The present invention relates to an aerosol-generating device comprising semiconductor heaters. The invention finds particular application as an electrically operated smoking system.

One type of aerosol-generating system is an electrically operated smoking system.

Known handheld electrically operated smoking systems typically comprise an aerosol-generating device comprising a battery, control electronics and an electric heater for heating an aerosol- generating article designed specifically for use with the aerosol-generating device. In some examples, the aerosol-generating article comprises an aerosol-generating substrate, such as a tobacco rod or a tobacco plug, and the heater contained within the aerosol-generating device is inserted into or around the aerosol-generating substrate when the aerosol-generating article is inserted into the aerosol-generating device. In an alternative electrically operated smoking system, the aerosol-generating article may comprise a capsule containing an aerosol-generating substrate, such as loose tobacco.

It would be desirable to provide an aerosol-generating device providing improved control over heating of an aerosol-generating article.

According to a first aspect of the present invention there is provided an aerosol-generating device comprising an electrical power supply, a cavity for receiving an aerosol-generating article, and a plurality of semiconductor heaters positioned within the cavity. Each semiconductor heater comprises a substrate layer and a heating layer providing on the substrate layer, wherein the heating layer is a continuous layer. The aerosol-generating device further comprises a controller configured to control a supply of electrical power from the electrical power supply to each of the semiconductor heaters.

Aerosol-generating devices according to the present invention comprise a plurality of semiconductor heaters positioned within a cavity for receiving an aerosol-generating article. Advantageously, a plurality of semiconductor heaters may provide improved control over the heating of an aerosol-generating article received within the cavity. The temperature and duration of heating using each semiconductor heater may be more accurately controlled when compared to conventional resistive heaters comprising a metallic or ceramic resistive heating element.

Advantageously, providing each semiconductor heater with a continuous heating layer can simplify the manufacture of the aerosol-generating device compares to known devices in which heating elements comprise a patterned electrically conductive layer.

A plurality of semiconductor heaters may advantageously facilitate heating of discrete portions of an aerosol-generating article, which may provide an improved release of aerosol from the aerosol-generating article. Heating discrete portions of an aerosol-generating article may facilitate a user consuming a first portion of the aerosol-generating article over a first time period and consuming a second portion of the aerosol-generating article over a later second time period. Advantageously, heating discrete portions of an aerosol-generating article using a plurality of semiconductor heaters may facilitate a more accurate estimation of the level of consumption of an aerosol-generating article.

Advantageously, a plurality of semiconductor heaters may be positioned within the cavity with a geometric distribution that more closely matches the shape and size of a portion of an aerosol-generating article when compared to conventional aerosol-generating devices comprising metallic or ceramic resistive heating elements. Matching the geometric distribution of the plurality of semiconductor heaters to the shape and size of a portion of an aerosol-generating article may advantageously provide a more uniform heating of the aerosol-generating article. A more uniform heating of an aerosol-generating article may increase the total aerosol delivery from the aerosol-generating article.

Each of the plurality of semiconductor heaters may be provided on an internal surface of the cavity.

The cavity may comprise a substantially planar wall, wherein the plurality of semiconductor heaters is provided on the substantially planar wall. Advantageously, providing the plurality of semiconductor heaters on a substantially planar wall may facilitate heating of a substantially planar aerosol-generating article. Advantageously, providing the semiconductor heaters on a substantially planar wall may simplify the manufacture of the aerosol-generating device.

Preferably, each of the semiconductor heaters is substantially planar. Advantageously, providing substantially planar semiconductor heaters may simplify the manufacture of both the semiconductor heaters and the aerosol-generating device. Advantageously, substantially planar semiconductor heaters may facilitate optimised contact between each semiconductor heater and a portion of an aerosol-generating article when the aerosol-generating article is received within the cavity.

Each of the semiconductor heaters comprises a substrate layer and a heating layer provided on the substrate layer. Each heating layer may be provided on a separate substrate layer. Preferably, the plurality of semiconductor heaters comprises a common substrate layer and a plurality of heating layers spaced apart from each other and each provided on the common substrate layer, wherein each heating layer forms a semiconductor heater. Advantageously, using a common substrate layer may simplify the manufacture of the plurality of semiconductor heaters and the aerosol-generating device. A suitable material for forming the substrate layer is silicon. The substrate layer may be a silicon wafer.

Each heating layer may have a convex polygonal shape. That is, the shape of each heating layer may be such that no line segment between any two points on the boundary of the heating layer extends outside of the heating layer. Suitable shapes include circular, oval, elliptical, triangular, rectangular, square, pentagonal, and so forth. Each heating layer may comprise polycrystalline silicon. Each heating layer may comprise one or more dopants to provide the polycrystalline silicon with a desired electrical resistance. A suitable dopant is phosphorous.

Each heating layer may be provided directly on the substrate layer, so that there are no intervening layers between the heating layer and the substrate layer.

Each semiconductor heater may further comprise one or more intermediate layers provided between the heating layer and the substrate layer. Each semiconductor heater may comprise an insulating layer positioned between the heating layer and the substrate layer. In embodiments in which the plurality of semiconductor heaters comprises a common substrate layer, the insulating layer may be a common insulating layer overlying the common substrate layer and underlying a plurality of heating layers. A suitable material for forming the insulating layer is silicon nitride.

Each semiconductor heater may comprise one or more electrodes electrically connected to the heating layer. Each electrode is preferably formed from an electrically conductive material. Each electrode may be formed from at least one metal. The at least one metal may comprise copper, zinc, aluminium, silver, gold, platinum, and combinations thereof.

Each semiconductor heater may comprise a passivation layer provided on the heating layer. Advantageously, a passivation layer may prevent oxidation of the heating layer during operation of the heater. A suitable material for forming the passivation layer is silicon dioxide.

Each of the semiconductor heaters may be configured to operate at a temperature of between about 200 degrees Celsius and about 400 degrees Celsius. Each of the semiconductor heaters may be configured to operate at a voltage of between about 3 volts and about 6 volts.

Each of the semiconductor heaters may extend over a total area of less than about 7 square millimetres. In embodiments in which each of the semiconductor heaters comprises a heating layer and one or more electrical contacts provided on a common substrate layer or a common insulating layer, the heating layer and the one or more electrical contacts preferably extends over a total area of less than about 7 square millimetres. Advantageously, providing a plurality of semiconductor heaters each having a size of less than about 7 square millimetres may facilitate accurate heating of discrete portions of an aerosol-generating article.

Each of the semiconductor heaters may overlie a portion of the internal surface of the cavity, wherein the surface area of each portion of the internal surface is less than about 7 square millimetres. Advantageously, providing a plurality of semiconductor heaters each having a size of less than about 7 square millimetres may facilitate improved control over heating of an aerosol- generating article.

Each semiconductor heater may directly overlie a portion of the internal surface of the cavity having a surface area of less than about 7 square millimetres. One or more intervening layers may be positioned between each semiconductor heater and the internal surface of the cavity so that each semiconductor heater indirectly overlies a portion of the internal surface of the cavity having an area of less than about 7 square millimetres.

In embodiments in which each semiconductor heater comprises a heating layer and one or more electrodes positioned on a common substrate layer, the heating layer and the one or electrodes may overlie a portion of the common substrate layer having a surface area of less than about 7 square millimetres. That is, the heating layer and the one or more electrodes indirectly overlie a portion of the internal surface of the cavity having a surface area of less than about 7 square millimetres.

In embodiments in which each semiconductor heater comprises a heating layer and one or more electrodes positioned on a common insulating layer, the heating layer and the one or electrodes may overlie a portion of the common insulating layer having a surface area of less than about 7 square millimetres. That is, the heating layer and the one or more electrodes indirectly overlie a portion of the internal surface of the cavity having a surface area of less than about 7 square millimetres.

The aerosol-generating device may further comprise at least one gas sensor. The aerosol-generating device may comprise a plurality of gas sensors. Advantageously, a gas sensor may be used to monitor the operation of the aerosol-generating device. For example, the presence of an oxidising gas or a reducing gas may indicate the depletion of an aerosol-forming substrate from an aerosol-generating article being heated by the aerosol-generating device. The presence of an oxidising gas or a reducing gas may indicate that an aerosol-generating article is being heated by the aerosol-generating device to a temperature that is higher than the operating temperature of the aerosol-generating article.

Each gas sensor may be a semiconductor gas sensor.

Preferably, each semiconductor gas sensor is positioned proximate at least one of the semiconductor heaters. Advantageously, positioning each gas sensor proximate at least one of the semiconductor heaters may eliminate the need to provide one or more additional heaters for heating each gas sensor during operation of the gas sensor. Preferably, the controller is configured to activate each gas sensor when a semiconductor heater proximate the gas sensor is activated. The controller may be configured to monitor an amount of at least one gas using each activated semiconductor gas sensor. That is, the controller may monitor an amount of at least one gas within the cavity using each activated semiconductor gas sensor. The controller may be configured to control a supply of electrical power to the semiconductor heater proximate the activated semiconductor gas sensor in response to an amount of the at least one gas determined with the activated semiconductor gas sensor, or in response to a change in the amount of the at least one gas determined with the activated semiconductor gas sensor. For example, the controller may be configured to reduce a supply of electrical power to the semiconductor heater proximate the activated semiconductor gas sensor when the determined amount of the at least one gas increases. The controller may be configured to deactivate the semiconductor heater proximate the activated semiconductor gas sensor when an amount of the at least one gas exceeds a predetermined threshold. The controller may be configured to monitor an electrical resistance or change of electrical resistance of the sensor. The electrical resistance or change of electrical resistance of the sensor is indicative of the presence of a reducing or oxidising gas.

At least one semiconductor gas sensor may overlie one of the semiconductor heaters. That is, the semiconductor heater underlying the semiconductor gas sensor may be a gas sensor heater. The controller may be configured to control a supply of electrical power from the electrical power supply to the gas sensor heater to heat the semiconductor gas sensor. That is, the controller may be configured to control a supply of electrical power from the electrical power supply to the heating layer of the gas sensor heater. The controller may be configured to simultaneously measure the electrical resistance of the semiconductor gas sensor to determine an amount of at least one gas within the cavity. The controller may be configured to control the supply of electrical power to the gas sensor heater in response to the determined amount of the at least one gas within the cavity or a change in the determined amount of the at least one gas within the cavity. For example, the controller may be configured to reduce the supply of electrical power to the gas sensor heater when the determined amount of the at least one gas increases. The controller may be configured to terminate the supply of electrical power to the gas sensor heater when the determined amount of the at least one gas within the cavity exceeds a predetermined threshold.

Each semiconductor gas sensor may be a metal-oxide gas sensor. In one example, the gas sensor is a N-type semiconductor gas sensor, and in particular a tin-oxide gas sensor. N- type semiconductor sensors decrease in electrical resistance in the presence of a reducing gas, such as carbon monoxide (CO) or ammonia, and increase in electrical resistance in the presence of oxidizing gas, such as oxygen, nitric oxide (NO), or nitrogen dioxide (NO2). A P-type semiconductor gas sensor can also be used. P-type semiconductor gas sensors behave in the opposite manner, so they increase in electrical resistance in the presence of a reducing gas and decrease in electrical resistance in the presence of oxidizing gas.

At least one of the plurality of semiconductor heaters may be configured to function as the at least one gas sensor. The heating layer of at least one of the semiconductor heaters may be configured to function as a gas sensor. That is, the heating layer may function as a combined heating and gas sensing layer. The controller may be configured to measure at least one electrical property of the heating layer to determine the presence or absence of one or more gases. The controller may be configured to measure at least one electrical property of the heating layer to measure an amount of at least one gas. The controller may be configured to measure the electrical resistance of the heating layer. The controller may be configured to simultaneously: control a supply of electrical power from the electrical power supply to the combined heating and gas sensing layer to heat the combined heating and gas sensing layer; and measure the electrical resistance of the combined heating and gas sensing layer to determine an amount of the at least one gas within the cavity. The controller may be configured to control the supply of electrical power to the combined heating and gas sensing layer in response to the determined amount of the at least one gas within the cavity or a change in the determined amount of the at least one gas within the cavity. For example, the controller may be configured to reduce the supply of electrical power to the combined heating and gas sensing layer when the determined amount of the at least one gas increases. The controller may be configured to terminate the supply of electrical power to the combined heating and gas sensing layer when the determined amount of the at least one gas within the cavity exceeds a predetermined threshold.

Each semiconductor heater configured to function as a gas sensor may comprise one or more first electrodes electrically connected to the heating layer for supplying electrical power from the electrical power supply to the heating layer for heating the heating layer. The one or more first electrodes may comprise at least two first electrodes. Each semiconductor heater configured to function as a gas sensor may comprise one or more second electrodes electrically connected to the heating layer for measurement of at least one electrical property of the heating layer by the controller. The one or more second electrodes may comprise at least two second electrodes.

Each gas sensor may be configured to operate at a temperature of between about 200 degrees Celsius and about 400 degrees Celsius. Gas sensors, such as semiconductor gas sensors, operate by virtue of a chemical reaction that takes place when the gas directly contacts the sensor. At temperatures between about 200 degrees Celsius and about 400 degrees Celsius the sensor is more sensitive because the chemical reaction rate is increased.

In embodiments in which the aerosol-generating device comprises a plurality of gas sensors, at least two of the gas sensors may be configured to be sensitive to different gases. One sensor may be configured to detect reducing gases and another may be configured to detect oxidising gases. Both gas sensors may be sensitive to reducing gases but may be differently tuned (by changing the composition, fabrication or doping of the gas sensing layer) to be particularly sensitive to different gases. For example, one gas sensor may be tuned to sense CO while another may be tuned to be sensitive to NO2.

Preferably, the controller is configured to sequentially activate and deactivate the plurality of semiconductor heaters. That is, the controller may be configured to control the supply of electrical power from the electrical power supply to each of the semiconductor heaters sequentially. The controller may be configured to activate and deactivate the plurality of semiconductor heaters one at a time. The controller may be configured to activate the plurality of semiconductor heaters in two or more groups, wherein all of the semiconductor heaters within a group are activated at the same time. The controller may be configured to activate the next heater or group of heaters after the previous heater or group of heaters has been activated but before the previous heater or group of heaters has been deactivated.

The electrical power supply may comprise a direct current (DC) source. In preferred embodiments, the electrical power supply comprises a battery. The electrical power supply may comprise a Nickel-metal hydride battery, a Nickel cadmium battery, or a Lithium based battery, for example a Lithium-Cobalt, a Lithium-Iron-Phosphate or a Lithium-Polymer battery.

An aerosol-generating article may be received within the cavity of the aerosol-generating device so that the article and the device together form an aerosol-generating system. As described herein, providing an aerosol-generating device comprising a plurality of semiconductor heaters may facilitate providing the heaters in a geometric distribution that more closely matches the shape and size of a portion of an aerosol-generating article when compared to conventional aerosol-generating devices comprising metallic or ceramic resistive heating elements.

According to a second aspect of the present invention there is provided an aerosol- generating device comprising an electrical power supply, a cavity for receiving an aerosol- generating article, and a plurality of semiconductor heaters positioned within the cavity. The aerosol-generating device further comprises a controller configured to control a supply of electrical power from the electrical power supply to each of the semiconductor heaters.

Each of the semiconductor heaters may comprise a substrate layer and a heating layer provided on the substrate layer. Each heating layer may be a substantially continuous layer. Each heating layer may form a pattern on the substrate layer. Advantageously, providing a heating layer that forms a pattern on the substrate layer may provide a desired temperature distribution across the semiconductor heater during operation of the heater.

Aerosol-generating devices according to the second aspect of the present invention may comprise any of the optional or preferred features described herein with respect to the first aspect of the present invention.

According to a third aspect of the present invention, there is provided an aerosol- generating system comprising an aerosol-generating article and an aerosol-generating device according to the first or second aspect of the present invention, in accordance with any of the embodiments described herein. The aerosol-generating article comprises at least one aerosol- forming substrate, wherein the plurality of semiconductor heaters is configured to heat the at least one aerosol-forming substrate when the aerosol-generating article is received within the cavity.

The aerosol-generating article may comprise a base layer, wherein the at least one aerosol-forming substrate is positioned on a surface of the base layer. In embodiments in which the plurality of semiconductor heaters is provided on a substantially planar wall of the cavity, the base layer is preferably substantially planar. The at least one aerosol-forming substrate may comprise an aerosol-forming substrate configured to overlie at least two of the semiconductor heaters when the aerosol-generating article is received within the cavity. The at least one aerosol-forming substrate may be a single aerosol- forming substrate configured to overlie all of the semiconductor heaters when the aerosol- generating article is received within the cavity.

The at least one aerosol-forming substrate may comprise a plurality of aerosol-forming substrates, wherein each aerosol-forming substrate is configured to overlie at least one of the semiconductor heaters when the aerosol-generating article is received within the cavity. The number of aerosol-forming substrates may be the same as the number of semiconductor heaters, wherein each aerosol-forming substrate is configured to overlie one of the semiconductor heaters when the aerosol-generating article is received within the cavity.

In embodiments in which each of the semiconductor heaters overlies a portion of the internal surface of the cavity, wherein the surface area of each portion of the internal surface is less than about 7 square millimetres, preferably each of the aerosol-forming substrates is positioned on a surface of the base layer, wherein the surface area of each portion of the surface of the base layer is less than about 7 square millimetres. Advantageously, providing a plurality of aerosol-forming substrates each overlying a surface area of less than about 7 square millimetres may facilitate uniform heating of each aerosol-forming substrate by the corresponding semiconductor heater.

According to a fourth aspect of the present invention, there is provided an aerosol- generating article comprising a base layer and a plurality of aerosol-forming substrates positioned on a surface of the base layer. Each aerosol-forming substrate overlies a portion of the surface of the base layer, wherein the surface area of each portion of the surface of the base layer is less than about 7 square millimetres. Preferably, the base layer is substantially planar.

The following optional and preferred features of the aerosol-generating device apply to the third aspect of the present invention. The following optional and preferred features of the aerosol- generating article apply to both the third and fourth aspects of the present invention.

Preferably, the aerosol-generating article comprises a removable cover layer overlying and secured to the base layer so that the one or more aerosol-forming substrates are sealed between the removable cover layer and the base layer. The removable cover layer may comprise a non-porous polymeric film.

In embodiments in which the aerosol-generating article comprises a plurality of aerosol- forming substrates, the plurality of aerosol-forming substrates may comprises a plurality of first aerosol-forming substrates positioned on the base layer and a plurality of second aerosol-forming substrates positioned on the base layer, wherein the second aerosol-forming substrates are different from the first aerosol-forming substrates. The controller may be configured to sequentially activate the plurality of semiconductor heaters so that the first aerosol-forming substrates are heated separately from the second aerosol-forming substrates. The controller may be configured to activate the plurality of semiconductor heaters so that at least some of the first aerosol-forming substrates are heated simultaneously with at least some of the second aerosol-forming substrates.

Both the first and second aerosol-forming substrates may each comprise a solid aerosol- forming substrate. Both the first and second aerosol-forming substrates may each comprise a liquid aerosol-forming substrate. Each of the first aerosol-forming substrates may comprise a solid aerosol-forming substrate and each of the second aerosol-forming substrates may comprise a liquid aerosol-forming substrate. Each of the first aerosol-forming substrates may comprise a liquid aerosol-forming substrate and each of the second aerosol-forming substrates may comprise a solid aerosol-forming substrate.

In embodiments in which at least one of the first and second aerosol-forming substrates comprises a liquid aerosol-forming substrate, each of the aerosol-forming substrates may comprise a porous substrate material positioned on the base layer and the liquid aerosol-forming substrate sorbed onto the porous substrate material. Preferably, the porous substrate material has a density of between about 0.1 grams/cubic centimetre and about 0.3 grams/cubic centimetre.

Preferably, the porous substrate material has a porosity of between about 15 percent and about 55 percent.

The porous substrate material may comprise one or more of glass, cellulose, ceramic, stainless steel, aluminium, polyethylene (PE), polypropylene, polyethylene terephthalate (PET), poly(cyclohexanedimethylene terephthalate) (PCT), polybutylene terephthalate (PBT), polytetrafluoroethylene (PTFE), expanded polytetrafluoroethylene (ePTFE), and BAREX ^® .

Preferably, the porous substrate material is chemically inert with respect to the liquid aerosol-forming substrate.

In embodiments comprising at least one solid aerosol-forming substrate, the solid aerosol- forming substrate may comprise tobacco. The solid aerosol-forming substrate may comprise a tobacco-containing material containing volatile tobacco flavour compounds which are released from the substrate upon heating. The solid aerosol-forming substrate may comprise a non- tobacco material. The solid aerosol-forming substrate may comprise tobacco-containing material and non-tobacco containing material.

The solid aerosol-forming substrate may include at least one aerosol-former. Suitable aerosol-formers include, but are not limited to: polyhydric alcohols, such as propylene glycol, triethylene glycol, 1 ,3-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate Preferred aerosol formers are polyhydric alcohols or mixtures thereof, such as propylene glycol, triethylene glycol, 1 ,3-butanediol and, most preferred, glycerine.

The solid aerosol-forming substrate may comprise a single aerosol former. Alternatively, the solid aerosol-forming substrate may comprise a combination of two or more aerosol formers.

The solid aerosol-forming substrate may have an aerosol former content of greater than 5 percent on a dry weight basis.

The solid aerosol-forming substrate may have an aerosol former content of between approximately 5 percent and approximately 30 percent on a dry weight basis.

The solid aerosol-forming substrate may have an aerosol former content of approximately 20 percent on a dry weight basis.

In embodiments comprising at least one liquid aerosol-forming substrate, the liquid aerosol-forming substrate may comprise a nicotine solution. The liquid aerosol-forming substrate preferably comprises a tobacco-containing material comprising volatile tobacco flavour compounds which are released from the liquid upon heating. The liquid aerosol-forming substrate may comprise a non-tobacco material. The liquid aerosol-forming substrate may include water, solvents, ethanol, plant extracts and natural or artificial flavours. Preferably, the liquid aerosol- forming substrate further comprises an aerosol former.

In embodiments in which the aerosol-generating article comprises a plurality of first aerosol-forming substrates and a plurality of second aerosol-forming substrates, the plurality of first aerosol-forming substrates may each comprise a nicotine solution and the plurality of second aerosol-forming substrates may each comprise an acid.

The acid may comprise an organic acid or an inorganic acid.

Preferably, the acid comprises an organic acid, more preferably a carboxylic acid, most preferably an alpha-keto or 2-oxo acid or lactic acid.

Advantageously, the acid comprises an acid selected from the group consisting of 3- methyl-2-oxopentanoic acid, pyruvic acid, 2-oxopentanoic acid, 4-methyl-2-oxopentanoic acid, 3- methyl-2-oxobutanoic acid, 2-oxooctanoic acid, lactic acid and combinations thereof. Advantageously, the acid comprises pyruvic acid or lactic acid. More advantageously, the acid comprises lactic acid.

In embodiments in which the aerosol-generating article comprises a plurality of first aerosol-forming substrates each comprises a nicotine solution and a plurality of second aerosol- forming substrates each comprising an acid, preferably the controller is configured to activate the plurality of semiconductor heaters so that at least some of the first aerosol-forming substrates are heated simultaneously with at least some of the second aerosol-forming substrates. Advantageously, simultaneously heating the liquid nicotine solution and the acid may generate a nicotine aerosol and an acid aerosol that react in the gas phase to form an aerosol comprising nicotine salt particles. At least one of the aerosol-forming substrates may comprise a flavourant. Suitable flavourants include, but are not limited to, menthol.

The invention is further described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a cross-sectional view of an aerosol-generating device according to an embodiment of the present invention;

Figure 2 shows a top view of the plurality of semiconductor heaters of the aerosol- generating device of Figure 1 ;

Figure 3 shows a detailed cross-sectional view of one of the semiconductor heaters of the aerosol-generating device of Figure 1 ;

Figure 4 shows a perspective view of an aerosol-generating article according to a first embodiment of the present invention;

Figure 5 shows a perspective view of an aerosol-generating article according to a second embodiment of the present invention; and

Figure 6 shows a cross-sectional view of the aerosol-generating article of Figure 5 combined with the aerosol-generating device of Figure 1 to form an aerosol-generating system.

Figure 1 shows a cross-sectional view of an aerosol-generating device 10 according to an embodiment of the present invention. The aerosol-generating device comprises a housing 12 defining a cavity 14 for receiving an aerosol-generating article. An air inlet 16 is provided at an upstream end of the cavity 14 and a mouthpiece 18 is provided at a downstream end of the housing 12. An air outlet 20 is provided in the mouthpiece 18 in fluid communication with the cavity 14 so that an airflow path is defined through the cavity 14 between the air inlet 16 and the air outlet 20. During use, a user draws on the mouthpiece 18 to draw air into the cavity 14 through the air inlet 16 and out of the cavity 14 through the air outlet 20.

The aerosol-generating device 10 further comprises a plurality of semiconductor heaters

22 provided on a planar wall 24 of the cavity 14. Each of the semiconductor heaters 22 comprises a heater package 26 provided on a common support layer 28. The plurality of semiconductor heaters 22 form a heater array 30, which is shown more clearly in Figure 2.

Figure 3 shows a cross-sectional view of an individual semiconductor heater 22. Each semiconductor heater 22 comprises a heater package 26 provided on a common support layer 28. The common support layer 28 comprises a common substrate layer 32 and a common insulating layer 34 overlying the common substrate layer 32. The common substrate layer 32 is a silicon wafer and the common insulating layer comprises silicon nitride.

Each heater package 26 comprises a heating layer 36 overlying a portion of the common insulating layer 34 and at least two electrodes 38 electrically connected to the heating layer 36. Each heater package 26 overlies a portion of the common insulating layer 34 have a surface area of less than about 7 square millimetres. The heating layer 36 comprises polycrystalline silicone doped with phosphorous to provide the heating layer 36 with a desired electrical resistance. The electrodes 38 comprise a metal, such as platinum.

The aerosol-generating device 10 further comprises an electrical power supply 40 and a controller 42 positioned within the housing 12. During operation of the aerosol-generating device 10, the controller 42 controls a supply of electrical current from the electrical power supply 40 to each semiconductor heater 22 via the corresponding electrodes 38 to activate the semiconductor heater 22. The controller 42 is configured to activate the plurality of semiconductor heaters 22 in groups, with each group being activated and deactivated sequentially.

The controller 42 is further configured to measure and monitor the electrical resistance of the heating layer 36 of each semiconductor heater 22 when the heater is activated to measure and monitor an amount of at least one gas. In this way, each semiconductor heater 22 also functions as a gas sensor. For example, each heating layer 36 may be sensitive to a gas that is generated when an aerosol-forming substrate on an aerosol-generating article is heated to a temperature above the operating temperature of the aerosol-generating article. In this scenario, the controller may be configured to deactivate the semiconductor heater 22 when the measured electrical resistance of the heating layer 36 of the semiconductor heater 22 is indicative of the presence of the gas.

Figure 4 shows an aerosol-generating article 50 according to a first embodiment of the invention. The aerosol-generating article 50 comprises a base layer 52 and an aerosol-forming substrate 54 provided on the base layer 52. The aerosol-forming substrate 54 comprises a substantially continuous layer of a solid tobacco-containing material. A removable cover layer 56 is secured to the base layer 52 to seal the aerosol-forming substrate 54 between the base layer 52 and the removable cover layer 56. The removable cover layer is formed from a non-porous polymeric film.

During use, the removable cover layer 56 is removed from the base layer 52 and the aerosol-generating article 50 is inserted into the cavity 14 of the aerosol-generating device 10 shown in Figure 1 to form an aerosol-generating system. The controller 42 then sequentially activates and deactivates groups of the semiconductor heaters 22 to sequentially heat discrete portions of the aerosol-forming substrate 54.

Figure 5 shows an aerosol-generating article 60 according to a second embodiment of the invention. The aerosol-generating article 60 comprises a base layer 52 and a cover layer 56 identical to the base layer 52 and the cover layer 56 of the aerosol-generating article 50 shown in Figure 4. However, the aerosol-generating article 60 comprises a plurality of discrete aerosol- forming substrates 64 positioned on the base layer 52 and sealed between the base layer 52 and the cover layer 56. Each of the aerosol-forming substrates 64 comprises a porous substrate material and a liquid aerosol-forming substrate sorbed onto the porous substrate material. Each of the aerosol-forming substrates 64 overlies a portion of the base layer 52 having a surface area of less than about 7 square millimetres.

The plurality of aerosol-forming substrates 64 is divided into three groups: a plurality of first aerosol-forming substrates 68 each comprising a liquid nicotine solution; a plurality of second aerosol-forming substrates 70 each comprising a volatile acid; and a plurality of third aerosol- forming substrates 72 each comprising a flavourant.

During use, the removable cover layer 56 is removed from the base layer 52 and the aerosol-generating article 60 is inserted into the cavity 14 of the aerosol-generating device 10 shown in Figure 1 to form an aerosol-generating system 80, as shown in Figure 6. The arrangement of the aerosol-forming substrates 64 is such that each aerosol-forming substrate 64 overlies a semiconductor heater 22 when the aerosol-generating article 60 is received within the cavity 14.

The controller 42 then sequentially activates and deactivates groups of the semiconductor heaters 22 to sequentially heat the discrete aerosol-forming substrates 64. At each stage of the sequential activation, the controller 42 activates the appropriate semiconductor heaters 22 to simultaneously heat one of the first aerosol-forming substrates 68, one of the second aerosol- forming substrates 70 and one of the third aerosol-forming substrates 72. The nicotine vapour released from the heated first aerosol-forming substrate 68 and the acid vapour released from the heated second aerosol-forming substrate 70 react in the gas phase to form an aerosol comprising nicotine salt particles for delivery to the user through the air outlet 20. The flavourant released from the heated third aerosol-forming substrate 72 imparts a flavour to the aerosol delivered to the user.