Abstract:
A nanotube field emission display. The nanotube field emission display includes a nanotube field emission cell, an active device, and a capacitor. The nanotube field emission cell includes a cathode, a gate, and an anode, wherein the cathode has nanotubes for field emission where the gate is used. The active device includes a first electrode, a second electrode, and a control electrode, wherein the second electrode is coupled to the gate of the nanotube field emission cell.  
     The capacitor is coupled between the gate of the nanotube field emission cell and a voltage source to store gate voltage to control illumination and gray level of the nanotube field emission cell.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a carbon nanotube field emission display, and particularly to an active control applied in a carbon nanotube field emission display.  
           [0003]    2. Description of the Related Art  
           [0004]    Carbon nanotube field emission display (CNT-FED) is one kind of FED with matrix driving. Each pixel has a cathode, lower plate, with a layer of nanotube for electron emission source and an anode, upper plate, with a transparent electrode for electron attraction. The cathode accelerates electrons to bombard fluorescent material for fluorescence. Arrangements of FEDs of pixels show images. CNT-FED is a high voltage device implemented by two electrodes, anode and cathode, or by three electrodes, anode, cathode, and gate. CNT-FED takes advantage of high voltage and low current to achieve high illumination. In addition to the light and slim properties of LCD (liquid crystal display), CNT-FED has the high illumination properties of CRT (cathode ray tube) displays, making CNT-FED a commercially valuable flat display. The illumination mechanism in three-electrode CNT-FED is briefly described as follows. Fluorescent material is disposed at an anode, and an electron emission source, CNT, with point discharge, is disposed at a cathode. A gate is arranged between anode and cathode to attract electrons from tips of CNTs by high electrical fields and accelerating electrons to bombard the cathode to fluoresce. Therefore, the operating voltage of three-electrode CNT-FED is reduced.  
           [0005]    Conventional control of CNT-FED is active or passive control. Passive control is provided by two or three electrodes. FIG. 1 shows a structure of a two-electrode CNT-FED, which has CNT  7 , cathode  6 , and anode  8 . In operating mode, anode  8  provides high voltage inducing field emission at cathode  6 , and electrons bombard anode  8  to phosphoresce. In passive control, each pixel is lit instantly by scanning, and duration of providing high voltage at anode  8  is controlled by a PWM (pulse width modulation) signal for adjusting gray level. CNT-FED must provide instantaneous illumination in a short time such that required average illumination is achieved in a frame. The operation of FIG. 1 reduces lifecycle of CNT-FED and requires high control voltages. FIG. 2 shows a structure of three-electrode CNT-FED, which includes CNT  7 , cathode  6 , anode  8 , and gate  15 . In normal-on mode, anode  8  provides high voltage to attract electrons from cathode  6 , and gate  15  reduces voltage, attracting electrons and achieving low control voltage. Gate  15  also provides a negative voltage to turn off CNT-FED. Gate  15  is utilized to limit the number of electrons to control phosphoresce at anode  8  and controls gray levels. The operation in FIG. 2 still requires PWM signals to control and achieve transient visual, and it also reduces lifecycle.  
           [0006]    [0006]FIG. 3 shows a structure of active control applied in two-electrode CNT-FED, which includes an active device T 1 , such as TFT, and a two-electrode CNT-FED, including CNT  7 , cathode  6 , and anode  8 . FIG. 4 shows an equivalent circuit of FIG. 3. In operating mode, a scan line  25  provides voltage to turn on the active device T 1 , and a signal line  27  is coupled to cathode  6  by the active device T 1  to control field emission, inducing phosphoresce, at cathode  6 . The scan line  25  provides voltage to turn off the active device T 1 , thereby extinguishing phosphoresce at anode. Adjustments of gray level are achieved by controlling the duration of illumination. The structure of FIG. 3 has advantages of low control voltage and high anode voltage but still has disadvantages of high instantaneous illumination and short lifecycle.  
           [0007]    In order to eliminate disadvantages of high control voltage, high instantaneous illumination, and short lifecycle, there is a need for a combination of an active device, a capacitor, and a three-electrode CNT-FED, which has the advantages of active control, low control voltage, stored control voltage, preserved gray level, high illumination, and reduced power consumption. The CNT-FED of the present invention has a novel control scheme applied in various processes.  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore an object of the present invention to provide an active control for a nanotube field emission display.  
           [0009]    To achieve the above objects, the present invention provides a nanotube field emission cell with an active device and a capacitor. The nanotube field emission cell includes a cathode, a gate, and an anode. The cathode has nanotube for field emission. The active device includes a first electrode, a second electrode, and a control electrode. The second electrode is coupled to the gate of the nanotube field emission cell. The capacitor is coupled between the gate of the nanotube field emission cell and a voltage source to store gate voltage to control illumination and gray level of the nanotube field emission cell.  
           [0010]    According to the active device, the present invention discloses an active control matrix. The active control matrix includes scan lines, data lines, and active devices. Nanotube field emission cells, active devices, and capacitors are disposed at intersections of scan lines and data lines. Scan lines are coupled to control electrodes of active devices. Data lines are coupled to the first electrodes of active devices. The second electrodes of active devices and capacitors are coupled to gates of nanotube field emission cells.  
           [0011]    To realize active control, the present invention also provides a nanotube field emission cell with two diodes and a capacitor. A first diode has a first electrode and a second electrode, wherein the second electrode is coupled to the gate of the nanotube field emission cell. A second diode has a first electrode and a second electrode, wherein the first electrode is coupled to the gate of the nanotube field emission cell.  
           [0012]    A capacitor coupled between the gate of the nanotube field emission cell and a scan line to store gate voltage to control illumination and gray level of the nanotube field emission cell.  
           [0013]    According to the above active control device, the first diode and the second diode, the present invention also discloses an alternative active control matrix. The active control matrix includes scan lines, first data lines, second data lines, nanotube field emission cells, first diodes, second diodes, and capacitors. The nanotube field emission cells, first diodes, second diodes, and capacitors are disposed at intersections of scan lines and first data lines. The scan lines are coupled to corresponding capacitors. The first data lines are coupled to the first electrodes of first diodes. The second data lines are coupled to the second electrodes of second diodes. The first electrodes of the first diodes and the second electrodes of the second diodes are coupled to the capacitors and gates of the nanotube field emission cells. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The aforementioned objects, features and advantages of this invention will become apparent by referring to the following detailed description of the preferred embodiment with reference to the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 shows a structure of a two-electrode CNT-FED.  
         [0016]    [0016]FIG. 2 shows a structure of a three-electrode CNT-FED.  
         [0017]    [0017]FIG. 3 shows a structure of active control applied in two-electrode CNT-FED.  
         [0018]    [0018]FIG. 4 shows an equivalent circuit of FIG. 3.  
         [0019]    [0019]FIG. 5 is a schematic of a CNT-FED with active control in the first embodiment.  
         [0020]    [0020]FIG. 6 shows an active control matrix for a CNT-FED of the first embodiment.  
         [0021]    [0021]FIG. 7 shows a cross-section of the first embodiment.  
         [0022]    [0022]FIG. 8 is a schematic of a CNT-FED with active control in the second embodiment.  
         [0023]    FIGS.  9 A- 9 B shows operations of the second embodiment.  
         [0024]    [0024]FIG. 10 shows an active control matrix for CNT-FED of the second embodiment.  
         [0025]    [0025]FIG. 11 shows a cross-section of the second embodiment.  
         [0026]    [0026]FIG. 12 is a schematic of a CNT-FED with active control in the third embodiment.  
         [0027]    FIGS.  13 A- 13 B shows operations of the third embodiment.  
         [0028]    [0028]FIG. 14 shows an active control matrix for CNT-FED of the third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The First Embodiment  
         [0030]    [0030]FIG. 5 is a schematic of a CNT-FED with active control in the first embodiment. The CNT-FED with active control includes an active device T 1 , a capacitor C 1 , and a three-electrode CNT-FED  100  including cathode  6 , gate  9 , and anode  8 . The operating method follows.  
         [0031]    1. Anode  8  is fed a high voltage. Cathode  6  is coupled to ground. CNT-FED  100  is in “normally on” state in the abnormal operating mode.  
         [0032]    2. In a scan period, a scan signal is fed to the scan line  25 , such that the active device T 1  is turned on, and the data line is coupled to gate  9  to provide control voltage, thereby controlling field emission at cathode  8 , illumination, and gray level. The capacitor C 1  stores control voltage of gate  9 .  
         [0033]    3. In the next scan period, the scan line  25  is fed a voltage to turn off the active device T 1 . The data line  27  is isolated from gate  9 . The capacitor C 1  holds control voltage from the previous frame to preserve illumination and gray level.  
         [0034]    4. Processes 2 and 3 are repeated. The CNT-FED  100  writes data and preserves illumination and gray level.  
         [0035]    Gate  9  can be fed positive voltage or negative voltage. Fed positive voltage, gate  9  attracts electrons, cathode  8  emits electrons, and CNT-FED  100  is in “normally off” state of normal operating mode, that is, turned on only when the scan line is active. Fed negative voltage, the CNT-FED  100  is in “normally on” state in the abnormal operating mode, that is, turned off only when the scan line is active. By gate  9 , gray level is adjustable and control voltage is reduced.  
         [0036]    Compared with conventional active control of two-electrode CNT-FED in FIGS. 3 and 4, the CNT-FED  100  in the first embodiment stores control voltage by a capacitor and sustains a gray level and illumination in a set time frame. High instantaneous illumination is not required; therefore, CNT-FED has a long lifecycle. Compared with conventional passive control, the CNT-FED in the present embodiment has advantages of power saving, high yielding, long lifecycle, and fabrication at large substrate.  
         [0037]    [0037]FIG. 6 shows an active control matrix for CNT-FED of the first embodiment. As shown in FIG. 6, the active control matrix includes scan lines SL 1 ˜SLN, data lines DL 1 ˜DLM, active devices T 1  disposed at the intersection of scan lines SL 1 ˜SLN and data lines DL 1 ˜DLM, and CNT-FED. Operation process is described as follows. In the first scan period, the scan line SL 1  turns on active devices of the first column and data lines DL 1 ˜DLM feed control voltages of the active devices of the first column to capacitors of the first column respectively. Control voltages of the three-electrode CNT-FED  100  of the first column are stored in the capacitors of the first column. In the next scan period, the scan line is inactive and the capacitors of the first column sustain illumination of CNT-FED of the first column until the next time frame when control voltages are rewritten. The scan lines SL 2 ˜SLN repeats previous procedures thereby data of the entire frame is completely written.  
         [0038]    [0038]FIG. 7 shows a cross-section of the first embodiment. Fabrication process is as follows. The gate  3  of the active device T 1  is fabricated first, electrode  4  of the capacitor C 1 , and cathode  6  of the three-electrode CNT-FED. CNT  7  is formed on cathode  6 , and dielectric layer  10  later. Thickness of the dielectric layer  10  is adjustable for various applications. The dielectric layer  10  is applied for the active device T 1 , the capacitor C 1 , and the three-electrode CNT-FED  100 . Source  1  and drain  2  of the active device T 1  is fabricated, and gate  2 , and  9  of the three-electrode CNT-FED  100 . Semiconductor layer  5  of the active device T 1  is fabricated. The lower plate is fabricated completely. Phosphor  11  is disposed on the upper plate  8 . The upper plate  8  and the lower plate are assembled, packaged, and vacuumized thereafter the active control and gray-level adjustable CNT-FED is completed. The active device T 1  is fabricated by bottom gate, top gate, and trench gate.  
         [0039]    The Second Embodiment  
         [0040]    [0040]FIG. 8 is a schematic of a CNT-FED with active control in the second embodiment. The CNT-FED with active control includes a diode D 1 , a diode D 2 , a capacitor C 1 , and a three-electrode CNT-FED  100  including cathode  6 , gate  9 , and anode  8 . The operating method follows.  
         [0041]    1. As shown in FIG. 9A, in a scan period, a negative-voltage scan signal is fed to V-bias and a first data line DL 1  feeds a control voltage to turn on the diode D 1 . The control gate voltage  9  controls field emission at cathode  6  and achieves adjustment of illumination and gray level. The second data line RL 1  feeds a first bias voltage, a positive voltage, thereby the diode D 2  is reverse-biased. The capacitor C 1  stores control gate voltage  9 .  
         [0042]    2. As shown in FIG. 9B, in the next scan period, V-bias  30  is coupled to a ground potential and the diode D 1  and D 2  are reverse-biased. The capacitor C 1  holds control voltage in the previous scan period, so the CNT-FED  100  preserves illumination and gray level.  
         [0043]    3. As shown in FIG. 9C, the second data line RL 1  feeds a second bias voltage, a negative, the diode D 2  is forward-biased. The capacitor C 1  is discharged. The control gate voltage  9  is cleared.  
         [0044]    4. Procedures 1˜3 are repeated. The CNT-FED  100  achieves writing data and preserving illumination and gray level.  
         [0045]    [0045]FIG. 10 shows an active control matrix for CNT-FED of the second embodiment. As shown in FIG. 10, the active control matrix includes the first data lines DL 1 ˜DLM, the second data lines RL 1 ˜RLN, SCAN lines SL 1 ˜SLN, CNT-FED, capacitor C 1 , and diodes D 1  and D 2  disposed at the intersection of the first data lines DL 1 ˜DLM and the second data lines RL 1 ˜RLN. Operation procedures are described as follows. In the first scan period, the scan line SL 1  is fed a negative voltage, and the second data line RL 1  is fed a first bias voltage, a positive voltage, to turn on the diodes D 1  and turn off the diodes D 2  of the first column. The data lines DL 1 ˜DLM feed control voltages to capacitors of the first column through the diodes D 1  respectively. Control voltages of the three-electrode CNT-FED  100  of the first column are stored in the capacitors of the first column. In the next scan period, the first scan line SL 1  is at a ground potential and the diodes D 1  of the first column are turned off and control voltages of the first column are stored in the capacitors of the first column, thereby sustaining illumination of the three-electrode CNT-FED  100  of the first column until the next time frame when control voltages are rewritten. The scan lines SL 2 ˜SLN repeat previous procedures, thereby data of the entire frame is written completely.  
         [0046]    [0046]FIG. 11 shows a cross-section of the second embodiment. Fabrication process is as follows. An electrode plate  4  of the capacitor C 1  is fabricated first, the cathode  6  of the three-electrode CNT-FED  100 , and dielectric layer  10  later. Thickness of the dielectric layer  10  is adjustable for various applications. The dielectric layer  10  is applied for the capacitor C 1  and the three-electrode CNT-FED  100 . P-type semiconductor layers  13  and  18  of the diodes D 1  and D 2  respectively are fabricated. N-type semiconductor layers  14  and  17  of the diodes D 1  and D 2  respectively are fabricated. The following are the gate  2  and  9  of the three-electrode CNT-FED  100 . The electrodes  12 ,  15 ,  16 ,  19  of the diodes D 1  and D 2  are fabricated. The lower plate is fabricated completely. Phosphor  11  is disposed on the upper plate  8 . The upper plate  8  and the lower plate are assembled, packaged, and vacuumized thereafter the active control and gray-level adjustable CNT-FED is completed. The diodes D 1  and D 2  are fabricated by bottom gate, top gate, and trench gate.  
         [0047]    The Third Embodiment  
         [0048]    [0048]FIG. 12 is a schematic of a CNT-FED with active control in the third embodiment. The CNT-FED with active control includes a diode D 1 , a diode D 2 , a capacitor C 1 , and a three-electrode CNT-FED  100  including cathode  6 , gate  9 , and anode  8 . The operating method follows.  
         [0049]    1. As shown in FIG. 13A, in a scan period, a positive scan signal is fed to V-bias  30  and a first data line DL 1  feeds a control voltage to turn on the diode D 1 . The control gate voltage  9  controls field emission at cathode  6  and achieves adjustment of illumination and gray level. The second data line RL 1  feeds a first bias voltage, a negative voltage, thereby the diode D 2  is reverse-biased. The capacitor C 1  stores control gate voltage  9 .  
         [0050]    2. As shown in FIG. 13B, in the next scan period, V-bias  30  is coupled to a ground potential and the diode D 1  and D 2  are reverse-biased. The capacitor C 1  holds control voltage in the previous scan period, so the CNT-FED  100  preserves illumination and gray level.  
         [0051]    3. As shown in FIG. 13C, the second data line RL 1  feeds a second bias voltage, a positive, the diode D 2  is forward-biased. The capacitor C 1  is discharged. The control gate voltage  9  is cleared.  
         [0052]    4. Procedures 1˜3 are repeated. The CNT-FED  100  achieves writing data and preserving illumination and gray level.  
         [0053]    [0053]FIG. 14 shows an active control matrix for the CNT-FED of the third embodiment. As shown in FIG. 14, the active control matrix includes the first data lines DL 1 ˜DLM, the second data lines RL 1 ˜RLN, SCAN lines SL 1 ˜SLN, CNT-FED, capacitor C 1 , and diodes D 1  and D 2  disposed at the intersection of the first data lines DL 1 ˜DLM and the second data lines RL 1 ˜RLN. Operation procedures are described as follows. In the first scan period, the scan line SL 1  is fed a positive voltage, and the second data line RL 1  is fed a first bias voltage, a negative voltage, to turn on the diodes D 1  and turn off the diodes D 2  of the first column. The data lines DL 1 ˜DLM feed control voltages to capacitors of the first column through the diodes D 1  respectively. Control voltages of the three-electrode CNT-FED  100  of the first column are stored in the capacitors of the first column. In the next scan period, the first scan line SL 1  is at a ground potential and the diodes D 1  of the first column are turned off and control voltages of the first column are stored in the capacitors of the first column thereby sustaining illumination of CNT-FED of the first column until the next time frame when control voltages are rewritten. The scan lines SL 2 ˜SLN repeat previous procedures, thereby data of the entire frame is written completely.  
         [0054]    Although the present invention has been described in its preferred embodiments, it is not intended to limit the invention to the precise embodiments disclosed herein. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.