Cathode structure with reduced capacitance

A cathode structure of an electron gun for a cathode ray tube includes: a substrate (51); cathode electrode layers (52) formed on the substrate (51) and spaced apart from each other at predetermined intervals; a plurality of metal tips (53); an insulating layer (54) formed on the cathode electrode layers (52) and the substrate (51) to isolate each of the metal tips (53) from each other; a gate electrode layer having a first gate electrode portion (56) having a gate through which the metal tips (53) are exposed and formed on top of the insulating layer (54), and a second gate electrode portion (57) extending horizontally from said first gate portion (56) and divided into several parts by a plurality of gaps (60) for reducing the capacitance between the cathode electrode layers (52) and the second gate electrode portion (57).

BACKGROUND OF THE INVENTION 
The present invention relates to a cathode structure of an electron gun for 
a cathode ray tube (CRT), and more particularly, to a cathode ray tube 
adopting a field emission device. 
An indirectly heated or directly heated cathode is used as a thermion 
emission source in the CRT of existing monitors and televisions. Since the 
thermion emission material of the conventional cathode structure is heated 
by a special heating source to emit thermions, certain inherent problems 
arise. First, the period for emitting the normal thermions is relatively 
long since the thermion emission material is heated after support members 
for supporting the thermion emission material is heated with heat 
generated by the heating source. Accordingly, the time required to form a 
CRT image increases, for example, up to 8 or 9 seconds. 
Second, a thermal drift phenomenon occurs due to the thermal expansion of 
the support member for supporting the thermion emission material. 
Third, to heat the thermion emission material, power consumption of between 
2 and 4 watts is required. 
To overcome the above-mentioned problems, a cathode structure adopting a 
field emission device has been devised. Referring to FIG. 1, the cathode 
structure is comprised of an electrode member 11, an insulating member 12 
combined with the electrode member, and a cell 20 installed on the 
insulating member and having field emission devices. 
As shown in FIGS. 2 and 3, the cell 20 comprises a substrate 21 and three 
cathode layers 22 formed in a predetermined pattern on the substrate 21. A 
plurality of metal tips 23 for emitting electron beams corresponding to 
red, green, and blue signals are formed on the cathode layer 22. Each of 
the metal tips 23 is isolated from each other by an insulating layer 24 
having openings 24a, and a gate electrode layer 25, having gates 25a 
through which the metal tips 23 are exposed, is formed on top of the 
insulating layer 24. 
In the operation of the conventional cathode structure as constituted 
above, the amount of electrons emitted from the metal tips 23 is 
controlled by applying negative or zero voltages to the cathode layer 22 
and a positive voltage to the gate electrode layer 25. Since the gate 
electrode layer 25 is formed on the overall surfaces of the substrate 21, 
the driving signal is distorted by the capacitance between the gate 
electrode layer 25 and cathode layer 22. 
SUMMARY OF THE INVENTION 
To solve the above problems, it is an object of the present invention to 
provide a cathode structure of an electron gun for a CRT in which 
distortion of a driving signal for controlling an electron beam emitted 
from the metal tips can be prevented by reducing the capacitance between a 
gate electrode and a cathode electrode. 
Accordingly, to achieve the above object, there is provided a cathode 
structure of an electron gun for a CRT, comprising a substrate; cathode 
electrode layers formed on the substrate spaced at predetermined 
intervals; a plurality of metal tips for emitting electrons formed on the 
upper surface of the cathode electrode layer; an insulating layer formed 
on the cathode electrode layer and the substrate to isolate each of the 
metal tips from each other; a first gate electrode layer formed on top of 
the insulating layer and having gates through which the metal tips are 
exposed; and a second gate electrode layer formed on top of the insulating 
layer, extending horizontally from the first gate electrode layer, and 
divided into several parts a gap therebetween. 
Also, it is preferable that a cathode lead portion connected to the cathode 
electrode layer is formed between the insulating layer where the gap is 
situated.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 4 through 6 show an embodiment of the cathode structure of an 
electron gun for a CRT according to the present invention. Referring to 
the drawings, the cathode structure includes a substrate 51, three cathode 
electrode layers 52 formed on the substrate 51 spaced apart from each 
other at predetermined intervals, and a plurality of metal tips 53 formed 
on the cathode electrode layer 52 for emitting electrons. Here, cathode 
lead portions 52a extend laterally from the cathode electrode layer 52. 
An insulating layer 54 having openings 54a is formed on the substrate 51 on 
which the cathode electrode layer 52 is formed such that the insulating 
layer 54 isolates each metal tip 53. Also, a first gate electrode layer 56 
having gates 56a formed so as to expose the metal tips 53 therethrough and 
a second gate electrode layer 57 extending laterally from the first gate 
electrode layer 56 are formed on top of the insulating layer 54. The 
second gate electrode layer 57 is divided into several parts by a gap 60 
through which the insulating layer 54 is exposed. The cathode lead portion 
52a is formed between the lower surface of the insulating layer 54 where 
the gap 60 is situated and the upper surface of the substrate 51, as shown 
in FIG. 6. 
In the operation of the cathode structure of an electron gun for a CRT 
according to the present invention as constituted above, a predetermined 
voltage or a ground voltage is applied to each cathode electrode layer 52 
and a positive voltage is applied to the first and the second gate 
electrode layers 56 and 57, thereby controlling the amount of electrons 
emitted from the metal tips 53. 
According to the present invention, the second gate electrode layer 57 is 
divided into several parts and there is a gap 60 in the second gate 
electrode layer 57 over the cathode electrode layer 52 and the cathode 
lead portion 52a so that the capacitance of the gate electrode layer can 
be reduced. In particular, since the cathode electrode layer 52 is not 
formed on the lower surface of the insulating layer 54, a charge of 
electrons can be prevented from accumulating, thereby reducing the 
capacitance. The reduction in the capacitance of the gate electrode 
prevents distortion of the driving signal for controlling the electron 
beam generated from the metal tips. 
It is noted that the present invention is not limited to the preferred 
embodiment described above, and it is apparent that variations and 
modifications by those skilled in the art can be effected within the 
spirit and scope of the present invention defined as in the appended 
claims.