A thin-film transistor circuit having a load device and a driver transistor and a method of producing the same

Abstract: A thin-film transistor circuit for a logic gate circuit includes: an amorphous silicon layer; a driver transistor having a source region, a drain region, and a channel region, the source, drain, and channel regions being formed in the amorphous silicon layer; and a load device formed in the amorphous silicon layer and made of n⁻ amorphous silicon, the load device being connected to the driver transistor.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a thin-film transistor circuit utilized as a driver circuit for a liquid crystal display apparatus or for an imaging sensor, and particularly, to a thin-film transistor circuit suitable for a driver-monolithic active matrix type liquid crystal display apparatus. 2. Description of the Related Art: A logic gate circuit, such as NOT, NOR, NAND, etc., consisting of thin-film transistors is conventionally used as a driver for a liquid crystal display apparatus. Figure 5A is a schematic plan view illustrating a conventional thin-film transistor circuit 530 . Figure 5B is a schematic cross sectional view of the thin-film transistor circuit 530 taken along a line A-A' in Figure 5A of the thin-film transistor circuit 530 . The thin-film transistor circuit 530 is a NOT-type logic gate circuit. The thin-film transistor circuit 530 has a load transistor 521 formed on an insulating substrate 501 and a driver transistor 522 formed on the insulating substrate 501 . The load transistor 521 consists of a gate electrode 502 formed on the insulating substrate 501 , a gate insulating film 504 formed on the insulating substrate 501 and the gate electrode 502 , a channel region 505 formed on the gate insulating film 504 , a source region 507 formed on the gate insulating film 504 , and a drain region 508 formed on the gate insulating film 504 . The channel region 505 is made of intrinsic amorphous silicon. The source region 507 and the drain region 508 are made of highly doped n-type (n + ) amorphous silicon. The driver transistor 522 consists of a gate electrode 503 formed on the insulating substrate 501 , the gate insulating film 504 formed on the insulating substrate 501 and the gate electrode 503 , a channel region 506 formed on the gate insulating film 504 , a source region 509 formed on the gate insulating film 504 , and a drain region 510 formed on the gate insulating film 504 . The channel region 506 is made of intrinsic amorphous silicon. The source region 509 and the drain region 510 are made of n + amorphous silicon. The gate electrode 502 of the load transistor 521 is electrically connected to a supply line 511 through a contact hole 512 formed in the gate insulating film 504 . The source region 507 of the load transistor 521 is electrically connected to the supply line 511 . Both the drain region 508 of the load transistor 521 and the source region 509 of the driver transistor 522 are electrically connected to an output line 513 . The drain region 510 of the driver transistor 522 is electrically connected with a ground line 514 . Because the gate electrode 502 is electrically connected to the source region 507 , the load transistor 521 is in an on state at any time and functions as a resistor. In the thin-film transistor circuit 530 , when a low-level voltage is applied to the gate electrode 503 of the driver transistor 522 , the driver transistor 522 turns off. A current, then, flows from the supply line 511 to the output line 513 via the load transistor 521 . When a high-level voltage is applied to the gate electrode 503 , the driver transistor 522 turns on. Then, a current flows from the supply line 511 to the ground line 514 , and the output line 513 has the same voltage as the voltage of the ground line 514 . Thus, the output line 513 can output a high-level voltage during the off state of the driver transistor 522 and a low-level voltage during the on state of the driver transistor 522 by adjusting the resistance of the load transistor 521 . In the aforementioned conventional thin-film transistor circuit 530 , the load transistor 521 serves as a resistor device. In order to make the thin-film transistor circuit 530 work as a logic gate circuit, the current gain of the driver transistor 522 must be fifty times larger than that of the load transistor 521 . Since thin-film transistors made of amorphous silicon generally have a small current gain, the driver transistor 522 must have a large channel region 506 so that the current gain of the driver transistor 522 is sufficiently large. Therefore, the driver transistor 522 must have a large area, which prevents the thin-film transistor circuit 530 from being made small. Other logic gate circuits than the NOT-type logic gate circuit may also have the same problems as mentioned above. The present invention overcomes the aforementioned shortcomings associated with the conventional technique and provides a thin-film transistor circuit having a small area and good device characteristics. JP-A-60-182168 describes the manufacture of an amorphous silicon inverter circuit in which an n + α-Si film is deposited and patterned, so as to constitute a load resistor, after the formation of an FET. JP-A-4-100270 describes a circuit in which, in order to improve static electricity protection performance, an input resistor is formed of a discrete portion of an amorphous silicon layer from which layer a thin-film transistor is also patterned and formed. SUMMARY OF THE INVENTION In one aspect, this invention provides the thin-film transistor circuit as defined by claim 1 , whose preamble reflects the state of the art according to JP-A-60-182168. In another aspect, this invention provides the method defined by claim 6 of producing a thin-film transistor circuit. Claims 7 and 8 are directed to embodiments of the method of claim 6. According to the thin-film transistor circuit of the present invention, a load device consists of n - amorphous silicon having a high resistance. Thus, the invention offers the advantage of providing a thin-film transistor circuit which has a small area and is driven with a low operating current. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic plan view and a cross sectional view of a thin-film transistor circuit for a NOT-type logic gate of the present invention. Figures 2A to 2E illustrate a method for producing a thin-film transistor circuit as shown in Figure 1 . Figure 3 is a schematic plan view of a thin film transistor circuit for a NOR-type logic gate of the present invention. Figure 4 is a schematic plan view of a thin-film transistor circuit for a NAND-type logic gate of the present invention. Figure 5A is a schematic plan view of a thin-film transistor circuit for a NOT-type logic gate according to a conventional technique. Figure 5B is a schematic cross sectional view taken along a line A-A' of Figure 5A of the thin-film transistor circuit. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Figure 1 schematically shows a plan view and a cross sectional view of a thin-film transistor circuit 100 of the present invention. The thin-film transistor circuit 100 is a NOT-type logic gate circuit. The thin-film transistor circuit 100 has a load device 105 and a driver transistor 120 , both formed on an insulating substrate 101 . The driver transistor 120 consists of a gate electrode 102 formed on the insulating substrate 101 , a gate insulating film 103 formed on the gate electrode 102 and the insulating substrate 101 , a channel region 107 formed on the gate insulating film 103 , and n + regions 106 and 108 both formed on the gate insulating film 103 . The channel region 107 is made of intrinsic amorphous silicon, and the n + regions 106 and 108 are made of n + amorphous silicon. The n + regions 106 and 108 function as a source region and a drain region of the driver transistor 120 , respectively. The n + regions 106 and 108 are electrically connected to an output line 110 and a ground line 111 , respectively. The load device 105 is formed adjacently to the n + region 106 . The load device 105 is made of n - amorphous silicon which is lightly doped with n-type impurity. The load device 105 is electrically connected to a supply line 109 through an n + silicon region 104 . Referring to Figure 1 , operation of the thin-film transistor circuit 100 is now explained. When a low-level voltage is applied to the gate electrode 102 , the driver transistor 120 turns off. A resistance between the n + regions 106 and 108 becomes much higher than that of the load device 105 . Since the supply line 109 supplies a high-level voltage, a current flows from the supply line 109 to the output line 110 via the load device 105 . The output line outputs a high-level voltage. When a high-level voltage is applied to the gate electrode 102 , the driver transistor 120 turns on. The resistance between the n + regions 106 and 108 becomes sufficiently lower than that of the load device 105 . Then, a current flows from the supply line 109 to the ground line 111 , and the potential of the output line 110 becomes a ground level. Thus, the output line 110 outputs a low-level voltage. Referring to Figures 2A to 2E , a method for producing a thin-film transistor circuit 100 of the present invention is now explained. As is shown in Figure 2A , a tantalum film (not shown) having a thickness of 300 nm is formed on an insulating substrate 101 by a sputtering method. The tantalum film is etched using a resist pattern (not shown) and a gate electrode 102 is formed on the insulating substrate 101 . The insulating substrate 101 is preferably made of a clear material, such as glass. As is shown in Figure 2B , a gate insulating film 103 is formed on the gate electrode 102 and the insulating substrate 101 by a CVD (chemical vapor deposition) method. The gate insulating film 103 has a thickness of 300 nm, and is made of silicon nitride. An intrinsic amorphous silicon layer 124 is successively formed on the gate insulating film 103 by a CVD method. The intrinsic amorphous silicon layer 124 has a thickness of 30 nm. After coating a photoresist (not shown) over an entire upper surface of the insulating substrate 101 , the photoresist is exposed with the gate electrode 102 as a mask by radiating light from the back side of the insulating substrate 101 . As is shown in Figure 2C , by developing the exposed photoresist, a resist pattern 125 is formed on the intrinsic amorphous silicon layer 124 . The resist pattern 125 has substantially the same pattern as that of the gate electrode 102 . Phosphorus ion, as an n-type impurity, is implanted into the intrinsic amorphous silicon layer 124 using the resist pattern 125 as a mask. It is preferable to utilize an ion shower doping method, for example, a method described in Akihisa Yoshida et al., "Fabrication of a-Si:H TFT's by a Large Area Ion Doping Technique", Extended Abstracts of the 1990 International Conference on Solid State Devices and Materials, Sendai, 1990, pp 1197-1198, so that the n-type impurity is implanted uniformly into a large area of the intrinsic amorphous silicon layer 124 . Because the resist pattern 125 is formed using the gate electrode 102 as a mask, an n - region 126 is formed in self-alignment with the gate electrode 102 . The n - region 126 preferably has a high resistance, specifically, has a sheet resistance in the range of 10 7 ohm per square to 10 10 ohm per square. In the case where the driver transistor has a resistance of 10 6 ohm in an on state and a resistance of 10 12 ohm in an off state, the n - region 126 more preferably has a sheet resistance in the range of 10 8 ohm per square to 10 9 ohm per square. In order to obtain such a high sheet resistance, phosphorus ion is implanted with a dose of about 10 13 cm -2 . The resist pattern 125 covers a part of the intrinsic amorphous silicon layer 124 , which is not doped with phosphorus ion and becomes a channel region 107 consisting of intrinsic amorphous silicon. As is shown in Figures 2C and 2D , a resist pattern 127 , then, is formed on the n - region 126 without removing the resist pattern 125 . Phosphorus ion is implanted into the n - region 126 using the resist patterns 125 and 127 as a mask with a dose of about 10 15 cm -2 . This ion implantation is also preferably performed by the ion shower doping method. After ion implantation, n + regions 106 and 108 are formed on the gate insulating film 103 in self-alignment with the gate electrode 102 . An n + region 104 is simultaneously formed on the gate insulating film 103 . The portion covered with the resist pattern 127 becomes a load device 105 made of n - amorphous silicon. After removing the resist patterns 125 and 127 , as is shown in Figure 2E , the amorphous silicon semiconductor layer is patterned so that unnecessary portions are removed. A titanium film (not shown) having a thickness of 300 nm is deposited over the entire surface. Then, a supply line 109 , an output line 110 , and a ground line 111 are formed on a part of the n + regions 104 , 106 , and 108 by patterning the titanium film. As is explained above, in the thin-film transistor circuit 100 of the present invention, an n - region is formed as a load device 105 adjacently to a driver transistor 120 . The n - region can be formed in intrinsic amorphous silicon layer by an ion shower doping method such that the n - region has a high sheet resistance. Therefore, the load device 105 of the present invention can have a small area, compared to a conventional load device consisting of a thin-film transistor. For example, the thin-film transistor circuit 100 has an area of 50 µm x 100 µm, which is one-third that of a conventional thin-film transistor circuit. The load device 105 having a high resistance can also reduce an operating current of the thin-film transistor circuit of the present invention. These advantages can make the integration of a thin-film transistor circuit of the present invention more densely. Moreover, because a source region and a drain region are formed in self-alignment with a gate electrode, a gate-source capacitance and a gate-drain capacitance can be reduced. Therefore, the thin-film transistor circuit has good device characteristics. The thin-film transistor circuit of the present invention can be made by the same manufacturing process as that used for producing driver transistors for an active matrix-type liquid crystal display apparatus. Therefore, it is appreciated that the present invention can be applied to a driver-monolithic active matrix type liquid crystal display apparatus. Example 2 Figure 3 shows a schematic plan view of a thin-film transistor circuit 300 of the present invention. The thin-film transistor circuit 300 is a NOR-type logic gate circuit. The thin-film transistor circuit 300 has a load device 303 and driver transistors 320 and 321 . The driver transistor 320 has a gate electrode 301 formed on a substrate 322 , a channel region 304 formed on a gate insulating film (not shown), and n + regions 307 and 308 both formed on the gate insulating film. The driver transistor 321 has a gate electrode 302 formed on the substrate 322 , a channel region 305 formed on the gate insulating film (not shown), and the n + region 307 and an n + region 309 both formed on the gate insulating film. The channel regions 304 and 305 are made of intrinsic amorphous silicon. The n + region 307 functions as a source region for both the driver transistors 320 and 321 . The n + regions 308 and 309 funcas drain regions for the driver transistors 320 and 321 , respectively. The n + region 307 is electrically connected to an output line 311 . Both the n + regions 308 and 309 are electrically connected to a ground line 312 . Thus, the driver transistors 320 and 321 are electrically connected in parallel between the output line 311 and the ground line 312 . The load device 303 is formed adjacently to the n + region 307 and made of n - amorphous silicon. The load device 303 is electrically connected to a supply line 310 though an n + region 306 . Referring to Figure 3 , operation of the thin-film transistor circuit 300 is now explained. When a low-level voltage is supplied to both the gate electrodes 301 and 302 , both the driver transistors 320 and 321 turn off. Both a resistance between the n + regions 307 and 308 and a resistance between the n + regions 307 and 309 become much higher than that of the load device 303 . Since the supply line 310 applies a high-level voltage to the load device 303 , a current flows from the supply line 310 to the output line 311 via the load device 303 . The output line 311 outputs a high-level voltage. When a high-level voltage is supplied to at least one of the gate electrodes 301 and 302 , at least one of the driver transistors 320 and 321 turns on. At least one of the resistances between the n + regions 307 and 308 and between the n + regions 307 and 309 is sufficiently lower than that of the load device 303 . Thus, a current flows from the supply line 310 to the ground line 312 , and the potential of the output line 311 becomes a ground level. Thus, the output line 311 outputs a low-level voltage. The thin-film transistor circuit 300 is produced by the same method as that for producing the thin-film transistor circuit 100 in Example 1. As explained in Example 1, the load device 303 made of the n - amorphous silicon is formed by implanting an n-type impurity into the intrinsic amorphous silicon layer formed on the gate insulating film. The channel region 304 and the n + regions 307 and 308 of the driver transistor 320 , and the channel region 305 and the n + regions 307 and 309 of the driver transistor 321 are also formed in the intrinsic amorphous silicon layer. Example 3 Figure 4 shows a schematic plan view of a thin-film transistor circuit 400 of the present invention. The thin-film transistor circuit 400 is a NAND-type logic gate circuit. The thin-film transistor circuit 400 has a load device 403 and driver transistors 420 and 421 . The driver transistor 420 has a gate electrode 401 formed on a substrate 422 , a channel region 404 formed on a gate insulating film (not shown), and n + regions 407 and 408 both formed on the gate insulating film. The driver transistor 421 has a gate electrode 402 formed on the substrate 422 , a channel region 405 formed on the gate insulating film (not shown), and the n + region 408 and an n + region 409 both formed on the gate insulating film. The channel regions 404 and 405 are made of intrinsic amorphous silicon. The n + region 407 functions as a source region for the driver transistor 420 . The n + region 408 functions as a drain region for the driver transistor 420 and as a source region for the driver transistor 421 . The n + region 409 functions as a drain region for the driver transistors 421 . The n + region 407 is electrically connected to an output line 411 . The n + region 409 is electrically connected to a ground line 412 . Thus, the driver transistors 420 and 421 are electrically connected in series between the output line 411 and the ground line 412 . The load device 403 is formed adjacently to the n + region 407 and made of n - amorphous silicon. The load device 403 is electrically connected to a supply line 410 though an n + region 406 . Referring to Figure 4, operation of the thin-film transistor circuit 400 is now explained. When a low-level voltage is supplied to at least one of the gate electrodes 401 and 402 , at least one of the driver transistors 420 and 421 turns off. The resistance between the n + regions 407 and 409 becomes much higher than that of the load device 403 . Since the supply line 410 supplies a high-level voltage to the load device 403 , a current flows from the supply line 410 to the output line 411 via the load device 403 . The output line 411 outputs a high-level voltage. When a high-level voltage is supplied to both the gate electrodes 401 and 402 , both the driver transistors 420 and 421 turn on. The resistance between the n + regions 407 and 409 is sufficiently lower than that of the load device 403 . A current flows from the supply line 410 to the ground line 412 , and the potential of the output line 411 becomes a ground level. Thus, the output line 411 outputs a low-level voltage. The thin-film transistor circuit 400 is produced by the same method as that for producing the thin-film transistor circuit 100 in Example 1. As explained in Example 1, the load device 403 made of the n - amorphous silicon is formed by implanting an n-type impurity into the intrinsic amorphous silicon layer formed on the gate insulating film. The channel region 404 and the n + regions 407 and 408 of the driver transistor 420 , and the channel region 405 and the n + regions 407 and 409 of the driver transistor 421 are also formed in the intrinsic amorphous silicon layer. As is explained in Examples 1, 2, and 3, the thin-film transistor circuit of the present invention can have a smaller circuit area and better device characteristics than those made from a conventional technique. The thin-film transistor circuit of the present invention can be suitably included in a liquid crystal display apparatus as a driver circuit, and can improve display quality of the liquid crystal display apparatus. Some exemplary values for impurity concentrations and other dimensions are mentioned above. It will be appreciated that other values which will enable operation of the invention described also may be used. The invention may be embodied in other specific forms and manners without departure from the scope thereof as defined by the claims.

Claims: A thin-film transistor circuit for a logic gate circuit, comprising: a driver transistor (120) having a source region (106), a drain region (108), and a channel region (107), each formed of amorphous silicon; and a load device (105) formed of amorphous silicon, electrically connected to said driver transistor; characterised by: a continuous amorphous silicon layer, the source, drain and channel regions of the driver transistor (120) being formed in said continuous layer, and an adjoining n - amorphous silicon portion of said continuous layer forming the load device (105), said n - amorphous silicon portion of the layer forming the load device being doped with an n-type impurity at a lower concentration than that of the source and drain regions. A thin-film transistor circuit according to claim 1, further comprising: a supply line (109;310;410) electrically connected to said load device, for applying a supply voltage to said load device; an output line (110;311;411) electrically connected to said supply line through said load device, for outputting logic level voltages; and a ground line (111;312;412) electrically connected to said output line through said driver transistor, for applying a ground voltage to said driver transistor; said driver transistor (120;320;420) having a gate electrode (102;301;401), and being switched based on a voltage applied to said gate electrode. A thin-film transistor circuit according to claim 2, further comprising a second driver transistor (321) having a gate electrode (302), a source region (307), a drain region (309), and a channel region (305), said source, drain and channel regions being formed in said continuous amorphous silicon layer, said second driver transistor (321) being connected to said first driver transistor (320) in parallel, and said output line (311) being connected to said ground line (312) through said first and second driver transistors. A thin-film transistor circuit according to claim 2, further comprising a second driver transistor (421) having a gate electrode (402), a source region (408), a drain region (409), and a channel region (405), said source, drain and channel regions being formed in said continuous amorphous silicon layer, said second driver transistor (421) being connected to said first driver transistor (420) in series, and said output line (411) being connected to said ground line (412) through said first and second driver transistors. A thin-film transistor circuit according to any preceding claim, wherein said amorphous silicon layer has a substantially uniform thickness. A method of producing a thin-film transistor circuit with a thin-film driver transistor electrically connected to a load device, comprising the steps of: (a) forming an amorphous silicon layer (124) on an insulator (103); (b) forming a first resist pattern (125) on said amorphous silicon layer; (c) implanting an n-type impurity into said amorphous silicon layer using said first resist pattern as a mask so as to form at least a load device (105) made of n - amorphous silicon in said amorphous silicon layer; (d) forming a second resist pattern (127) on said amorphous silicon layer masking said load device; and (e) implanting an n-type impurity into said amorphous silicon layer using said first (125) and second (127) resist patterns as a mask so as to form an n + region (106,108) in said amorphous silicon layer adjoining said load device, said n + region forming a source/drain region of said driver transistor, said amorphous silicon layer extending continuously between said n - region and said n + region. A method according to claim 6, wherein said steps (c) and (e) are performed by an ion implantation method. A method according to claim 6 or claim 7, wherein said amorphous silicon layer (124) is formed in said step (a) using a CVD method.

Inventors: KATAOKA YOSHIHARU, KATAYAMA MIKIO, KONDO NAOFUMI, SAKURAI TAKEHISA, SHIMADA YOSHINORI, TAKAHAMA MANABU, KATAOKA, YOSHIHARU, KATAYAMA, MIKIO, KONDO, NAOFUMI, SAKURAI, TAKEHISA, SHIMADA, YOSHINORI, TAKAHAMA, MANABU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L27
IPC codes: H01L27, H01L29, G02F1