US3482119A - Regulated nucleating position twovalley electron transfer effect device - Google Patents

Description

3,482,119 EcTRoN Dec. 2. 1969 w.1-ANTRAPORN ETAI- REGULATED NUCLEATING POSITION TWO-VALLEY EL TRANSFER EFFECT DEVICE 5 Sheets-Sheet 1 Filed May 13, 1958 Dec. 2, 1969 w, TANTRAPQRN ETAL 3,482,119

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REGULATED NUCLEATING POSITION Two-VALLEY ELECTRON TRANSFER EFFECT DEVICE 5 Sheets-Sheet 5 Filed May 13, 1968 f m M A M 6 d l. a 0 2v 3 0. .c w w M 0 M 2 RSS bwnmt 0 was heQEL SS. SS SQ I Dec. 2, 1969 w. TANTRAPORN ETAL 3,482,119 REGULATED NUCLEATING POSITION TwovALLEY ELECTRON TRANSFER EFFECT DEVICE Filed May 13, 1968 5 Sheets-Sheet 4.

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REGULATED NUCLEATING POSITION TWO-VALLEY ELECTRON TRANSFER EFFECT DEVICE Filed May 13, 1968 5 Sheets-Sheet 5 'PM/oa of .maar JMU asm/mow aM/rwry 11n/ts (trans/ time) (doma/I7 format/vn movement) fr? Vehz'sor's: W/rq/na Zentraporn, Se Puan Ya,

7772/2 4 Joffrey United States Patent O 3,482,119 REGULATED NUCLEATING POSITION TWO- VALLEY ELECTRON TRANSFER EFFECT DEVICE Wirojana Tantraporn and Se Puan Yu, Schenectady,

N.Y., assignors to General Electric Company, a corporation of New York Filed May 13, 1968, Ser. No. 728,411 Int. Cl. H03k 3/26, 19/08; H011 3/00 U.S. Cl. 307-299 Claims ABSTRACT OF THE DISCLOSURE A new Gunn effect diode for higher frequency, higher power applications comprises an n-type semiconductor crystalline material of substantially constant cross section having a controlled longitudinal donor doping density with a dominant feature providing a regulated or chosen :preferred nucleating position for launching the high field domain in the interelectrode space without the use of external tuned circuits. In one embodiment the donor profile has an increase, then a decrease about the ambient level; and in the second embodiment the profile has a smooth minimum. The latter diode is voltage tunable to change the frequency of oscillation as determined by the net result of a new effect of displacing the effective domain nucleating position and the known effect of changing the domain transit time.

This invention relates to two-valley electron transfer effect devices, or Gunn effect devices, and more particularly to new devices of this type wherein the high field domain is launched at a regulated or chosen position in the interelectrode space, and to a mode of operating the same. The regulated nucleating position diode has a controlled longitudinal donor density profile, and in one embodiment of the invention employs a new voltage tuning effect to change the frequency of oscillation of the output RF current.

The generation of coherent microwave current oscillations in homogeneous crystals of n-type gallium arsenide when the sample is subjected to high D-C voltages exceeding the threshold voltage was first observed by J. B. Gunn. The Gunn effect has been observed in other semiconductor materials such as n-type indium phosphide and cadmium telluride having a similar or closely related electronic structure. iIt is now generally accepted that the Gunn effect is associated with the transfer of hot electrons between conduction-band valleys separated in energy by a fraction of an electron-volt. The lowest energy conduction-band Valley is the normal electron conduction band, and a high electric field (the applied D-C voltage divided by the length of the sample is the electric field) causes the hot electrons to transfer from the low energy, high mobility valley to the unfilled higher energy, low mobility valley where they become less effective in the conduction process. The transferred electron mechanism gives rise to a voltage controlled bulk negative differential resistance which causes the output current to decrease even though the applied electric field is held steady or increased. Because of the attachment of ohmie contacts to either end of the n-type semiconductor crystal, as a result of which each end of the crystal is heavily doped, the conventional Gunn diode has a donor doping density profile of N+-N-N+ between the anode and cathode. Although doping gradients and random inhomogeneities do occur, the crystal has an approximately constant doping density over the major portion of the length of the diode. When the threshold voltage is exceeded, a high field Space charge dipole domain tends to form somewhere 3,482,119 Patented Dec. 2, 1969 ICC within the interelectrode space. The high field domain usually nucleates in the vicinity of the cathode and grows continuously larger as it propagates toward the anode, and as it is collected at the anode a new high field domain is again nucleated near the other electrode. The period of the resulting current oscillations is proportional to the transit time for the moving high field domain to traverse the length of the device. This is the original mode of operation discussed by Gunn and is known as the Gunn mode. The conventional Gunn diode, since it is a transittime device, must be kept thin to achieve the higher microwave frequencies, and this limits its power capabilities. Moreover, the transit time and thus the frequency of the RF current is practically indepedent of the voltage.

Certain subsequent developments in Gunn effect devices will be referred to briefly to facilitate the understanding and characteristics of the invention. Another mode of operation which has been suggested is the quenched domain mode, and is characterized by quenching or extinguishing the high field domain after launching before it has completely traversed the length of the sample. A radio frequency voltage produced by an external tuned circuit having a frequency greater than the transit time frequency is superimposed on the D-C biasing voltage such that the total voltage oscillates from values above the threshold voltage to values below the quenching voltage. In each cycle the high field domain is launched at the cathode, and quenched in the interelectrode space. Operation shifts in each cycle between the static characteristic, wherein the device follows Ohms law and the current increases as the voltage increases, and the dynamic characteristic which applies while the high field domain exists. The RF current has a considerably higher frequency than obtained using the Gunn mode. In the limited space-charge accumulation (LSA) mode of operation, an externally tuned radio frequency circuit is also used in a similar manner to obtain higher frequencies, but the power output is higher than when operated in the Gunn mode or quenched domain mode. When the sample is oscillating in the LSA mode, the electric field across the diode rises from below the threshold value to a value more than twice the threshold field so quickly that the space charge distribution associated with a high field domain does not have time to form. The ratio of frequency-to-doping level of the sample must be within a predetermined range.

It has been suggested that the high field domain be launched in the interelectrode space by reducing the cross section of the semiconductor crystal at a chosen point along its length or by abruptly changing the donor density doping level by including in the sample a small section of higher resistivity material than the remainder of the sample. In this case the high field domain again traverses only a portion of the length of the crystal and higher frequencies of operation are obtained. It is to an improvement of this general type of diode and mode of operation that the present application is directed. Another aspect of the invention is voltage tunability of the device to change the frequency of oscillation of the RF current due to changing the applied voltage. It is well known that the propagation velocity of a high field space charge domain decreases as the average electric eld applied to the diode is increased, and will be referred to herein as voltage tuning effect A. It has further been shown that the voltage tuning range can be accentuated by effectively introducing a monotonically tapered electrical conductance gradient in a Gunn diode, and can be implemented mechanically by tapering the cross-sectional area of a diode which has a nominally constant donor doping density profile. This was also done on a diode of constant cross-sectional area by introducing a temperature gradient from anode to cathode, the electrical conductivity then being higher at the anode end. In this type of diode, a new high field domain is formed before the previous domain can reach the anode, and the original domain dies away in favor of the new one. While higher frequencies can be obtained than with the conventional Gunn diode, the frequency of oscillation increases as the biasing voltage decreases but with smaller and noisier current fluctuations. Consequently, this diode is not suitable for high power, high frequency applications. This and other instances of voltage tuning of Gunn effect devices reported in the literature can be defined in terms of voltage tuning effect A, namely, that the domain transit time increases as the terminal voltage is increased, although the detailed causes may differ.

Accordingly, an object of the invention is to provide a new and improved two-valley electron transfer effect device, or Gunn effect device, having a controlled longitudinal donor doping profile whose gross shape provides a nucleation position for the launching of a high field domain at a regulated position other than the cathode.

Another object is the provision of a new regulated nucleating position diode of the foregoing type for higher power, higher frequency applications which does not need to be operated in an electrically tuned circuit and is not subject to the power limitations of the transittime solid state microwave devices.

Yet another object of the invention is to provide a new and improved Gunn effect device, and a mode of operating the same, characterized by being voltage tunable by a new voltage tuning effect.

A further object is to provide a new and improved two-valley electron transfer effect device of constant cross section having a smooth minimum in the longitudinal donor doping density profile determining grossly the nucleating position of the high field domain and the frequency of oscillation, and which is further voltage tunable to fine adjust the frequency by the net result of the aforementioned voltage tuning effect and a new voltage tuning effect.

In accordance with the invention, a device for generating a radio frequency current includes a diode comprising a body of n-type semiconductor material inherently capable of producing Gunn effect oscillations and having a substantially constant cross section and a cathode and anode electrode respectively at opposing ends thereof defining a longitudinal direction for the propagation of high field domains. Circuit means are coupled to these electrodes including means applying to the diode a predetermined bias voltage in excess of the threshold voltage for launching a high eld domain. More particularly, the body of semiconductor material has a substantially constant cross-sectional donor doping density and a controlled variable longitudinal donor doping density. In one embodiment, the longitudinal donor doping density has a nominally constant ambient doping level and is characterized by an increase in doping density above the ambient level followed by a decrease below the ambient level at a regulated position between the electrodes which provides a dominant feature in the longitudinal donor doping density profile that is a preferred nucleation position for the launching of high field domains at the regulated position within the interelectrode space. In a second embodiment, the longitudnal donor doping density has a smooth minimum at the regulated position, and preferably changes continuously and gradually from the cathode region to the minimum, and from the minimum to the anode region. The initial domain is formed at the smooth minimum position, and the second and subsequent domains are nucleated at a position displaced from the minimum toward the anode by an amount that is a function of the applied bias voltage. For this reason the latter diode is voltage tunable to produce higher frequency oscillations at higher output power levels..

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of several preferred embodiments of the invention, as illustrated in the accompanying drawings wherein:

FIG. 1 is a simplified schematic circuit diagram of a circuit for producing microwave oscillations comprising a regulated nucleating position Gunn effect diode constructed in accordance with the teaching of the invention;

FIG. 2 shows a characteristic curve of net donor concentration or donor doping density vs. length for a Gunn effect diode according to a first embodiment of the invention wherein the donor doping is increased and then decreased at a chosen position;

FIG. 3 is an enlarged schematic isometric View of the diode per se illustrating several constant doping density planes;

FIGS. 4a and 4b show curves of donor density and electric eld in the diode vs. length for the donor doping density profile of FIG. 2, respectively taken at different time intervals after the launching of the high -field domain;

FIG. 5 is a curve of donor doping density vs. length for a Gunn effect diode according to a second embodiment of the invention wherein the donor density has a smooth minimum;

FIGS. 6a and 6b are plots of electric field intensity vs. length for the diode having the donor doping density profile of FIG. 5 to show graphically the nucleation and collection of the first high field domain and the nucleation and collection of the second and subsequent domains;

FIG. 7 shows a set of output RF current vs. time characteristic curves for the FIG. 5 profile diode;

FIG. S is a curve of the period of the steady state oscillations vs. applied bias voltage for the RF current oscillations illustrated in FIG. 7; and

FIG. 9 is a characteristic curve of frequency of the output current vs. applied bias voltage for a diode of the type referred to in FIG. 5, further illustrating the voltage tuning characteristics.

Before proceeding to a discussion of the internal features of the new regulated nucleating position Gunn effect diode, the typical circuit for operating the diode shown in FIG. 1 will be explained. The new Gunn effect diode 10 is connected in series circuit relationship with a load 11 across the terminals of an adjustable D-C voltage source 12 for producing square or rectangular pulses. In this manner, the magnitude of the voltage applied to the new Gunn effect device 10 can be constant or can be adjusted to a preselected value, and the output RF current fiows through the load 11 which is connected between terminals 13 and 14. Although pulsed operation of the Gunn effect diode is preferred, continuous operation of the diode is feasible when it is properly designed. In any event, it will be understood that the magnitude of the bias voltage applied to the diode 10 exceeds the threshold voltage in order to produce Gunn oscillations. The device 10 comprises a crystal 15 of n-type gallium arsenide, or other semiconductor material in which the Gunn effect is observed, between two electrodes 16 and 17. The regulated nucleating position Gunneffect diode 10 is realized through a controlled arrangement of the longitudinal electron donor density profile of the crystal 15.

It is elementary that an n-type semiconductor crystalline material such as n-type gallium arsenide contains excess electrons or donor charge carriers available for the conduction process which are obtained by doping the semiconductor material with a donor impurity. By varying the amount of donor impurity added to the semiconductor material during manufacture, the semiconductor material can be heavily or lightly doped, or to any degree therebetween, and the number of donor charge carriers added to the crystal lattice by the doping process is changed in like manner. The regulating nucleating position diode 10 has a longitudinal donor doping density profile of the general type shown in FIG. 2, wherein the net electron donor concentration is plotted against diode length as the abscissa. The cathode electrode 16 and the anode electrode 17 of the diode 10- are commonly made of a metal such as tin which acts as a donor impurity for the semiconductor crystal 15, and because of the conventional techniques used to add the electrodes or ohmic contacts to opposing ends of the semiconductor crystal, the cathode region and the anode region are both heavily doped as indicated by the steeply rising portions at each end of tthe donor doping densiy profile curve 20 in FIG. 2. The major portion of the curve 20` between these two heavily doped end regions is substantially constant, as is the case with the conventional transit time Gunn diode. While as to its gross features the doping density can be considered to be relatively constant, in actuality there are naturally occurring random variations and possibly local concentrations of greater or lesser doping variations as shown within the small circle 21, wherein the curve 20 is drawn to a considerably larger scale. In accordance with the invention, the donor doping density prole within the interelectrode space includes appropriate additional variations of sucient magnitude to establish at a chosen position in the interelectrode space a preferred region of high eld domain nucleation. Thus the high iield domain nucleates at a regulated or chosen position other than the cathode as compared to the conventional Gunn diode in which the high eld domain normally nucleates only at the cathode.

In the rst embodiment of the invention shown in FIG. 2, the new arrangement of the donor doping density profile curve 20I takes the form of an increase in the donor density above the ambient level in one region 22, and a decrease below the ambient level in another region 23 adjacent to or closely following the region 22. The region of preferred high field domain nucleation occurs approximately at the line 24 between the two regions 22 and 23. Assuming that the line 24 is at a distance L1 from the cathode 16 for a diode having a total length L0, then the frequency of oscillation of the RF output current produced by the regulated nucleating position diode is Lo L0-L1 times that of a conventional transit-time Gunn diode of the same physical length.

The precise size and shape of the longitudinal donor doping variations introduced into the donor doping density profile curve 20 are not critical, within the restriction that there be an increase in donor doping followed by a decrease in donor doping about the ambient level as a reference proceeding from the cathode 16 to the anode 17, so long as these variations form a dominant feature in the curve 20 within the interelectrode space. Since it is a dominant feature, the high field domain nucleates in the vicinity of the line 24 rather than at some point of smaller local variation having a magnitude comparable to the random or naturally occurring fluctuations in the ambient level of doping. There may be some electric eld disturbance in the vicinity of the cathode, but this unwanted disturbance decays rapidly. A signicant feature of the invention is that the doped semiconductor crystal has a constant or substantially constant crosssectional area, and the donor doping density in any particular cross-sectional plan is also substantially constant. This is illustrated in FIG. 3. Although not limited to this shape, the crystal 15 is preferably a rectangular parallelepiped and the dashed line doping profile curve is drawn on this gure to show the doping variations in the longitudinal direction of the crystal between the cathode and the anode. Two constant doping planes or sections 25 and 26 are illustrated to demonstrate that in any crosssectional plane the donor doping level is approximately constant, whereas in the longitudinal direction the donor doping levels are different inasmuch as the plane 25 is within the region 22 of increased donor doping density while the plane 26 is within the region 23 of decreased donor doping density.

The regulated nucleating position diode shown in FIG. 3 is conveniently fabricated by vapor phase epitaxial crystal growth techniques. Well-known gas phase epitaxy techniques can be used with the modification that a smal-l amount of acceptor dopant gas (e.g., a cadmium-bearing gas) and then a small amount of a donor dopant gas (e.g., a selenium-bearing gas) be added to the gas stream of the epitaxy system at an appropriate time. This occurs when the diode crystal has grown to a thickness of L0-L1. Liquid phase epitaxy techniques can also be used to fabricate regulated nucleating position diodes through a sequence of operations which are analogous to those just described. This sequence of doping Variations assumes that the epitaxial growth of gallium arsenide takes place from the anode region, which is the first to be deposited, to the cathode region. Variations on these crystal growth and doping procedures are possible and will not be discussed further in view of the fact that these techniques are well known in the art. Other semiconductor materials such as cadmium telluride and zinc selenide can also be used for regulated nucleating position diodes with suitably chosen doping agents.

The formation and propagation of the high eld domain at two time periods after its nucleation is shown graphically in FIGS. 4a and 4b. At a relatively short time after its nucleation, the normalized donor density Icharacteristic denoted as n/no and the normalized electric eld intensity characteristic denoted as E/EO exhibit a peak followed by a dip, and vice versa, bearing a relation to the controlled variations introduced into the donor doping prole. At a substantially later time as shown in FIG. 4b, the high field domain 27 has fully formed and is growing continuously larger as it propagates toward the anode, while in the remainder of the material there is a low field region. At this point, the electric eld intensity distribution in the diode is approximately the same as if the diode were a conventional transit time Gunn device with a substantially constant longitudinal donor doping density wherein the high field domain nucleates in the vicinity of the cathode rather than at the regulated position L1. The donor density characteristic curve n/n:o is also much the same as if the diode were a conventional Gunn diode. Thus, although FIG. 4a is dissimilar to corresponding curves for a conventional Gunn diode because of the appearance of both a peak and a dip about the ambient level, it is seen that after the high eld domain 27 has formed and propagated a portion of the distance toward the anode, the regulated nucleating position diode 10 acts like the conventional transit time Gunn diode.

As has been pointed out, a Gunn effect diode having a longitudinal donor doping density profile as shown in FIG. 2 has a domain transit time which can be chosen to be considerably smaller than the domain transit time in a conventional Gunn diode of the same physical length. Thus, for a given radio frequency output power and diode terminal impedance, the frequency of oscillation can be chosen to be appreciably higher than that which is obtainable with the transit time Gunn diode. As evident by a review of FIG. 1, this important increase in operating frequency is obtained without the use of the external tuned radio frequency circuits needed to realize the previously mentioned limited space charge accumulation (LSA) or quenched domain mode of operation. Moreover, by making L1 approach L0, this new Gunn effect diode has the same practical operating advantage as the LSA diode in not being limited `by the known power-impedance-frequency product limitation which applies to transit time solid state microwave devices.

In the above relation, EM is the breakdown electric field strength and VS is the charge carrier saturation drift velocity of the material from which the device is c011- structed. The increase in output power is obtained without the need for the electrically tuned circuit required to operate the LSA diode.

In a second embodiment of the invention, the new Gunn effect diode has a longitudinal donor doping profile of the type shown by curve 30 in FIG. 5, which is characterized by having a smooth minimum along its length determining grossly the position at which the high field domain is nuoleated. Furthermore, the donor doping density changes gradually and continuously between the cathode and the minimum, and the minimum and the anode. The curve 30 comprises two oppositely sloped parabolas intersecting at and having a minimum at 0.9 L0, but this is probably not an optimum shape for providing the best set of operating characteristics. The general shape of the doping profile in FIG. 5 is of importance, and the significant feature of the new donor density profile is the existence of a smooth minimum in the doping density. This minimum can be located anywhere in the interelectrode space, but the actual location should be at least one high field space charge dipole domain width away from both the cathode 16 or the anode 17. IOf course, the highest frequency of oscillation can be obtained with the minimum in doping density located in the vicinity of the anode, preferably at approximately 0.8 to 0.9 of the physical length of the diode measuring from the cathode. As was the case with the first embodiment, there are naturally occurring variations in donor doping density along the curve 30, and the smooth minimum position is the dominant or gross feature of the curve so that a clearly defined preferred position for the nucleation of the initial high field domain is provided.

The formation and movement of the initial high field domain is shown graphically in FIG. 6a by curves 1 to 4. When a constant bias voltage is applied to the diode, the electric field intensity at first (see curve 1) exhibits a maximum at the point of minimum donor doping density. A high field space charge dipole domain nucleates near this region of highest electric field and grows in size as it propagates toward the anode 17 as indicated by curves 2 and 3. As this initial high field domain is collected by the anode (curve 4), a second high field domain is formed in the vicinity of the minimum in the doping density profile, as shown by curve 5 in FIG. 6b. The second domain (curve S) also grows in size as it propagates toward the anode where it is collected (curve 6). This cycling continues, thus producing the radio frequency oscillation of electric current through the diode. It will be noted from a comparison of curves 3 and 5 that the exact position of nucleation of the second and all subsequent domains in the time sequence is closer to the anode 17 than the position of nucleation of the rst domain. 'In practical terms, the first radio frequency cycle can be regarded as a start-up transient, and steady state operation is reached on the second and following current cycles. In the steady state, the effective domain nucleation position is on the anode side of the minimum in the donor doping profile, which in FIG. 5 is at 0.9 L0. This behavior can be expected since the electric field intensity distribution over the diode is different during the nucleation of the first high field domain as compared to the field intensity distribution present during the nucleation of the second and subsequent domains.

When the bias voltage applied to the diode is changed as for example by the circuit shown in FIG. 1, the nucleation position of the second and subsequent domains as compared to that of the first domain will be changed. That is to say, the nucleation position of the initial high field domain will be the same regardless as to the magnitude of the applied voltage, but the nucleation position of the second and subsequent domains changes as the applied voltage is changed. More particularly, the nucleation position of the second and subsequent domains is closer to the anode electrode as the applied voltage is increased. Thus, in the absence of other voltage tuning effects the frequency of oscillation for the regulated nucleating position diode having the doping profile of FIG. 5 increases as the terminal voltage across the device increases. This voltage tuning effect is different from the previously mentioned voltage tuning effect A and will be referred to as voltage tuning effect B or the domain formation movement tuning effect. It will be recalled that voltage tuning effect A relates to the fact that a propagation velocity of a high field space charge domain decreases as the average field applied to the diode is increased. Since the transit time of the domain increases as the voltage applied to the diode increases, this can also be referred to as the transit time tuning effect. Voltage tuning effect A is known in the art whereas voltage tuning effect B is believed to be novel. Both of these voltage tuning effects are effective when the voltage applied to the diode is increased and can in fact compete with one another, since the transit time tuning effect tends to decrease the frequency of oscillation whereas the domain formation movement tuning effect tends to increase the frequency of oscillation.

The net results of these two competing voltage tuning processes is made clearer by an examination of the series of curves 1 to 5 shown in FIG. 7 representing the output current waveform as a function of time for increasing magnitudes of applied bias voltage V as denoted adjacent the respective curves in normalized units. FIG. 7 should be studied in conjunction with FIG. 8, wherein the period of steady state oscillations is plotted as a function of applied bias voltage. In FIG. 7 the vertical scale of electric current is displaced for convenience. It will be observed that as the bias voltage is increased, the first current maximum moves to the right in accordance with voltage tuning effect A demonstrating that the period of the initial transient current oscillation is increased. In each of the curves 1 to 5', the second and subsequent current maxima have the same period. Voltage tuning effect B now occurs simultaneously and is competing with voltage tuning effect A, and tends to decrease the period of the second and su-bsequent current oscillations as the applied voltage is increased. The net result on the period of steady state current oscillations as shown in FIG. 8 is that voltage tuning effect A dominates for lower applied voltages, and voltage tuning effect B dominates for the higher applied voltages. Consequently, as the terminal voltage across the diode increases, the period of steady state oscillations at first increases and then decreases.

The characteristic curve of frequency vs. bias voltage drawn in FIG. 9 has, of course, the converse general shape. Although in the lower frequency ranges on this curve the same frequency can be obtained with two different values of applied voltage, one in the region where tuning effect A dominates and the other in the region where tuning effect B dominates, a higher power output is obtained when a selected frequency occurs due to applying the higher voltage. Furthermore, the higher frequencies shown on this curve either cannot be obtained when the applied voltage is low, or if obtained by assuming the curve to be extrapolated to the left, produce very weak output power. This characteristic curve of frequency vs. voltage was obtained using a regulated nucleating position diode having a more optimum doping density profile than is shown in FIG. 5, although not necessarily the most optimum profile from the standpoint of maximum voltage tuning range and higher power output since all cases were not studied. The gross frequency of oscillation of the diode is obtained by adjusting the position of the smooth minimum in the doping density profile. By means of the net result of the voltage tuning effects which have been described, the frequency can be increased by greater than 2O percent by increasing the bias voltage. Thus, having selected the gross frequency of oscillation by suitably arranging the donor density doping profile when the diode is manufactured, the frequency can be fine adjusted by changing the applied bias voltage.

The advantages of the new Gunn effect diode having the donor profile of FIG. 5 are similar to those for a FIG. 2 profile diode. Briefiy by way of summary, these are that the frequency of the generated RF current can be chosen to be appreciably higher than is obtainable with a conventional transit time Gunn diode by regulating the active length of the device to a selected portion of its physical length. Thus, for a given RF output power and diode terminal impedance, there is a considerable increase in operating frequency. The circuit needed to operate the diode can be relatively simple as the higher frequency is obtained without the use of external tuned radio frequency circuits. Voltage tuning of the diode is made practical, since external tuned circuits are also not needed for this purpose and the frequency of oscillation can be varied over a wide range by changing the magnitude of the D-C biasing voltage. For these several reasons the new Gunn effect diode is useful in higher frequency, higher power applications.

While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A device for generating a radio frequency current including the combination of a diode comprising a body of n-type semiconductor material inherently capable of producing Gunn effect oscillations and having a substantially consant crosssection and a cathode and anode electrode respectively at opposing ends thereof defining a longitudinal direction for the propagation of high field domains, and

circuit means coupled to said electrodes including means for applying to said diode a bias voltage in excess of the threshold voltage for launching a high field domain, wherein said body of semiconductor material has a substantially constant cross-sectional donor doping density and a controlled variable longitudinal donor doping density having a nominally constant ambient level doping density characterized by, at a regulated position between the electrodes, an increase above the ambient level followed by a decrease below the ambient level providing a dominant feature in the longitudinal donor doping density profile which is the preferred nucleating position for the launching of high field domains at the regulated position within the interelectrode space.

2. A device as set forth in claim 1 wherein said circuit means further includes load means connected in series circuit relationship with said diode.

3. A voltage tunable device for generating a radio frequency current including the combination of a diode comprising a body of n-type semiconductor material inherently capable of producing Gunn effect oscillations and having a substantially constant cross section and a cathode and anode electrode respectively at opposing ends thereof defining a longitudinal direction for the propagation of high field domains, and

circuit means coupled to said electrodes including means for applying to said diode a bias voltage in excess of the threshold voltage for launching a high field domain, wherein said body of semiconductor material has a substantially constant cross-sectional donor doping density and a controlled variable longitudinal donor doping density characterized by a smooth minimum at a regulated position between the electrodes providing a dominant feature in the longitudinal donor doping 5 density profile which is the preferred nucleating position for the launching of an initial high field domain at the regulated position within the interelectrode space, the second and subsequent high field domains being launched at a position displaced from the minimum toward the anode electrode by an amount that is a function of the applied bias voltage.

4. A device as set forth in claim 3 wherein the longitudinal donor doping density as to its gross features continuously changes between the region of the cathode electrode and the region of the anode electrode, and the minimum in the longitudinal donor doping density is spaced at least one high field domain width from both of the electrodes.

5. A device as set forth in claim 3 wherein the longitudinal donor doping density has approximately the same magnitude in the region of the cathode electrode and in the region of the anode electrode, and as to its gross features continuously and gradually decreases from the cathode electrode region to the minimum, and continuously and gradually increases from the minimum to the anode electrode region, and the minimum is spaced at least one high field domain width from both of the electrodes.

6. A device as set forth in claim 3 wherein the longitudinal donor doping density has approximately the same magnitude at the cathode electrode re-gion and the anode electrode region, and as to its gross features continuously and gradually changes between the cathode and anode electrode regions, the minimum in the longitudinal donor doping density occurring at approximately 0.9 of the physical length of the diode measuring from the cathode electrode.

7. A device as set forth in claim 3 wherein said circuit means further includes load means connected in series circuit relationship with said diode, and

said means for applying a bias voltage comprises an adjustable source of bias voltage.

8. A device as set forth in claim 3 wherein the semiconductor material is gallium arsenide, and

said means for applying to the diode a bias voltage cornprises an adjustable source of unidirectional pulses.

9. A device as set forth in claim 3 wherein the semiconductor material is gallium arsenide, and

the longitudinal donor doping density as to its gross features changes continuously between the region of the cathode electrode and the minimum, land between the minimum and the region of the anode electrode.

10. A device as set forth in claim 3 wherein the semi- 55 conductor material is gallium arsenide,

the longitudinal donor doping density as to its gross features changes continuously from the region of the cathode electrode to the minimum, and from the minimum to the region of the anode electrode according to parabolic curve distributions, and

the minimum in the longitudinal donor doping density occurs at approximately 0.9 of the physical length of the diode measuring from the cathode electrode.

References Cited 6D UNITED STATES PATENTS 3,377,566 4/1968 Lanza 317-234 OTHER REFERENCES IBM Tech. Discl Bul., Microwave Oscillator, by

Dumke, vol. 8, No. ll, April 1966, pp. 1646-47.

JERRY D. CRAIG, Primary Examiner U.S. Cl. X.R.

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