CN111103056B - Thin film transistor integrated optical sensor - Google Patents
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- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
- G01J1/44—Electric circuits
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- G—PHYSICS
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- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/02—Details
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/02—Details
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- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
- G01J1/44—Electric circuits
- G01J2001/4446—Type of detector
- G01J2001/4473—Phototransistor
Abstract
A thin film transistor integrated optical sensor comprises a photoelectric conversion part, a voltage conversion part and a voltage adjustment part, wherein the photoelectric conversion part is used for sensing external light and outputting a photocurrent signal corresponding to the illumination intensity of the external light, the voltage conversion part is used for converting the photocurrent signal into a voltage signal, the voltage signal comprises a voltage difference between a positive phase output voltage signal and a negative phase output voltage signal, and the voltage adjustment part is used for adjusting the positive phase output voltage signal and the negative phase output voltage signal according to the illumination intensity of the external light so as to amplify the voltage signal. The output voltage signal amplification of the sensor is controlled by the voltage adjusting part through the induction of the illumination intensity of the external light, so that the responsivity of the sensor is improved, and the driving capability of the sensor is enhanced. And has a definite photoelectric response threshold, above which the sensitivity of photoelectric response is high.
Description
Technical Field
The present application relates to the field of optical sensor technology, and more particularly, to an integrated optical sensor of a thin film transistor.
Background
An active array based on a Thin Film Transistor (TFT) is the basis of modern display technology, and the design, preparation and driving of the TFT array are key technical problems of modern display, no matter for the current mainstream TFT liquid crystal display (TFT-LCD) or TFT-OLED display. In a conventional display active array, the TFTs are simply used as switching elements to control the programming and holding of data voltages on the display pixel electrodes. As TFT technology has developed and matured, TFT arrays are being used to implement more and more new optical and electrical functions, not just the switching functions of active arrays. On the other hand, the application range of the display is increasingly expanded, and the additional value of the display can be remarkably improved by integrating a sensing device and a circuit based on the thin film transistor, so that the user experience effect of the display is better in the application scenes of the 5G Internet of vehicles and the Internet of things.
The relationship between the electrical characteristics of the thin film transistor, for example, the transfer characteristics (Ids to Vgs) of the TFT and the light irradiation is large. Taking an amorphous silicon thin film transistor (a-Si TFT) as an example, under the condition that the illumination intensity of external light is increased, the subthreshold current and the off-state current of the amorphous silicon thin film transistor are increased by 10 times or more. This is mainly because the band structure of the active layer material (a-Si layer) of the TFT is modulated by light, and under the influence of strong external light, more defect-state electrons in the band tail state are excited into on-state electrons, which corresponds to a more significant photo-generated current. Thus, by utilizing the relationship between the current of the TFT and the external light, a proper sensor circuit can be designed to quantitatively characterize the intensity of the external light. Since such a sensor is integrated by TFTs, the sensor can be fabricated with TFT structures within the display array to form a "substrate-integrated photosensor".
The light irradiation mainly affects the subthreshold current and the leakage current of the TFT (for example, a-Si TFT), and does not easily affect the value of the on current. Therefore, in the TFT-integrated photosensor structure, the current in the subthreshold region and the leakage region is often used, but not the current in the on region. However, the current values of the sub-threshold region and the leakage region are actually small, and the current of the conduction region is large, so that the driving capability of the conventional TFT integrated optical sensing circuit is weak, and the response speed is slow. In particular, there is a large parasitic capacitance between the gate-source and gate-drain of the elements of the a-Si TFT, which further results in a slow response speed of the TFT integrated photosensor.
In a conventional TFT photosensor circuit, TFTs are used only as photo-electric conversion elements. Under the action of different ambient light, the photosensitive TFT outputs different current/voltage values, and then the obtained weak photoelectric signal is enhanced through an external amplifying circuit. The conventional TFT sensing circuit has a problem in that an effective photoelectric signal is easily interfered due to a large amount of noise and dark current inside the TFT sensing circuit. Since the amplifying circuit is located outside the TFT active array, i.e. the amplifying circuit is far from the photo-sensing circuit, this causes both noise and photo-electric signal to be amplified at the same time, and the effective photo-electric signal is difficult to pick up.
Disclosure of Invention
The application provides a thin film transistor integrated optical sensor, can need external amplifier circuit, through responding to external light intensity with the weak voltage signal amplification of sensor output.
According to a first aspect of the present application, there is provided a thin film transistor integrated light sensor comprising:
a photoelectric conversion part for sensing external light and outputting a photocurrent signal corresponding to the illumination intensity of the external light;
a voltage converting part for converting the photocurrent signal into a voltage signal, the voltage signal including a voltage difference between a positive-phase output voltage signal and a negative-phase output voltage signal;
and the voltage adjusting part is used for adjusting the positive phase output voltage signal and the negative phase output voltage signal according to the illumination intensity of the outside light so as to amplify the voltage signals.
In one possible implementation manner, the voltage adjustment part includes a pull-up unit and/or a pull-down unit;
the pull-up unit is used for pulling up the voltage signal according to the illumination intensity of the outside light;
the pull-down unit is used for pulling down the voltage signal according to the illumination intensity of the outside light.
In one possible implementation manner, the voltage adjustment part further includes an illumination threshold unit;
and the illumination threshold value unit is used for setting a preset illumination threshold value and controlling the pull-up unit and/or the pull-down unit to work according to the illumination intensity of the outside light and the preset illumination threshold value.
In one possible implementation manner, the voltage adjusting part further includes a positive feedback unit;
and the positive feedback unit is provided with a positive feedback mechanism and is used for controlling the pull-up unit and/or the pull-down unit to work according to the illumination intensity of the outside light and the positive feedback mechanism.
In one possible implementation, the photoelectric conversion section includes a transistor T1 and a transistor T4, and the voltage conversion section includes a transistor T2 and a transistor T3, in which a transistor T3 and a transistor T2 are light-shielded;
the first pole of the transistor T1 receives the high voltage VDDA second pole of the transistor T1 is connected to the first pole of the transistor T3 and a positive phase voltage output terminal for outputting the positive phase output voltage signal;
the second pole of the transistor T3 receives the low voltage VSS；
The first pole of the transistor T2 receives the high voltage VDDA second pole of the transistor T2 is connected to the first pole of the transistor T4 and a negative phase voltage output terminal for outputting the negative phase output voltage signal;
the second pole of the transistor T4 receives the low voltage VSS；
The control electrodes of the transistors T1, T2, T3 and T4 all receive a predetermined voltage signal VBIASThe preset voltage signal VBIASFor controlling the transistor T1, the transistor T2, the transistor T3, and the transistor T4 to be in a sub-threshold region or a leakage region.
In one possible implementation, the voltage adjustment part includes a transistor T51, a transistor T61, and a transistor T71; wherein the transistor T51 and the transistor T61 are light-shielded;
the first pole of the transistor T51 receives the high voltage VDDA second pole of the transistor T51 is connected to the first pole of the transistor T71 and the control pole of the transistor T61, and the control pole of the transistor T51 is connected to the negative phase voltage output end;
the second pole of the transistor T71 is connected to the control pole of the transistor T71 and the negative phase voltage output end;
a first electrode of the transistor T61 is connected to the positive phase voltage output terminalThe second pole of T61 receives a low voltage VSS。
In one possible implementation, the voltage adjustment part includes a transistor T52, a transistor T62, and a transistor T72; wherein the transistor T52 and the transistor T62 are light-shielded;
the first pole of the transistor T52 receives the high voltage VDDA second pole of the transistor T52 is connected to the first pole of the transistor T72 and the control pole of the transistor T62, and the control pole of the transistor T52 and the control pole of the transistor T72 receive the preset voltage signal VBIAS；
The second pole of the transistor T72 receives the low voltage VSS；
A first electrode of the transistor T62 is connected to the non-inverted voltage output terminal, and a second electrode of the transistor T62 receives the low voltage VSS。
In one possible implementation, the voltage adjustment part includes a transistor T53 and a transistor T63; wherein the transistor T53 and the transistor T63 are light-shielded;
a control electrode of the transistor T53 is connected to the positive phase voltage output terminal, a first electrode of the transistor T53 is connected to the negative phase voltage output terminal, and a second electrode of the transistor T53 receives the low voltage VSS；
The control electrode of the transistor T63 is connected to the negative phase voltage output terminal, the first electrode of the transistor T63 is connected to the positive phase voltage output terminal, and the second electrode of the transistor T63 receives the low voltage VSS。
In one possible implementation, the voltage adjustment part includes a transistor T54 and a transistor T64; wherein, the transistor T64 is shielded from light;
a control electrode of the transistor T54 is connected to the positive phase voltage output terminal, a first electrode of the transistor T54 is connected to the negative phase voltage output terminal, and a second electrode of the transistor T54 receives the low voltage VSS；
A control electrode of the transistor T64 is connected to the negative phase voltage output terminal, a first electrode of the transistor T64 is connected to the positive phase voltage output terminal, and a second electrode of the transistor T64 receives the low voltage VSS。
In one possible implementation manner, the pull-up unit is further configured to pull up the positive-phase output voltage signal when the illumination intensity of the external light is strong;
the pull-down unit is further used for pulling down the negative phase output voltage signal when the illumination intensity of the external light is strong, and pulling down the positive phase output voltage signal when the illumination intensity of the external light is weak.
The beneficial effect of this application is:
the invention provides a thin film transistor integrated optical sensor, which comprises a photoelectric conversion part, a voltage conversion part and a voltage adjustment part, wherein the photoelectric conversion part is used for sensing external light and outputting a photocurrent signal corresponding to the illumination intensity of the external light, the voltage conversion part is used for converting the photocurrent signal into a voltage signal, the voltage signal comprises a voltage difference between a positive phase output voltage signal and a negative phase output voltage signal, and the voltage adjustment part is used for adjusting the positive phase output voltage signal and the negative phase output voltage signal according to the illumination intensity of the external light so as to amplify the voltage signal. The output voltage signal amplification of the sensor is controlled by the voltage adjusting part through the induction of the illumination intensity of the external light, so that the responsivity of the sensor can be improved, and the driving capability of the sensor is enhanced.
Drawings
Fig. 1 is a schematic structural diagram of a tft-integrated optical sensor according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a differential TFT optical sensor according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a transient response result obtained by SPICE equivalent simulation of the differential TFT optical sensor in FIG. 2;
FIG. 4 is a schematic diagram of another thin film transistor integrated optical sensor according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of transient response results obtained from SPICE equivalent simulation of the thin film transistor integrated optical sensor of FIG. 4;
FIG. 6 is a schematic diagram of another thin film transistor integrated optical sensor according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of another thin film transistor integrated optical sensor according to an embodiment of the present invention;
fig. 8 is a schematic structural diagram of another tft-integrated optical sensor according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the following detailed description and accompanying drawings.
The transistors in this application may be bipolar transistors or field effect transistors. When the transistor is a bipolar transistor, the control electrode refers to a base electrode of the bipolar transistor, the first electrode refers to a collector electrode or an emitter electrode of the bipolar transistor, and the corresponding second electrode refers to the emitter electrode or the collector electrode of the bipolar transistor; when the transistor is a field effect transistor, the control electrode refers to a gate electrode of the field effect transistor, the first electrode may be a drain electrode or a source electrode of the field effect transistor, and the corresponding second electrode may be a source electrode or a drain electrode of the field effect transistor. In an N-type transistor, the voltage of the drain should be greater than or equal to the voltage of the source, so the position of the source and the drain will vary with the bias state of the transistor. Since a transistor used in a display is generally a Thin Film Transistor (TFT), the embodiment of the present application does not take the thin film transistor as an example for description, and a drain and a source of the transistor in the embodiment of the present application may vary according to a bias state of the transistor.
Example one
As shown in fig. 1, an embodiment of the present invention provides a thin film transistor integrated optical sensor, which includes a photoelectric conversion portion 01, a voltage conversion portion 02, and a voltage adjustment portion 03, which will be described in detail below.
And a photoelectric conversion part 01 for sensing external light and outputting a photocurrent signal corresponding to the illumination intensity of the external light.
In the embodiment of the present invention, the photoelectric conversion portion 01 includes a single or a plurality of thin film transistors, and the thin film transistors perform a photoelectric effect under the influence of strong external light to generate a photocurrent signal.
A voltage converting part 02 for converting the photocurrent signal into a voltage signal including a voltage difference between a positive phase output voltage signal and a negative phase output voltage signal.
In the embodiment of the present invention, when the photoelectric conversion portion 01 generates a photocurrent signal due to the photoelectric effect, a voltage signal corresponding to the photocurrent signal is generated at the connection between the voltage conversion portion 02 and the photoelectric conversion portion 01, so as to convert the photocurrent signal into the voltage signal.
In one possible implementation manner, as shown in fig. 2, the photoelectric conversion portion 01 includes a transistor T1 and a transistor T4, and the voltage conversion portion 02 includes a transistor T2 and a transistor T3, wherein the transistor T3 and the transistor T2 are shielded from light, which is described in detail below.
The first pole of the transistor T1 receives the high voltage VDDA second pole of the transistor T1 is connected to the first pole of the transistor T3 and a positive phase voltage output terminal for outputting the positive phase output voltage signal VOUTPThe second pole of the transistor T3 receives a low voltage VSSThe first pole of the transistor T2 receives the high voltage VDDA second pole of the transistor T2 is connected to the first pole of the transistor T4 and a negative phase voltage output terminal for outputting the negative phase output voltage signal VOUTNThe second pole of the transistor T4 receives a low voltage VSSThe control electrodes of the transistors T1, T2, T3 and T4 all receive a predetermined voltage signal VBIASThe preset voltage signal VBIASFor controlling the transistor T1, the transistor T2, the transistor T3, and the transistor T4 to be in a sub-threshold region or a leakage region.
It should be noted that, under the action of ambient light and electrical stress, the electrical characteristics of the thin film transistor TFT are prone to drift, which makes the output of the sensor unstable. For example, the threshold voltage of an a-Si TFT increases with time under longer gate-source voltage or longer on-current. This causes variations in the output of the TFT-integrated sensor over time. Meanwhile, the performance of the Thin Film Transistor (TFT) in different batches and different sample wafers has certain difference.
FIG. 2 illustrates an embodiment of a differential TFT light sensorThe middle transistor T1 and the transistor T3 form a positive phase output branch circuit, and the positive phase output voltage signal V output by the positive phase voltage output terminal is controlled according to the illumination intensity condition of the outside lightOUTPThe transistor T1 and the transistor T3 form a negative phase output branch circuit, and the negative phase output voltage signal V output by the negative phase voltage output end is controlled according to the illumination intensity condition of the outside lightOUTN. The transistor T1 and the transistor T4 are light-sensitive elements whose leakage current or sub-threshold current value is modulated by ambient light, and the transistor T2 and the transistor T3 are reference elements whose device surfaces are subjected to light-shielding treatment and whose leakage current or sub-threshold current value is hardly affected by ambient light. The gates of the four TFTs are all subjected to a preset voltage signal VBIASThereby operating in either the sub-threshold region or the leakage region. When the intensity of light is increased, the values of the leakage currents of the transistor T1 and the transistor T4 are increased, so that the equivalent impedances of the transistor T1 and the transistor T4 are decreased, and the positive phase output voltage signal V is increasedOUTPCorrespondingly increased, while the negative phase output voltage signal VOUTNAnd is reduced accordingly. The differential TFT photosensor shown in fig. 2 has the advantages that: the common mode noise amount brought by temperature, manufacture procedure and voltage fluctuation can be effectively offset, the output has higher signal-to-noise ratio, and the structure is simple. However, the output signal value of the structure of fig. 2 is small, and the driving capability of the photoresponse output is weak, which is described in detail below with reference to fig. 3.
Fig. 3 illustrates the transient response results from SPICE equivalent simulation of the differential TFT photosensor shown in fig. 2, where the illumination intensity is multiplied and the photosensor is able to reach a stable voltage value substantially within 0.5 s. Here, the output voltage signal of the photo sensor takes the difference between the positive phase voltage output terminal and the negative phase voltage output terminal. From the simulation results of SPICE in fig. 3, it can be seen that the output of the differential TFT optical sensor in fig. 2 is not ideal: 1) the saturation is very easy, and the output voltage is close to the maximum voltage value above 32 times of illumination intensity; the response discrimination for strong illumination is poor. 2) At lower light intensities, there is no apparent light intensity threshold. Therefore, the light intensity response range of the light sensor is narrow, and the digitization process of the light sensor is difficult. Further, it is considered that the actual leakage current of the thin film transistor TFT is a result of the multi-physical field coupling control, which is not only affected by the intensity of light but also related to the operating temperature of the device, the stress (long time operation) accumulation time, and the like. To achieve a stable, controllable photo-electric response, it is generally necessary to digitize the output of the photo-sensor. If the digitisation process is handled entirely by the peripheral amplifier circuitry, the drift in the output of the light sensor after passing through the peripheral amplifier will be greater in the event of drift in its characteristics. Therefore, there is a need to have digitized features in the TFT photosensor itself: 1) a relatively stable photoelectric response threshold value is obtained; 2) above the threshold of the photoelectric response, the sensitivity of the photoelectric response is high. These requirements are not met by the TFT photosensor represented by fig. 2.
Considering that the weak driving capability of the TFT optical sensor shown in fig. 2 needs to be connected to the amplifying circuit again, the output result of the final circuit is inaccurate. For this purpose, the present application provides a voltage adjustment section 03, which is described in detail below.
And the voltage adjusting part 03 is configured to adjust the positive-phase output voltage signal and the negative-phase output voltage signal according to the illumination intensity of the external light, so as to amplify the voltage signals.
In the embodiment of the present invention, the voltage adjustment portion 03 includes one or more thin film transistors, and amplifies the voltage signal according to the illumination intensity of the external light, so as to improve the responsivity of the optical sensor, enhance the driving capability of the optical sensor, and avoid an error caused by the amplification of the voltage signal output by the optical sensor by the peripheral amplification circuit.
In one possible implementation manner, the voltage adjustment section 03 includes a pull-up unit and/or a pull-down unit, which is described in detail below.
The pull-up unit is used for pulling up the voltage signal according to the illumination intensity of the outside light. The pull-down unit is used for pulling down the voltage signal according to the illumination intensity of the outside light.
In the embodiment of the present invention, when the illumination intensity of the external light is strong, the pull-up unit may be used to pull up the positive-phase output voltage signal, when the illumination intensity of the external light is strong, the pull-down unit may be used to pull down the negative-phase output voltage signal, and when the illumination intensity of the external light is weak, the pull-down unit may be used to pull down the positive-phase output voltage signal. The positive phase output voltage signal is pulled down when the illumination intensity of the external light is weak, so that the voltage signal output by the optical sensor is reduced, and the negative phase output voltage signal is pulled up when the illumination intensity of the external light is strong, so that the voltage signal output by the optical sensor is increased. That is, when the illumination intensity of the external light is weak, the voltage signal output by the optical sensor is decreased, and when the illumination intensity of the external light is strong, the voltage signal output by the optical sensor is increased.
In one possible implementation manner, the voltage adjustment section 03 further includes an illumination threshold unit. And the illumination threshold value unit is used for setting a preset illumination threshold value and controlling the pull-up unit and/or the pull-down unit to work according to the illumination intensity of the outside light and the preset illumination threshold value.
Further, the light threshold unit may include a transistor T51, a transistor T61, and a transistor T71, wherein the transistor T51 and the transistor T61 are shielded from light, for example: the top and bottom regions of the TFT element are patterned with metal layers, so that the electrical characteristics of the TFT element are not affected by external light, as described in detail below.
The first pole of the transistor T51 receives the high voltage VDDA second pole of the transistor T51 is connected to the first pole of the transistor T71 and the control pole of the transistor T61, the control pole of the transistor T51 is connected to the negative phase voltage output terminal, the second pole of the transistor T71 is connected to the control pole of the transistor T71 and the negative phase voltage output terminal, the first pole of the transistor T61 is connected to the positive phase voltage output terminal, and the second pole of the transistor T61 receives the low voltage VSS。
Referring to fig. 4, the light sensing element includes a transistor T1, a transistor T4, and a transistor T71, and the non-light sensing element includes a transistor T2, a transistor T3, a transistor T51, and a transistor T61. Among them, the transistors T1 to T4 constitute a basic differential photosensitive structure shown in the differential TFT photosensor of fig. 2. The transistor T1 and the transistor T3 constituteSeries connection structure for controlling positive phase output voltage signal VOUTPTransistor T2 and transistor T4 form a series arrangement that controls the negative phase output voltage signal VOUTNTo output of (c). As can be understood from the analysis of fig. 2, in the digital driving, the output of the optical sensor is required to satisfy a certain threshold relationship, that is, a certain illumination threshold is reached, and the voltage signal V is outputted in the positive phaseOUTPOnly a certain amount of positive voltage is output, and when the illumination intensity does not reach the illumination threshold value, the positive phase output voltage signal VOUTPThe output value of (c) is still low. Ideally, the illumination threshold can be set flexibly, and the specific value should be in a certain relationship with the size of the element.
Compared with the scheme in fig. 2, in the TFT photosensor embodiment shown in fig. 4, an illumination threshold unit is added, that is, the TFT photosensor embodiment includes a transistor T51, a transistor T61, and a transistor T71, and a differential photosensitive structure formed by the transistors T1 to T4 is similar to that in fig. 2, and is not described again. The illumination threshold unit is described in detail below, and when the illumination intensity of the external light is weak, the negative phase output voltage signal VOUTNThe voltage is higher, the conduction degree of the transistor T51 is higher, so that the gate of the transistor T61 is pulled high by the conduction of the transistor T51, the transistor T61 is turned on, and the transistor T61 outputs the positive phase output voltage signal VOUTPPulling towards the low voltage Vss. The negative phase output voltage signal V can be output only when the illumination intensity of the external light reaches a certain amountOUTNPulled low, the transistor T61 is turned off, and the positive phase output voltage signal V is obtained after the transistor T61 is turned offOUTPCan be pulled high. Quantitatively, the on state of the transistor T61 is determined by the voltage distribution between the transistor T51 and the transistor T71, i.e., the gate voltage of the transistor T61 is at the high voltage VDDAnd a negative phase output voltage signal VOUTNIn the meantime. Under the condition of weak illumination intensity, the negative phase output voltage signal VOUTNHas a value of about (V)DD+VSS)/2. At this time, the gate potential of the transistor T61 is higher than (V)DD+VSS) /2, so the gate-source voltage difference of the transistor T61 is (V)DD-VSS) (v 2) VTH. Thus, the positive phase output voltage signal VOUTPQuilt crystalTube T61 is pulled down approximately to a low voltage VSS。
Assuming that the illumination intensity is increased by k times, the negative phase outputs a voltage signal VOUTNIs lowered to about VDDK, then approximately the gate voltage of transistor T61 is reduced to VDDK is the sum of the values of k and k. When the transistor T61 is turned off due to the influence of light, the transistor T61 is no longer the positive phase output voltage signal VOUTPSo that the photosensor begins to respond more strongly to light. Thus, the corresponding critical condition here is VDD/k＝Vth. In other words, the preset illumination threshold is k ═ VDD/Vth. For example, high voltage VDDValues of 9V, VthIn the case of 2, the light intensity is about 4.5 × 200lux to 900lux, and the positive phase output voltage signal V appearsOUTPIs increased. Here, when the light intensity of 200lux corresponds to the above, the transistor T71 is turned on as compared with the transistor T51.
Referring to fig. 5, fig. 5 illustrates the transient response results obtained by SPICE equivalent simulation of the thin film transistor integrated photosensor shown in fig. 4, in which the illumination intensity is increased by multiple times, and it can be seen that under 8 times of illumination intensity, the output value of the photosensor, i.e., the voltage signal, is almost 0 or negative. Here, the reason why the negative value of the output voltage signal occurs is that the positive phase output voltage signal VOUTPIs less than the negative phase output voltage signal VOUTN. Above 16 times the illumination intensity, the output of the light sensor shows greater sensitivity. Therefore, the thin film transistor integrated optical sensor illustrated in the present application has a good light intensity threshold control capability, and the illumination threshold can be precisely controlled by adjusting the width-to-length ratios of the transistor T51, the transistor T61 and the transistor T71.
In one possible implementation, the illumination threshold unit includes a transistor T52, a transistor T62, and a transistor T72, wherein the transistors T52 and T62 are shielded from light, which is described in detail below.
The first pole of the transistor T52 receives the high voltage VDDThe second pole of the transistor T52 is connected to the first pole of the transistor T72The control electrodes of the transistors T62, T52 and T72 receive a predetermined voltage signal VBIASThe second pole of the transistor T72 receives a low voltage VSSA first electrode of the transistor T62 is connected to the positive phase voltage output terminal, and a second electrode of the transistor T62 receives the low voltage VSS。
The thin film transistor integrated photosensor shown in fig. 6 is different from fig. 4 or fig. 2 in a light threshold cell including a transistor T52, a transistor T62, and a transistor T72. The transistor T52 and the transistor T73 are both at the predetermined voltage signal VBIASUnder the control of high voltage VDDAnd a low voltage VSSVoltage-operated voltage division, the intermediate potential V of which is generatedXFor controlling the conductive state of the transistor T62. Therefore, the positive phase output voltage signal VOUTPIncludes transistor T3 and transistor T62. This allows a positive output voltage signal V of the photosensor in the case of a stronger luminous fluxOUTPA larger voltage amplitude can be reached.
For example, in the photosensor constituted by the transistors T1 to T4 in fig. 2, when the light intensity is 200lux, the positive phase output voltage signal VOUTPIs 4.5V (where VDD is 9V and VSS is 0V). In contrast, in the thin film transistor integrated optical sensor shown in fig. 6, due to the pull-down effect of the transistor T62, when the illumination intensity is still 200lux, the positive phase output voltage signal VOUTPIs reduced to below 3V, only when the light intensity continues to increase to 350lux, due to the intermediate potential Vx being reduced to a low voltage V under the strong conduction action of the transistor T72SSThe transistor T62 is turned off, and the positive phase output voltage signal V is turned off after the transistor T62 is turned offOUTPThe potential of (2) rises to 4.5V or more.
For the thin film transistor integrated photosensor embodiments provided in fig. 4 and 6, by providing an illumination threshold unit to precisely control the threshold amount of illumination response, the sensitivity of the photo-electric response is higher only above the threshold amount of illumination response. This facilitates digitization, and suppresses the influence of noise current, light irradiation, and the like on digitization.
In one possible implementation manner, the voltage adjustment section 03 further includes a positive feedback unit, and the positive feedback unit has a positive feedback response mechanism, and is configured to control the operation of the pull-up unit and/or the pull-down unit according to the illumination intensity of the external light and the positive feedback mechanism.
In the embodiment of the invention, the positive feedback unit is used for enabling the voltage signal output by the optical sensor to be consistent with the control information direction of the external light received by the optical sensor, and the output of the voltage signal of the optical sensor can be promoted or enhanced. That is, when the illumination intensity of the external light is strong, the optical sensor outputs a voltage signal, and the voltage signal is amplified by the positive feedback unit.
In one possible implementation, the positive feedback unit includes a transistor T53 and a transistor T63, wherein the transistor T53 and the transistor T63 are light-shielded. This will be explained in detail below.
A control electrode of the transistor T53 is connected to the positive phase voltage output terminal, a first electrode of the transistor T53 is connected to the negative phase voltage output terminal, and a second electrode of the transistor T53 receives the low voltage VSSA control electrode of the transistor T63 is connected to the negative phase voltage output terminal, a first electrode of the transistor T63 is connected to the positive phase voltage output terminal, and a second electrode of the transistor T63 receives the low voltage VSS。
Referring to fig. 7, fig. 7 is different from fig. 2 in this positive feedback unit; the transistors T1-T4 form a basic differential structure, similar to fig. 2, and are not described again. The transistors T53 and T63 are light-insensitive elements, and the transistors T53 and T63 are cross-coupled and amplify the differential voltage signal output by the photosensor. When the light intensity reaches a certain degree, the voltage signal V is output in positive phaseOUTPIs pulled up and a negative phase outputs a voltage signal VOUTNIs pulled down. Due to positive phase output voltage signal VOUTPAnd a negative phase output voltage signal VOUTNThe turn-on of the transistor T53 and the turn-on of the transistor T63 are different.
In order to enhance the light response sensitivity of the light sensor, the transistors T1 to T4 should be turned offAnd (4) a zone. I.e. the preset voltage V received by the gate electrodes of the transistors T1-T4BIASShould be relatively negative, for example below 0V. The working principle of the thin film transistor integrated light sensor of fig. 7 is analyzed more quantitatively as follows:
1) in the non-exposure period, the equivalent impedances of the transistors T1 to T4 are all approximately equal, and therefore the positive phase output voltage signal VOUTPNegative phase output voltage signal VOUTN＝VDD/2。
2) In the exposure period, the leakage currents of the transistor T1 and the transistor T4 increase. As can be read from the current-voltage characteristic, the increase in leakage current is more than 10 times in the case of strong illumination. Equivalently, the equivalent impedances of T1 and T4 were reduced by a factor of 10. According to partial pressure relation, approximately
VOUTP＝VDD*RT2/(RT1+RT2)～VDD，VOUTN＝VDD*RT4/(RT3+RT4)～0
Here, the transistor T53 and the transistor T63 form a cross-coupled structure, and further amplify and enhance the output of the photosensor on the basis of the basic operating point of the photosensor shown in fig. 2. At moderate light intensity, VOUTP＞VDD/2＞VOuTNTherefore, the on current of the transistor T63 is larger than the on current of the transistor T53. This further results in a negative phase output voltage signal VOUTNIs pulled down to a lower, positive phase output voltage signal VOUTPWill rise as the conduction of transistor T53 decreases. Thus, a positive feedback mechanism is formed, which is advantageous for amplifying the positive phase output voltage signal VOUTPAnd a negative phase output voltage signal VOUTNThe voltage difference between them.
In one possible implementation manner, the positive feedback unit includes a transistor T54 and a transistor T64, wherein the transistor T64 is shielded from light, a control electrode of the transistor T54 is connected to the positive phase voltage output terminal, a first electrode of the transistor T54 is connected to the negative phase voltage output terminal, and a second electrode of the transistor T54 receives the low voltage VSSA control electrode of the transistor T64 is connected to the negative phase voltage output terminal, and a first electrode of the transistor T64 is connected toConnected to the positive voltage output terminal, the second pole of the transistor T64 receives the low voltage VSS。
Referring to fig. 8, fig. 8 is different from fig. 2 in that a positive feedback unit is provided, and the transistors T1 to T4 form a basic differential structure, which is similar to fig. 2 and will not be described again. Fig. 8 is different from fig. 7 in that the transistor T54 is a light receiving element, and the transistor T64 is a light non-receiving element. On the one hand, this cross-coupled structure of the transistor T54 and the transistor T64 amplifies the differential signal output of the illumination stimulus. On the other hand, since the transistor T54 is again a light receiving element, it will further increase the output response of the photoelectric effect.
The embodiment of the invention has the following characteristics:
the invention provides a thin film transistor integrated optical sensor, which comprises a photoelectric conversion part, a voltage conversion part and a voltage adjustment part, wherein the photoelectric conversion part is used for sensing external light and outputting a photocurrent signal corresponding to the illumination intensity of the external light, the voltage conversion part is used for converting the photocurrent signal into a voltage signal, the voltage signal comprises a voltage difference between a positive phase output voltage signal and a negative phase output voltage signal, and the voltage adjustment part is used for adjusting the positive phase output voltage signal and the negative phase output voltage signal according to the illumination intensity of the external light so as to amplify the voltage signal. The output voltage signal amplification of the sensor is controlled by the voltage adjusting part through the induction of the illumination intensity of the external light, so that the responsivity of the sensor can be improved, and the driving capability of the sensor is enhanced. And has a definite photoelectric response threshold, above which the sensitivity of photoelectric response is high.
The present invention has been described in terms of specific examples, which are provided to aid understanding of the invention and are not intended to be limiting. Numerous simple deductions, modifications or substitutions may also be made by those skilled in the art in light of the present teachings.
Claims (9)
1. A thin film transistor integrated light sensor, comprising:
a photoelectric conversion part for sensing external light and outputting a photocurrent signal corresponding to the illumination intensity of the external light;
a voltage converting part for converting the photocurrent signal into a voltage signal, the voltage signal including a voltage difference between a positive phase output voltage signal and a negative phase output voltage signal;
the voltage adjusting part is used for adjusting the positive phase output voltage signal and the negative phase output voltage signal according to the illumination intensity of the outside light so as to amplify the voltage signals;
the photoelectric conversion section includes a transistor T1 and a transistor T4, the voltage conversion section includes a transistor T2 and a transistor T3, wherein a transistor T3 and a transistor T2 are light-shielded;
the first electrode of the transistor T1 receives the high voltage VDDA second pole of the transistor T1 is connected to the first pole of the transistor T3 and a positive phase voltage output terminal for outputting the positive phase output voltage signal;
the second pole of the transistor T3 receives the low voltage VSS；
The first pole of the transistor T2 receives the high voltage VDDA second pole of the transistor T2 is connected to the first pole of the transistor T4 and a negative phase voltage output terminal for outputting the negative phase output voltage signal;
the second pole of the transistor T4 receives the low voltage VSS；
The control electrodes of the transistors T1, T2, T3 and T4 all receive a predetermined voltage signal VBIASThe preset voltage signal VBIASFor controlling the transistor T1, the transistor T2, the transistor T3, and the transistor T4 to be in a sub-threshold region or a leakage region.
2. The optical sensor according to claim 1, wherein the voltage adjustment section includes a pull-up unit and/or a pull-down unit;
the pull-up unit is used for pulling up the voltage signal according to the illumination intensity of the outside light;
the pull-down unit is used for pulling down the voltage signal according to the illumination intensity of the outside light.
3. The light sensor according to claim 2, wherein the voltage adjustment section further comprises an illumination threshold unit;
and the illumination threshold value unit is used for setting a preset illumination threshold value and controlling the pull-up unit and/or the pull-down unit to work according to the illumination intensity of the outside light and the preset illumination threshold value.
4. The optical sensor according to claim 2, wherein the voltage adjusting section further comprises a positive feedback unit;
and the positive feedback unit is provided with a positive feedback mechanism and is used for controlling the pull-up unit and/or the pull-down unit to work according to the illumination intensity of the outside light and the positive feedback mechanism.
5. The optical sensor according to claim 4, wherein the voltage adjustment section includes a transistor T51, a transistor T61, and a transistor T71; wherein the transistor T51 and the transistor T61 are light-shielded;
the first pole of the transistor T51 receives the high voltage VDDA second pole of the transistor T51 is connected to the first pole of the transistor T71 and the control pole of the transistor T61, and the control pole of the transistor T51 is connected to the negative phase voltage output end;
the second pole of the transistor T71 is connected to the control pole of the transistor T71 and the negative phase voltage output end;
a first electrode of the transistor T61 is connected to the positive phase voltage output terminal, and a second electrode of the transistor T61 receives the low voltage VSS。
6. The optical sensor according to claim 5, wherein the voltage adjustment section includes a transistor T52, a transistor T62, and a transistor T72; wherein the transistor T52 and the transistor T62 are light-shielded;
the first pole of the transistor T52 receives the high voltage VDDA second pole of the transistor T52 is connected to the first pole of the transistor T72 and the control pole of the transistor T62The control electrode of the transistor T52 and the control electrode of the transistor T72 receive a preset voltage signal VBIAS；
The second pole of the transistor T72 receives the low voltage VSS；
A first electrode of the transistor T62 is connected to the positive phase voltage output terminal, and a second electrode of the transistor T62 receives the low voltage VSS。
7. The optical sensor according to claim 6, wherein the voltage adjusting section includes a transistor T53 and a transistor T63; wherein the transistor T53 and the transistor T63 are light-shielded;
a control electrode of the transistor T53 is connected to the positive phase voltage output terminal, a first electrode of the transistor T53 is connected to the negative phase voltage output terminal, and a second electrode of the transistor T53 receives the low voltage VSS；
The control electrode of the transistor T63 is connected to the negative phase voltage output terminal, the first electrode of the transistor T63 is connected to the positive phase voltage output terminal, and the second electrode of the transistor T63 receives the low voltage VSS。
8. The optical sensor according to claim 7, wherein the voltage adjustment section includes a transistor T54 and a transistor T64; wherein, the transistor T64 is shielded from light;
a control electrode of the transistor T54 is connected to the positive phase voltage output terminal, a first electrode of the transistor T54 is connected to the negative phase voltage output terminal, and a second electrode of the transistor T54 receives the low voltage VSS；
The control electrode of the transistor T64 is connected to the negative phase voltage output terminal, the first electrode of the transistor T64 is connected to the positive phase voltage output terminal, and the second electrode of the transistor T64 receives the low voltage VSS。
9. The light sensor according to any one of claims 2 to 4,
the pull-up unit is also used for pulling up the positive phase output voltage signal when the illumination intensity of the external light is strong;
the pull-down unit is further used for pulling down the negative phase output voltage signal when the illumination intensity of the external light is strong, and pulling down the positive phase output voltage signal when the illumination intensity of the external light is weak.
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| CN101377438A (en) * | 2007-08-31 | 2009-03-04 | 卡西欧计算机株式会社 | Optical sensor, and display apparatus provided with the optical sensor |
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