JP3563433B2 - Thin film phototransistor - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a TFT phototransistor, and more particularly to a highly sensitive double-gate polysilicon phototransistor.
[0002]
[Prior art]
A double gate type TFT in which two gate electrodes are provided on one semiconductor thin film layer is described in JP-A-60-58676. This is for suppressing the off current of the thin film transistor for liquid crystal, that is, the leakage current, and is configured to suppress the leakage current using the back electrode of the two gate electrodes provided above and below the semiconductor thin film layer. It was done.
[0003]
The double-gate TFT phototransistor is described in the technical report of the Institute of Television Engineers of Japan (ITE Technical Report Vol. 18, No. 4, pp. 19-24, published on January 21, 1994).
[0004]
[Problems to be solved by the invention]
A TFT that provides higher light sensitivity than conventional TFT phototransistors, easily obtains intermediate gradations, and widens the range of the same pairing characteristics in the differential method of an image reading circuit using a TFT. There is a strong demand for phototransistors.
[0005]
An object of the present invention is to provide a high-sensitivity TFT phototransistor capable of satisfying the above demand.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, as shown in FIG. 1, for example, a rear gate electrode 2 is provided on a Si single crystal substrate 1, and a rear gate insulating film 3 is formed thereon. On this rear gate insulating film 3, a source region 7, an active silicon layer 4, and a drain region 8 made of polysilicon are formed. Then, a front gate insulating film 5 is formed on the active silicon layer 4, and a front gate electrode 6 is provided thereon. In this case, the thickness of the active layer 4 is set to 500 ° or less. Then, the TFT phototransistor is formed by providing the interlayer insulating layer 9 and the electrodes 10 and 10. The light is irradiated from above in FIG. 1, and accordingly, a current flows between the source region 7 and the drain region 8.
[0007]
[Action]
In the present invention, the polarities of the electrodes applied to the front gate electrode 6 and the rear gate electrode 2 are reversed.
[0008]
For example, when the active layer 4 is an N-channel, a positive voltage is applied to the front gate electrode 6 while a negative voltage is applied to the rear gate electrode 2. Conversely, when the active layer is a P-channel, a negative voltage is applied to the front gate electrode 6 and a positive voltage is applied to the rear gate electrode 2. With this configuration, a very sensitive thin film phototransistor can be obtained.
[0009]
【Example】
One embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a structural view of an embodiment of the present invention, FIG. 2 is an explanatory view of a thin film phototransistor of the present invention (part 1), FIG. 3 is an explanatory view of a thin film phototransistor of the present invention (part 2), FIG. 4 is a graph showing the gate voltage dependence of the N-channel thin film phototransistor of the present invention, FIG. 5 is the graph showing the gate voltage dependence of the P-channel thin film phototransistor of the present invention, and FIG. FIG. 7 shows a characteristic diagram of the layer thickness and the current value, and FIG. 7 shows an energy band diagram of a double-gate thin-film phototransistor.
[0010]
In FIG. 1, 1 is an insulating film such as a Si single crystal substrate, 2 is a rear gate electrode, 3 is a rear gate insulating film, 4 is an active silicon layer, 5 is a front gate insulating film, 6 is a front gate electrode, 7 Is a source region, 8 is a drain region, 9 is an interlayer insulating layer, and 10 and 10 are electrodes. Note that an insulating substrate such as a glass substrate, a quartz plate, or an alumina plate may be used instead of the insulating film 1. In this case, when a glass substrate or an alumina plate is used, it is desirable to form a thin SiO ₂ film thereon. When a Si single crystal substrate is used, SiO ₂ may be formed on the surface.
[0011]
The manufacturing process of the double-gate type thin film phototransistor according to the present invention will be described with reference to FIGS.
(1) First, a polysilicon layer 2 'made of P-doped polysilicon or B-doped polysilicon is formed on the insulating film 1 made of a Si single crystal substrate to a thickness of about 1000 (see FIG. 2A).
[0012]
At this time, for example, the film forming conditions of P doping are as follows.
SiH ₄ gas 10 to 50 SCCM
5% PH ₃ / H ₂ gas 5-20 SCCM
Pressure 0.1-1 Torr
Temperature 600-650 ° C
(2) Next, the polysilicon layer 2 'formed by the above (1) is patterned to form the rear gate electrode 2 (see FIG. 2B).
[0013]
(3) Then, a rear gate insulating film 3 made of SiO ₂ is formed with a thickness of about 100 to 1000 °. In this case, desirably, a thinner one can apply a larger electric field (see FIG. 2C).
[0014]
The film forming conditions at this time are as follows.
TEOS (tetraethoxysilane) gas 10-50 SCCM
O ₂ 500 SCCM
Pressure 0.5 to 1.0 Torr
Temperature 750 ℃
(4) An amorphous silicon film 21 is formed thereon at about 1500 °, and then annealed to obtain a polysilicon active silicon layer by solid phase growth (see FIG. 2 (D)).
[0015]
The film forming conditions at this time are as follows.
Si ₂ H ₆ gas 100-500 SCCM
He 500SCCM
Pressure 0.5 to 1.0 Torr
Temperature 750 ℃
The annealing conditions for forming the amorphous silicon film 21 into polysilicon by solid phase growth are as follows.
[0016]
N ₂ 1SLM
Temperature 600 ° C
Processing time 8 to 48 hours
(5) The polysilicon film is patterned to form polysilicon films 21 'and 21' (see FIG. 2E).
[0017]
(6) The polysilicon films 21 ', 21' formed by the above (5) are thermally oxidized to form a front gate insulating film 5 made of SiO _{2 with} a thickness of about 1000 ° (see FIG. 2F). ). The polysilicon films 21 'and 21' shown in FIG. 2F become a source region, an active silicon layer, and a drain region as described later.
[0018]
The film forming conditions at this time are as follows.
O ₂ 2.7SLM
Temperature 1000 ° C
Processing time 150min
(7) On the front gate insulating film 5, a P-doped polysilicon 22 for a front gate electrode is formed to a thickness of about 1000 ° (see FIG. 2 (G)).
[0019]
The film forming conditions at this time are as follows.
SiH ₄ gas 10 to 50 SCCM
5% PH ₃ / H ₂ gas 5-20 SCCM
Pressure 0.1-0.5 Torr
Temperature 600-650 ° C
(8) The front gate electrodes 6, 6 are formed by patterning the front gate electrode polysilicon 22 formed in the above (7) (see FIG. 2H).
[0020]
(9) After forming the front gate electrodes 6 in this way, doping is performed to form the polysilicon films 21 ′, 21 ′ in the source region 7, the drain region 8 and the active silicon layer 4. When forming the active silicon layer 4 with an N channel, for example, P is implanted (see FIG. 3A).
[0021]
(10) When the active silicon layer 4 is formed with a P channel, for example, B is implanted. In this case, when one active silicon layer 4 is formed by an N channel and one active silicon layer 4 is formed by a P channel, B may be implanted by masking the N channel side with a resist 23 (FIG. 3). (B)).
[0022]
(11) After these implantations, heat treatment is performed at about 600 ° C. for 12 hours to activate.
(12) Then, an interlayer insulating layer 9 made of SiO _{2 is} formed at about 4000 ° (see FIG. 3C).
[0023]
The film forming conditions at this time are as follows.
TEOS gas 10-50 SCCM
O ₂ 500 SCCM
Power 50-300W
Pressure 0.5 Torr
Temperature 400 ° C
(13) A hole 24 is formed in the interlayer insulating layer 9 for wiring to a source region and a drain region (see FIG. 3D).
[0024]
(14) Then, aluminum 25 serving as an electrode metal is deposited (see FIG. 3E).
(15) The aluminum 25 is patterned to form the electrodes 10 and 10 of the source region 7 and the drain region 8, and wiring is performed to obtain a double-gate polysilicon TFT phototransistor (see FIG. 3F). .
[0025]
The operating characteristics of the thin-film phototransistor of the present invention obtained as described above will be described.
FIG. 4 shows threshold voltages of light irradiation (Photo) and light irradiation (Dark) when a positive to negative voltage is applied to the rear gate electrode 2 when the active silicon layer is an N channel, that is, a front part. It shows the magnitude of the positive threshold voltage to be applied to the gate voltage. In this figure, circles indicate values when light is not irradiated, and triangles indicate values when light is irradiated.
[0026]
As is clear from FIG. 4, when a positive voltage is applied to the rear gate electrode, the characteristics of both are the same. However, it can be seen that when a negative voltage is applied to the rear gate electrode, the threshold voltage is smaller in the case where light is applied (Photo) than in the case where light is not applied (Dark). That is, it is understood that the TFT phototransistor is turned on when the voltage to be applied to the front gate electrode is small.
[0027]
In FIG. 5, when the active silicon layer is a P-channel, the negative voltage to be applied to the front gate electrode is smaller in the light irradiation state (Photo) when a positive voltage is applied to the rear gate electrode. It can be seen that the conduction state occurs at a threshold voltage smaller than that in the case (Dark).
[0028]
As described above, in the double-gate thin film phototransistor of the present invention, the electric fields to be applied to the front gate electrode and the rear gate electrode preferably have opposite polarities.
FIG. 6 shows the relationship between the thickness of the active silicon layer and the current value in the thin-film phototransistor of the present invention. And the drain current value without light irradiation (Dark). In this example, the active silicon layer is an N channel, the source / drain voltage is 1.5V, the voltage applied to the front gate electrode is V _G = 5V, and the value applied when the voltage applied to the rear gate electrode is −15V is The value at −10 V is indicated by a circle, and the value at −10 V is indicated by a cross.
[0029]
As is clear from this, when the thickness of the active silicon layer is 500 ° or less, the value of the difference increases.
The reason why good characteristics are obtained when an electric field of opposite polarity is applied to the front gate electrode and the rear gate electrode as in the present invention will be described with reference to FIG.
[0030]
FIG. 7A shows an energy band diagram of a cross section of the thin film transistor when an electric field of the same polarity is applied to the front gate electrode and the rear gate electrode, and FIG. 7B shows the energy when an electric field of the opposite polarity is applied. The band diagram is shown.
[0031]
When an electric field of the same polarity is applied to the front gate electrode and the rear gate electrode, in FIG. 7A, the hole electrons generated when the active silicon layer is irradiated with light are converted into the active silicon layer of the polysilicon layer. Therefore, it is trapped by the crystal grain boundary therearound, and the threshold value does not change.
[0032]
On the other hand, when an electric field of opposite polarity is applied to the front gate electrode and the rear gate electrode, as shown in FIG. 7 (B), an electric field is applied due to the opposite polarity, so that holes generated by light irradiation are generated. Electrons are polarized, the threshold value is easily changed, and a highly sensitive one is obtained.
[0033]
FIG. 8 illustrates a differential circuit for detecting an optical signal using the thin film phototransistor of the present invention. In FIG. 8, PET and DTFT are both double-gate thin film phototransistors constructed according to the present invention. The TFT1 and TFT2 used are both thin-film phototransistors for switching, and a control voltage for ON / OFF is applied to their gate electrodes. The PTFT, DTFT, TFT1, and TFT2 constitute a light detection unit.
[0034]
Output currents of the thin-film phototransistors TFT1 and TFT2 are input to current / voltage converters D1 and D2, converted into voltages corresponding to current values, and input to a differential amplifier DA.
[0035]
When light is incident on the thin-film phototransistor PTFT, a corresponding current is input to the current / voltage converter D1 through the thin-film TFT1 and converted into a voltage. At this time, the output current of the light-shielded thin film phototransistor DTFT is similarly input to the current / voltage converter D2 and converted into a voltage. These output voltages are input to the differential amplifier DA, and an output voltage Vout corresponding to the incident light, in which the voltage corresponding to the light shielding current is subtracted, is obtained.
[0036]
In this case, if the sensitivity of the thin-film phototransistor PTFT is low, if the characteristics with the thin-film phototransistor DTFT are not uniform, that is, if the pairing is not good, the voltage values at the time of light blocking become uneven, and the accurate value of the incident light becomes I can't get it. However, in the present invention, since the sensitivity of the thin-film phototransistor is high, the output at the time of light incidence is increased, and the influence of the non-uniformity of the characteristics at the time of light blocking on the reading accuracy can be reduced.
[0037]
According to the present invention, it is possible to obtain a phototransistor having a photosensitivity about 50 times higher than that of a conventional thin film phototransistor. The read time of a double-gate type phototransistor published by the ITE Technical Report is about 800 μs, while the present invention can improve the read time to about 100 μs.
[0038]
Note that the present invention is not limited to the Si single crystal substrate 1, and an insulating substrate such as a glass substrate, a quartz plate, and an alumina plate may be used. In that case, a thin SiO ₂ film is preferably formed on the glass substrate and the alumina plate. An SiO ₂ film may also be formed on a Si single crystal substrate.
[0039]
In the present invention, the use state of the front gate electrode and the rear gate electrode is reversed from the above, a constant voltage is applied to the rear gate electrode, and a voltage having a polarity opposite to that of the front gate electrode is applied to the front gate electrode. It can also be used as a thin film phototransistor.
[0040]
【The invention's effect】
According to the present invention, it is possible to provide a thin film phototransistor having extremely excellent light sensitivity.
[0041]
According to the present invention, the hole electrons generated in the active polysilicon layer are hardly trapped and are polarized, so that the threshold value is easily changed and a highly sensitive one is obtained.
[0042]
According to the present invention, the current ratio (current difference) between photo and dark can be increased, and a high sensitivity can be obtained. In addition, when the thin film effect is thick, an electric field is less likely to be applied.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an embodiment of the present invention.
FIG. 2 is an explanatory view (1) of a manufacturing process of the thin film phototransistor of the present invention.
FIG. 3 is an explanatory view (2) of a manufacturing process of the thin-film phototransistor of the present invention.
FIG. 4 is a graph showing gate voltage dependence characteristics of an N-channel thin film phototransistor according to the present invention.
FIG. 5 is a graph showing gate voltage dependence characteristics of a P-channel thin film phototransistor according to the present invention.
FIG. 6 is a characteristic diagram of the thickness of the active silicon layer and the current value of the N-channel thin film phototransistor of the present invention.
FIG. 7 is an energy band diagram of a double-gate thin film phototransistor.
FIG. 8 is an example of a differential circuit for detecting an optical signal using the thin-film phototransistor of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Single crystal silicon substrate 2 Rear gate electrode 3 Rear gate insulating film 4 Active silicon layer 5 Front gate insulating film 6 Front gate electrode 7 Source region 8 Drain region 9 Interlayer insulating layer 10 Electrode

Claims (2)

In a thin film phototransistor provided with an active layer irradiated with light between a source region and a drain region of the thin film transistor,
Said active layer is composed of the following polysilicon Con thickness 500 Å, thin film phototransistor, characterized in that a active layer of the polysilicon con between the two gate electrodes. Of said two gate electrodes, to a voltage you applied to the gate electrode of the photo output side, that the applied voltage was opposite polarity to the voltage applied to the gate electrode of the output side to the gate electrode of the opposite The thin film phototransistor according to claim 1, wherein:

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