The present invention relates to an integrated sensor device. In particular, the present invention relates to a hybrid form of semiconductor devices combining a field effect transistor with a bipolar junction transistor, the field effect transistor being connected to a sensing electrode by a plurality of contacts/vias and metal layers.
Over the recent years a growing interest has been seen in the area of highly sensitive semiconductor devices that can be used for charge detection in liquids (e.g. ion sensitive field-effect transistor (ISFET)-type for hydrogen ions, as applied in the determination of pH-concentrations, detection of biomolecules, studies of DNA replication and genome sequences, etc.), or for the detection of ions or polarisable molecules in gaseous mixtures. Hybrid forms of such semiconductor devices have previously been proposed and demonstrated (S-R. Chang et al. Sensors 9, 2009, pp. 8336-8348 and H. Yuan et al. Biosensors and Bioelectronics 28, 2011, pp. 434-437), Ref. 1-2. These hybrids combine in a single integrated circuit an ISFET charge-sensitive device in parallel with a lateral bipolar junction transistor (LBJT). Such hybrid devices beneficially combine the high sensitivity of the ISFET with the additional amplification provided by the LBJT.
The term “ISFET” employed herein includes various equivalent forms of such devices (P. Bergveld, Sensors and Actuators B 88, 2003, pp. 1-20), see also Ref. 3-6. For example, the conductive gate electrode may be of metal or other suitable conductive material such as highly doped polycrystalline silicon. Similarly, the gate insulating material may be an oxide, such as silicon dioxide, but may as well comprise oxynitride or even high-k dielectrics.
Arrangements are known where the gate-electrode is brought in contact with the gas or liquid to be analysed via a consecutive arrangement of contacts/metal-lines and vias/metal-lines encapsulated by suitable passivation materials, as for example silicon oxide, silicon nitride or a sandwich of oxide/nitride. The metal film in the electrode in closest proximity to the liquid or gas may or may not be enclosed by said passivation. The metal itself may be a standard metal used in the manufacturing of semiconductor devices, such as aluminium, palladium, platinum or gold. The electrode may alternatively be in some form of metal/metal-oxide (Al2O3, Ta2O5, HfO2).
Disclosed in U.S. Pat. No. 8,283,736 is an ion sensing device constituting an ISFET connected to a lateral bipolar device integrated on same semiconductor chip. As disclosed, a pchannel ISFET, is located in an n-well with a lateral pnp bipolar transistor connected in parallel with emitter/source and drain/collector, respectively, in common and with a separate base connection. When an appropriate amount of ions are accumulated (or depleted) at the gate electrode, as a result of bias applied to the reference electrode immersed in the electrolyte, the channel conduction in the ISFET is altered. That, in turn, affects the conduction of the lateral bipolar device.
A drawback of this particular architecture is the parallel arrangement of the two devices, which requires an additional terminal. To this comes the inherently low gain of the gated lateral bipolar transistor. At a bias lower than the turn-on voltage of the bipolar device, the sub-threshold characteristics resemble those of the metal-oxide-semiconductor field-effect transistor (MOSFET) device, i.e. ISFET in this context. The transconductance enhancement obtained in the hybrid configuration primarily occurs above the threshold voltage of the MOSFET. As a consequence, the amplification of this structure will be very low.
The parasitic vertical pnp-transistor indicated in the referenced patent, is common to all n-well based CMOS processes. It is formed by the source/emitter of the ISFET, the externally connected n-well base with the p-type substrate as collector. It is not part of the sensing device, since a conductivity change in the ISFET does not influence said parasitic component. Active use of the parasitic vertical pnp-transistor could eventually cause reliability problems, e.g. latch-up. A further concern is the requirement of additional terminals for external connection of the individual devices in the desired configuration. Such additional wiring is likely to introduce unwanted signal noise.
U.S. Pat. No. 5,126,806 describes a lateral insulated gate bipolar transistor (IGBT), which is particularly well suited for high power switching applications. Disclosed is an enhancement-MOSFET device having its source and drain electrodes connected to the base and emitter, respectively, of a lateral bipolar transistor. When an appropriate gate input voltage, here in the form of a positive charge, is applied to the MOSFET, the channel conducts, thus biasing the bipolar transistor into conduction. The applied charge on the gate electrode can be used to control a large current through the bipolar device, which is of particular interest in power applications. Safe switching operation at high voltages, however, requires a very wide base and a low gain in the bipolar transistor. Various forms of said devices have been integrated in modern CMOS processes as described by Bakeroot et al. in IEEE EDL-28, pp. 416-418, 2007, Ref. 7. Relevant in this context is also a report by E. Kho Ching Tee entitled “A review of techniques used in Lateral Insulated Gate Bipolar Transistor (LIGBT)” in Journal of Electrical and Electronics Engineering, vol. 3, pp. 35-52, 2012, Ref. 8. While this type of device is potentially quite useful for various forms of power switching, with its requirements of high voltage capability and low internal gain, it is disadvantageous for a device incorporated in a circuit intended for charge detection (of particularly hydrogen ions) in liquids or gaseous mixtures.
A prior-art ISFET-gated LBJT is described with reference to
The gated LBJT 10 has an enclosed gate electrode 18, between p+-doped regions, on top of a gate dielectric layer 17. In addition, a base contact 14 is formed in an n+-doped region in the n-well 12. Likewise, a p+-doped region 16, outside the n-well is provided as substrate contact.
The gate electrode 18 and the p+-doped regions on adjacent sides, which function as source/drain contacts, constitute a p-type MOSFET device.
The floating gate electrode 18 is electrically connected by a plurality of contacts/vias and metal layers 21 to a hydrogen ion sensing electrode 19 above the gated LBJT. The surface of the sensing electrode 19 is in contact with an ion-containing solution 22 to which a reference gate-electrode 20 is attached.
In the prior art of
The MOSFET source region and the bipolar emitter region are likewise connected since they are formed by the same p+-type semiconductor region 13. In the particular structure depicted in
Referring now to
Applying proper bias to the source/drain terminals of the MOSFET and to the reference electrode will result in a lateral current in the MOSFET device.
Forward biasing of the emitter-base junction will add a lateral current, which is picked up by the lateral collector ring (15) in
Any change in the reference potential will affect both the MOSFET current as well as the current passing through the bipolar device(s).
For the described prior art device, any change of potential or charge in the electrolyte part is primarily sensed by the parallel arrangement 5 of the MOSFET transistor and the lateral pnp-bipolar transistor.
The fact that the active layers are shared between the p-type MOSFET and the lateral pnp-transistor, respectively, leads to a non-optimised low current-gain pnp-transistor.
In addition, the substrate current from the vertical parasitic pnp-transistor 6 is disadvantageous from a device isolation point and does not provide information with respect to changes in the electrolyte part.
When a positive potential is applied to the gate electrode 45, the conductivity of a surface portion of the p-region 50 under the gate dielectric 40 is inverted to form an n-type channel. Electrons from the n+-region 60 can then pass through the channel, into the n-layer 35 and on to the p+-region 70 from which positive holes are injected. Thereby the n-layer 35, having a high resistivity, is conductivity-modulated to provide a low resistance path between the anode (C) and cathode (E) in
Numerous modifications of the above described embodiment, with emphasis on improved switching performance, exist, some of which are covered in a report by E. Kho Ching Tee, Ref. 8.
A vertical parasitic npn-transistor that has its base connected to the collector of the lateral pnp-transistor is included in
Obviously an improved sensor device based on hybrid form of semiconductor devices combining a field effect transistor with a bipolar junction transistor is needed, with reference to
The object of the present invention is to provide a highly sensitive integrated sensor device, with internal amplification, that can be used for charge detection in liquids or for the detection of ions or polarisable molecules in gaseous mixtures. This is achieved by the device as defined in claim 1.
The present innovation provides a floating gate MOSFET intimately merged with a vertical bipolar junction transistor, BJT, and characterised by having a very high internal amplification, a high signal-to-noise ratio and operating at low supply voltages. In the preferred embodiment, the device can be realised in a standard low-voltage CMOS process as provided by semiconductor foundries worldwide.
The integrated sensor according to the present invention comprises a lateral Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and a Bipolar Junction Transistor (BJT) arranged in a semiconductor substrate wherein the MOSFET is connected in series to the base of the vertical BJT. The MOSFET is arranged to be connected at the semiconductor substrate surface and the emitter of the BJT is located at the semiconductor substrate surface. The drain-drift region of said MOSFET is part of the base-region of the BJT within the semiconductor substrate, the drain-drift region thus making electrical contact to the base of the BJT. The distance from the drain-drift-region of the MOSFET to the emitter of the BJT exceeds the vertical distance between the emitter and any buried layer serving as collector. Hence, the breakdown voltage of the device is determined by the BVCEO of the vertical BJT.
The sensor device according to one embodiment of the invention further comprises an ion-sensitive electrode electrically connected by a plurality of contacts/vias and metal layer(s) to the floating gate-electrode. The surface of said ion-sensing electrode is in contact with an ion-containing solution to which a reference gate-electrode is attached, wherein electrodes are arranged in ohmic contact with a third region in contact with the buried layer serving as collector of the BJT, a fourth region serving as an emitter of the BJT, a fifth region serving as the source of the MOSFET, and a sixth region providing ohmic contact to the third region regions for the purpose of external device connection and signal extraction.
The integrated sensor device according to a preferred embodiment comprises:
According to one embodiment an MOSFET is provided, with its drain connected in common to the extrinsic base of a vertical BJT as described above, wherein the MOSFET is of n-type and said BJT is of pnp-type.
The integrated sensor has a broad range of applications at the molecular level, such as, but not limited to, medical diagnostics devices, environmental and bioprocess analysis devices and food processing and chemical process monitoring devices.
Areas of application should however not be limited to those listed above since it is obvious to those skilled in the art that the proposed device, without an ion sensing electrode, can be used as an amplifier in many types of electronic circuits.
One advantage of the present invention is that the sensor device can be built from methods and means well established within the microelectronics field. The manufacturing costs will therefore correspond to what is to be expected for standard integrated circuits. Furthermore, the design only has such features that make sensing chips consisting of a multitude of said MOSFET/BJT devices easily manufacturable at facilities already commercially available.
A maximum signal-to-noise output can only be obtained if amplification is applied as close to the signal source as possible. In the present invention this is achieved automatically in that the first amplification stage is merged with the sensor itself.
The high gain and excellent signal-to-noise properties of the invention obviate the need for heretofore costly sample enrichments by providing sensing chips for which the manufacturing costs are still held at the low level typical of IC manufacturing.
The invention will now be described in detail with reference to the drawing figures, wherein:
The function of the floating gate structure can be illustrated by the following application example, where DNA strands of a known base-sequence are immobilized to a desired surface density on the bottom metal surface of the electrolyte vessel in
The type and shape of the electrolyte vessel shown in the Figures is only intended as an example, the electrode in the bottom of the vessel and its role in gating the MOSFET being the important aspects for the current context. Actual forms of the sensing part are intimately related to respective application and clear to those versed in respective field. Furthermore, it should be clear to those versed in the field of semiconductor technology that the electrolyte vessel and/or any individual compartment(s) holding the molecules in place above the sensing element(s) can be designed with dimensions in a range from macro to micro. Practical limits in the micro range are set by the design requirements arising from the state-of-the-art of semiconductor manufacturing technology. One consequence of this is that the vessel-sensor assembly can be repeated in amounts numbering millions on a single semiconductor chip. The Reference Electrode can also be designed in a multitude of ways, as known by those versed in the field. One example, relevant to the invention, is a silver film placed on the surface of the semiconductor chip itself and appropriately converted to silver chloride. In this context it is worth noticing that the floating extended gate structure in
The functionality of the inventive device is outlined with references to
The electronic sensor according to the invention can be adapted for detecting various charged and/or polarized substances in a variety of applications, examples including, but not being limited to, the liquid detection of ions (e.g. H+, Na+, Ca++) for biomedical and food quality monitoring applications, detection of biomolecules by the arrangement outlined above, as well as gas monitoring applications. For the latter, a sample vessel is not a necessity, but surface functionalisation is still a key step in order to attain selectivity and specificity in sensing.
The emitter, base and collector of the vertical BJT, that is part of the sensor device, are built up by means of a vertical stacking of laterally extending doped layers on a semiconductor substrate. The base layer of the BJT has a vertically extending portion that reaches the surface next to the emitter and forms the drain of the laterally oriented MOSFET. Proceeding along the surface, the drain is followed by the channel and the source regions of the MOSFET. The collector region of the vertical BJT is located below the base region, where it forms a lateral band. The collector is of a conductivity type opposite to that of both the base and the substrate so as to form the necessary junctions. Above, and in direct contact with, outer parts of the lateral collector band is a well of same conductivity type. The well is thus adjacent to the base region and extends laterally along the surface so as to allow for a connection to the collector region.
Preferably, the device is constructed in such a way that it has mirror symmetry vis-à-vis an imaginary vertical plane passing through the emitter region of the BJT and perpendicular to the plane of the paper, thereby providing a double combined MOSFET/BJT.
In
The integrated sensor device 100 in
Starting from the bottom in
An oxide isolation 119, stretching from the surface approximately 0.3 μm into the p-well 125, encloses the emitter region 145. Here, the p-region 125 acts as the base, the n-band 120 as the collector and the n+ region 145 as the emitter in the vertical BJT.
An n-region 130, forming an n-well, with a typical thickness in the order of 1 μm that is vertically in contact with the n-band and stretches to the surface is formed.
A gate structure comprising a gate electrode 156, a gate oxide 157 and insulators 158, is formed on top of the surface at the border between the n-region 130 and the p-region 125. Said gate electrode 156 and gate oxide 157 stretches across the border formed by the n-region 130 and the p-region 125. The insulators 158 providing insulation from contact metal layer 150. Alternatively, the gate oxide 157 consists of another type of dielectric material, such as so called “high-k dielectrics”, for example hafnium or zirconium oxides or silicates.
Said n-region 130 extends in the lateral direction under the MOSFET and has a portion 131 extending in the vertical direction towards the top surface of the device. The surface of the vertical portion 131 of the n-region 130 also forms the n-type doped channel region of the p-type MOSFET.
A portion 126 of the p-region 125 extends in the vertical direction towards the surface of the device from a level below that of the oxide isolation 119 until adjacent to the corresponding portion of the n-region 131. Said vertical portion 126 of the p-region 125 forms the drain-drift-region of the MOSFET.
Moreover there is provided a p+-doped drain region 141, adjacent to the gate electrode 156 of the MOSFET, extending from the surface and thereinto approximately 0.2 μm with a typical surface concentration in the range of 1·1019 to 1·102C cm−3. Said p+-drain region 141 is located within part of the p-region 125 serving as low ohmic shunt to a part of said drain-drift-region 126.
The oxide isolation 119 that separates the p+-doped drain region 141 in the p-region 125 from the n+-region 145, the latter being the emitter of the BJT, will improve the characteristics of the emitter-base/drain diode. The addition of the p+-doped drain region 141, being adjacent to the oxide ring, will likewise improve the base resistance and the performance of the BJT device.
At least partly enclosed by the n-region 130 is a contacting n+-region 135 extending from the surface and thereinto approximately 0.2 μm and with a typical surface concentration in the range of 1·1019 to 1·102C cm−3. The n-region 130 with its n+-doped contact region 135 serves as the body of the p-type MOSFET. The n+ contact region 135 is separated by an oxide isolation region 117, stretching from the surface approximately 0.3 μm into the n-region 130, from the p+-doped source region 140 of the MOSFET. Said p+-source region being formed within the n-type region 130 is extending from the surface and thereinto approximately 0.2 μm and has a typical surface concentration in the range of 1·1019 to 1·102C cm−3. A metal layer 150 connects to the contact n+-region 135 as well as to the p+-source region 140, thus forming a combined body/source connection.
The gate structure can connect with a sample vessel 160, adapted to hold, for example, an electrolyte 161 and functionalized bio-molecules 162. A reference electrode 170 is immersed in the electrolyte in sample vessel 160. The sample vessel 160 may be connected to the gate 156 via a metal layer 163 at its bottom, an intermediate dielectric layer 164, e.g. a Si3N4-layer and one or several sandwiched metal layers 165. Alternatively, if the electronic sensor is adapted to detect for example gases, the sample vessel and its bottom metal layer are replaced with an appropriately functionalized surface.
As indicated in
In
In general, the structure of the floating base composite BJT-MOSFET charge-sensitive devices 101 and 201 in
In
The structure of the floating base composite BJT-MOSFET charge-sensitive device 301 in
An optional addition of a second n-region 127 extending from and in electrical contact with the n-band layer 120 stretching towards the emitter n+-region 145 allows dopant profile tailoring and device optimization of the BJT see
Referring now to
The MOSFET device 401 thus appears in series with the base of the vertical npn-BJT 402, the n-type emitter region 145 of the latter being at the semiconductor surface.
It is likewise observed that a vertical (parasitic) pnp-transistor 403 is formed by the p-type region 125 serving as emitter, the n-band 120 acting as base and the p-type substrate 115 functioning as global collector. To reduce the current gain of said parasitic transistor so as to avoid latch-up, a high doping level and a large layer thickness are used in realization of the n-band.
In this configuration, the channel current generated by the MOSFET 401 is also the base current in the BJT 402. The signal on the gate can therefore, by means of the channel current, be amplified in the BJT 402 and therefore emerge from the sensor in the form of a strong collector current response at the collector terminal. By operating the p-type MOSFET 401 in the sub-threshold regime for best sensitivity and linearity, a change in the surface potential of the floating gate, as caused by, e.g., charged molecules at the bottom of sample vessel 160, will lead to a corresponding change in the amplified collector current. The operating point of the MOSFET is set by the application of a bias between the reference electrode and the source terminal of the MOSFET in
A plurality of above described, floating base composite BJT-MOSFET charge-sensitive devices 101, 201 and 301 on same substrate 115 can be connected in parallel, each device including an ISFET directly merged to a BJT and a floating gate 156 connected via a plurality of contacts/vias and metal layers 165 to an electrode 163 in contact with the electrolyte 161, so as to improve sensitivity.
Likewise a plurality of said integrated sensor devices 101, 201 and 301 on same substrate 115 can be connected via a plurality of contacts/vias and metal layers 165 in an array configuration with each floating gate connected to an individual electrode 163 in contact with the electrolyte 161, so as to sequentially monitor the collector current of each individual integrated sensor device.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/SE2014/050743 | 6/17/2014 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
61837383 | Jun 2013 | US |