JP3982090B2

JP3982090B2 - Semiconductor ion sensor

Info

Publication number: JP3982090B2
Application number: JP36396198A
Authority: JP
Inventors: 幸男飯高
Original assignee: Matsushita Electric Works Ltd
Current assignee: Panasonic Electric Works Co Ltd
Priority date: 1998-12-22
Filing date: 1998-12-22
Publication date: 2007-09-26
Anticipated expiration: 2018-12-22
Also published as: JP2000187018A

Description

【０００１】
【発明の属する技術分野】
本発明は、半導体イオンセンサに関するものである。
【０００２】
【従来の技術】
従来より、溶液（測定液）中のイオンの濃度（活量）を電気信号に変換するデバイスとしてＩＳＦＥＴ（Ion Sensitive Field Effect Transistor）と呼ばれる半導体イオンセンサが提供されている。この種の半導体イオンセンサは、図３に示すように、ｐ形シリコン基板１の主表面側にｎ⁺層よりなるドレイン領域２とｎ⁺層よりなるソース領域３とが離間して形成され、両領域２，３間のチャネル部４上にゲート絶縁膜５を介してイオン感応膜６が形成されている。また、ドレイン領域２上にはドレイン電極７が形成され、ソース領域３上にはソース電極８が形成されており、各電極７，８上には保護膜１０が形成されている。なお、図３中の９はフィールド酸化膜を示す。
【０００３】
要するに、図３に示す構成のＩＳＦＥＴは、電界効果型トランジスタのドレイン・ソース間のチャネル部上にゲート絶縁膜を介して形成されたゲート電極に相当する部分に、イオン感応膜６を形成してある。したがって、ＩＳＦＥＴでは、イオン濃度の大きさに応じてチャネル部４の電位が変わってチャネル部４の電気抵抗が変化することになり、ドレイン・ソース間の電流が変化する。
【０００４】
この種の半導体イオンセンサでは、ＩＳＦＥＴと別体の参照電極３０をＩＳＦＥＴのソース電極８に可変電圧源Ｖａを介して接続し、溶液（測定液）の電位を参照電極３０を通してＩＳＦＥＴのソース電位に対して一定の値にすることにより、ソース・ゲート間にはイオンの濃度に応じた電圧が加わるから、ドレイン電極７・ソース電極８間に電圧源Ｖ２を介して挿入された電流計４０による測定電流によりイオン濃度を知ることができる。
【０００５】
【発明が解決しようとする課題】
ところで、上記従来構成の半導体イオンセンサでは、製品ごとのＩＳＦＥＴのしきい値電圧Ｖｔｈの違いや温度特性などにより、参照電極３０へ印加する最適な電圧が製品ごとに異なっており、ＩＳＦＥＴのしきい値電圧Ｖｔｈのばらつきや温度変化に関わらず一定の感度および精度を保つために、可変電圧源Ｖａによる参照電極３０への印加電圧をその都度調整する必要があり、手間がかかるという不具合があった。
【０００６】
本発明は上記事由に鑑みて為されたものであり、その目的は、参照電極への印加電圧を調整する作業が不要で安定した感度特性を有する半導体イオンセンサを提供することにある。
【０００７】
【課題を解決するための手段】
請求項１の発明は、上記目的を達成するために、半導体基板の主表面側にドレイン領域とソース領域とが離間して形成され、半導体基板においてドレイン領域とソース領域との間に介在する領域上に絶縁膜を介してイオン感応膜が形成され、ドレイン領域上にドレイン電極が形成され、ソース領域上にソース電極が形成されたＩＳＦＥＴと、ＩＳＦＥＴのソース電極に接続された参照電極と、該参照電極とＩＳＦＥＴのソース電極との間に挿入されＩＳＦＥＴのしきい値電圧と略同じしきい値電圧を有するＦＥＴであってドレインおよびゲートが参照電極に接続され、ソースがＩＳＦＥＴのソース電極に接続された定電圧用ＦＥＴと、定電圧用ＦＥＴのドレイン・ソース間に並列接続される電流制御用抵抗と電圧源との直列回路とを備えることを特徴とするものであり、参照電極の電位をＩＳＦＥＴのしきい値電圧に略一致させることができるので、従来のように参照電極に印加する電圧を調整するような作業が不要で、半導体イオンセンサの感度特性が温度変化やＩＳＦＥＴのしきい値電圧Ｖｔｈのばらつきなどがあっても変わることなく、製品ごとの感度特性のばらつきが小さい安定した感度特性を有する。
【０００８】
請求項２の発明は、請求項１の発明において、定電圧用ＦＥＴとＩＳＦＥＴとは隣接して同一半導体基板に形成され、両ＦＥＴのソース領域が共通化されているので、両ＦＥＴのしきい値電圧を容易に一致させることができ製品ごとの感度特性のばらつきがより小さくなるとともに、センサの小型化を図ることができる。
【０００９】
【発明の実施の形態】
（実施形態１）
本実施形態の半導体イオンセンサの基本構成は図３に示した従来構成と略同じであって、図１に示すように、参照電極３０とＩＳＦＥＴのソース電極８との間に、ＩＳＦＥＴのしきい値電圧と略同じしきい値電圧を有するＭＯＳＦＥＴであってドレインおよびゲートが参照電極３０に接続されソースがＩＳＦＥＴのソース電極８に接続された定電圧用ＭＯＳＦＥＴ２０を設けるとともに、電流制御用抵抗Ｒと電圧源Ｖ１との直列回路を定電圧用ＭＯＳＦＥＴ２０のドレイン・ソース間に並列接続した点に特徴がある。なお、図３に示した従来構成と同様の構成要素には同一の符号を付して説明を省略する。
【００１０】
ここにおいて、定電圧用ＭＯＳＦＥＴ２０は、ゲート・ドレイン間が短絡され、ドレイン・ソース間に上記直列回路が並列接続されているので、定電圧源として機能し、参照電極３０の電位を常にＩＳＦＥＴのドレイン電流が流れ始める電位に保つことができる。
【００１１】
しかして、本実施形態の半導体イオンセンサでは、参照電極３０の電位をＩＳＦＥＴのしきい値電圧に略一致させることができるから、従来のように参照電極３０に印加する電圧を可変電圧源Ｖａ（図３参照）によって調整するような作業が不要で、温度変化やＩＳＦＥＴのしきい値電圧Ｖｔｈのばらつきなどがあっても感度特性が変わることなく、製品ごとの感度特性のばらつきが小さくなり、感度特性が安定する。
【００１２】
（実施形態２）
本実施形態の半導体イオンセンサの基本構成は図１に示した従来構成と略同じであって、図２に示すように、実施形態１の定電圧用ＭＯＳＦＥＴ２０をＩＳＦＥＴが形成されたｐ形シリコン基板１に形成し、ソース領域３を共通化した点に特徴がある。すなわち、本実施形態では、ＩＳＦＥＴと定電圧用ＭＯＳＦＥＴ２０とが１チップ内に隣接して形成されている。なお、実施形態１と同様の構成要素には同一の符号を付して説明を省略する。
【００１３】
定電圧用ＭＯＳＦＥＴ２０は、ＩＳＦＥＴが形成されたｐ形シリコン基板１のの主表面側にｎ⁺層よりなるドレイン領域２’とソース領域３とが離間して形成され、両領域２’，３間のチャネル部４’上にゲート絶縁膜１５を介してゲート電極１６が形成されている。また、ドレイン領域２’上にはドレイン電極７’が形成され、ソース領域３上にはソース電極８が形成されており、各電極７，８，１６上には保護膜１０が形成されている。ここに、定電圧用ＭＯＳＦＥＴ２０のソース領域３およびソース電極８はＩＳＦＥＴと共通である。
【００１４】
しかして、本実施形態の半導体イオンセンサでは、ＩＳＦＥＴと定電圧用ＭＯＳＦＥＴ２０とが同一チップに形成されソース領域３が共通化されているので、ＩＳＦＥＴのしきい値電圧と定電圧用ＭＯＳＦＥＴ２０のしきい値電圧とを容易に一致させることができ製品ごとの感度特性のばらつきがより小さくなるとともに、両ＦＥＴを別々のチップに形成する場合やソース領域３を共通化しない場合に比べてセンサの小型化を図ることができる。
【００１５】
【発明の効果】
請求項１の発明は、半導体基板の主表面側にドレイン領域とソース領域とが離間して形成され、半導体基板においてドレイン領域とソース領域との間に介在する領域上に絶縁膜を介してイオン感応膜が形成され、ドレイン領域上にドレイン電極が形成され、ソース領域上にソース電極が形成されたＩＳＦＥＴと、ＩＳＦＥＴのソース電極に接続された参照電極と、該参照電極とＩＳＦＥＴのソース電極との間に挿入されＩＳＦＥＴのしきい値電圧と略同じしきい値電圧を有するＦＥＴであってドレインおよびゲートが参照電極に接続され、ソースがＩＳＦＥＴのソース電極に接続された定電圧用ＦＥＴと、定電圧用ＦＥＴのドレイン・ソース間に並列接続される電流制御用抵抗と電圧源との直列回路とを備えるので、参照電極の電位をＩＳＦＥＴのしきい値電圧に略一致させることができるから、従来のように参照電極に印加する電圧を調整するような作業が不要で、半導体イオンセンサの感度特性が温度変化やＩＳＦＥＴのしきい値電圧Ｖｔｈのばらつきなどがあっても変わることなく、製品ごとの感度特性のばらつきが小さい安定した感度特性を有するという効果がある。
【００１６】
請求項２の発明は、請求項１の発明において、定電圧用ＦＥＴとＩＳＦＥＴとは隣接して同一半導体基板に形成され、両ＦＥＴのソース領域が共通化されているので、両ＦＥＴのしきい値電圧を容易に一致させることができ製品ごとの感度特性のばらつきがより小さくなるとともに、センサの小型化を図ることができるという効果がある。
【図面の簡単な説明】
【図１】実施形態１を示す概略構成図である。
【図２】実施形態２を示す概略構成図である。
【図３】従来例を示す概略構成図である。
【符号の説明】
１ｐ形シリコン基板
２ドレイン領域
３ソース領域
４チャネル部
５ゲート絶縁膜
６イオン感応膜
７ドレイン電極
８ソース電極
９フィールド酸化膜
１０保護膜
２０定電圧用ＭＯＳＦＥＴ
３０参照電極
Ｒ電流制御用抵抗
Ｖ１電圧源[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor ion sensor.
[0002]
[Prior art]
Conventionally, a semiconductor ion sensor called an ISFET (Ion Sensitive Field Effect Transistor) has been provided as a device that converts the concentration (activity) of ions in a solution (measurement solution) into an electrical signal. In this type of semiconductor ion sensor, as shown in FIG. 3, a drain region 2 made of an n ⁺ layer and a source region 3 made of an n ⁺ layer are formed on the main surface side of a p-type silicon substrate 1 separately from each other. An ion sensitive film 6 is formed on the channel portion 4 between the two regions 2 and 3 via a gate insulating film 5. A drain electrode 7 is formed on the drain region 2, a source electrode 8 is formed on the source region 3, and a protective film 10 is formed on each of the electrodes 7 and 8. In FIG. 3, 9 indicates a field oxide film.
[0003]
In short, the ISFET having the configuration shown in FIG. 3 has an ion sensitive film 6 formed in a portion corresponding to a gate electrode formed on a channel portion between the drain and source of a field effect transistor via a gate insulating film. is there. Therefore, in the ISFET, the potential of the channel portion 4 changes according to the magnitude of the ion concentration, and the electric resistance of the channel portion 4 changes, and the current between the drain and the source changes.
[0004]
In this type of semiconductor ion sensor, a reference electrode 30 separate from the ISFET is connected to the source electrode 8 of the ISFET via the variable voltage source Va, and the potential of the solution (measurement solution) is made to the source potential of the ISFET through the reference electrode 30. On the other hand, since a voltage corresponding to the ion concentration is applied between the source and the gate by setting a constant value, measurement by the ammeter 40 inserted between the drain electrode 7 and the source electrode 8 via the voltage source V2 is performed. The ion concentration can be known from the current.
[0005]
[Problems to be solved by the invention]
By the way, in the semiconductor ion sensor having the above-described conventional configuration, the optimum voltage to be applied to the reference electrode 30 differs depending on the product due to the difference in threshold voltage Vth of the ISFET for each product, temperature characteristics, and the like. In order to maintain constant sensitivity and accuracy regardless of variations in the value voltage Vth and temperature changes, it is necessary to adjust the voltage applied to the reference electrode 30 by the variable voltage source Va each time, which is troublesome. .
[0006]
The present invention has been made in view of the above reasons, and an object of the present invention is to provide a semiconductor ion sensor having a stable sensitivity characteristic that does not require an operation of adjusting a voltage applied to a reference electrode.
[0007]
[Means for Solving the Problems]
According to a first aspect of the present invention, in order to achieve the above object, a drain region and a source region are formed apart from each other on the main surface side of a semiconductor substrate, and the region interposed between the drain region and the source region in the semiconductor substrate. An ISFET having an ion sensitive film formed thereon via an insulating film, a drain electrode formed on the drain region, a source electrode formed on the source region, a reference electrode connected to the source electrode of the ISFET, An FET that is inserted between the reference electrode and the source electrode of ISFET and has a threshold voltage substantially the same as the threshold voltage of ISFET, the drain and gate are connected to the reference electrode, and the source is connected to the source electrode of ISFET And a series circuit of a current control resistor and a voltage source connected in parallel between the drain and source of the constant voltage FET. Since the potential of the reference electrode can be made to substantially match the threshold voltage of the ISFET, there is no need to adjust the voltage applied to the reference electrode as in the prior art. The sensitivity characteristic of the sensor does not change even if there is a change in temperature or a variation in threshold voltage Vth of the ISFET, and the sensor has a stable sensitivity characteristic with a small variation in sensitivity characteristic for each product.
[0008]
According to a second aspect of the present invention, in the first aspect of the invention, the constant voltage FET and the ISFET are formed adjacent to each other on the same semiconductor substrate, and the source regions of both FETs are made common. The value voltages can be easily matched, and variations in sensitivity characteristics among products can be further reduced, and the size of the sensor can be reduced.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
(Embodiment 1)
The basic configuration of the semiconductor ion sensor of the present embodiment is substantially the same as the conventional configuration shown in FIG. 3, and as shown in FIG. 1, the threshold of the ISFET is between the reference electrode 30 and the source electrode 8 of the ISFET. A constant voltage MOSFET 20 having a threshold voltage substantially the same as the value voltage and having a drain and gate connected to the reference electrode 30 and a source connected to the source electrode 8 of the ISFET is provided, and a current control resistor R and It is characterized in that a series circuit with the voltage source V1 is connected in parallel between the drain and source of the constant voltage MOSFET 20. It should be noted that the same components as those in the conventional configuration shown in FIG.
[0010]
Here, the constant voltage MOSFET 20 is short-circuited between the gate and drain, and the series circuit is connected in parallel between the drain and source. Therefore, the constant voltage MOSFET 20 functions as a constant voltage source, and the potential of the reference electrode 30 is always set to the drain of the ISFET. It can be kept at a potential at which current begins to flow.
[0011]
Thus, in the semiconductor ion sensor of the present embodiment, the potential of the reference electrode 30 can be made substantially equal to the threshold voltage of the ISFET, so that the voltage applied to the reference electrode 30 can be set to the variable voltage source Va ( (See Fig. 3) Adjustment is not necessary, and even if there is a change in temperature or variation in threshold voltage Vth of ISFET, the sensitivity characteristics do not change, and the variation in sensitivity characteristics from product to product is reduced. The characteristics are stable.
[0012]
(Embodiment 2)
The basic configuration of the semiconductor ion sensor of this embodiment is substantially the same as the conventional configuration shown in FIG. 1, and as shown in FIG. 2, the constant voltage MOSFET 20 of Embodiment 1 is a p-type silicon substrate on which an ISFET is formed. 1 in that the source region 3 is shared. That is, in this embodiment, the ISFET and the constant voltage MOSFET 20 are formed adjacent to each other in one chip. In addition, the same code | symbol is attached | subjected to the component similar to Embodiment 1, and description is abbreviate | omitted.
[0013]
The constant voltage MOSFET 20 is formed on the main surface side of the p-type silicon substrate 1 on which the ISFET is formed, with a drain region 2 ′ made of an n ⁺ layer and a source region 3 separated from each other, and between the regions 2 ′ and 3. A gate electrode 16 is formed on the channel portion 4 ′ via a gate insulating film 15. A drain electrode 7 ′ is formed on the drain region 2 ′, a source electrode 8 is formed on the source region 3, and a protective film 10 is formed on each electrode 7, 8, 16. . Here, the source region 3 and the source electrode 8 of the constant voltage MOSFET 20 are common to the ISFET.
[0014]
Therefore, in the semiconductor ion sensor of this embodiment, the ISFET and the constant voltage MOSFET 20 are formed on the same chip and the source region 3 is shared, so that the threshold voltage of the ISFET and the threshold of the constant voltage MOSFET 20 are the same. The value voltage can be easily matched and the variation in sensitivity characteristics of each product becomes smaller, and the size of the sensor is smaller than when both FETs are formed on separate chips or when the source region 3 is not shared. Can be achieved.
[0015]
【The invention's effect】
According to the first aspect of the present invention, the drain region and the source region are formed on the main surface side of the semiconductor substrate so as to be separated from each other, and ions are interposed on the region interposed between the drain region and the source region in the semiconductor substrate via the insulating film. A sensitive film is formed, a drain electrode is formed on the drain region, a source electrode is formed on the source region, a reference electrode connected to the source electrode of the ISFET, the reference electrode and the source electrode of the ISFET, A constant voltage FET having a threshold voltage substantially the same as the threshold voltage of the ISFET and having a drain and a gate connected to the reference electrode and a source connected to the source electrode of the ISFET; Since it has a series circuit of a current control resistor and a voltage source connected in parallel between the drain and source of the constant voltage FET, the potential of the reference electrode is set to ISF Since it can be made substantially equal to the threshold voltage of T, there is no need to adjust the voltage applied to the reference electrode as in the prior art, and the sensitivity characteristics of the semiconductor ion sensor are affected by temperature changes and the threshold of ISFET. Even if there is a variation in the voltage Vth or the like, there is an effect that there is a stable sensitivity characteristic with a small variation in sensitivity characteristic among products.
[0016]
According to a second aspect of the present invention, in the first aspect of the invention, the constant voltage FET and the ISFET are formed adjacent to each other on the same semiconductor substrate, and the source regions of both FETs are made common. The value voltages can be easily matched to each other, so that variations in sensitivity characteristics among products are further reduced, and the sensor can be miniaturized.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a first embodiment.
FIG. 2 is a schematic configuration diagram showing a second embodiment.
FIG. 3 is a schematic configuration diagram showing a conventional example.
[Explanation of symbols]
1 p-type silicon substrate 2 drain region 3 source region 4 channel portion 5 gate insulating film 6 ion sensitive film 7 drain electrode 8 source electrode 9 field oxide film 10 protective film 20 constant voltage MOSFET
30 Reference electrode R Current control resistor V1 Voltage source

Claims

A drain region and a source region are formed apart from each other on the main surface side of the semiconductor substrate, and an ion sensitive film is formed through an insulating film on a region interposed between the drain region and the source region in the semiconductor substrate. An ISFET having a drain electrode formed on the region, a source electrode formed on the source region, a reference electrode connected to the source electrode of the ISFET, and an ISFET inserted between the reference electrode and the source electrode of the ISFET A FET having a threshold voltage substantially equal to the threshold voltage, the drain and gate of which are connected to the reference electrode, the source of which is connected to the source electrode of the ISFET, and the drain of the constant voltage FET A semiconductor ion sensor comprising: a current control resistor connected in parallel between sources; and a series circuit of a voltage source.

2. The semiconductor ion sensor according to claim 1, wherein the constant voltage FET and the ISFET are formed adjacent to each other on the same semiconductor substrate, and the source regions of both FETs are made common.