WO2009043370A1

WO2009043370A1 - A voltage controlled switching device

Info

Publication number: WO2009043370A1
Application number: PCT/EP2007/060386
Authority: WO
Inventors: Spartak Gevorgyan; Per Thomas Lewin; Andrei Vorobiev; Martin Norling; John Berge
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-10-01
Filing date: 2007-10-01
Publication date: 2009-04-09

Abstract

A capacitive device (100, 200) comprising first (110) and second (130) layers of electrically conductive material, each of which has a first and a second main surface. The device also comprises a third layer (120) of a paraelectric phase material between the first (110) and second (120) layers, so that one of said first or second surfaces of the first and second layers face the third layer. The device (100, 200) additionally comprises a non-conducting supporting structure (160) on which the device rests, and comprises means (DC) for applying a DC voltage across the third layer (160), so that the device by means of varying the applied DC voltage may be made to alter its electrical characteristics between that of a capacitor, a short circuit or an open circuit.

Description

TITLE

A voltage controlled switching device.

TECHNICAL FIELD The present invention discloses a voltage controlled switching device.

BACKGROUND

Switches and frequency selective filters are among the critical components in microwave technology, especially in communications systems. In many cases, the performance of these components defines the overall performance of the systems in which they are used. Performance parameters which may be mentioned are, for example, electrical performance and cost.

In applications in future and advanced systems, such as microwave systems, the technical requirements for components such as switches and frequency selective filters will be rather high, which implies that such available components (for example switches and tuneable filters based on technologies such as MEM, FET, and PiN-diodes) may fail to meet these requirements. Parameters where the requirements are expected to become more stringent are, for example, size, cost and electrical performance.

One major problem with existing switch technologies are relatively high leakage currents, particularly in semiconductor switches, complex design and low speed, particularly in MEM switches, and that they are bulky and rather power consuming (particularly switches based on ferromagnetic materials).

Tuneable frequency selective components, e.g. filters, based on the switching technologies enumerated above exhibit similar problems.

SUMMARY

As explained above, there is thus a need for a switching technology which can combine low costs, low power leakage, high switching speeds, and small /uuA uy

size. Preferably, it should also be possible to base a tuneable frequency selective component on the switch in question.

Such a switch is offered by the present invention in that it discloses a capacitive device which comprises a first and a second layer of electrically conductive material, each of which has a first and a second main surface.

The device also comprises a third layer of a paraelectric phase material positioned between the first and second layers, so that one of the first or second surfaces of the first and second layers face the third layer.

In addition, the device comprises a non-conducting supporting structure on which said layers rest, and also comprises an "acoustic mirror" below the first, second and third layers, as well as comprising means for applying a DC voltage across the third layer, so that the device by means of varying the applied DC voltage may be made to alter its electrical and acoustic characteristics between that of a capacitor, a short circuit or an open transmission line.

Preferably, the "acoustic mirror" may be a membrane of a dielectric material or a metal material, positioned above a cavity in the supporting structure, or a Bragg reflector.

Suitably, the material for the third layer, i.e. the layer of paraelectric material is a Perovskite or similar ferroelectric material having paraelectric state in a desired temperature range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows a first embodiment of the invention, and Fig 2 shows a second embodiment of the invention, and Figs 3-5 show equivalent circuits, and Fig 6 shows a first device comprising the invention, and Fig 7 shows the transmission characteristics of the device of fig 6, and Fig 8 shows the reflection characteristics of the device of fig 6, and Fig 9 shows a second device comprising the invention, and Fig 10 shows the transmission characteristics of the device of fig 9, and Fig 11 shows the reflection characteristics of the device of fig 9, and Fig 12 shows a third device comprising the invention, and Fig 13 shows the transmission characteristics of the device of fig 12, and Fig 14 shows the reflection characteristics of the device of fig 12, and Fig 15 shows a third device comprising the invention, and Fig 16 shows the transmission characteristics of the device of fig 15, and Fig 17 shows the reflection characteristics of the device of fig 15, and Fig 18 shows a bias-resonance curve.

DETAILED DESCRIPTION

Fig 1 shows a first embodiment 100 of the invention. As can be seen in fig 1 , the device 100 comprises a first 110 and a second 130 layer of an electrically conductive material, and a third layer 120 which is made of a paraelectric phase material positioned ("sandwiched") between the first 110 and second 130 layers.

As also shown in fig 1 , the device 100 additionally comprises a non- conducting supporting structure 160 on which the device rests.

According to the invention, the device 100 comprises means ("DC") for applying a DC voltage across the third layer 120, and the device 100 can, as will be explained in more detail in the following, be made to alter its electrical and acoustic characteristics between that of capacitor, a series resonator (short circuit) or a parallel resonator (open circuit), by means of varying the applied DC voltage. As is also shown in fig 1 , the device 100 may additionally comprise a fourth layer 140 which is positioned between the supporting structure 160 and the first, second and third layers, i.e. "below" the first, second and third layers. The purpose of the fourth layer 140 is to acoustically isolate the structure comprised of the first, second and third layers from the supporting structure, i.e. the fourth layer 140 will act as an "acoustic mirror" between the supporting structure 160 and the rest of the device. This is done in conjunction with a cavity 150 in the supporting structure 160. The cavity 150 may also be replaced by a material with a low acoustic impedance.

Suitably, the device 100 consists of a single crystal or polycrystalline paraelectric film as the third layer 120, which is sandwiched between top and bottom films 110, 130 with metallic conductivity. This parallel-plate design can be supported by the thin membrane 140, made of, for example, SiO2 or a similar dielectric or metal, which is supported by the substrate 160 made of for example silicon, with the cavity 150 positioned below the membrane 140.

Before the exact function of the inventive device is described in further detail, an alternative embodiment 200 will be described with the aid of fig 2: features which the embodiment 200 has in common with the embodiment 200 of fig 1 have been given the same reference numerals as in fig 1. Thus, as can be seen from fig 2, the main difference is that the acoustic mirror in this embodiment comprises a layer 240, which is a so called Bragg mirror. Bragg mirrors as such are well known to those skilled in the field, and will thus not be described in detail here.

Turning now to a more detailed description of the inventive device 100 or 200, it can be pointed out that under DC bias, the crystalline structure of the paraelectric layer 120 becomes non-centre-symmetric, leading to an induced piezoelectric effect, i.e. the generation of acoustic waves under an applied microwave signal. 2ϋϋ u9 2

In contrast to known piezoelectric resonators (such as those based on e.g. AIN, ZnO, BaTiO3 etc.) the piezoelectric effect in the paraelectric phase capacitor such as the one shown in figs 1 and 2 is DC bias induced.

The acoustic waves reflect from the metal plates 110,130 (and the Bragg reflector for acoustic waves) and form standing waves (acoustic resonance) at frequencies given by the expression:

f=(vac/2t)n

where vac is the acoustic velocity in the ferroelectric layer 120, t is the thickness of the layer, and n is an integer (=1 ,2,3...).

The acoustic velocity as such is DC bias dependent. In principle, the devices 100, 200, will have two resonant frequency series, one at which the devices will exhibit low impedance and one at which they will exhibit high impedance.

Under different DC bias voltages, the acoustic resonances will appear at different electrical frequencies. This is symbolically shown in figs 3a-3c, in which equivalent circuits are used to show how the device will appear under different DC bias conditions:

- Fig 3a shows the device without DC bias, where the device is a simple capacitance.

- Fig 3b shows the device with a DC bias at a frequency close to series resonance, where the device in principle will exhibit a very low impedance

- Fig 3c shows the device with a DC bias at a frequency close to parallel resonance, where the device in principle will exhibit very high impedance. 2007 09 26 6

Thus, turning the DC bias voltage on and off causes the resonances to appear and disappear, which means that the device 100, 200 can be used as a switch. This will be illustrated with the aid of figs 4a-b and 5a-b, which show that at a given frequency, the device 100, 200, will appear as a:

• simple capacitance, fig 4a, or as a short at the frequency of the series resonance, fig 4b, and

• as a simple capacitance fig 5a, or open (very high impedance), fig 5b, at the frequency of the parallel resonance.

For different DC bias voltages, the resonances which cause series or parallel impedances (ON/OFF) will appear at different frequencies, which means that the device of the invention can be used as a frequency selective switch. However, some additional elements, inductive, and in some embodiments also resistive, may be used in order to suppress one of the two resonant frequencies.

In figs 6-15, some alternative embodiments of the invention will be shown, said embodiments also being described in the text below.

Fig 6 shows a device 600 of the invention, which comprises a tuneable switch 610 ("X5") similar to those shown and described as 100, 200 above, said switch being connected to ground via respective impedances 620 ("Z1") and 630 ("Z2"), which may represent the load on the input/output sides of the device 600.

Fig 7 shows the transmission (S21 ) characteristics of the device 600 in the

ON and OFF states of the switch 610, and fig 8 shows its reflection (S21) characteristics in the ON and OFF states of the switch 610. As can be seen from these figures, under DC bias of the switch 610 (ON state), the device 2007 09 26 7

600 has higher transmission at 4.5 GHz, in other words at this frequency the transmission increases, and the reflection decreases.

Fig 9 shows another embodiment 900 of a frequency selective switching device based on a switch 910 ("X5") such as the ones shown as 100 and 200 above. As shown in fig 9, the switching device 900 comprises a circuit with an additional inductor 940 ("L1") which has a parasitic resistance 950 ("R1"), said circuit being connected in parallel with the switch 910. The purpose of the inductor 940 with its parasitic resistance 950 is to suppress one of the two resonant frequencies of the switch 910, thus improving the isolation characteristics of the device 900.

In similarity to the device 700 of fig 7, the switching device 900 is shown as being connected to ground via respective load impedances 920 ("Z1") and 930 ("Z2").

Fig 10 shows the transmission characteristics of the device 900 in the ON and OFF states of the switch 910, and fig 11 shows the reflection characteristics of the device 900 in those states. As can be seen, at a certain DC bias voltage, the device 900 exhibits highly increased transmission centred around a frequency of approximately 4.5 GHz, and conversely, a highly decreased transmission centred around the same frequency when the DC bias voltage is removed, i.e. in the OFF state. As can be expected, and as shown in fig 11, the reflection around the centre frequency is highly decreased with the DC bias voltage present, i.e. ON

Fig 12 shows another switching device 1200 based on a switch of the invention. As shown in fig 12, the device 1200 comprises two switches such as those shown in figs 1 and 2, both of which are connected in parallel with an inductance which has a parasitic loss resistance. One of the switches is included in a circuit 1240 which is series connected in the device 1200, and 2007 09 26

the other switch is included in a device 1250 which is shunt connected to ground in the circuit 1200.

Fig 13 shows the transmission characteristics of the device 1200, and fig 14 shows its reflection properties. As can be seen from fig. 14, in both the ON and the OFF state, the reflection is very low, that is to say the device is "matched". This is a feature which could be useful for some applications, since the transmission may be turned ON and OFF without affecting the reflection.

The ON state for the device 1200 referred to in figs 13 and 14 refer to a state in which the circuit 1240 is ON (DC bias present) and the circuit 1250 is OFF. Conversely, the OFF state for the device 1200 is a state in which the circuit 1240 is OFF and 1250 is ON (DC bias present.)

More than one switch of the invention may be included in a device in order to obtain multiple ON frequencies at one and the same DC bias. An example of such a device 1500 is shown in fig 15: as can be seen in fig 15, the device 1500 includes two switches 1540 ("X11") and 1550 ("X12") of the invention, which are connected in parallel to each other, the device 1500 also including an inductance 1510 ("L3") with a parasitic impedance ("R7"), said inductance (with its impedance) being connected in parallel with the two switches 1540, 1550.

In similarity to the devices shown above, the device 1500 is also connected to ground via respective impedances (loads) 1520 ("Z1") and 1530 ("Z2").

Fig 16 shows the transmission characteristics of the device 1500, and fig 17 shows its reflection characteristics. As can be expected, the device exhibits two ON states at two different frequencies for two given DC bias, the ON states being the respective ON states for each of the switches 1540, 1550, which in the example of fig 15 is approximately 4.3 GHz and 4.7 GHz, respectively.

Similarly, the device 1500 exhibits two reflection "lows", one at each of the ON frequencies of the switches 1540, 1550, i.e. 4.3 GHz and 4.7 GHz.

An alternative way of utilizing a switch of the invention is to let the switching be achieved by changing the DC bias of the switch, so that the switch goes from series ("open") to parallel ("open") resonance for one and the same frequency.

An example of this is shown by means of a graph in fig 18: at the frequency 4.182 GHz, a certain device is in parallel resonance at a DC bias voltage of 2 V. By increasing the DC bias voltage to 15 V, the same device is in series resonance at the same frequency, i.e. 4.182 GHz. The graph of fig 18 also shows how one and the same device will be at series or parallel resonance with varying DC bias voltage and varying frequency.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims.

Claims

2007 09 26CLAIMS

1. A capacitive device (100, 200) comprising a first (110) and a second (130) layer of electrically conductive material, each of which has a first and a second main surface, the device comprising a third layer (120) of a paraelectric phase material positioned between said first (110) and second (120) layers, so that one of said first or second surfaces of the first and second layers face the third layer, the device (100, 200) additionally comprising a non-conducting supporting structure (160) on which said layers rest, the device being characterized in that it additionally comprises an "acoustic mirror" (140,150; 240) below said first, second and third layers, the device also comprising means (DC) for applying a DC voltage across the third layer (160), so that the device by means of varying the applied DC voltage may be made to alter its electrical and acoustic characteristics between that of a capacitor, a short circuit or an open transmission line.

2. The device (100) of claim 1 , in which the acoustic mirror is a membrane (140) of a dielectric material or a metal material, positioned above a cavity in the supporting structure.

3. The device (200) of claim 2, in which the acoustic mirror (240) is a Bragg reflector.

4. The device (100, 200) of any of the previous claims, in which the material for the third layer (120), i.e. the layer of paraelectric material, is a ferroelectric material.

5. The device (100, 200) of any of the previous claims, in which the material for the third layer (120), i.e. the layer of paraelectric material, is a Perovskite ferroelectric material. ₆ ^

6. The device (100, 200) of claim 4, in which the Perovskite material (120) is chosen from one of the following

• Ba_xSr_{1 -x}TiO₃ • SrTiO3,

• KTaO3,

• K_xLii_-xTaO₃,

• Na_xKi -χNbO₃

7. The device (100, 200) of any of the previous claims, in which the means (DC) for applying a DC voltage across the third layer (160) comprises input means connected to the first (110) and third (130) layers, respectively.

8. An electrical circuit (600, 900, 1200, 1500) comprising a first voltage controlled switch (100, 200), said first switch being the device of any of claims 1-7.

9. The circuit (900) of claim 8, with said first switch (100, 200) being connected in parallel with a second circuit which comprises an inductance (940) and a resistance (950) which are serially connected to each other.

10. The circuit (1200) of claim 9, comprising a second switch (1250) of any of claims 1-7, said second switch (1250) being shunt connected to the circuit, so that said second switch is also connected to ground, and is connected in parallel to an inductance and a resistance which are serially connected to each other.