CIRCUIT PROTECTION DEVICE

Abstract: A threshold device for protecting electrical circuits from voltage transients e.g. the electromagnetic pulse or eletro­ static discharge transients is formed from an amorphous selenium/arsenic/germanium composition, preferably hav­ ing 15 to 75 atomic % selenium, 10 to 65 atomic % arsenic and 5 to 35 atomic % germanium. The devices can switch from a high resistance state to a low resistance state with a very fast switch time and so pass the transient pulse to earth before reverting to their high resistance state. The devices can withstand very high electrical energies before they latch in their low resistance state.

Description: CIRCUIT PROTECTION DEVICE This invention relates to circuit protection devices, and especially to devices for protecting electrical circuits against voltage transients that are caused by an electromagnetic pulse, e.g. lightning, and also the transients that are caused by electrostatic discharge. Electrostatic discharge and electromagnetic pulses can all induce very high currents and voltages on cables and structures such as aeroplanes and ships, which can penetrate the electrical systems and damage them, either causing hardware damage such as semiconductor burnout, or electronic upset, e.g. transmission loss or loss of stored data. As semiconductor technology advances, the energy needed to cause such damage becomes less. Electrostatic discharges are extremely fast discharges from a capacitor such as a human body. These discharges, because they can be so local, present an extreme threat to the individual electronic component. The magnitude of the threat is illustrated by reference to IEC Technical Committee TC65, WG4 draft specification or the SAE-AE-4 proposed specification on ESD. In these documents the maximum threat is a double exponential pulse of rise time 5 nanoseconds with a 30 nanosecond fall time. The peak voltage is given as 15,000 volts with a current of 70 amps. The energy storage capacitance is given as 150 pico farads giving a maximum energy of 33 millijoules. Induced electrical transients caused by lightning represent a threat to electrical/electronic equipment especially in aerospace vehicles. The characteristics of the induced pulses are best described in the SAE AE4L Committee Report AE4L-81-22. Test Wave Forms and Techniques for Assessing the Effects of Lightning -Induced Transients. This specification describes the idealised versions of the lightning stroke waveform and of those which occur when equipment cables are excited by the EM environment produced. The specification specified a range of transients which may be produced during normal operation. The waveforms expected on each line depends on a number of factors but the two basic types are (waveform 3) with a frequency of between 1 and 10 MHz, a rise time of 45 nanoseconds and a peak current of 24 amps. The alternative wave form (Wave Form 4A) is a decaying exponential with a rise time of 100 nanoseconds and a width of 2 microseconds with a peak voltage of 300 volts and peak current of 60 amps. The most severe threat to electrical and electronic equipment is the electromagnetic pulse although all effects can generate large voltage or current spikes with very short rise times. The exact characteristics of the pulse are complicated although a typical pulse will exhibit a field strength of about 105 Vm-l with an accompanying H field intensity of 130 Am-l (based on free space impedance of 377 ohms), a pulse width of a few microseconds and a rise time of a few nanoseconds. In general damaging amplitudes in the pulse occur within the frequency range of 10KHz to 100MHz. In order to protect electrical circuits from such threats it would be desirable to incorporate therein a circuit protection device which, in normal operation, has a high resistance but which, when the circuit is subjected to a voltage surge, quickly changes to a low resistance state in order to short the current surge to earth, and, after the current surge has finished, immediately reverts to its high resistance state. Based on the electrical characteristics of the pulse and the nature of the equipment it is intended to protect, it is possible to determine what characteristics of a circuit protection device are necessary for it to be able successfully to protect the circuit against the pulse damage. For example, one set of guidelines published by H.R. Philipp and L.M. Levinson in J. Appl Phys 52(2) February 1981 pp. 1083 to 1090, the disclosure of which is incorporated herein by reference, specifies inter alia the following requirements for a surge arrestor material that is intended to short a voltage transient to earth: Threshold voltage 100V Switching delay time < 1 nanosecond Current capacity up to 400 A Insertion loss < 0.4 dB at 0 to 200 MHz < 0.8 dB at 200 to 500MHz Size limit lem The insertion loss, or attenuation of an electrical signal, at any frequency may be determined from the capacitance of the device, and it is this value of attenuation which is required to determine the suitabi lity of any protection device. Since the switching device will comprise a normally insulating switching element located between a pair of conductive electrodes, the capacitance of the device (assuming parallel plate configuration) is given by C = EoEr.A.t#l where C is the capacitance; E0 is the absolute permitivity of free space (8.85 x 10-12#-2; sr is the relative permitivity of the dielectric; A is the cross-sectional area (in metres), and t is the thickness of the dielectric (in metres); and the insertion loss, measured in decibels, is given by: Insertion loss = 20.log10[V1/V2] where V1/V2 is the ratio of the signal intensity without the device to that with the device, and is given by: V1/V2 = (1 + (2aiC)2li where f is the frequency of the signal (in Hertz); and R is the characteristic impedance In addition to these requirements, the device will need a resistance in normal operation of at least about 106 ohms in order to allow the equipment to function correctly. One class of material that has been proposed for use in the manufacture of circuit protection devices in general are the chalcogenide glasses, by which is meant glasses formed from elements of group VIB of the periodic table (IUPAC 1965 revision) together with other elements, especially those of groups IVB and VB, for example as described in U.S. Patent No. 3,271,591 to Ovshinsky. Certain of these glasses can be used to form "threshold" devices by which is meant devices that will change from a high resistance state to a low resistance state on application of a high voltage (the lowest such voltage being referred to as the n#threshold voltage") but which will remain in their low resistance state only for as long as a small "holding" current is maintained. Other chalcogenide glasses can be used to form "memory" devices which will change from a high resistance state to a low resistance state on application of a high voltage and which will remain in the low resistance state, even when no voltage is applied, until an appropriate, different, voltage pulse is applied. As will be appreciated, only threshold devices are appropriate for the production of circuit protection devices. The chalcogenide glass materials have the advantage that they exhibit very short switching times between their high and low resistance states where the voltage transient that causes switching is significantly higher (e.g. about 50 V or more) than the threshold voltage, typically less than 1 nanosecond, which is sufficiently fast for protecting circuits from the transient. However, they suffer from a number of disadvantages that, hitherto, have prevented their use for such purposes. The shortcomings of such devices may be seen by comparing the properties of a typical glass with the properties required of a circuit protection device to meet the guidelines mentioned above (although it will be appreciated that other guidelines may require different properties). For example a typical chalcogenide glass material (for example Te48As30Si12Ge10 described in U.S. Patent No. 3,271,591) has the following properties: Dielectric constant 10 High resistance resist- eoff 107 ohm cm ivity low resistance resist- eon 0.07 ohm Cm ivity threshold field strength Vth 4x105 Vcm-1 where the threshold field strength is the field strength across the device required to switch the glass to its low resistance (on resistance) state. The minimum area needed for a device comes from a knowledge of the maximum current density that the materials can stand and the maximum current flow in a transient, and is given by: Minimum Area = EMP Current max. current density The maximum current density is of the order of 2x104Acm-2 (see D. Adler et al, J. Appl Phys Vol 51(6) 1980 P. 3289 to 3309) whilst the maximum current is, as mentioned above from Philipp and Levinson, up to 400A. Hence, for example, the device would need a crosssection of at least 2mm2 to exhibit the required current capacity of 400A given by the guidelines but for a required current capacity of 200A, a minimum area of lmm2 is needed. In practice, the smallest area that is compatible with the current density is preferred in order to minimise the capacitance of the device and hence the insertion loss. Also, since the threshold field strength of the glass is 4 x 105 Vcm#1, the device would need a maximum thickness of 2.5 micrometres in order to exhibit a threshold voltage of 100V. However, such a device would have a normal high resistance state resistance (off resistance) of 2.5x105 ohms which is far too low for normal operation of the equipment, and would also exhibit an insertion loss for frequencies up to 200 MHz of 6dB. If the thickness of the device were increased to give an acceptable insertion loss, the threshold voltage would be raised to 1000V, while if the thickness were increased to give an acceptable off resistance the threshold voltage would be raised to 10000V. Although it is possible to reduce the threshold field strength of the glass by appropriate choice of formulation, we have found that, in the previously proposed materials, other properties of the material are seriously impaired. According to one aspect, the invention provides a circuit protection device for protecting an electrical circuit from a voltage transient, which comprises a threshold switching device having a switching element that exhibits an energy to latch of at least 40, preferably at least 60 and especially at least 100 mJ (for a 10 micrometre thickness) and is formed from an amorphous composition comprising germanium, selenium and arsenic, the composition having a figure of merit (as hereinafter defined) of at least 5 kgmSs-2A-l, We have found that it is possible to form threshold switches from such chalcogenide glasses that have a balance of properties that enable them to be used successfully for protecting equipment from voltage transients. For example, we have found that the threshold field strength of these glasses are, in general, lower than 4 x 105 Vcm-l and can be in the order of 1x105 Vcm#1. Furthermore the off resistivity of the glasses can be in the order of 108 ohm cm as compared with 107 ohm cm for the previously proposed glasses. Thus it is possible to form circuit protection devices that are acceptable by appropriate choice of the glass optionally together with dimensional control of the device, for example by increasing the thickness of the chalcogenide glass switching element e.g. to 10 to 30 micrometres. The threshold field strength of the element combined with the other properties thereof causes the threshold voltage of the device to remain at an acceptable value e.g. from 70 to 300V, while the increased off resistivity of the element enables an acceptable normal resistance value of about 106 ohms to be achieved. Indeed, the fact that these glasses act as threshold devices at all is contrary to previous references to these materials (R. Pinto J. Non-Crystalline Solids 6(1971) 187-196, R. Pinto Thin Solid Films 7(1971) 391, J.P. deNeufville et al 5th International Conference on Amorphous and liquid semiconductors 1973 Vol 2 pp. 737-743 and U.S. patent No. 3,530,441 to Ovshinsky) in which the glasses are described only as memory materials. In addition, by the use of materials defined above it is possible to overcome one significant problem that has hitherto been largely ignored or has not been appreciated in connection with circuit protection: although such devices will often revert to their high resistance state as soon as the transient voltage has subsided, it is easily possible for a device to be forced permanently into its low resistance state if the energy of the transient is large or if a number of rapid transients are experienced. In fact, the energy needed to latch the previously proposed devices permanently into their conductive state is relatively low and is often less than the expected energy of the transient against which the device is intended to provide protection. Thus, although the electrical and electronic equipment would be protected against the transient, the equipment would not then operate until the protection device is replaced or reset. In contrast, the materials used in the devices according to the present invention exhibit a relatively high energy to latch and often considerably higher than that exhibited by any of the previously proposed materials. The "energy to latch" or "latching energy" is defined herein as the electrical energy required to force the switching element or the glass composition permanently into its low resistance state. Some values for the energy to latch are quoted herein in units of energy (mJ), and such values refer to glass compositions of 10 micrometre thickness and lmm2 area. It is believed that the energy to latch is substantially proportional to the thickness of the glass, at least at thicknesses of this order of magnitude, and independent of the area, at least for areas of this order of magnitude, and so other values herein may be quoted in units of energy per thickness of the glass. Preferably the materials have an energy to latch of at least 2000, more preferably at least 5000 and especially at least 10,000 Jm-l (or, more correctly, microjoules micrometre#1). The energy to latch should be determined using electrical energy of 1MHz frequency. The particular value desired for any property of the material used in the device according to the invention will depend, at least to some extent on the other properties of the material, and the choice of material may often represent a compromise between conflicting properties. We have found, however, that it is possible to define a property of the material, called herein the "figure of merit" which distinguishes those materials that may be used to form useful circuit protection devices from those that cannot be used. In some cases the figure of merit may be used alone to determine the suitability of a material although in many cases, depending on the expected threat to the equipment, the figure of merit should be used together with one or more other criteria. The figure of merit for the glass compositions is defined herein as follows: Figure of merit = EL . eoff Vth ¯ Er where EL is the energy required to latch the materia#l in its low resistance state (measured in Joules per metre (of thickness) or microjoules per micrometre); poff is the resistivity of the material in its high resistance state (measured in ohm metres); Vth is the threshold field strength of the material (measured in volts per metre); and Er is the dielectric constant of the material) The materials used in the devices according to the present invention generally have a figure of merit of at least 5 units whereas those proposed in the prior art generally have a figure of merit below 0.3 and often belgw 0.1 unit. Preferably the material exhibits a figure of merit of at least 10, and more preferably at least 20 units (a unit being 1 kgm3s-2A-1#, Alternatively or in addition, a figure of merit for the circuit protection device, rather than for the glass, may be defined as follows: Figure of merit = EL'.R V' C where EL' is the energy to latch of the switching element (measured in joules); R is the high resistance state resistance of the switching element (measured in ohms); V'is the threshold voltage of the switching ele ment (measured in volts); and C is the capacitance of the switching element (measured in farads). The device preferably has a figure of merit of at least 5x1012, more preferably at least 1x1013 and especially at least 2x1013 JA-lF-l. Preferably the composition contains at least 15 atomic % selenium but preferably not more than 75 atomic * selenium. The arsenic content is preferably at least 10 atomic % but preferably not more than 65 atomic %. In addition or alternatively, the composition preferably contains at least 5 atomic % germanium but not more t##han 42 atomic % germanium. These glass compositions are particularly preferred, and so, according to another aspect, the invention provides a circuit protection device for protecting an electrical circuit which includes a threshold switching element formed from an amorphous composition comprising: (a) 15 to 75 atomic % selenium; (b) 10 to 65 atomic % arsenic ; and (c) 5 to 42 atomic % germanium or, if the com position contains less than 35 atomic % selemium, 5 to 35 atomic % germanium in which the proportions of (a), (b) and (c) (based on the total molar quantity of (a), (b) and (c)) add up to 100%. Preferably the composition contains not more than 35 atomic % germanium, more preferably not more than 30 atomic % germanium and especially not more than 25 atomic % germanium. Also, the composition preferably contains at least 20 atomic % selenium and especially at least 30 atomic % selenium but preferably not more than 70 atomic % selenium and especially not more than 60 atomic % selenium. The composition preferably contains at least 20 atomic % arsenic and especially at least 25 atomic % arsenic, but preferably not more than 60 atomic % arsenic and especially not more than 55 atomic % arsenic. Thus the composition preferably comprises: (a) 20 to 70 atomic % selenium; (b) 20 to 60 atomic % arsenic, and (c) 5 to 30 atomic % germanium and most preferably comprises: (a) 30 to 60 atomic % selenium; (b) 25 to 55 atomic % arsenic; and (c) 5 to 25 atomic % germanium. All the proportions of the components (a), (b) and (c) are based on the total molar quantity of (a), (b) and (c) alone and total 100%. It is possible for quantities e.g. up to 10% or sometimes more, of other materials to be present in the compositions used in the devices according to the invention, for example minor amounts of the elements antimony, bismuth, silicon, tin, lead, halogens and some transition metals provided that the presence of such materials does not deleteriously affect the properties, such as the energy to latch and/or off resistivity, to a significant degree. It is preferred, however, for the compositions to contain substantially no tellurium since the presence of tellurium has been found to reduce the off resistivity of the materials severely, although, in certain circumstances, small quantities of tellurium may be tolerated, e.g. up to 10 atomic %, but preferably less than 5 atomic %. The switching element is usually formed by a vapour deposition method in which a vapour of the appropriate composition is deposited onto one of the electrodes. The vapour may be generated by heating an appropriate mixture of the components (not necessarily having the same composition as the intended glass) or the separate components may simultaneously be heated. Without wishing to be bound to any particular theory, it is currently believed that one factor in determining the quality of the switch is the contact resistance between the chalcogenide glass material and the electrodes, and that a reduction in the contact resistance can increase the energy to latch of the switch. Although many glass compositions falling within the range quoted above exhibit threshold characteristics irrespective of the electrode contact resistance, it appears that in some cases, and especially at the edges of the three component composition defined above, the improvement in the energy to latch caused by a substantial reduction in electrode contact resistance may even effectively cause a composition to change from one that exhibits memory characteristics to one that exhibits threshold characteristics, and it is accordingly conjectured that the memory characteristics previously observed by Pinto and Ovshinsky et al could, in fact, be caused by a high electrode contact resistance. Thus, according to a further aspect, the invention provides a cricuit protection device for protecting an electrical circuit from a voltage transient, which comprises a threshold switching element that is formed from an amorphous composition comprising germanium, selenium and arsenic, and a pair of electrodes in contact with the composition, the electrodes having a contact resistance with the composition that is sufficiently low to cause the device to exibit an energy to latch of at least 40 mJ. Preferably the electrode contact resistance is sufficiently low that the overall electrode-toelectrode resistance of the device is not more than 10 ohms and especially not more than 5 ohms, the most preferred resistance being less than 1 ohm. Low electrode contact resistance may be achieved for example by depo siting the glass material on the electrode by a vacuum method for example by evaporation. If desired, materials such as indium may be provided as a layer between the electrode and the glass and/or incorporated in at least part of the glass in order to reduce the contact resistance. The dimensions of the switching element used in the device according to the invention will depend on the particular chalcogenide glass composition that is used to form it, although the thickness of the switching element will usually be not more than 40 micrometres, preferably not more than 20 micrometres, but usually at least 5 and preferably at least 10 micrometres. The cross-sectional area of the switching element, in a plane normal to the direction of current flow therethrough, will depend on the maximum current flow. It is preferably at least 0.5mm2, the preferred size being about lmm2 for discrete devices and for the maximum pulse level, at least 2mm2. The devices may be incorporated in an electrical circuit in any suitable position, normally being connected between a current carrying line and earth, (the term "earth" in this context including any structure having an appropriate shape and/or capacity so that it can absorb the charge generated by the transient, and includes for example connection to the chassis and the like in vehicles such as aircraft), and, of course, more than one such device may be employed in the electrical circuit. The devices are conveniently incorporated in other electrical components for example electrical connectors, in which case the device will usually be connected between a current carrying element of the device and a terminal or other part of the device to be earthed e.g. a conductive housing. Although in most instances the device will revert to its high resistance state as soon as the transient voltage has subsided, it is still possible for the device to be forced into a permanent low resistance state, for example if the voltage transient is unduly large or if a number of rapid transients are experienced. As mentioned above, whether or not the device will become permanently conductive depends on the amount of energy absorbed by the device from the transient. In some applications, for example in some ground installations, it may be desirable for the protection device to fail in this way, that is to say, so that the equipment is still protected against transients but will not function until the protection device is replaced or reset. In other applications it may be desirable for the device to fail in a high resistance (open circuit) state so that the equipment will carry on functioning although unprotected from subsequent transients. Thus in some cases the device may be connected in series to means that will exhibit a high resistance to the intended electrical circuit current at least when the switching element has become permanently conductive. Thus, for example, the switching element may be connected to the current carrying line or to earth via a fuse or switch that is capable of transmitting currents passed through it when the switching element is in its threshold mode but will change to a high resistance state when the switching device has become permanently conductive. Alternatively or in addition, the device may include a capacitor to ensure that the device exhibits a high resistance to all frequencies below the cut-off limit of the capacitor. The use of a capacitor in series with the switching element has a number of advantages as mentioned in our copending British Patent Application entitled Overvoltage protection Device: filed on even date herewith (Agent's reference no. RK269), the disclosure of which is incorporated herein by reference. Briefly, the use of a capacitor of appropriate size, for example from l0pF to 2 microfarads in series with the switching element will enable the transient current to be transmitted to earth, since most of the power of the transient current occurs at frequencies above 10 kHz, but will exhibit a high impedance to the intended currents in the circuit which will have significantly lower frequencies or will be a direct current. Also, the use of a capacitor will prevent or significantly reduce the possibility of the switching element latching in its low resistance state after a transient has occurred. Such latching may occur in the absence of a capacitor due to current flow through tshe switching element keeping the switching element in its low resistance state. If a capacitor is used it is possible, in many cases, for only for a single capacitor to be employed in order to protect a number of lines if it is connected on the earth side of the switching elements, thereby allowing a greater degree of space saving. Several embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a side view, partly in section, of a BNC coaxial connector that incorporates a circuit pro tection device according to the invention; Figure 2 is a side view, partly in section, of a flat cable mass termination connector and wafers that incorporate a circuit protection device according to the invention; Figure 3 is an enlarged view of part of the connector shown in figure 2; Figure 4 is a perspective view of a modification of the wafers shown in figure 2; Figure 5 is a schematic view of a capacitor that incor porates a circuit protection device according to the invention; and Figure 6 is a diagram of a circuit used to determine the energy to latch of the materials and devices according to the invention. Figure 7 is a perspective view of a chip carrier employing the device according to the invention; Figure 8 is a view, partly in section, of a modifi cation of the chip carrier of figure 7; Figure 9 is a section showing a further modification of the carrier shown in figure 7; Referring to figure 1 of the accompanying drawings, a connection arrangement for connecting two coaxial cables comprises a connector shell 1 and a male connector 2. The male connector 2 comprises a pin 3, the central and rear portion of which is hollow for receiving the central conductor of a coaxial cable to be connected (not shown). The pin has a fluxed solder ring 4 and a number of apertures (not shown) beneath the solder ring which communicate between the solder ring 4 and the hollow interior of the pin 3. The rear end 10 of the pin is firmly located in a connector housing 5 by means of an electrically insulating plastics spacer 6. The housing 5, which provides the electrical connection between the shields of the cables to be connected, has a termination portion 7 on which is mounted a solder impregnated braid 8 and solder ring 9. The rear end 10 of the pin is provided on its outer surface with an electrode, e.g. a molybdenum electrode followed by a 10 micrometre thick layer 11 of a selenium-germanium-arsenic glass described above that has been deposited thereon by a vapour deposition method, and the outer surface of the glass layer 11 has been provided with a further thin (about 5 micrometres thick) electrode, e.g. formed from molybdenum by a d.c. sputtering method. The molybdenum electrode is electrically connected to the housing 5 by means of a column or wire 12 of solder or other suitable conductive material. In order to connect a coaxial cable to the connector, the outer jacket, shield and dielectric are cut back by appropriate amounts and the cable inserted into the connector so that the exposed end of the internal conductor is located within the hollow interior of the pin 3, the dielectric abuts the rear end of the spacer 6 and the exposed shield is located within the solder impregnated braid 8. The connector is then briefly heated, for example by means of a hot-air gun, to fuse the solder rings 4 and 9 and to form solder connections between the pin 3 and central conductor and between the braid 8 and coax cable shield. The connector will function exactly as a standard coaxial connector until the connected cable experiences a voltage transient whereupon the potential difference across the thickness of the glass layer 11 will cause the glass to become electrically conductive and form a closed circuit between the central conductor and the shield. Referring to figures 2 and 3, a mass termination connector such as that described in British Patent No. 1,522,485 (the disclosure of which is incorporated herein by reference) is schematically shown. The connector comprises a connector housing 21 and a pair of connector wafers 22 and 23 that can be inserted into the housing. Each wafer 22,23 has a number of (usually 25 or 50) metallic electrical conductors 24 extending therethrough which terminate at one end either in the form of pins 25 or complementary "tuning fork" female contacts and at the other end in the form of contacts 26 for connection to a flat cable or to a number of small single conductor wires. The particular means used for connecting the conductors 24 to the wires or flat cable is not shown but usually comprises one or more solder devices for example as described in U.S. Patent Specification No. 3,852,517. In each of the wafers 22 and 23 a stepped recess 27 is made that extends across the width of the entire wafer to expose each of the conductors. in one embodiment of this connector, a molybdenum electrode is deposited onto the individual conductors 24 followed by a 10 micrometre thick layer 28 of the selenium-germaniumarsenic glass described above, and a thin electrode 29 formed e.g. from molybdenum, gold or aluminium is deposited on top of the glass layer 28. An additional conductive layer 30 or "ground plane" of gold or aluminium is deposited on the wafer material in the stepped recess 27, the ground plane being electrically earthed for example to the metallic housing of the connector or to an earth pin. Each electrode 29 is connected to the ground plane by means of a wire 31 formed from e.g. gold or aluminium and bonded to the electrode 29 and ground plane 30 by conventional wire bonding tech niques. Alternatively, a single layer 28 of the glass and electrode 29 may be deposited accross the entire width of the wafer in which case only a single wire 31 is necessary for connection to the ground plane, or the ground plane and wire can even be dispensed with if one of the conductors 24 is earthed. In an alternative construction, the seleniumgermanium-arsenic glass layer and electrodes are deposited onto the common ground plane 30, and the wires 31 connect the conductors 24, after any appropriate surface preparation if necessary, with the electrode of the glass layer. Figure 4 shows schematically a further modification of the wafer shown in figures 2 and 3. In this form of wafer the glass layers 28 and electrodes 29 are deposited on the conductors 24 as described above and are electrically connected to a ground plane 30 by means of wires 31. In addition, however, a 100 nanofarad capacitor 40 is located in the recess 27 and is connected between the ground plane and an earth terminal or housing of the connector. In this form of device any transient current having a frequency spectrum above about 1 MHz is conducted directly to earth while any direct currents or alternating currents of frequencies significantly lower than about 1 MHz are blocked by the capacitor. This modification of the device has the advantage that it reduces or eliminates the possibility of the glass switching layers 28 being held in their low resistance state by the direct currents in the electrical system after the transient has been transmitted to earth. Figure 5 shows schematically a circuit protection device according to the invention that may be incorporated into any larger electrical component. The device comprises a capacitor 51 having a capacitance for example of 0.5 to 3 microfarads (although larger or smaller capacitances e.g. 0.1 to 1 microfatads may be desired in many circumstances) which is provided with terminals 52 and 53. A 10 micrometre thick layer 54 of the selenium-germanium-arsenic glass switching material described above is deposited on the terminal 52 for example by a vapour deposition method and then a thin electrode 55 for example formed from molybdenum is deposited on the glass layer 54 e.g. by a d.c. sputtering technique. Additional metal layers may be provided on top of the electrode 55 in order to improve the formation of solder joints if desired. As described above and in our copending Patent Application entitled "overvoltage protection device" filed on even date herewith (Agent's ref. RK269) the glass layer will become conductive when the circuit is exposed to a transient having a high frequency spectrum, e.g. above 1 MHz, to allow the transient to be passed to earth, but the capacitor will exhibit a high impedence to lower frequency signals and therefore prevent such signals latching the glass layer 54 in its low resistance state. Figure 7 shows the use of a threshold circuit protection device according to the present invention on a chip carrier 71 containing a chip 72. The conductive I/O pads of chip 72 are wire bonded by wires 73 to conductive pads 74 in the chip carrier. The threshold circuit pretection device 75 comprises a glass layer which contacts the conductive pads 74 of the chip carrier and an electrode layer in contact with the glass which can be grounded to earth through an appropriate earthpad in the chip carrier or any other appropriate point in the chip package. The threshold circuit protection device 75 covers a portion of the pad 74 of chip carrier 71 leaving a portion of each pad 74 available for wire bonding of the chip 72 and wires 73. The threshold circuit protection device 75 can be formed in situ on chip carrier 71 by first depositing the glass layer, for example by vapour deposition, on the appropriate areas of the conductive pads 74 of the chip carrier and then depositing the electrode layer on top of the glass, which in turn can be connected to earth through a separate wire bond to an earth pad or chip package frame. Alternatively the glass layer can be omitted from an appropriate pad on the chip carrier thereby permitting the electrode layer to be deposited on the glass and on and in contact with that pad which will serve as an earthing pad. The circuit protection device in the chip carrier allows all of the in/out circuits to be in contact with the glass layer which glass is in turn in contact with a ground plane. Thus any overvoltage which enters the package through any lead or I/O pad can immediately pass through the glass layer to the ground plane. As mentioned above, any appropriate elements such asthe capacitor of figures 4 or 5 may also be built into the combination of the circuit protection device and the chip carrier. The circuit protection device 75 can be connected to pads 74 by conductive adhesive or other appropriate means. Figure 8 shows a further modification of figure 7, here to frame 120 used to support the circuit protection device is multilayered. A pattern of electrodes 121,122 is produced on both top an bottom faces of the frame to match electrodes 123 of the chip carrier 124. The two electrodes 121,122 on opposite faces are interconnected by a conducting channel or via 125. A circuit protection device 126 is built up on the top electrode pattern 121 of the frame 120 and when completed the frame can be fitted into the chip carrier as shown in figure . As shown in figure 9, it is also possible to incorporate within the frame material the elements of a capacitor 127 within the structure of the via 125. This system of assembly provides for a more robust package since the circuit protection device is separated from the electrode tracks of the chip carrier by the conducting vias. The following Examples illustrate the invention: Samples of a number of chalcogenide glass compositions were prepared by mixing the components, which were of at least 29.99% purity, and melting the mixture in a silica ampoule under a vacuum or under reduced argon pressure. During melting, which was carried out at tempertures of up to 10000C and for periods of up to 48 hours depending on the#composition, the ampoule was rocked and rotated in order to ensure that a homogeneous melt was obtained. A 10 micrometre thick film of the glass so prepared was deposited onto a lower molybdenum electrode by vapour deposition at a pressure of 10-3 to 10-4 Pa using either a resistance heated, or an electron beam heated, source. Deposition rates of 0.3 to 1.0 micrometres per minute were employed. An upper molybdenum electrode of lmm2 area was deposited by d.c. sputtering in argon at a pressure of 10 Pa and at a deposition rate of 20 to 30 micrometres per minute. The glasses had the compositions shown in Table I. TABLE I Components (Parts by mole) Example Si Ge As Se Te 1. (Comparison) 12 10 30 48 2. (comparison) 20 80 3. (comparison) 40 60 4. 7 37 56 5. 7 56 37 6. 10 20 70 7. 14 34 52 8. 14 52 34 9. 20 20 60 10. 20 60 20 11. 22 30 48 12. 27 51 22 13. 40 19 41 14. (comparison) 14 34 52 15. (comparison) 14 43 43 16. (comparison) 14 34 26 26 17. (comparison) 40 40 20 18. (comparison\ 40 20 40 19. 39 16 45 20. 30 32 38 21. 22 27 51 22. 13 28 59 23. 23 14 63 24. 17 20 63 25. (comparison) 13 23 64 26. (comparison) 55 32 13 The electrical properties of the compositions are shown in Table II and of the devices formed from them are shown in Table III. TABLE II Example Energy to Energy #off threshold field #r Figure of merit Latch (Jm-1) (ohm m) strength (MVm-1) (kgm3s-2A-l) 1.(comparison) 100 1X105 45 10 0.02 2.(comparison) 2000 1X106 7.5 6.7 39 3.(comparison) 2000 lXlQ6 7.0 6.8 42 4. 5000 3X105 10.0 7.0 22 5. 7000 1X106 11.0 6.9 92 6. 4000 1.5x106 9.0 6.8 98 7. 15000 1.5X106 13.0 > 6.4 270 8. 20000 6X105 14.0 6.6 130 9. 5000 1X106 10.0 6.4 78 10. 20000 1x106 16.0 6.7 186 11. 30000 1X106 25.0 6.7 179 12. 18000 1.2X106 16.0 6.8 198 13. 5000 1x106 14.0 6.8 52 14.(comparison) 3000 l0X104 9.0 12.2 0.3 15.(comparison) 6000 3X103 14.0 13.3 0.1 16.(comparison) 6000 1X104 12.0 9.5 0.6 17.(comparison) 2000 1X105 8.0 8.7 2.9 18.(comparison) 50 5X104 10.0 9.7 0.3 19. 4000 7x105 12.0 6.8 34 20. 22000 2X105 10.0 6.8 65 21. 16000 1.2X106 18.0 6.7 159 22. 5000 l.0X106 12.0 6.5 64 23. 4500 5X105 10.5 6.5 33 24. 5000 1X106 11.0 6.8 67 25. 500 1X105 6.5 6.8 11 26. 2000 2X105 10.0 7.0 9 TABLE III Example Energy to off resistance Threshold Capacitance Figure Latch (mJ) (M ohm) Voltage (V) (pF) of Merit 1.(comparison) 1 1 450 8.8 .25 2.(ccmparison) 20 10 75 5.9 452 3.(comparison) 20 10 70 6.0 476 4. 50 3 100 6.2 242 5. 70 10 110 6.1 1043 6. 40 15 90 6.0 1111 7. 150 15 130 5.6 3091 8. 200 6 140 5.8 1478 9. 50 10 100 5.6 893 10. 200 10 160 5.9 2119 11. 300 10 250 5.9 2034 12. 180 12 160 6.0 2250 13. 50 10 140 6.0 595 14.(comparison) 30 0.1 90 10.7 3.1 15.(comparison) 60 0.03 140 11.7 1.1 16.(comparison) 60 0.1 120 8.4 6.0 17.(comparison) 20 1 80 7.7 32.5 18.Ccomparison) 5 0.5 100 8.5 2.9 19. 40 7 120 6.0 388 20. 220 2 100 6.0 733 21. 160 12 180 5.9 1808 22. 50 10 120 5.7 731 23. 45 5 105 5.7 376 24. 50 10 110 6.0 756 25. 5 1 65 6.0 13 26. 20 3 100 6.2 97 The energy to latch of the device, and hence of the glass material, was measured by means of a circuit as shown in figure 6. Single shot pulses generated by means of a pulser 1 were passed to the switching element 2 connected in series with a current limiting resistor 3 having a resistance R2 of from 40 to 100 ohms and the voltage across the switching element was observed by means of oscilloscope 4. The voltage generated by the pulser 1 was gradually increased (about 5 to 10 pulse$ being passed for each voltage level from the pulser) until the switching element latched in its low resistance state (determined by subsequently measuring its resistance. The energy to latch the device, EL was determined by the equation: EL = J. V 2, R1 (1000) (R1+R2) where J is the energy available from the pulser; V is the peak voltage from the pulser when latching occurred; R1 is the internal output impedance of the pulser (5 ohms); and is isthe resistance of the current limiting resistor, This equation gives very good agreement with values obtained by integrating the current and voltage curves of the pulses. Examples 27 to 29 A 10 micrometre thick layer of a Gel4 As34 Se52 glass formed by the method of the previous Examples was provided with a number of electrodes of varying electrode contact resistance, and the energy to latch of the switch devices so formed was determined as described above. In Example 27 a gold spring probe of 1 mm diameter was applied to the top surface of the glass layer with a force of lg.wt., the opposite surface being provided with a-copper film electrode. In Example 28 copper- electrode was provided on each surface of the glass layer by a vacuum evaporation method. However the electrodes were allowed partially to oxidize by readmission of air into the vacuum chamber while the electrodes were hot. In Example 29 a 0.5 micrometre thick layer of the same glass composition but containing 15% indium was provided on each surface of the glass layer and 0.1 micrometre thick layer of indium was provided between each indium containing glass and (unoxidized) vacuum deposited copper electrodes. The results are shown in table IV. TABlE I#7 EMI34.1 <tb> Example <SEP> Threshold <SEP> Energy <SEP> to <SEP> Voltage <SEP> across <SEP> Electrodes <tb> <SEP> Voltage(V) <SEP> Latch <SEP> (N3) <SEP> (in <SEP> low <SEP> resistance <SEP> state) <SEP> (V) <tb> <SEP> 27 <SEP> (comp) <SEP> 80 <SEP> (5 <SEP> 20 <tb> <SEP> 28 <SEP> <SEP> (comp) <SEP> 110 <SEP> 25 <SEP> 20 <tb> <SEP> 29 <SEP> llQ <SEP> <SEP> 200 <SEP> < 2 <tb> The resistance across the device was determined by replacing the device with standard resistances and comparing the voltage/time curve with that of the device when subjected to standard pulses. It was observed that the peak current flowing through the device was approximately 1 ampere in all cases so that the resistance across the device (in ohms) is numerically equal to the voltage across the electrodes (in volts).

Claims: CLAIMS: 1. A circuit protection device for protecting an electrical circuit from a voltage transient, which comprises a threshold switching device having a switching element that exhibits an energy to latch of at least 40mJ (for a 10 micrometre thickness) and is formed from an amorphous composition comprising germanium, selenium and arsenic, the composition having a figure of merit of at least 5 Kgm3S-2A'1 2. A device as claimed in claim 1, wherein the switching element exhibits an energy to latch of at least 60 mJ preferably at least 100 mJ. 3. A device as claimed in claim 1 or claim 2, wherein the composition has an electrical resistivity in its high resistance state of at least 107 ohm cm. 4. A circuit protection device for protecting an electrical circuit which includes a threshold switching element formed from an amorphous composition comprising: (a) 15 to 75 atomic % selenium; (b) 10 to 65 atomic % arsenic; and (c) b 5 to 42 atomic % germanium or, if the com position contains less than 35 atomic 8 selenium, 5 to 35 atomic % germanium in which the proportions of (a), (b) and (c) (based on the total molar quantity of (a), (b) and (c)) adds up to 100%. 5. A device as claimed in claim 4, wherein the switching element is formed from a composition comprising: (a) 20 to 70 atomic % selenium; (b) 20 to 60 atomic % arsenic; and (c) 5 to 30 atomic % germanium in which the proportions of (a), (b) and (c) (based on the total molar quantity of (a), (b) and (c)) add up to 100%. 6. A device as claimed in claim 5, wherein the switching element is formed from a composition comprising: (a) 30 to 60 atomic % selenium; (b) 25 to 55 atomic % arsenic; and (c) 5 to 30 atomic % germanium in which the proportions of (a), (b) and (c) (based on the total molar quantity of (a), (b) and (c)) add up to 100%. 7. A device as claimed in claim 6, wherein the switching element is formed from a composition that comprises; (a) about 52 atomic % selenium; (b) about 34 atomic % arsenic; and (c) about 14 atomic % germanium 8. A circuit protection device for protecting an electrical circuit from a voltage transient, which comprises a switching element that is formed from an amorphous composition comprising germanium, selenium and arsenic, and a pair of electrodes in contact with the composition, the electrodes having a contact resistance with the composition that is sufficiently low to cause the device to exibit an energy to latch of at least 40 mJ. 9. A device as claimed in claim 8, which has an electrode-to-electrode resistance of not more than 1 ohm. 10. A device as claimed in claim 8 or claim 9, wherein the switching element has a composition comprising: (a) 15 to 75 atomic % selenium; (b) 10 to 65 atomic % arsenic; and (c) 5 to 42 atomic % germanium; or if the com position contains less than 35 atomic % selenium, 5 to 35 atomic % germanium, in which the proportions of (a), (b) and (c) (based on the total molar quantity of (a), (b) and (c)) add up to 100%. 11. A device as claimed in any one of claims 1 to 10, wherein the thickness of the switching element in the direction of current flow therethrough is less than 50 micrometres preferably not more than 25 micrometres. 12. A device as claimed in any one of claims 1 to 11, wherein the thickness of the switching element in the direction of current flow is at least 5 micro metres. 13. A device as claimed in any one of claims 1 to 12, wherein the cross-sectional area of the switching element in a plane normal to the direction of current flow therethrough is at least 0.5 mm2. 14. A device as claimed in any one of claims 1 to 13, wherein the crossssectional area of the switching element in a plane normal to the direction of current flow therethrough is not more than 2 mm2. 15. A device as claimed in any one of claims 1 to 14, which includes means that will exhibit a high resistance if the switching element becomes permanently electrically conductive. - 16. An electrical circuit which includes a device as claimed in any one of claims 1 to 15 connected between a current quarrying line of the circuit and earth. 17. An electrical component which includes a current carrying element, an element or terminal to be earthed, and, eleetrically connected between the elements, a device as, claimed in any one of claims 1 to 16. 18. An electrical component as claimed in claim 17 which is an electrical connector. 19. The use of an amorphous composition comprising: (a) 15 to 75 atomic % selenium; (b) 10 to 65 atomic % arsenic; and (c) 5 to 42 atomic % germanium or, if the com position contains less than 35 atomic % selenium, 5 to 35 atomic % germanium in which the proportions of (a), (b) and (c) (based on the total molar quantity of (a), (b) and (c)) add up to 100% as a threshold switching element for protecting an electrical circuit against voltage transients.

Inventors: CROFTS, DAVID, MOORE, ANTHONY JAMES, PORTER, CHRISTOPHER JOHN, VINSON, JOHN REGINALD
Applicants: RAYCHEM LIMITED
Application year: 1986
Main classification: H02H9
IPC codes: H02H9, H01L45