GB2531347A

GB2531347A - High efficiency low thickness antenna device

Info

Publication number: GB2531347A
Application number: GB1418462.6A
Authority: GB
Inventors: Walaa Sahyoun; Philippe Le Bars
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2014-10-17
Filing date: 2014-10-17
Publication date: 2016-04-20
Anticipated expiration: 2034-10-17
Also published as: GB201418462D0; GB2531347B

Abstract

An antenna device 601, or a method of making an antenna, comprises: a first layer which includes at least two antenna branches 603, 604 separated by a separation area and a second layer 607 which comprises a ground plane below the at least two branches 603, 604 with a non-conducting part 605 disposed below the separation area. The separation area may be a rectangular shape and the non-conducting part 605 of the ground plane 607 may be a slot with an area which is 1 to 4.8 times that of the separation area. A metal track 606 may be formed in the slot 605, which is isolated from the ground plane 607. The first and second layers may be made of metal which are separated by a dielectric layer. The antenna branches may be separated by a distance of a quarter to a half wavelength of an operating frequency of the antenna. The antenna arrangement may also include an electromagnetic radiation focusing lens 608. The antenna arrangement may be formed in a printed circuit or a CMOS integrated circuit. A communication device comprising a field effect transistor (FET) combined with the antenna arrangement may be used to provide a low thickness, efficient, microwave to Terahertz frequency operational arrangement suitable for recovering a signal sampling frequency.

Description

TITLE OF THE INVENTION

High efficiency low thickness antenna device.

FIELD OF THE INVENTION

The invention relates to an antenna device for wireless data communication.

In particular, the present invention has applications in high speed and wide bandwidth communication devices.

BACKGROUND OF THE INVENTION

Designing antennas with small dimensions has proved to be difficult.

In particular, the results obtained so far for integrated antennas show negative gains. The antenna gain is usually defined as the ratio between the power produced by the antenna from a far-field source on the antenna's beam axis and the power produced by a theoretical lossless isotropic antenna (such theoretical antenna is equally sensitive to signals from all directions).

An antenna can have a negative gain when its directivity does not compensate for its losses.

The design of such antennas is even more difficult when specific requirements have to be met. For example, it may be desirable to provide a ratio of relative bandwidth (i.e. the ratio of the bandwidth at -10dB over the central frequency) above 5 % of the central frequency. It may also be desirable to provide a thickness of the substrate (i.e. the layer separating the antenna from the ground) lower than half the wavelength of the wave associated with the central frequency. In prior art solutions, in order to operate properly, the thickness of the substrate must be greater than half the wavelength.

Several authors have published results in this field which are summarized in Table 1 below (the frequency is the "centre frequency" and the -dB BW is the bandwidth at -10dB of the return loss over the central frequency): a) >, > -00) a m C L.Q) C) rn -Th -- o 0)0 C = SN N - = C) C Ct0 c- --o E ens a.. -C- = .tfl 0ffl 1)Ct cC c( (O w a.ot-(D2-w Capolino 180 nm Leaky 10 94 10 10.6 -2.5 2x1.3 _________ BiCMOS wave _______ _____ _____ ______ _____ __________ Rebeiz lSOnm Elliptical 11 90 2.5 2.8 -6 1.7x1.1 ____________ ___________ slot __________ _______ _______ ________ _______ _____________ Capolino 180 nm slot 10 140 5 3.6 -2 1.2x0.6 ________ BiCMOS _______ _______ _____ _____ ______ _____ _________ Flynn 130 nm slot NA 9 NA NA -10 0.SxO.5

________ CMOS _______ _______ _____ _____ _____ _____ _________

Wentzloff 130 nm patch NA 60 0.81 1.3 -3.3 1.58x1.22

________ CMOS _______ _______ _____ _____ _____ _____ _________

Rebeiz 180 nm slot 10 360 NA NA -2.2 0.25x0.2 ________ BiCMOS _______ _______ _____ _____ ______ _____ _________ Ngoya 250 nm patch 10 79 4 5 -1.3 0.95x0.85 ________ BiCMOS _______ _______ _____ _____ ______ _____ _________ Madinian 130 nm SIW NA 410 12 2.9 -0.5 0.5x0.2

________ CMOS _______ _______ _____ _____ _____ _____ _________

Knapp 45 nm patch 4 410 NA NA -1.5 0.2x0.2

________ CMOS _______ _______ _____ _____ _____ _____ _________

Fetterman 65 nm patch NA 1000 24 2.4 1.5 0.068x0.045

________ CMOS _______ _______ _____ _____ _____ _____ _________

Ando 180 nm patch <10 60 1.5 2.5 -14.5 1.15x1.15x

________ CMOS _______ _______ _____ _____ _____ _____ _________

Hella 130 nm patch 16.6 300 19 6.3 5 0.4x0.21

________ CMOS _______ _______ _____ _____ _____ _____ _________

Hella 65 nm patch 5 300 24 8 -2.4 0.24x0.31

________ CMOS _______ _______ _____ _____ _____ _____ _________

When the frequency is high it becomes easier to reduce the thickness while obtaining positive gains. This is because in the prior art, the thickness of the substrate must be higher than half of the wavelength and when the frequency is high the wavelength is low.

However, even when reaching the high frequency domains, the thickness required is still too large for integrated applications. For example, at 300 GHz, the wavelength is 1 mm which not compatible with integrated applications. In the terahertz domain, thicknesses about 200 pm may be obtained but this is still too large.

In the semiconductor industry, the dimensions of the circuits tend to decrease. Transistors thus tend to have reduced dimensions and increased cutoff frequencies.

Therefore, faster transistors can be designed that are capable of working in wireless communication systems with small wavelengths However, prior art antennas cannot be used in such a circuit because their design imposes large substrate thickness.

There is thus a need for antennas with low thickness substrates. In particular, there is a need for such antennas that can operate at microwave or terahertz frequencies.

The present invention lies within this context.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided an antenna device comprising: -a first layer comprising at least two antenna branches separated by a separation area, -a second layer comprising a ground section plane disposed below said at least two antenna branches, wherein, said ground section has a non-conducting part facing said separation area.

An antenna device according to the first aspect may work for microwave or terahertz applications for example.

An antenna device according to the first aspect provides high performance in low thickness circuits.

An antenna device according to the first aspect may be embodied in a circuit with a low substrate thickness. For example, in such circuits, the distance from the metallic parts or the antenna (e.g. branches) to the ground of the antenna device is less than half of the wavelength of the electromagnetic wave the antenna device emits or receives.

The invention provides a design of antenna devices that is not complex and that it efficient. In particular, large bandwidth and/or positive gain may be provided.

An antenna device according to the first aspect may be embodied in planar circuits such as PCB circuits.

An antenna device according to the first aspect may also be embodied in integrated circuits (for example CMOS circuits, Ill-V semiconductor circuits etc.).

In integrated circuits, an antenna device can advantageously be operated at frequencies above the cut-off frequency and the transition frequency of the integrated circuit. In such case, a direct demodulation may be performed using one or many plasma FETs (Field Effect Transistor).

In particular, the second layer may act as an AC ground. This may be the case when the second layer is polarized. The ground may be a floating ground.

According to embodiments, said non-conducting pad has an area at least equal to said separation area.

For example, the area of said non-conducting is four times the separation area.

According to embodiments, the area of said non-conducting part is in a range between 3.2 and 4.8 times the separation area.

For example, said non-conducting pad is superposed relative to said branches along a distance between 10 % and 20 % of the distance separating said branches in a direction separating the branches.

According to embodiments, said non-conducting pad has a slot shape.

For example, said separation area has a rectangular shape.

According to embodiments, said non-conducting part is made by matter removal in said ground section.

For example, said non-conducting part comprises a reinforcement part.

According to embodiments, said reinforcement pad is a metal track electrically isolated from said ground section.

For example, said reinforcement pad has an area which is at least 30 % of the area of the non-conducting pad.

According to embodiments, said first layer and said second layer are separated by a distance less than half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.

For example, said at least two antenna branches are separated by a distance of a quarter of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.

Said wavelength may be the "guided wavelength" which depends on the actual permittivity of a dielectric layer separating the first and second layers.

According to embodiments, said at least two antenna branches have a length of half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.

For example, said first and second layer are metal layers.

According to embodiments, said first and second layers are separated by a dielectric layer.

For example, the antenna device further comprises a lens for focusing electromagnetic radiations to be received and/or emitted by said antenna device.

The antenna device may further comprise an additional layer, said second layer being situated between said first layer and said additional layer and said lens being formed on said additional layer.

According to embodiments, the first and second layers are separated by a dielectric layer and said dielectric layer and the additional dielectric layer are separated by said second layer.

For example, said additional layer is a dielectric layer.

The antenna device may be embodied in a planar technology circuit.

The antenna device may be embodied in a printed circuit board.

The antenna device may be embodied in an integrated circuit.

The antenna device may be embodied in a CMOS circuit.

According to a second aspect of the invention there is provided communication device comprising:

-a field effect transistor, and

-an antenna device according to the first aspect connected to a

gate of the field effect transistor.

The communication device may be configured for receiving an incident electromagnetic wave carrying a signal, the sampling frequency of which is to be recovered.

For example, a drain of the field effect transistor is biased differently from a source of said field effect transistor, thereby providing between said source and drain a signal representing said sampling frequency of the signal carried by said received incident electromagnetic wave.

According to embodiments, a plasma wave oscillates in the channel of the field effect transistor at a plasma frequency above the field effect transistor's cut-off frequency. It may also oscillate above its transition frequency.

For example, a plasma wave oscillates in the channel of the field effect transistor at a plasma frequency in the range of an incident electromagnetic wave's frequency.

According to embodiments, at least one of the source and the drain is grounded.

For example, the communication device further comprises at least one first biasing voltage generator for biasing the source or the drain of the field effect transistor.

According to embodiments, the communication device further comprises at least one second biasing voltage generator for biasing the gate of

the field effect transistor.

For example, said field effect transistor is disposed between two branches of said antenna device.

According to embodiments, a first branch of said antenna device is connected to the gate of the transistor and a second branch of said antenna device is connected to the source of said transistor.

According to a third aspect of the invention there is provided a method of manufacturing an antenna device comprising: -forming a first layer comprising at least two antenna branches separated by a separation area, -forming a second layer comprising a AC ground section plane disposed below said at least two antenna branches, and -forming in said AC ground section a non-conducting part disposed below said separation area.

For example, said non-conducting part has an area at least equal to said separation area.

According to embodiments, the area of said non-conducting part is four times the separation area.

For example, the area of said non-conducting part is in a range between 3.2 and 4.8 times the separation area.

According to embodiments, said non-conducting part is superposed relative to said branches along a distance between 10 % and 20 % of the distance separating said branches in a direction separating the branches.

For example, said non-conducting part has a slot shape.

According to embodiments, said separation area has a rectangular shape.

For example, said non-conducting part is made by matter removal in said ground section.

According to embodiments, the method further comprises forming a reinforcement part in said non-conducting.

For example, said reinforcement part is a metal track electrically isolated from said ground section.

According to embodiments, said reinforcement part has an area which is at least 30 % the area of the non-conducting part.

For example, said first layer and said second layer are separated by a distance less than half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.

B

According to embodiments, said at least two antenna branches are separated by a distance of a quarter of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.

For example, said at least two antenna branches have a length of half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.

According to embodiments, said first and second layer are metal layers.

The method may further comprise forming a dielectric layer separating said first and second layers.

According to embodiments, the method further comprises forming a lens for focusing electromagnetic radiations to be received and/or emitted by said antenna device.

The method may further comprise forming an additional layer, said second layer being situated between said first layer and said additional layer, said lens being formed on said additional layer.

For example, said additional layer is a dielectric layer.

According to embodiments, the method is carried out in planar technology.

For example, the method is carried out in integrated circuit technology.

According to embodiments, the method is carried out in CMOS technology.

The objects according to the second and third aspects of the invention provide at least the same advantages as those provided by the detector according to the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which: -Figures Ia and lb illustrate an antenna design according to general embodiments, -Figures 2a and 2b show exemplary dimensions for an antenna design according to embodiments in planar technology, -Figures 3a and 3b show simulation results for the antenna design of Figures 3a-3b, -Figure 4a illustrates an antenna design according to embodiments in CMOS technology, -Figure 4b show exemplary dimensions for the antenna design of Figure 4a, -Figures 5a and 5b show simulation results for the antenna design of Figure 4b, -Figure 6 illustrates an antenna design according to embodiments using a lens, -Figures 7a and 7b illustrate an exemplary use of antennas according to embodiments in a communication system, -Figure 8 schematically illustrate a structure of a FET that can be used in the system of Figures 7a and 7b, and -Figures 9a and 9b are schematic models of the FET transistor of Figure 8.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Figures Ia and lb the design of antennas according to general embodiments is described.

In the antenna device of Figure la, a dipole antenna having two branches 101 and 102 is disposed on a top metal layer. The dipole antenna may have any shape such as two rectangular branches (as illustrated in Figure Ia and Ib) or a "bow tie" shape or any other shape. The two branches of antenna are separated but current may flow from one branch to another.

The top metal layer (and therefore the antenna) is separated from a bottom metal layer 103 that acts as the ground (hereinafter "the ground").

The top metal layer and the ground are separated by a dielectric layer 105 (in Figures la and lb the dielectric layer is made transparent in order to show the ground).

According to the invention, a part 104 of the ground is made non-conductive. For example, part 104 is a slot formed in the ground.

Part 104 provides a discontinuity in current conduction in the ground.

Current conduction is thus interrupted in part 104. The current flowing in the ground thus sees an obstacle when reaching part 104.

Part 104 is formed in the ground in a zone facing the separation between the two branches of the antenna. In the example of Figure la, part 104 is in a zone below the separation between the two branches of the antenna.

In other words, part 104 is (fully or partially) superposed relative to the separation area (it could also be said that the separation area is fully or partially superposed relative to part 104). Part 104 and the separation area may also be said to be aligned or in register one with the other.

Part 104 may be formed by removing matter in the ground layer by known manufacturing processes.

Figure lb is a top view of the dipole antenna of Figure 1 a.

As illustrated in Figure la, since current coming from branch 102 of the antenna seeks the shortest conduction path and since part 104 just below branch 102 is non-conductive, the current (and the electrical field) flows directly to branch 101 of the antenna as illustrated by arrow 106. Thus, the current is not directly dissipated in the ground and the two branches of the antenna are thereby mutually coupled in order to act as a wireless communication antenna.

According to prior art antennas, without part 104, the ground is completely conductive. Still according to prior art antennas, when the thickness of the dielectric layer is less than half of the wavelength the antenna is supposed to receive or emit, the antennas' bandwidth and overall efficiency drastically decreases. The effect of the ground is to increase the intensity of radiation at some vertical angles and to decrease it at others. The higher the antenna is placed above the ground, the more effective the radiation for communication is.

According to the invention, part 104 makes it possible to increase the bandwidth and the efficiency of the antenna.

In the absence of pad 104, in prior ad antennas, the current flows through the dielectric layer to the ground and energy is thus dissipated in the two antenna branches by thermal effect.

In the presence of pad 104, according to the invention, the current density presents a high magnitude in the separation area between the two branches of the antenna. This creates an electromagnetic radiation instead of a thermic radiation.

In Figure 1 b, part 104 has a rectangular shape. The branches of the antenna are also rectangular. The branches are separated by a separation area having a rectangular shape. However, for each element, other shapes may be envisaged.

Part 104 has a length "L" and a width "I". Therefore, it has a surface Si = lxL (in the example of the rectangular shape). Each branch has a width b' and a length c'. The branches of the antenna are separated by a distance "d".

The direction of the separation is orthogonal to the length of the branches. The separation area between the branches thus has a surface S2 = dxb (in the example of the rectangular shape). Part 104 extends over branch 101 along a distance "e2" in the direction separating the branches. Part 104 extends over branch 102 along a distance el" in the direction separating the branches.

Preferably, the projection of the separation area between the branches of the antenna (in a direction orthogonal to the layers) is contained in the area of part 104. In other words, part 104 is at least as large as the separation area (d «= I) and at least as long as the separation area (b «= L).

However, according to embodiments, part 104 may be shorter than the separation area in length (i.e. in the direction orthogonal to the direction in which the branches are separated, L «= b).

Part 104 may also be shorter than the separation area in width (i.e. in the direction in which the branches are separated, I «= d). However, parts 104 at least as wide as the separation area (the width being measured in the direction separating the branches) are preferred. For example, part 104 may extend over the branch of the antenna along a distance in the direction separating the branches between 10 % and 20% of the distance separating the branches. In other words, part 104 may be superposed relative to the branches of the antenna in the direction separating the branches along a distance between 10 % and 20 % of the distance separating the branches. In Figure lb, this means that the sum el + e2 is in a range within 10 % and 20 % of distance d.

Preferably, the surface area Si of part 104 is four times the surface area S2 of the separation area between the branches (Si = 4x52). However, surface areas Si of part 104 within a range of +1-20 % around 4xS2 are also preferred (i.e. a surface area of part 104 between 3.2 and 4.8 times the surface area of the separation area, 3.2xSi «= 4.8xS2).

For example, the antenna branches are separated by a distance d of a quarter of the wavelength of an electromagnetic radiation to be received and/or emitted. Also, the antenna branches may have a length c of half of said wavelength. This makes it possible to maximize the bandwidth at 50 Ohms impedance (impedance of the antenna). More generally, this makes it possible to maximize the bandwidth at impedances between 50 and 75 Ohms.

As illustrated in Figures 2a and 2b the invention may be embodied in planar layouts.

Figure 2a illustrates an antenna according to embodiments using planar technology, for example with a Rogers substrate. The antenna branches 201 and 202 have dimensions of 10 mm x 4mm. The ground 203 has dimensions of 50 mm x 50 mm. The non-conductive part 204 below the branches separation, here a slot, has dimensions of 5 mm x 30 mm. The current flows from branch 202 to branch 201 as shown by arrow 209.

The slot width can be greater or smaller than the distance between the antenna branches. A width greater than the distance between the antenna branches can however be preferred.

Figure 2b is a cross section view of the antenna of Figure la. Top and bottom metal layers 205 and 206 are separated by a dielectric layer 207.

Layer 206 acts as the ground.

Layer 207 has a thickness of 0.2 mm. For a frequency of 10 GHz, the wavelength is 3 cm. Thus, the dielectric thickness is smaller than half of wavelength. According to prior art designs (without slot 204), the efficiency of the antenna, in particular the bandwidth would be poor.

However, by the effect of slot 204 in the present embodiment, as illustrated by the simulation results of Figures 3a and 3b, performance of the antenna is enhanced.

Simulation has been performed using the CST Microwave Studio ® software. Figure 3a is a graph showing the reflection parameter in ordinates, also referred to as the "return loss' (Si i-Parameter) 301 of the antenna at a port impedance of 75 ohms and the frequency in abscissa.

The band pass is below -10 dB in a range of 1.87 GHz around a centre frequency of 6.8 GHz (27.5% of the centre frequency).

Figure 3b is a graph showing the radiation pattern of the antenna for a phase Phi equal to 900 at 6.8 GHz. A gain of 5.9 dB and an efficiency of 99.4% are obtained with an angular width of 69°.

A prior art antenna with the same dimensions as shown in Figure 2a and 2b, but without slot 204 has been simulated (results not represented). No matching has been found in the band. Also, the efficiency found was 1%.

Therefore, the presence of the slot 204 provides better performance.

Another simulation using a 65 nm process CMOS circuit with a 5 pm substrate and a 0.6 xO.5 mm antenna has provided the following results. For a centre frequency of 300 GHz, the reflection parameter has a bandwidth of 44 GHz for a magnitude less than -10 dB. The relative bandwidth is 30 % and the gainisl.5.

All the results presented are to be compared with the results of Table 1 of the background section. The present invention provides better results than

in the prior art.

The invention may also be embodied in integrated circuits such as CMOS layouts. Such embodiments are illustrated in Figures 4a and 4b.

Figure 4a shows an antenna device 401 comprising two antenna branches 404 and 403 disposed in a top metal layer. A bottom metal layer 407 acts as the ground. In particular it may act as the AC and DC ground. The top and bottom metal layers are separated by a dielectric layer (not represented for the sake of clarity of the Figure). A silicon layer 402 is disposed under the ground layer. According to some CMOS technologies, the silicon layer is of 5 pm and the bottom metal layer is of 300 pm thickness.

A non-conductive part 405, here a slot, is provided below the separation between the two antenna branches. The current flows from branch 404 to branch 403 as shown by arrow 408.

A reinforcement part 406, here metal track, is provided within the non-conductive part 405. The metal track 406 reinforces the ground layer. In CMOS layouts, filling at least in 30% of the slot areas is a requirement. The reinforcement track is electrically isolated from the ground. Thus, current from one branch cannot flow to the ground through it.

Preferably, the reinforcement part 406 is less wide than the separation area between the branches of the antenna.

The reinforcement part 406 may be deposited by a known manufacturing process after removing matter for forming the non-conductive part. Alternatively, it may be formed simultaneously with the non-conductive part by matter removal around the reinforcement part.

Figure 4b is a top view of the antenna device of Figure 4a showing its dimensions.

The metal layers and the silicon layers are 650x650 pm squares. The branches are 200x200 pm squares. The slot is a 100x600 pm rectangle and the metal track is a SOx45Opm rectangle.

The antenna of Figures 4a and 4b has been simulated using the CST Microwave Studio® software. The results are shown in Figures 5a and Sb.

Figure 5a and 5b represent the results of the simulation of the CMOS antenna.

Figure 5a is a graph showing in ordinates the reflection parameter (S-parameter) 501 of the antenna and in abscissa the frequency. The reflection parameter has a bandwidth of 44 GHz for a magnitude less than -10 dB. The centre frequency is around 290 GHz.

Figure Sb is a graph showing the radiation pattern 502 of the antenna for a phase Phi equal to 90° at 300 GHz. A gain of 1 dB with an angular width of 38.5° is obtained. The efficiency of the antenna is 30%. The antenna efficiency decreases with comparison with the embodiment of Figures 2a-2b because of the silicon layer under the ground layer.

A prior art patch in CMOS technology with the dimensions of the antenna of Figures 4a-4b but without the slot has been simulated (results not represented). Such prior art antenna has a bandwidth of 24 GHz around 300 GHz with a negative gain of -2.4 dB and an efficiency of 30%. Therefore, the presence of the slot 405 provides better performance.

Performance of antennas according to the invention may be even further enhanced by using a lens as illustrated in Figure 6.

Even though the lens makes the efficiency of the antenna decrease due to its resistivity, the lens makes it possible to obtain a larger bandwidth of the antenna and a higher gain.

The antenna 601 illustrated in Figure 6 is in the CMOS technology like the antenna of Figure 4a.

Antenna 601 comprises two branches 604 and 603 disposed in a top metal layer. A bottom metal layer 607 acts as the ground. In particular, it may act as the AC and DC ground. The top and bottom metal layers are separated by a dielectric layer (the substrate layer, not represented for the sake of clarity of the Figure). An additional layer 602, for example a silicon layer, is disposed under the ground layer. According to some CMOS technologies, the dielectric substrate layer is of 5 pm and the silicon layer is of 300 pm thickness. The additional layer differs from the dielectric layer (substrate layer) separating the branches of the antenna from the ground.

The invention is not limited to the CMOS technology. The semiconductor layers may be SiGe, AsGa, or any elements of the Ill-V columns of the Mendeleiev table. The substrate layer may be made of Si, AsGa or metal Oxyde.

A non-conductive part 605, here a slot, is provided below the separation between the two antenna branches. The current flows from branch 604 to branch 603 as shown by arrow 609.

A reinforcement part 606, here a metal track, is provided within the non-conductive part 605. The metal track 606 reinforces the ground layer. In CMOS layouts, filling at least in 30% of the slot areas is a requirement.

A lens 608 is disposed on the additional layer. The lens may be made of silicon. It may also be made of Poytetrafluoroethylene (Teflon®).

The lens focuses the waves received or emitted by the antenna. The lens thus enhances the directivity of the antenna device.

A simulation has been run (results not represented) and with the same dimensions as in Figure 4b and with comparison with the antenna without the lens of Figure 4a, the bandwidth of the antenna is larger up to 50 GHz and the gain is increased up to 3.5 dB. The efficiency decreases from 30% to 25% due to the resistivity of the silicon.

The antenna according to the invention may advantageously be used in combination with plasma FET5. This makes it possible to use the antenna at higher frequencies. For operating frequencies of the antenna device higher than the cut-off frequency and the transition frequency of the technology in use (for example CMOS 65 nm has a cut-off frequency around 200 GHz), a heterodyne system cannot be used for demodulating the received signal. A plasma FET after the antenna block may be used in order to perform a direct demodulation and to extract the base band signal.

The channel of a field effect transistor can act as a resonator for plasma waves. This has been disclosed in document Knap et al. "Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications", Journal of Infrared, Millimeter, and Terahertz Waves December 2009, Volume 30, Issue 12, pp 1319-1337 and in document Blin et al. "Plasma Waves Detectors for Terahertz Wireless Communications", IEEE electron device letters, vol 33, N1O, October 2012. The plasma frequency of such resonator depends on its dimensions. For example, with a gate of one micron or below (even about one nanometer) in length, the plasma frequency can reach the terahertz (1Hz) frequency range.

In the early 90s, Dyakonov and Shur predicted in "Phys. Rev. Letter 71(1993)2465', that a steady current flow in an asymmetric FET channel can lead to instability against the spontaneous generation of plasma waves. This can in turn produce the emission of electromagnetic radiations at the plasma wave frequency. This started a general interest for the applications of FET5 in THz spectroscopy.

Dyakonov and Shur also have shown in "Detection, Mixing, and frequency multiplication of terahertz radiation by two dimensional electric fluid", IEEE Trans. On Electron Devices vol. 43, N3 pp38O-387, March 1996, that the nonlinear properties of the 2D plasma in the transistor channel can be used for detection and mixing of THz radiations.

The plasma of charge carriers appears at the interface between materials, namely where the semiconductor crystal layer ends or where the metallic structures begin. The tightness of the bounds between atoms and electrons is smaller at such interface and therefore, the carriers have a greater mobility. This increased mobility is sometime referred to as "a gas of carriers", or "plasma". The oscillations that may appear within the plasma are sometimes referred to as "plasmons".

The FET behaves like a mixer because of the non-linearities of the wave plasma. Thus, it creates a rectified form of the alternative current that was induced by the incoming electromagnetic waveform (the limited development of a nonlinear function often contains some squared components of its variable.

This voltage appears when there is some asymmetry between the drain and the source of the FET. The voltage appears between the source and the drain of the FET and is proportional to the power of the incoming electromagnetic waveform (or radiation). Without the asymmetry, the rectified current would be the same at the drain and at the source and no voltage would appear. In order to have the incoming radiation detected, there must be asymmetry between the source and the drain.

Figure 7a schematically illustrates systems according to embodiments combining antennas according to variants of the invention with plasma FETs. The FET connecting points may be placed in the metal track between the branches of the antenna in the circuit layout. The gate is connected to one branch and the source to the other branch.

A receiver circuit 703 comprises a plasma FET 701 and an antenna 705. The antenna comprises a non-conductive part (a slot for example) in a metal layer acting as the ground under a separation area between two branches of the antenna as described hereinabove. In particular, the metal layer may act as an AC ground. The antenna is used for example for clock recovery. A second circuit 704 is configured for detection of the base band signal emitted by a transmitting device 706 and a transmission antenna 707. Circuit 704 may comprise one or several plasma FETs 702. A circuit 704 according to embodiments is described with reference to Figure 7b.

The transmitting device 706 transmits the signal through an antenna 707. The antenna comprises a non-conductive part (a slot for example) in a metal layer acting as the ground under a separation area between two branches of the antenna as described hereinabove. The signal is carried by a THz electromagnetic wave 708.

The plasma FET 701 of circuit 703 is biased on its drain with bias voltage Vdd generated by a voltage generator 709. The source of the transistor is connected to the ground. The ground may be an AC ground layer. The gate of the transistor is connected to the antenna 705. Thus, when the THz wave 708 reaches antenna 705, it is fed to the gate-source contact of the transistor.

The gate of transistor 701 is also biased with a bias voltage Vg generated by a voltage generator 710. In order to isolate the DC and AC signals on the gate, a DC block can be added to Vg. The source and the drain are asymmetrically biased and therefore, a voltage VOS clock can be induced between the source and the drain. Plasma waves oscillate in the channel of the transistor at frequency foG_clock of the induced voltage (which also corresponds to the image at the receiver of the sampling frequency of the signal transmitted from the device 706). At the receiver, the amplitude of this clock signal is more important than the rectified signal corresponding to the base band signal. Thus, the base band signal detected is hidden because of its small energy.

Once the sampling frequency fos_0100k is known, it can be delivered to circuit 704 in order to have the envelope of the modulated signal extracted.

For example, circuit 704 has its own antenna 711 for receiving the signal modulated in THz wave 708. The antenna comprises a non-conductive part (a slot for example) in a metal layer acting as the ground under a separation area between two branches of the antenna as described hereinabove. Alternatively, circuits 703 and 704 may use a common antenna from which, each circuit is fed with the modulated signal. A wide band power divider operating at the carrier frequency can be used.

The clock recovery information given by circuit 703 enables circuit 704 to identify the sampling clock of the received modulated signal.

The plasma FET 702 in circuit 704 is configured not to oscillate until entering into resonance at the sampling frequency. Stability of the transistor or attenuation of the sampling clock frequency should be ensured in order to extract the base band signal.

Figure lb illustrates the system of Figure 7a in more detail. In particular, circuit 704 is described in more details. A radio front end unit 718 detects, amplifies and puts in shape the digital signal transporting the data received from antenna 711. In the present example, antenna 711 is independent from antenna 705. However, the antennas are located close enough to each other to be subjected to the same 1Hz wave.

A low noise amplifier 717 is used for shaping the received signal from the oscillating FET 701. The system comprises a multiphase clock generator 716 including delay units that feed a multiplexer 719. The frequency of the receiver does not differ from the frequency of the transmitter (not represented in Figure 7b) and the selection of the phase at the output of the multiplexer can be performed by a phase selection controller 712 at low speed. Advantageously, the frequency is tracked, only the phase has to be adjusted. Still advantageously, the phase detection is non-complex and the phase selection controller does not need to process as rapidly as in the prior art, which may also be useful for reducing the power consumption or the hardware footprint.

The output of the radio front end unit 718 is fed to a data sample unit 715 which outputs the recovered base band signal according to the recovered clock signal output by multiplexer 719. The output of the radio front end unit 718 is also fed to a proportional derivative unit 714 which also receives the clock signal output by multiplexer 719. The output of the proportional derivative unit 714 is fed to a digital filter 713 feeding the phase selection controller 712.

The plasma of the plasma FET 701 oscillates at a frequency in the range of the incident electromagnetic wave's frequency. As the plasma wave frequency is significantly larger than the FET cut-off frequency, the demodulation can be performed at very high THz frequency.

It is to be recalled that the cut-off frequency is the characteristic of an intrinsic transistor and is a measure of the intrinsic speed of the transistor. It is defined as the frequency at which the magnitude of the short-circuit current-gain is 1.The system of Figure 7b is not limitative. The antennas according to the invention may be used in other types of systems.

As illustrated by the system of Figures 7a and 7b, antennas according to embodiments can operate at a carrier frequency above the cut-off frequency and the transition frequency of the transistors in the circuit in which they are integrated (for example a CMOS integrated circuit). This provides an alternative to heterodyne systems usually used for demodulation. Instead, a plasma FET detector is used that operates in plasma mode and that makes it possible to directly demodulate the received signal.

Figure 8 shows a structure on a High Electron Mobility Transistor (HEMT) that can be used in the system of Figures 7a and 7b.However, other types of Field Effect Transistors may be used. Generally the structures of FET transistors are similar. The present structure is purely exemplary for the sake of the understanding of the normal and the plasmonic operation of this kind of transistor.

The transistor comprises a substrate layer 807 on top of which is deposited a spacer 806. The spacer separates the substrate layer from a barrier layer 805 comprising highly doped wide band gaps of the n type. The barrier layer acts as a donor supply layer.

The transistor further comprises a capping layer on top the barrier layer comprising non-doped narrow band gap material with no dopant impurities. A first part 804a of the capping layer has a metal layer 801 deposited on top of it to act as the drain pad of the transistor. A second part 804b of the capping layer has a metal layer 802 deposited on top of it to act as the source pad. The gate pad of the transistor is made of a metal layer 803 disposed on top of the barrier layer in an air gap between the two parts of the capping layer.

"Drain" and "source" are conventional names and are related to the polarization applied to the transistor and not to the structure itself. The source is the terminal through which the majority carriers enter the barrier layer, while the drain is the terminal through which the majority carriers leave the layer.

The substrate layer 807 may be made of GaAs, the spacer 806 may be made of undoped AIGaAs, the barrier layer 805 may be of AIGaAs and the capping layer 804 may be made of GaAs.

Since GaAs has a higher affinity for electrons than the AIGaAs, free electrons in the AIGaAs layer are transferred to the undoped GaAs layer where they form a two dimensional high mobility electron gas within 10 nanometers of the interface. The n-type AIGaAs layer of the HEMT is depleted completely of its carriers, from this first mechanism as well as from a second mechanism since the free electrons are also trapped by surface states. A similar mechanism can exist in Silicon based materials.

This high mobility electron gas gives the channel a very low resistivity (about 100 to 1k Ohms). A voltage applied to the gate affects this conductivity and allows the control of the flow of carriers going from drain to source, when these latter are biased by a voltage. The average mobility of this electron gas defines the cut off and transition frequency of the device. However, under some conditions, part of this gas can have a higher mobility, this being the basis of the plasmonic effect. Some electrons are more distant to a nucleus than others, especially when there is a discontinuity in the crystal structure, and therefore do not slow down like the one in the crystal.

The spacer layer reduces the scattering caused by the electrical interaction between the electrons and the donors. A semi-insulating material is used as a substrate to provide insulation between the adjacent devices.

The heavily doped GaAs capping layer provides a good ohmic contact. It reduces the device source resistance and protects the AIGaAs donor layer from surface oxidation and depletion. Variation on the thickness of the layers and on their concentration of doping impurities enables various adaptations to different applications.

Figures 9a-9b illustrate an HEMT model.

When a bias voltage is applied to the transistor, as represented in Figure 9a, a channel 905 of length L is created between the drain 904 and the source 903. Alternating voltage Ua represents the effect of an incident wave above the biasing voltage Vg arriving between the transistor's gate 901 and source 903. Under some conditions on the frequency and power of the incident wave, a voltage AU appears between the source and the drain, even if the frequency of the incident wave is well above the cut off frequency or the transition frequency of the transistor. This voltage comes from the rectification of the current exciting the part of the gas of electron in the transistor that is more mobile.

Figure 9b shows the transistor of Figure 9a with the symbolization 906 of an FET with plasma wave effect in the channel.

The antennas according to the invention are not limited to a combination with plasma FET transistors. The antennas may be used in baseband and/or heterodyne systems.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not restricted to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or an" does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

CLAIMS1. An antenna device comprising: -a first layer comprising at least two antenna branches separated by a separation area, -a second layer comprising a ground section plane disposed below said at least two antenna branches, wherein, said ground section has a non-conducting part facing said separation area.
2. An antenna device according to claim 1, wherein said non-conducting part has an area at least equal to said separation area.
3. An antenna device according to any one of the preceding claims, wherein the area of said non-conducting part is four times the separation area.
4. An antenna device according to any one of the preceding claims, wherein the area of said non-conducting part is in a range between 3.2 and 4.8 times the separation area.
5. An antenna device according to any one of the preceding claims, wherein said non-conducting part is superposed relative to said branches along a distance between 10 % and 20 % the distance separating said branches in a direction separating the branches.
6. An antenna according to any one of the preceding claims wherein said non-conducting part has a slot shape.
7. An antenna device according to any one of the preceding claims, wherein said separation area has a rectangular shape.
8. An antenna device according to any one of the preceding claims, wherein said non-conducting part is made by matter removal in said ground section.
9. An antenna device according to any one of the preceding claims, wherein said non-conducting part comprises a reinforcement part.
10. An antenna device according to claim 9, wherein said reinforcement part is a metal track electrically isolated from said ground section.
11. An antenna device according to any one of claims 9 and 10, wherein said reinforcement part has an area which is at least 30 % the area of the non-conducting part.
12. An antenna device according to any one of the preceding claims, wherein said first layer and said second layer are separated by a distance less than half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.
13. An antenna device according to any one of the preceding claims, wherein said at least two antenna branches are separated by a distance of a quarter of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.
14. An antenna device according to any one of the preceding claims, wherein said at least two antenna branches have a length of half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.
15. An antenna device according to any one of the preceding claims, wherein said first and second layer are metal layers.
16. An antenna device according to any one of the preceding claims, wherein said first and second layers are separated by a dielectric layer.
17. An antenna device according to any one of the preceding claims, further comprising a lens for focusing electromagnetic radiations to be received and/or emitted by said antenna device.
18. An antenna device according to claim 17, further comprising an additional layer, said second layer being situated between said first layer and said additional layer and wherein said lens is formed on said additional layer.
19. An antenna device according to any one of the preceding claims embodied in a planar technology circuit.
20. An antenna device according to claim 19 embodied in a printed circuit board.
21. An antenna device according to any one of claims 1 to 18 embodied in an integrated circuit.
22. An antenna device according to claim 21 embodied in a CMOS circuit.
23. A communication device comprising:-a field effect transistor, and-an antenna device according to any one of the preceding claims connected to a gate of the field effect transistor.
24. A communication device according to claim 23, configured for receiving an incident electromagnetic wave carrying a signal, the sampling frequency of which is to be recovered.
25. A communication device according to claim 24, wherein a drain of the field effect transistor is biased differently from a source of said field effect transistor, thereby providing between said source and drain a signal representing said sampling frequency of the signal carried by said received incident electromagnetic wave.
26. A device according to any one of claims 23 to 25, wherein a plasma wave oscillates in the channel of the field effect transistor at a plasma frequency above the field effect transistors cut-off frequency.
27. A device according to any one of claims 23 to 25, wherein a plasma wave oscillates in the channel of the field effect transistor at a plasma frequency in the range of an incident electromagnetic wave's frequency.
28. A device according to any one of claims 23 to 27, wherein at least one of the source and the drain is grounded.
29. A device according to any one of claims 23 to 28, further comprising at least one first biasing voltage generator for biasing the source orthe drain of the field effect transistor.
30. A device according to any one of claims 23 to 29, further comprising at least one second biasing voltage generator for biasing the gate ofthe field effect transistor.
31. A device according to any one of claims 23 to 30, wherein said field effect transistor is disposed between two branches of said antenna device.
32. A device according to claim 30, wherein a first branch of said antenna device is connected to the gate of the transistor and a second branch of said antenna device is connected to the source of said transistor.
33. A method of manufacturing an antenna device comprising: -forming a first layer comprising at least two antenna branches separated by a separation area, -forming a second layer comprising a ground section plane disposed below said at least two antenna branches, and -forming in said ground section a non-conducting part disposed below said separation area.
34. A method according to claim 33, wherein said non-conducting part has an area at least equal to said separation area.
35. A method according to any one of claims 33 to 34, wherein the area of said non-conducting part is four times the separation area.
36. A method according to any one of claims 33 to 35, wherein the area of said non-conducting part is in a range between 3.2 and 4.8 times the separation area.
37. A method according to any one of claims 33 to 36, wherein said non-conducting is superposed relative to said branches along a distance between 10 % and 20 % the distance separating said branches in a direction separating the branches.
38. A method according to any one of claims 33 to 37, wherein said non conducting part has a slot shape.
39. A method according to any one of claims 33 to 38, wherein said separation area has a rectangular shape.
40. A method according to any one of claims 33 to 39, wherein said non-conducting part is made by matter removal in said ground section.
41. A method according to any one of claims 33 to 40, further comprising forming a reinforcement part in said non-conducting part.
42. A method according to claim 41, wherein said reinforcement part is a metal track electrically isolated from said ground section.
43. A method according to any one of claims 41 and 42, wherein said reinforcement part has an area which is at least 30 % the area of the non-conducting part.
44. A method according to any one of claims 33 to 43, wherein said first layer and said second layer are separated by a distance less than half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.
45. A method according to any one of claims 33 to 44, wherein said at least two antenna branches are separated by a distance of a quarter of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.
46. A method according to any one of claims 33 to 45, wherein said at least two antenna branches have a length of half of a nominal wavelength of an electromagnetic radiation to be received and/or emitted by said antenna device.
47. A method according to any one of claims 33 to 46, wherein said first and second layer are metal layers.
48. A method according to any one of claims 33 to 47, further comprising forming a dielectric layer separating said first and second layers.
49. A method according to any one of claims 33 to 49, further comprising forming a lens for focusing electromagnetic radiations to be received and/or emitted by said antenna device.
50. A method according to claim 49, further comprising forming an additional layer, said second layer being situated between said first layer and said additional layer and wherein said lens is formed on said additional layer.
51. A method according to any one of claims 33 to 50 carried out in planar technology.
52. A method according to any one of claims 33 to 50 carried out in integrated circuit technology.
53. A method according to claim 52 carried out in CMOS technology.
54. An antenna device substantially as hereinbefore described with reference to, and as shown in, Figures la, lb, 2a, 2b, 4a, 4b, and 6 of the accompanying drawings.
55. A communication system substantially as hereinbefore described with reference to, and as shown in, Figures 7a and 7b of the accompanying drawings.