Biosensor

Abstract

A sample sensor (100) has a AIGaN / GaN high electron mobility transistor (HEMT) (102) comprising a semiconductor portion comprising an AIGaN / GaN heterojunction, a source (108) a drain (114) and a gate comprising a gate foot (140) with a functionalised gate head detection surface thereon, the gate head being offset from the source and drain, and the gate head with the patterned detection surface spans over the entire device area, wherein the detection surface has a capture reagent provided thereon, wherein the capture reagent binds or otherwise associates with a target antigen or analyte.

Description

BiosensorTechnical Field

[0001] The present invention relates to a biosensor comprising a semiconductor device (e.g., a diode or a transistor). More specifically, the present invention relates to a biosensor utilising a GaN-based high electron mobility transistor (HEMT). This novel design can also be used in other semiconductor based sensors to achieve higher sensitivities.Background Art

[0002] The twenty-first century has witnessed the outbreak of highly contagious and globally transmitted diseases that threaten human civilization. The demand of POCT (Point of Care Test) devices as efficient diagnostic tools today stand the highest and is increasing exponentially. The biosensor market is forecasted to reach $31.5 billion by 2024. The methods that can be relied upon for detection falls in 4 main categories:(1 ). ELISA tests (Enzyme Linked Immunosorbant Assay): This method even though used widely, is time consuming, leads to false detections and requires expertise with laboratory set up.(2). Spectrometric Techniques: This has the limitations of poor detection limits, faulty results due to interference, not yet standardized.(3). Fluorescence detection: Involves a lot of preprocessing, hence not suitable for rapid detection.(4). Electrochemical sensors: Even though they provide fast responses and high sensitivities performance drift is observed due to instability in solution.

[0003] Recent decades have seen the emergence of Field Effect Transistors (FETs) as electrochemical sensors where the electric signals generated through the chemical reaction or binding get amplified by the device gain leading to higher sensitivities and lower limits of detection. In such devices, molecules are detected when in contact with reagents on the gate terminal.

[0004] Such FET based sensors face limitations due to chemical degradation and they do not give reliable detection in solution especially when Si based. The problems faced by conventional electrochemical sensors can be overcome using wide bandgapbiocompatible materials like GaN. In addition to the inherent chemical stability of the material, the internal gain when AIGaN / GaN heterojunction-based transistors can provide is extremely high. This gain is combined with its high sensitivity to surface charges or reactions makes the device the most promising candidate for biosensing applications.

[0005] This type of device has gained the device huge research attention; however, they have a few limitations that needs to be addressed to make them suitable for clinical applications.

[0006] Cheng et ah discloses a Prostate specific antigen detection method using AIGaN / GaN HEMT. The sensors disclosed therein are open gated, which provides poor stability and limited surface area for bio immobilisation.

[0007] Kang et al2discloses an Au gated sensor, but with a very short gate, hence poor sensitivity due to limited surface area.

[0008] Yang et aP discloses a HEMT sensor with a disposable gate. This has a large surface area, but losses are high.

[0009] US 8,828,713 B2 discloses a sensor utilising a HEMT. In this device, a capture reagent is applied to the Au-containing gate. The reagent is configured to binds or otherwise associates with a cancer antigen. The source is connected to a fixed bias voltage, and the current from source to drain measured to determine the presence of the antigen. Sensitivity is poor.

[0010] The basic limitation that hinders research is the limited gate area and contaminations and short circuits created when the solution of interest is dropped on to the sensor surface. Solutions put forward to solve these limitations are using a gate pulsed structure or using a disposable gate.

[0011] However, both these solutions increase the design complexity and subsequent modifications. Further, using a disposable gate reduces the proximity of the channel 2DEG1Jia-dong Li, Jun-jie Cheng, Bin Miao, Xiao-wei Wei, Jie Xie, Jin-cheng Zhang, Zhi-qiang Zhang and Dong-min Wu, 2014. Detection of prostate-specific antigen with biomolecule-gated AIGaN / GaN high electron mobility transistors. J. Micromech. Microeng. 24 (2014) 075023 (6pp)2B. S. Kang, H. T. Wang, T. P. Lele, et al., 2007. Prostate specific antigen detection using AIGaN / GaN high electron mobility transistors. Appl. Phys. Lett. 91 , 1 12106 (2007).3Yang, S., Gu, L., Ding, X., Miao, B., Gu, Z., Yang, L., Li, J. and Wu, D., 2018. Disposable gate AIGaN / GaN high-electron-mobility sensor for trace-level biological detection. IEEE Electron Device Letters, 39(10), pp.1592-1595.(Two-Dimensional Electron Gas) from the sensing surface and as such leads to lower sensitivities and resolutions. The interconnects used can also lead to noise interference.

[0012] Another advancement seen in increasing the sensitivity and resolution is the use of nanoparticles to increase the surface to volume ratio as the gate area available for binding / detection is very small.

[0013] It is an aim of the present invention to overcome, or at least mitigate, the above problems.Summary of InventionAccording to a first aspect of the present invention there is provided a sample sensor comprising: a semiconductor device comprising: a semiconductor junction having an active surface; at least two contacts on the active surface, one of which comprises a head and a foot, the foot being in contact with the active surface, wherein the head comprises a detection surface having a capture reagent provided thereon, wherein the capture reagent binds or otherwise associates with a target antigen or analyte.

[0014] Advantageously, the present invention offers a solution that has a larger detection surface area for functionalization and detection. Using this technique helps in increasing the sensitivities and reduced signal losses as the patterned gate has a wider head with a narrow foot and is not separated from the sensor.

[0015] Reference herein to a capture reagent refers to any substance or reagent that can bind, reversibly or irreversibly, to the target analyte or antigen. In this manner, as will be appreciated by those skilled in the art, a sample having, or suspected of having, the target analyte or antigen which when added to the sensor is specifically recognized and bound by the capture reagent, such that it can be used to identify and / or quantitate the analyte

[0016] In a preferred embodiment, said capture reagent includes, but is not limited to, antibodies, aptamers and fragments thereof, oligonucleotides or other specific ligands or receptors comprising a specific binding partner for the target analyte or antigen.

[0017] Reference herein to an 'analyte' refers to any target of interest including, but not limited to, biological molecules or substances, chemical analytes, environmental substances or contaminants, or the like. For example, the analyte may be human or nonhuman, animal or non-animal, biological or non-biological substances, such as naturally occurring substances, artificial substances, metabolites, and / or reaction products, and refers without limitation to a molecule, substance or chemical constituent in a sample.As will be readily appreciated by the skilled reader, the term 'antigen' refers to a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that is recognised by, and is capable of binding to, the variable domain of a specific antibody or T-cell receptor.

[0018] Preferably, the semiconductor device is a transistor (such as a field effect transistor), and wherein the at least two contacts comprise: a source in contact with the active surface; a drain in contact with the active surface; and, a gate comprising at least one gate foot in contact with the active surface, and a gate head, wherein the gate head comprises the detection surface.

[0019] Preferably, the gate head is offset from and at least partially overlapping with the gate foot, the source and the drain.

[0020] Preferably, there is a sensor circuit connected to the at least two contacts and being configured to determine the presence of the target antigen or analyte based on a change in current through the semiconductor device.

[0021] Preferably, the functionalised detection surface comprises three-dimensional features.

[0022] Preferably, the functionalised detection surface comprises at least one of protrusions and recesses.

[0023] Preferably, the functionalised detection surface comprises an array of regularly spaced protrusions.

[0024] Preferably, the head comprises a peripheral wall extending away from the semiconductor junction.

[0025] Preferably, a first one of the drain and the source is shaped to mate with a second of the drain and the source.

[0026] Preferably, the first one of the drain and the source comprises a portion disposed within two spaced-apart portions of the second of the drain and the source.

[0027] Preferably, the second of the drain and the source is U-shaped and receives the portion of the first of the drain and the source therebetween.

[0028] Preferably, the first of the drain and the source is T-shaped.

[0029] Preferably, the gate foot comprises: a gate fin in contact with the semiconductor portion, the gate fin disposed between the source and drain; a via connecting the gate foot to the gate head.

[0030] Preferably, recesses are formed in the semiconductor portion at the point of contact with the gate fin, such that the gate fin and the semiconductor portion are engaged in a mating relationship.

[0031] Preferably, the gate fin and the semiconductor portion are interdigitated.

[0032] Preferably, there is provided a substrate onto which the semiconductor portion is formed, the substrate comprising silicon.

[0033] Preferably, the method comprises the steps of: forming a semiconductor device comprising a semiconductor junction, the device having an active surface; forming at least two contacts on the active surface, one of which comprises a head having a detection surface and a foot, the foot being in contact with the active surface; functionalising the detection surface, with a capture reagent configured to bind or otherwise associate with a target antigen or analyte.

[0034] Reference herein to a capture reagent refers to any substance or reagent that can bind, reversibly or irreversibly, to the target analyte or antigen. In this manner, as will be appreciated by those skilled in the art, a sample having, or suspected of having, the target analyte or antigen which when added to the sensor is specifically recognized and bound by the capture reagent, such that it can be used to identify and / or quantitate the analyte

[0035] In a preferred embodiment, said capture reagent includes, but is not limited to, antibodies, aptamers and fragments thereof, oligonucleotides or other specific ligands or receptors comprising a specific binding partner for the target analyte or antigen.

[0036] Reference herein to an 'analyte' refers to any target of interest including, but not limited to, biological molecules or substances, chemical analytes, environmental substances or contaminants, or the like. For example, the analyte may be human or nonhuman, animal or non-animal, biological or non-biological substances, such as naturally occurring substances, artificial substances, metabolites, and / or reaction products, and refers without limitation to a molecule, substance or chemical constituent in a sample.As will be readily appreciated by the skilled reader, the term 'antigen' refers to a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that is recognised by, and is capable of binding to, the variable domain of a specific antibody or T-cell receptor.

[0037] Preferably, the method comprises the steps of: forming a transistor from a semiconductor junction, the transistor having an active surface; forming a plurality of contacts on the semiconductor portion, the contacts comprising: a source; and a drain; and, a gate foot, providing a gate head being offset from the gate foot, the source and the drain.

[0038] Preferably, the head is at least partially overlapping the gate foot, the source and the drain.

[0039] Preferably, the method has the steps of: forming the semiconductor portion by growing heterojunction epilayers through Metal Organic Chemical Vapor Deposition (MOCVD).

[0040] Preferably, the method comprises the steps of: forming the contacts using e beam evaporation or sputtering.

[0041] Preferably, the method comprises the steps of: passivating the semiconductor portion with PECVD (Plasma Enhanced Chemical Vapor Deposition) SisN^

[0042] Preferably, the method comprises the steps of: metallizing the gate head with gold prior to functionalisation.

[0043] Preferably, the method comprises the steps of: using photoresist to form a well to hold solutions on the gate head.

[0044] Preferably, the method comprises the steps of: bonding thiol with the gate head detection surface; and, treating the surface with a capture reagent solution, optionally an antibody solution.

[0045] Preferably, the method comprises the steps of: providing a sample sensor according to the first aspect: introducing a sample fluid onto the detection surface; monitoring the drain current to determine the presence and / or concentration of a target antigen or analyte in the sample fluid.

[0046] Preferably, the target antigen is a biomarker.

[0047] Preferably, the biomarker is a prostate specific antigen.

[0048] Proposed here is a patterned FIN gated HEMT with the gate extended over the entire active area to provide larger surface area for functionalization and detection. Using this technique helps in increasing the sensitivities and reduced signal losses as the patterned gate has a wider top with a narrow foot and is not separated from the sensor as in the prior art. The present invention offers the following advantages:1 . Fin-HEMTs used for the first time as biosensorFin gates with small gate foot yield higher transconductance leading to higher sensitivities. In addition, they also provide larger surface are for bioimmobilization and functionalization.2. Lowest ever LOD (Limit of Detection):The basic limitation of existing HEMTs is their poor resolution. The proposed technology presents a high-performance sensor with optimized epi layer and dimensions that can be used for label free detection of biomarkers for early- stage detections.3. On-wafer integrated packagingA novel packaging technology is provided for sensors is presented here with wells created for ease of functionalization and detection. This packaging also helps in miniaturization and development of low-cost sensors.4. Reusable sensorsThese sensors are reusable and can be reused after cleaning with protein wash buffers.5. Low cost GaN sensor / Lowest cost biodetectionThe basic limitation that hinders the GaN based sensors from commercial breakthrough is the large development costs involved. The proposed sensors here when developed through on wafer integration will be cheap and cost a few pounds per sensor.6. Universal sensor platformThe application of Fin HEMT sensors are not limited to cancer detections the gate material and functionalization can be modified for:Bio-detection: Kidney injury, breast cancer, prostate cancer, SARS CoV-2, Zika virus, cardiac troponin, lactic acid, DNA, transplant rejection, glucose etc.Chemical detection: Gases, cations, anions, pH etcPhysical detection: Terahertz waves, magnetic field, pressure, strain etcBrief Description of Drawings

[0049] An embodiment of the present invention will now be described with reference to the following figure in which:FIGURE 1 is a perspective view of part of a sensor in accordance with the present invention;FIGURE 2 is a detail view of area II in Figure 1 ;FIGURE 3 is a side view of the sensor of Figure 1 ;FIGURE 4 is a plan view of the sensor of Figure 1 ;FIGURES 5a and 5b are views of results from a test of a device according to the invention; and,FIGURES 6a to 6c are views of results from a test of a device according to the invention.Description of the first embodimentConfiguration

[0050] Referring to Figure 1 , a sensor 100 according to the present invention comprises a HEMT 102. The sensor of Figure 1 is shown in a coordinate system having X, Y and Z axes. The sensor is generally laminated and planar in form in the XY plane. The HEMT 102 comprises a substrate layer 104, on top of which GaN epitaxial layers 106 are disposed. The layers comprise a planar AIGaN / GaN heterojunction forming a 2D electron gas (2DEG) as is known in the art.

[0051] On the surface of the AIGaN layer there is provided a source 108 in the shape of a "T" with a first portion 1 10 and a second portion 1 12 normal thereto. A U-shaped drain 1 14 is also provided having a base portion 1 16 and two legs 1 18, 120.

[0052] The legs 1 18, 120 of the drain 1 14 are positioned either side of the first portion of the source 108 such that the two formations essentially are mating. Two areas are thereby formed between each side of the first portion of the source 108 and each respective leg of the drainl 14.

[0053] Within each area fin-gate trench formations 122, 124 are provided, each comprises a plurality of trenches 126 extending between the drain and source.

[0054] A gates 128 is provided, comprising two gate feet 129, 131 , a single gate head 140 and a pair of vias 136, 138. The vias are constructed from gold in this embodiment.

[0055] Each gate foot 129, 131 comprises a downwardly depending fin 132 and a crosspiece 134 normal thereto (Fig. 2). The free end of the fin 132 is shaped to engage with the fin-gate trench formations 126. The lower edge of the fin comprises a plurality of fingers that engage each trench. As such, the lower edge of the fin is interdigitated with the trench formation. In this way, the gate electrode has a much longer contact edge with the semiconductor.

[0056] The gate head 140 is patterned and spans over the active area of the device over the gate fingers and the source and drain areas. The gate head 140 is provided with a lower surface 142 and an upper (sensing) surface 144.

[0057] A pair of vias 136, 138 extend from the gate feet 129, 131 to the gate head 140.

[0058] A passivation layer 130 (SisIXU) is provided over the top layer of the uppermost heterojunction material (AIGaN). A thicker layer of dielectric 133 (SisNz is provided in which the vias 136, 138 are embedded. The gate head 140 is provided on the upper surface of the dielectric layer 133.

[0059] Referring to Figure 1 , the sensing surface 144 of the gate head 140 is patterned comprising an upstanding, endless peripheral wall 146 and a plurality of protrusions 148 formed to increase the surface area of the head 140.

[0060] Prior to use, the gate head 140 is functionalised with a capture reagent or biofunctional coating 150.

[0061] Referring to Figure 4, the sensor 100 is shown in plan view. An analyser device 152 is provided, having three terminals connected to each of the drain (D), source (S) and gate (G).Use

[0062] The analyser 152 is configured to apply a potential difference (e.g. 2V) across the DS terminals, whilst measuring the current. Liquid samples are placed in contact with the functionalised surface of the gate head 140. If the liquid sample contains the analyte or antigen corresponding to the capture reagent, such as an antibody, of the functionalised gate, the ID (drain current) will change- specifically increase.Manufacture

[0063] Si has been used as the substrate with focus on delivering low cost GaN HEMT sensors. The growth of heterojunction epilayers has been done through Metal Organic Chemical Vapor Deposition (MOCVD). The ohmic contacts of Ti / AI / Ni / Au has been deposited using e beam evaporation. The devices were then passivated with PECVD (Plasma Enhanced Chemical Vapor Deposition) SisIXL which is later etched using RIE (Reactive Ion Etching) to form the gate foot prior to patterning and deposition of gate head. A thicker (1 micron) layer of SisIXL has been used to support the gate head that extends over the device active area. Patterning on nitride before gold gate head metallization makes trenches on the gate surface that yields very large surface areas for bioimmobilizations.

[0064] Functionalization began with 10 mM ethanolic cysteamine in dark condition for 3 hours. This leads to thiol-gold formation. The thiol functionalized devices were then treated with 1 .25% aqueous glutaraldehyde solution at room temperature for 3 hours. An antibody solution of specific antibodies is dropped on the sensing surface and preserved over a full night and day at 4 degrees to ensure binding of antibody forming the bio-functionalization layer. After 24 hours, the sensors were rinsed in DI water and dried in N2 flow.Test

[0065] GaN based transistors were developed according to the above specification. The output and transfer characteristics of the fabricated device is presented in Fig. 5a and 5b. The currents plotted are normalized currents per 100 ii m. The device exhibits ON current of ~55 A with peak transconductance of 269 mS / mm at VDS= 2V. Good sensor performance is guaranteed by this high ON current and transconductance of the device. 10 ii g / ml antibody solution of prostate specific antibody was used to functionalise the gate surface.

[0066] For sensitivity analysis PBS solution has been dropped on the sensor surface and the drain currents were plotted. This output current has been used as the reference current for sensitivity analysis. As test cases, to prove the functionality of the device, Antigen solutions (PSA) of different concentrations were prepared in PBS and dropped on the functionalized sensor and the output characteristics were plotted. 20 nl antigen sample solutions were dropped on the sensing surface and responses were plotted after waiting for 60 s, while the gate was constantly biased at 1 V. The results of this analysis are shown in Figures 6a to 6c. There is a clear increase in lswith PSA concentration, meaning that the presence of the PSA can be determined by the analyser 152.Variations

[0067] Although the above embodiment discloses the gate head 140 patterned with a three dimensional structure in the form of a peripheral wall and protrusions, it will be understood that other forms are possible. Atthe simplest end ofthe spectrum, a flat, planar XY surface may be provided. It is preferable however to provide functional features such as a wall to retain the fluid sample, and protrusions, recesses or other features configured to increase the surface area of the gate head and therefore the area in which the sample is in contact with the HEMT.

[0068] The above embodiment uses an AIGaN / GaN HEMT as the example sensing device. The present invention is not so limited, and instead may utilise transistors including (but not limited to): Bipolar Junction transistors (BJT), Metal Oxide Semiconductor Field Effect Transistors (MOSFET), Tunneling Field Effect Transitors (TFET), Insulated Gate Bipolar Transistors (IGBT), Fin gated Field Effect Transistors (FinFET) and HEMTs based on other semiconductor materials etc. Alternatively, the novel gate electrode topology presentedhere can be used to design the sensing electrode in diodes like schottky diodes, PIN diodes, PN junction diodes, tunnel diodes, etc.

[0069] In the above embodiment, the passivation material and the dielectric material are the same, although it will be noted that they can differ.

Claims

Claims1 . A sample sensor comprising: a semiconductor device comprising: a semiconductor junction having an active surface; at least two contacts on the active surface, one of which comprises a head and a foot, the foot being in contact with the active surface, wherein the head comprises a detection surface having a capture reagent provided thereon, wherein the capture reagent binds or otherwise associates with a target antigen or analyte.

2. A sample sensor according to claim 1 , wherein the semiconductor device is a transistor, and wherein the at least two contacts comprise: a source in contact with the active surface; a drain in contact with the active surface; and, a gate comprising at least one gate foot in contact with the active surface, and a gate head, wherein the gate head comprises the detection surface.

3. A sample sensor according to claim 2, wherein the gate head is offset from and at least partially overlapping with the gate foot, the source and the drain.

4. A sample sensor according to any of claims 1 to 3 comprising a sensor circuit connected to the at least two contacts and being configured to determine the presence of the target antigen or analyte based on a change in current through the semiconductor device.

5. A sample sensor according to any preceding claim, wherein the functionalised detection surface comprises three-dimensional features.

6. A sample sensor according to claim 5, wherein the functionalised detection surface comprises at least one of protrusions and recesses.

7. A sample sensor according to claim 7, wherein the functionalised detection surface comprises an array of regularly spaced protrusions.

8. A sample sensor according to any preceding claim, wherein the head comprises a peripheral wall extending away from the semiconductor junction.

9. A sample sensor according to claim 2 or any claim dependent thereon, wherein a first one of the drain and the source is shaped to mate with a second of the drain and the source.1 0. A sample sensor according to claim 9, wherein the first one of the drain and the source comprises a portion disposed within two spaced-apart portions of the second of the drain and the source.1 1 . A sample sensor according to claim 1 0, wherein the second of the drain and the source is U-shaped and receives the portion of the first of the drain and the source therebetween.

12. A sample sensor according to claim 1 0 or 1 1 , wherein the first of the drain and the source is T-shaped.

13. A sample sensor according to claim 2 or any claim dependent thereon, wherein the gate foot comprises: a gate fin in contact with the semiconductor portion, the gate fin disposed between the source and drain; a via connecting the gate foot to the gate head.

14. A sample sensor according to claim 13, wherein recesses are formed in the semiconductor portion at the point of contact with the gate fin, such that the gate fin and the semiconductor portion are engaged in a mating relationship.1 5. A sample sensor according to claim 1 4, wherein the gate fin and the semiconductor portion are interdigitated.1 6. A sample sensor according to any preceding claim, comprising a substrate onto which the semiconductor portion is formed, the substrate comprising silicon.1 7. A method of manufacturing a sample sensor, the method comprising the steps of: forming a semiconductor device comprising a semiconductor junction, the device having an active surface; forming at least two contacts on the active surface, one of which comprises a head having a detection surface and a foot, the foot being in contact with the active surface; functionalising the detection surface, with a capture reagent configured to bind or otherwise associate with a target antigen or analyte.

18. A method of manufacturing a sample sensor according to claim 1 7, the method comprising the steps of: forming a transistor from a semiconductor junction, the transistor having an active surface; forming a plurality of contacts on the semiconductor portion, the contacts comprising: a source; and a drain; and, a gate foot, providing a gate head being offset from the gate foot, the source and the drain.

19. A method of manufacturing a sample sensor according to claim 18, wherein the head is at least partially overlapping the gate foot, the source and the drain.

20. A method of manufacturing a sample sensor according to any of claims 17 to 19, comprising the steps of: forming the semiconductor portion by growing heterojunction epilayers through Metal Organic Chemical Vapor Deposition (MOCVD).

21. A method of manufacturing a sample sensor according to any of claims 17 to 20 comprising the steps of: forming the contacts using e beam evaporation or sputtering.

22. A method of manufacturing a sample sensor according to any of claims 17 to 21 , comprising the steps of: passivating the semiconductor portion with PECVD (Plasma Enhanced Chemical Vapor Deposition) SisN^23. A method of manufacturing a sample sensor according to any of claims 17 to 22, comprising the steps of: metallizing the gate head with gold prior to functionalisation.

24. A method of manufacturing a sample sensor according to any of claims 17 to 23, comprising the steps of: using photoresist to form a well to hold solutions on the gate head.

25. A method of manufacturing a sample sensor according to any of claims 17 to 24, wherein the step of functionalising comprises the steps of: bonding thiol with the gate head detection surface; and, treating the surface with a capture reagent solution.

26. A method of detecting a biomarker comprising the steps of: providing a sample sensor according to any of claims 1 to 16; introducing a sample fluid onto the detection surface; monitoring the drain current to determine the presence and / or concentration of a target antigen or analyte, in the sample fluid.

27. A method of detecting a biomarker according to claim 26, wherein the target antigen or analyte is a biomarker.

28. A method of detecting a biomarker according to claim 27 , wherein the biomarker is a prostate specific antigen.

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