode electrode were to extend into the active region, the exact point of domain nucleation would no longer be controlled accurately. The phenomenon of domain nucleation would also be affected by the damage directly under the contact. Also, the two extensions free the active region from doping level requirements to make good ohmic contacts. 
     FIG. 4 shows another, more typical doping radient profile 30 for a Gunn diode embodying the present invention. Observe that the length of the active region has been shortened to 20 μm. 
     FIG. 5A shows a three-terminal device 30, which is similar to the two-terminal device of FIG. 1A, except that a Schottky barrier gate 32 has been added. The active region 34 has the doping profile shown in FIG. 5B. As a Gunn domain travels through the Gunn diode active region from cathode 36 to anode 38, the electric field everywhere in the device rises. The current in the device is proportional to the domain sustaining electric field E s  at the edges of the domain, the doping density at the domain position, and the device width at the domain position. If at some time and location the electric field reaches the threshold level E th , at which domain nucleation occurs, a new domain will form there, and the old domain will be extinguished. In a two-terminal graded (or tapered) Gunn diode, this usually happens at or near the cathode, where the device is more resistive (in the case of a graded doping) or more narrow (in the case of a tapered cross section). By placing a Schottky barrier gate 32 across the Gunn diode near the cathode, the electric field in the device can be increased under the gate and domain nucleation can be made to occur in a controlled manner. As the gate voltage is made more negative, the depletion region depth is increased, constricting the channel height and increasing the electric field. The domain, hence, travels a shorter distance before the field under the gate reaches E th  and a new domain nucleates, extinguishing the old one. The decreasing range of domain travel leads to a higher frequency of oscillation of the current at the ohmic device terminals. Thus, the frequency of oscillation can be controlled by varying the gate voltage. 
     The Schottky barrier is typically a thin layer of aluminum or titanium/gold, and generally is patterned on the wafer in the last stage of fabrication. Standard photo lithographic technigues may be employed for this patterning. Typical gate width would be on the order of one half to one micron. FIG. 6 illustrates the process for making the three terminal device of FIG. 5A. It is identical to FIG. 2 except that a step 52 has been added, to take into account the patterning of the Schottky barrier. 
     The use of this Schottky barrier gate has several advantages. First, the frequency can be controlled with very low power applied to the gate. Secondly, the gate is electrically isolated from the cathode and anode of the diode. Third, a very small voltage change on the gate can produce a very large change in the oscillation frequency of the diode. Simulations and analyses indicate that a 10 millivolt variation in gate voltage can produce about a 10 GHz change in oscillation frequency. Simulations further show that such diodes may be fabricated with tunability ranges of 30-80 GHz. The Schottky barrier can be used to like effect in Gunn diodes having uniformly doped, active regions of tapered cross section. 
     The idea of having a single gate to control the extent to which the domain can travel along the graded channel can be obviously extended to more than one gate. The advantage of such an arrangement is that each gate can control the oscillation frequency in a certain band (determined by the location of the gate with respect to the graded doping profile). Having more than one gate can thus extend the tunability range of the oscillation frequency dramatically. A range of 500 MHz up to 100 GHz is then possible. 
     Having thus described the invention and multiple embodiments thereof, those skilled in the art will readily see various alterations, modifications and improvements to the disclosed embodiments which are not expressly stated herein. Such alterations, improvements and modifications are intended to be suggested, even if not expressly set forth, as they are within the spirit and scope of the invention. Accordingly, the invention is limited only by the appended claims and equivalents thereto, the disclosed embodiments being presented by way of example only.