Patent Document

BACKGROUND 
     1. Technical Field 
     The present invention relates to photo transistors, and more particularly to a driving method of a photo transistor, and a photo sensor and a flat panel display using the method. 
     2. Description of Related Art 
     Photo transistors and photo sensors using such photo transistors have the advantage of high sensitivity and have thus been widely used in various electronic devices, such as flat panel displays. 
     Referring to  FIG. 6 , a typical photo sensor  10  includes a photo transistor  11 , a resistor  12 , a first voltage terminal  13 , a second voltage terminal  14 , and a signal output terminal  15 . The photo transistor  11  is an amorphous silicon (a-Si) thin film transistor (TFT), and includes a gate electrode  111 , a source electrode  112 , a drain electrode  113 , and an a-Si layer (not shown). The gate electrode  111  is connected to the first voltage terminal  13 . The source electrode  112  is connected to the second voltage terminal  14 . The drain electrode  113  is connected to the signal output terminal  15  and to one end of the resistor  12 . The other end of the resistor  12  is grounded. 
     The first voltage terminal  13  and the second voltage terminal  14  output a first voltage and a second voltage to the gate electrode  111  and the source electrode  112  respectively, to drive the photo transistor  11 . The first voltage is five volts (5V) direct current (DC) voltage, and the second voltage is 1V DC voltage. 
     When the photo transistor  11  is exposed to light, the a-Si layer generates a plurality of photocarriers so as to produce photocurrent between the source electrode  112  and the drain electrode  113 . In operation, with variations in the light intensity in external environments, the quantity of photocarriers and photocurrent correspondingly change. Thereby, a resistance between the source electrode  112  and the drain electrode  113  varies, and a signal output by the signal output terminal  15  correspondingly varies. As a result, the variations of light intensity can be measured. 
     Due to the first voltage used for driving the gate electrode  111  of the photo transistor  11  being a positive DC voltage, a plurality of electrons attracted by the first voltage reside in the a-Si layer of the photo transistor  11 . These electrons can restrict the motion of the photocarriers, such that the photocurrent and the signal output by the signal output terminal  15  are weakened. Thus, the reliability of the photo sensor  10  using DC voltage to drive the photo transistor  11  is somewhat low. 
     What is needed is a driving method for a photo transistor which can overcome the limitations described, and a photo sensor using the driving method and a flat panel display using the photo sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of at least one embodiment. In the drawings, like reference numerals designate corresponding parts throughout the various views. 
         FIG. 1  is a circuit diagram of a photo sensor according to a first embodiment of the present invention, the photo sensor including a signal output terminal for outputting a signal. 
         FIG. 2  is a comparison diagram of signals output by the signal output terminal of the photo sensor of  FIG. 1 , and signals output by a signal output terminal of the photo sensor of  FIG. 6 , under the same external environmental light intensity. 
         FIG. 3  is a comparison diagram of signals output by the signal output terminals of the photo sensors of  FIG. 1  and  FIG. 6 , under gradually increasing light intensity of the external environment. 
         FIG. 4  is a circuit diagram of a photo sensor according to a second embodiment of the present invention. 
         FIG. 5  is a block diagram of a flat panel display according to the present invention. 
         FIG. 6  is a diagram of a conventional photo sensor, the photo sensor including a signal output terminal for outputting a signal. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings to describe exemplary embodiments of the present invention in detail. 
       FIG. 1  is a circuit diagram of a photo sensor  20  according to a first embodiment of the present invention. The photo sensor  20  includes a photo transistor  21 , a resistor  22 , a first voltage terminal  23 , a second voltage terminal  24 , and a signal output terminal  25 . The photo transistor  21  is an amorphous silicon (a-Si) TFT, and includes a gate electrode  211 , a source electrode  212 , a drain electrode  213 , and an a-Si layer (not shown) for generating photocarriers. The gate electrode  211  is connected to the first voltage terminal  23 . The source electrode  212  is connected to the second voltage terminal  24 . The drain electrode  213  is connected to the signal output terminal  25  and one end of the resistor  22 . The other end of the resistor  22  is grounded. 
     The first voltage terminal  23  outputs a first voltage to the gate electrode  111  and the second voltage terminal  24  outputs a second voltage to the source electrode  112 , to drive the photo transistor  21 . 
     The first voltage terminal  23  is an AC voltage output terminal. The first voltage is an AC voltage. In detail, the first voltage is a continuous alternating square signal, and includes a frequency, a first amplitude, and a second amplitude. The frequency of the first voltage is in the range from greater than 0 hertz (Hz) to less than or equal to 100 Hz, and is preferably 0.4 Hz. The first amplitude of the first voltage is in the range from greater than 0V to less than or equal to 20V, and is preferably 5V. The second amplitude of the first voltage is in the range from less than or equal to −20V to less than 0V, and is preferably −3V. The duty ratio of the first amplitude is in the range from 1:11 to 10:11, and is preferably 1:2. The duty ratio of the second amplitude is also in the range from 1:11 to 10:11, and is also preferably 1:2. 
     The second voltage terminal  24  is a DC voltage output terminal. The second voltage is in range from 1V to 10V, and is preferably 1V. 
     In operation, a period of the first voltage can be divided into two sub-periods T 1  and T 2 . A value of the first voltage is the first amplitude during sub-period T 1 . That is, the first voltage is a positive voltage, and a plurality of electrons are attracted by the first voltage and reside in the a-Si layer of the photo transistor  21 . In the subsequent sub-period T 2 , the value of the first voltage is the second amplitude. That is, the first voltage is a negative voltage. A plurality of positive holes are attracted by the first voltage and neutralize said plurality of electrons in the a-Si layer of the photo transistor  21 . As a result, most or even all of the photocarriers generated by the a-Si layer are not restricted by said plurality of electrons, and a corresponding signal output by the signal output terminal  25  can be steady. The light-electricity conversion efficiency of the photo transistor  21  and thus the photo sensor  20  is significant. 
     Referring to  FIG. 2 , a comparison of the signals output by the signal output terminals  15 ,  25  of the photo sensors  10 ,  20  under the same external environmental light intensity is shown. L 1  is a plot of the signals of the output terminal  15  of the photo sensor  10 . L 2  is a plot of the signals of the output terminal  25  of the photo sensor  20 . Under the same external environmental light intensity, the signal L 1  of the output terminal  15  begins to weaken from a tome about 5 minutes after the photo sensor  10  is enabled; whereas the signal L 2  of the output terminal  25  remains substantially steady even 13 hours after the photo sensor  20  is enabled. 
     Referring to  FIG. 3 , a comparison of the signals output by the signal output terminals  15 ,  25  of the photo sensors  10 ,  20  under gradually increasing external environmental light intensity is shown. In the illustration, the light intensity is increased in steps, with a ramp-up from one step to the next. L 3  is a plot of the signals of the output terminal  15  of the photo sensor  10 . L 4  is a plot of the signals of the output terminal  25  of the photo sensor  20 . Under the illustrated regime of gradually increasing external environmental light intensity, the signal L 3  of the output terminal  15  begins to exhibit weakening about 2 hours after the photo sensor  10  is enabled; whereas the signal L 4  of the output terminal  25  exhibits substantial steadiness even 3 hours after the photo sensor  20  is enabled. 
     In summary, the gate electrode  211  of the photo transistor  21  is provided with an AC voltage, and electrons attracted by the positive cycle of the AC voltage can be neutralized by the positive holes attracted by the negative cycle of the AC voltage. As a result, most or even all of the photocarriers are not restricted by said electrons, and the signal output by the signal output terminal  25  can be very steady. The light-electricity transfer conversion of the photo transistor  21  and the photo sensor  20  are improved. Accordingly, the reliability of the photo sensor  20  is also improved. 
     Referring to  FIG. 4 , a circuit diagram of a photo sensor  30  according to a second embodiment of the present invention is shown. The photo sensor  30  includes a first TFT  31 , a second TFT  35 , a first resistor  32 , a second resistor  36 , a first voltage terminal  33 , a second voltage terminal  34 , and a difference amplifier  37 . The first TFT  31  and the second TFT  35  are a-Si TFTs, and each includes a gate electrode (not labeled), a source electrode (not labeled), and a drain electrode (not labeled). The difference amplifier  37  includes two input terminals  371 ,  372  and an output terminal  373 . 
     The gate electrodes of the first and the second TFTs  31 ,  35  are connected to the first voltage terminal  33 . The source electrodes of the first and the second TFTs  31 ,  35  are connected to the second voltage terminal  34 . The drain electrode of the first TFT  31  is connected to the input terminal  371  and to one end of the first resistor  32 . The other end of the first resistor  32  is grounded. The drain electrode of the second TFT  35  is connected to the input terminal  372  and to one end of the second resistor  36 . The other end of the second resistor  36  is grounded. 
     The first voltage terminal  33  outputs a first voltage to the gate electrodes of the first and the second TFTs  31 ,  35 , and the second voltage terminal  34  outputs a second voltage to the source electrodes of the first and the second TFTs  31 ,  35 , to drive the first and the second TFTs  31 ,  35 . 
     The first voltage terminal  33  is an AC voltage output terminal. The first voltage is an AC voltage. In detail, the first voltage is a continuous alternating square signal, and includes a frequency, a first amplitude, and a second amplitude. The frequency of the first voltage is in the range from greater than 0 Hz to less than or equal to 100 Hz, and is preferably 0.4 Hz. The first amplitude of the first voltage is in the range from greater than 0V to less than or equal to 20V, and is preferably 5V. The second amplitude of the first voltage is in the range from less than or equal to −20V to less than 0V, and is preferably −3V. The duty ratio of the first amplitude is in the range from 1:11 to 10:11, and is preferably 1:2. The duty ratio of the second amplitude is also in the range from 1:11 to 10:11, and is also preferably 1:2. 
     The second voltage terminal  34  is a DC voltage output terminal. The second voltage is in the range from 1V to 10V, and is preferably 1V. 
     The first and the second resistors  32 ,  36  have the same impedance. The first TFT  31  serves as a photo transistor. The second TFT  35  serves as a comparing transistor, and is shaded by an object, such that light of the external environment is prevented from reaching the second TFT  35 . 
     In operation, due to the first TFT  31  serving as a photo transistor and the second TFT  35  being shaded, the first TFT  31  generates a quantity of photocarriers that the second TFT  35  does not. This causes the resistance between the source electrode and the drain electrode of the first TFT  31  to differ from that between the source electrode and the drain electrode of the second TFT  35 . Thereby, the signals of the two input terminals  371 ,  372  are correspondingly different, with a difference value therebetween considered as a light intensity signal of the external environment. Because the difference value is typically small, the difference amplifier  37  amplifies the difference value. 
     In summary, the gate electrode of the first TFT  31  is provided with an AC voltage, and electrons attracted by the positive cycle of the AC voltage can be neutralized by positive holes attracted by the negative cycle of the AC voltage. As a result, most or even all the photocarriers generated by the first TFT  31  are not be restricted by said electrons, and a signal of the output terminal  373  can be steady. Thus, the light-electricity conversion efficiency of the first TFT  31  and thus the photo sensor  30  may be considerable. Accordingly, the reliability of the photo sensor  30  is also improved. 
     Referring to  FIG. 5 , a block diagram of a flat panel display  1  according to the present invention is shown. The flat panel display  1  includes a brightness adjustment unit  2 , a controller  3 , an analog to digital (A/D) converter  4 , and a photo sensor  5 . The photo sensor  5  can be the same as the photo sensor  20  or  30 . 
     The photo sensor  5  provides a light intensity signal to the A/D converter  4 . The A/D converter  4  converts the light intensity signal to a digital signal. The controller  3  outputs an adjustment signal to the brightness adjustment unit  2  according to the digital signal. The brightness adjustment unit  2  adjusts the brightness of the flat panel display  1  accordingly. The improved performance of the photo sensor  20  or  30  provides corresponding improvement in the reliability of the flat panel display  1 . 
     It is to be understood, however, that even though numerous characteristics and advantages of the present embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the invention is illustrative only; and that changes may be made in detail, especially in matters of arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Technology Category: g