Fishing – trapping – and vermin destroying
Patent
1989-04-03
1993-10-26
Fourson, George
Fishing, trapping, and vermin destroying
437176, 437904, H01L 21265
Patent
active
052565793
DESCRIPTION:
BRIEF SUMMARY
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 .mu.m, and typical width, w, is about 10 .mu.m. A typical doping profile 18 is shown in FIG. 1B. There, the dose is varied linearly from 10.sup.13 to 3.times.10.sup.13 cm.sup.-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.sup.++ 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.degree. C. for about 20 minutes, in step 24. The annealing step may use a cap consisting of about 700 Angstroms of Si.sub.3 N.sub.4 and 3,000 angstroms of S.sub.i 0.sub.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.degree. C.
Focused ion implantation equipment and technigues are well-known and commercially available, t
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Ismail Khalid
Lezec Henri
Fourson George
Massachusetts Institute of Technology
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