Real-space charge-transfer device and method thereof

A real-space charge-transfer device is disclosed. In particular, a Gunn diode is disclosed having a conductive structure fabricated overlying its active region. A secondary signal, other than the normal Gunn diode signal, is generated by the Gunn diode based upon a characteristic of the overlying conductive structure. For example, when the conductive structure is a grate having N teeth the secondary signal will have N secondary oscillation cycles that occur during the duration of a single normal Gunn diode oscillation cycle.

BACKGROUND

1. Field of the Disclosure

The present application is related to electronic devices, and in particular to real-space charge-transfer devices and methods based on real-space charge transfer devices.

2. Related Art

Real-space charge-transfer devices, such as a Gunn diode, can be used to generate microwaves or millimeter waves. Such a device has an active region between an anode and a cathode that can be manufactured using a semiconductor material, such as a compound semiconductor in the case of a Gunn diode. It is the case with such devices that their electron mobility is large in a low electric field (several thousands of cm2N-sec) and that in response to being exposed to a sufficiently large electric field their electron mobility is decreased as accelerated electrons transit to a band of large effective mass. This decrease in mobility in high electric fields causes a negative differential mobility within their active region that is characterized by the generation of a p-n junction domain that transits across the active region of the device, from the cathode side to the anode side. This domain is referred to as a Gunn domain in Gunn diodes. Once the p-n junction domain completes its transit across the active region of a device, another p-n junction domain is generated and begins its transit across the device.FIG. 1illustrates a current-time graph illustrating the vibrating current at the anode of a Gunn diode that is the result of this phenomenon. As illustrated, the duration of the Gunn domain is represented by the time labeled DGd, and the period of the vibrating current of the Gunn Domain that results in the normal Gunn oscillation fGis represented by the time labeled PGd,osc.

The Oscillation frequency of a standard Gunn diode can be determined from the transit distance L of the domain, e.g., the length of the Gunn diode's active region, and the average drift velocity Vd of the electrons in the active region using the equation: ft=Vd/L. Thus, the energy relaxation time of the device, which consists of the time needed for the electron to increase and decrease energy at Γ valley, and the length of the device primarily determine the upper limit of the oscillating frequency in the millimeter wave range. For example, the relaxation time constant of GaAs is such that the upper limit of the oscillating frequency for a Gunn diode is between 60 and 70 GHz (gigahertz).

Efforts to increase the upper frequency limits of Gunn diodes include using compound semiconductor materials having faster relaxation time constants. In addition, the distance of transit has been short, e.g., 1 to 2 μm (micrometers).

In order to implement such efforts, measures have been taken with conventional Gunn diodes for millimeter waves such as employing a vertical diode structure having an anode and cathode at opposing surfaces, to use elements including the active layer of extremely small sizes, having diameters of approximately several tens of μm.

DETAILED DESCRIPTION OF THE DRAWINGS

A real-space charge-transfer device is disclosed herein by way of example in the context of a planar Gunn diode, wherein the Gunn diode is fabricated to have a conductive structure overlying its active region. During operation, the conductive structure, which is passive by virtue of not being actively driven to bias the active region, causes a generated Gunn diode signal to have a secondary output signal in addition to the normal Gunn diode signal. The secondary output signal has a fundamental frequency (f0) that is different from the fundamental frequency of the normal Gunn diode signal. For example, in one embodiment, a conductive structure formed overlying the active region of a Gunn diode having repetitive features, such as a grate structure having a plurality of teeth, causes a secondary output signal that modulates during each normal Gunn oscillation cycle to produce a moving charge image of the grate. For example, a Gunn diode having a grate having N teeth overlying its active region generates an output signal comprising not only the normal Gunn oscillation, but also a secondary signal with N secondary oscillation cycles that correspond to the N teeth. The ability to generate a secondary oscillation allows for the generation of higher frequency signals without having to reduce the length of the Gunn diode. In addition, the frequency characteristics of the secondary oscillation of a Gunn diode can be controlled by changing various characteristics of the overlying conductive layer based upon the requirements of various applications.FIGS. 2-11disclose particular embodiments of a real-space charge-transfer device having a conductive structure overlying its active region to effectuate this secondary oscillation. It will be appreciated that while the illustrated embodiments are described with reference to a Gunn diode, that similar techniques can apply to any real-space charge-transfer device.

FIGS. 2 and 3illustrate a planar Gunn diode that is disposed at a portion of a workpiece100in accordance with a particular embodiment of the present disclosure. In particular,FIG. 2illustrates a cross-sectional view of a Gunn diode120at a location of the workpiece100, andFIG. 2illustrates a plan view of the Gunn diode120. While the portion of workpiece100ofFIG. 1only illustrates a Gunn diode120, it will be appreciated that the workpiece can include other components and features.

Workpiece100is presumed to be a wafer at which various features of an integrated circuit device are formed during fabrication of the integrated circuit device, and is illustrated to include levels111-113, wherein level111is the lower-most level of the workpiece, level113is the upper-most level where an active layer resides, and level112resides between level111and level113. A layer121of workpiece100is a support layer that resides at level111to provide structural support to the workpiece100. By way of example, it is presumed layer121is a semiconductor layer, e.g., Silicon, Germanium, Gallium Arsenide (GaAs), Indium Phosphide (InP), and the like that may be doped or undoped.

A layer123of workpiece100includes a semiconductor material suitable for acting as the active region of a real-space charge-transfer device. Therefore, layer123can be referred to herein as the active layer123. For a Gunn diode, the active layer123can be a compound semiconductor material, such as a III-V or a II-VI semiconductor, an organic semiconductor material, and other semiconductor materials suitable for the purposes described herein. By way of example, it is presumed layer123is Indium Phosphide having a thickness of approximately 1 micron that has been doped with an n-type dopant to provide an appropriate conductivity at which to form the active region of a Gunn diode, such as a doping in the range of 1×1012ions/cm3to 1×1018ions/cm3.

Layer122is an intermediate layer that provides for an appropriate interface with the active layer123and the support layer111. For example, in a particular embodiment where layers121-123form a Semiconductor-On-Insulator (SOI) substrate, layer122can be a dielectric layer. Alternatively, layer122can be a semiconductor layer having a doping concentration that is greater than that of the active layer123. By way of example, it is presumed layer122is a dielectric layer.

An isolation region139, such as a dielectric region, resides at level131that is suitable to electrically isolate the active region123from adjacent active regions (not shown) that also reside at level131of the workpiece100. For example, the isolation region139can be a shallow-trench isolation structure.

Features132and133are anode/cathode electrodes of the Gunn diode120that are in electrical contact with the active region123. During operation, by way of example, feature132is presumed to be a cathode, referred to as cathode123, and feature133is presumed to be an anode, referred to as anode123. Typically, the resistivity of the electrodes132and133is lower than that of the semiconductor material of active region123. For example, the electrodes132and133can be a metal silicide, a semiconductor region that is more highly n-doped than the active region123, e.g., 1×1019ions/cm2or greater, and the like.

A conductive structure is formed overlying the active region123, e.g., above level113. As will be described in greater detail below, the conductive structure can abut the active region123, or a dielectric can reside between the conductive structure and the active region, thereby isolating the conductive portion from the active region123.

In the illustrated embodiment, the conductive structure is a grate130having a plurality of teeth134. The conductive structure can be a passive structure in that it is electrically isolated from interconnect structures that are capable of being actively driven to provide a bias signal to the tooth. For example, each tooth134of the grate130is a passive tooth, wherein the term “passive” is intended to indicate that a structure that overlies a portion of the active region of a Gunn diode is electrically isolated from interconnect structures that are capable of being actively driven to provide a bias signal to the tooth. For example, the teeth134can be isolated from any interconnect structures, e.g., such as when the upper most surface and side surfaces of tooth134are completely surrounded by a dielectric material (not shown), e.g., the teeth are not connected to any interconnect structures. Alternatively, the teeth134can be connected to an interconnect structure that is not capable of biasing the tooth during normal operation. For example, the conductive structure can have a conductance that is greater than the conductance of the material of the active region123, but is not connected to any active components, such as a transistor, or to any input/output terminals that would be capable of driving the teeth134to provide a bias signal. In a particular embodiment, the teeth of a grate can be electrically connected together, or electrically isolated from each other. In contrast, the term “active tooth” is intended to refer to a structure that is electrically connected to an interconnect structure that is capable of being actively driven to a desired voltage, such as ground, or to otherwise provide a bias signal to the tooth. The passive nature of the teeth disclosed herein is illustrated the figures by the label “z”. In other embodiments the teeth may be actively driven.

In the plan view ofFIG. 3, the Gunn diode120is shown to have a length102and a width103as defined by the active region as bound by the dielectric region139and the terminals132and133. The term “length” as used with respect to a Gunn diode, or its features, is intended to refer to a shortest dimension of the active region of the Gunn diode residing between the anode and cathode as measured in a length direction. The term “length direction” as used herein with respect to a Gunn diode is intended to mean the predominate direction of current flow between the cathode and anode. The term “width” as used herein with respect to a Gunn Diode, or its features, is intended to refer to a dimension of the active region of the Gunn diode that is perpendicular to the length direction of the Gunn diode.

The teeth134are illustrated to have a length191, while the width of each tooth134has the same actual width103as the width of the Gunn diode120, by virtue terminating at the isolation regions139. Note that a tooth that terminates past the isolation regions139would have a larger actual width that an illustrated tooth134, but would have the same effective width as an illustrated tooth134, wherein the effective width is limited by the width of the active region123. Conversely, a tooth that terminates directly over the active region123, e.g., prior to the isolation regions139, has a width that is less than that of the active region123. A center location of the active region123, represented by line101, directly underlies a portion of the center tooth134. As specifically illustrated, the center location of the middle tooth134is also represented by line101, and therefore the tooth and the active region are commonly centered. Dimension194ofFIG. 3represents the grate period, which is the distance between adjacent teeth134of the grate130. Dimension192ofFIG. 3represents the distance from the cathode to the center of a closest tooth134. Dimension193ofFIG. 3represents the distance from the anode133to the center of a closest tooth134.

FIG. 4is a current-time graph illustrating the operation of Gunn diode120. In particular, it has been discovered that in addition to the normal Gunn oscillation, an additional modulation occurs during Gunn domain transit that is based upon a characteristic of the overlying conductive layer (e.g., the number of teeth134overlying the active region123). In particular, a number of cycles of the secondary oscillation occurring during each normal Gunn oscillation cycle is equal in number to the number of teeth134in the grate130. Thus, the three-tooth grate ofFIG. 3produces the three oscillation cycles illustrated atFIG. 4, wherein each one of the three oscillation cycles are associated with one of three oscillation cycles and one of the three teeth134of grate130.

The secondary oscillation illustrate atFIG. 4is referred to herein as the Grate oscillation of the Gunn diode. The period of time294from the start of one secondary oscillation to the start of an adjacent secondary oscillation is based upon the distance between adjacent gates, e.g., the period of the grate, and can be referred to herein as the grate oscillation period of the Gunn diode (PGg,osc). The period of time293of the secondary oscillation is based upon the distance from the last gate134and the anode133. The period of time292is passed upon the distance from the cathode132and the first gate134.

A duration211represents the duration of the domain of the secondary oscillation, e.g., that portion of PGg,oscduring which the secondary oscillation is actively transitioning. This duration can be referred to herein as DGg,osc.

It will be appreciated that when the teeth134of grate130are periodic, the duration294will be based upon the distance between adjacent teeth, e.g., distance194. Thus, inFIG. 4, the first secondary oscillation is associated with the two teeth134closest the cathode132, and the second secondary oscillation is associated with the two teeth134furthest from cathode132. The duration214associated with these two oscillation periods can be generally characterized by the equation:
PGg,osc=(PGd,osc/ngt)*(Pg*(ngt−1))/GLWhere:PGd,osc—is the period of the normal Gunn oscillation; andngtis the number of teeth in the grate.Pg*(ngt−1) is the distance from the first tooth to the last tooth of the grate; wherePgis the tooth period of the grate (e.g., 194)ngis the number of teeth in the grate; andGLis the length of the Gun Diode.

The duration293of third secondary oscillation, however, is based upon the distance193from the last tooth134, e.g., the tooth furthest from cathode132, to the anode133. Similarly the duration292from the start of the normal Gunn oscillation until the start of the first grate oscillation is based upon the distance192, which is the distance from the cathode to the first tooth134

FIG. 5illustrates a cross-sectional view of a workpiece300that includes an embodiment of a Gunn diode320having a grate with six (6) teeth334. Gunn diode320ofFIG. 5has a length102, which is the same length as Gunn diode120ofFIGS. 2 and 3. The period314of adjacent teeth334is approximately one-half the period194of adjacent teeth134ofFIGS. 2 and 3. Features of workpiece300that are presumed to be the same as those previously described with reference to workpiece100are identically numbered.

Because the Gunn diode320has six (6) teeth, its current-time graph, illustrated atFIG. 6, shows a corresponding six (6) secondary oscillations occurring during each Normal Gunn oscillation period. The period314of the grate oscillation for Gunn diode320, however, is substantially shorter than the oscillation of Gunn diode120by virtue of the tooth period314of Gunn diode320being shorter than the tooth period194of Gunn diode120. For clarity of illustration, it is presumed that the distance392from the cathode132to the first tooth334is sufficiently long so that the first grate oscillation does not overlap with the Gunn domain. Thus, the duration from when the Gunn domain is generated until the Gunn domain reaches the first grate is greater than DGd(FIG. 1). It will be appreciated, that in an alternate embodiment, the six (6) oscillations could be more equally spaced across the entire Normal Gunn oscillation period, which would result in the first grate oscillation and the signal of the Gunn domain occurring concurrently.

FIG. 7illustrates a cross-sectional view of another embodiment of a Gunn diode520having a grate with three (3) teeth534. Gunn diode520ofFIG. 7has the same length as Gunn diodes120and320. The period of adjacent teeth434is dimension194, which is the same as Gunn diode120, which also has three (3) teeth. The length491of teeth534is approximately twice the length191of the teeth134of Gunn diode120. Features of workpiece500presumed to be the same as those previously described with reference to workpiece100ofFIG. 1are identically numbered.

Because the Gunn diode520has three (3) teeth434, its current-time graph, illustrated atFIG. 8, shows three (3) corresponding secondary oscillations that occur during a normal Gunn oscillation period. The period of the grate oscillation for Gunn diode420is substantially the same as for Gunn diode320. However, while teeth having a greater length do not substantially change the duration of PGg,osc, the harmonic components of each oscillation is believed to change. In particular, a Gunn diode having a grate with longer teeth, but the same period as a Gunn diode having the same grate with shorter teeth, is understood to result in a secondary oscillation having a fundamental frequency with relatively more power than its harmonic frequencies. This results in the secondary oscillation being less “spikey”, e.g., more sinusoidal, when teeth with a greater length are used. Thus, in applications where it is desirable to work with the highest possible frequency of the Gunn diode's grate oscillation a shorter tooth length may be desirable, as the result will be grate oscillations that have more significant higher order harmonics, e.g., the oscillations are more “spikey”. Conversely, in applications where it is desirable to work directly with the fundamental frequency of the secondary signal, a Gunn Diode having a longer tooth length could be desirable, as the result will be a first harmonic with a higher power component.

After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of various embodiments, and that there are other embodiment and details.

For example conductive features, such as teeth134, can be considered conductive by virtue of having at least a portion that is more conductive than the active region123, for example, or by virtue of having a portion that is more conductive than a material that resides between conductive features134. For example, the conductive features134can be more conductive than active region123by at least an order of magnitude, and more conductive than the material between the conductive features, where the material between the conductive features134can be either less conductive or more conductive than active region123; the conductive features134can be less conductive than active region123, but more conductive than the material between conductive features134.

In addition, the number of location of the conductive teeth can vary. For example, more or fewer teeth can be used, included a single tooth. The length of a tooth can vary significantly. For example, where a single tooth is formed overlying the active region the tooth can have a length that is from near zero percent of the length of the active region to a length that is near 100 percent of the length of the active region. Other ranges for the length of a tooth include: between 5000 nm and 50 nm; between 2500 and 50 nm; between 1250 and 50 nm; between 600 and 50 nm; between 300 and 50 nm; between 100 and 50 nm; between 1250 nm and 500 nm.

In addition, it will be appreciated that the manner and materials used to form the conductive structures can vary to incorporate any one of various embodiments.FIG. 9illustrates a specific embodiment of a conductive tooth134that directly abuts the active region123. For example, a metal layer, a doped poly-silicon layer, or other conductive layer can be formed directly at the surface of active region123. Alternatively, as illustrated atFIG. 10, a conductive tooth134is spaced apart from the active region by a less conductive layer682, e.g., a dielectric layer682. It will be appreciated that the dielectric portion needs have sufficiently small thickness dimension to ensure the conductive tooth134is close enough to the active region123to effectuate the secondary oscillation described herein. For example, the thickness of the dielectric portion182can be 2500 Angstroms or less. In addition, after formation of the teeth134, a dielectric material (not shown) can be formed over the active region123and tooth134, wherein the dielectric material would reside between adjacent teeth of the device.

In addition, it will be appreciated that conductive structures other than the grate specifically described can be used. For example, a conductive structure can be a meta-material having a plurality of features overlying the active region that are isolated from each other. For example,FIG. 11illustrates a meta-material formed by a plurality of circular conductive features634that are arranged in an offset array pattern. It will be appreciated that other meta-materials can be used that may or may not be organized in arrays, and have feature that are the same or different than those illustrated atFIG. 11.

FIG. 12illustrates a system device for implementing a particular application using a Gunn diode in accordance as described herein. In particular, the system ofFIG. 12includes a Gunn diode810, application circuitry820, and a voltage reference terminal, labeled “V”. For convenience, a label or reference numeral associated with a particular node or terminal can also be used to refer to a signal present at that node. Therefore, it will be appreciated that cathode of the Gunn diode810is connected to the voltage reference terminal labeled V, which during operation provides a voltage V having sufficient magnitude to effectuate generation of a Gunn domain as described above.” The anode of the Gunn transistor810is connected to a node811that is also connected to application circuitry820.

The application circuitry820includes filter circuitry821and is associated with implementing a particular application based on the secondary signal received from the Gunn diode810. According to an embodiment, filter circuitry812is connected to the input node811to receive the signal generated by the Gunn diode810, which includes the normal Gunn oscillation and the secondary oscillation described herein. During operation, filter820can filter the received signal and provide a filtered signal at its output, which is connected to node822. Node822can be connected to other circuits of the application circuitry820, such as other filters, amplifiers, mixers, switches, and data processor devices that are used to implement a particular application. A typical frequency used in such an application is in excess of 1 GHz, and can include frequencies that are greater than the fundamental frequency of the normal Gunn oscillation. For example, it is anticipated that frequencies in excess of 100 GHz can be realized, such as frequencies up to 150 GHz, 200 GHz, or more.

In accordance with an embodiment, the filtered signal provided by filter821for use by the application circuitry820is based upon the secondary oscillation. For example, the filter821can provide one or more of the fundamental or harmonic frequencies of the secondary signal for use by a specific application. In another embodiment, the filter821can attenuates one or more of the fundamental or harmonic frequencies of the normal Gunn oscillation signal.

As used herein, a particular frequency of signal from the diode810is said to be “associated with the secondary signal but not the normal Gunn diode oscillation” if the amount of energy of the secondary signal from the Gunn diode at that frequency is greater than the amount of energy of the normal Gunn diode oscillation at that frequency. Similarly, a particular frequency of signal from the diode810is said to be “associated with the normal Gunn oscillation but not the secondary signal” if the amount of energy at the particular frequency is greater in the normal Gunn diode oscillation than in the secondary oscillation. It will be appreciated that whether a particular signal is associated with the secondary signal can be further limited by a relative difference in power of the particular frequency. For example, whether a particular frequency is associated with one signal but not the other can be qualified by an amount. For example, a particular frequency can be said to be associated with the secondary signal but not the normal Gunn oscillation signal in response to the particular frequency of secondary signal having some greater amount of energy than does that particular frequency at the normal Gunn oscillation. For example, the amount of energy can be a relative difference, such as at least 3 dB, 6 dB, 9 db, 10 db, or more.

It will be appreciated therefore, that the application implemented by application circuitry820may be designed to use the fundamental and harmonic frequency characteristics of the secondary oscillation, as opposed to using the fundamental and harmonic frequency characteristics of the normal Gunn oscillation. For example, the application circuitry may be designed to use the higher fundamental frequency of the secondary signal, wherein the fundamental frequency of the normal Gunn diode oscillation is attenuated, e.g., by an amount as listed above. Another reason an application may designed to use a frequency associated with the secondary signal would be because the Gunn Diode is designed to generate a secondary oscillation that is more sinusoidal than the normal Gunn oscillation.

It will be further appreciated that the application circuitry820can include circuitry to implement an oscillator, a microwave source, and the like. In other embodiments, in addition to implementing an oscillator or microwave, the resulting oscillations or microwaves can be used in a variety of applications. Examples of such applications include: intrusion alarms, radars, microwave test equipment; power applications; airborne collision avoidance systems; sensors for monitoring flow of traffic; car radar detectors; traffic signal controllers; automatic door opener; automatic traffic gates; speed sensors; anti-lock brakes; motion detectors; and the like.

It will be appreciated that real-space charge-transfer devices, such as the described Gunn Diode, may include features other than those illustrated. For example, a region of the active region near the anode may have a lower doping level than the other portions of the active region to facilitate initiation of the Gunn domain. Other real-space charge-transfer devices can include conductive features as described herein, such as IMPATT diodes (IMPact ionization Avalanche Transit-Time diode), and a Read diodes.