Charge transfer device having a bent transfer channel

Abstract: A charge transfer device has a transfer section (11) for transferring a signal charge along a transfer channel (Y-Y'), and a pickup section (12) connected to the transfer section (11) for converting the signal charge received from the transfer section (11) to a voltage signal, both sections being formed on a substrate (10). The transfer channel (Y-Y') is bent generally at a right angle between the transfer section (11) and the pickup section (12).

Description: 1. Field of the Invention The present invention relates to a charge transfer device (CTD) for use in solid-state image pickup devices, delay lines, and the like. 2. Description of the Prior Art An example of currently available charge transfer devices (CTD) for use in a solid-state image pickup device is shown in Fig. 3. This charge transfer device has a transfer section 101, a pickup section 102, and an amplification section 103, all provided on a substrate 100. Transfer gates G1 - G4 at the stages of the transfer section 101 are driven by two-phase clocks  1 ,  2 applied thereto alternately from external and transfer a signal charge along a linear transfer channel X-X' toward the pickup section 102. In the figure, the hatched portions represent barrier regions which function to determine the direction in which the signal charge is transferred. The signal charge deriving from the transfer section 101 passes under an output gate OG, to which a DC voltage Vog is being applied, and enters a floating diode FD, which constitutes the pickup section 102. Then a potential change of the floating diode FD corresponding to an amount of the signal charge is transferred to the amplification section 103. The amplification section 103, made up of MOS transistors Tr 1 , Tr 2 , Tr 3 , and Tr 4 provided between the power supply OD and ground, amplifies the voltage signal coming from the floating diode FD and provides its output signal OS to a connecting point of the MOS transistors Tr 2 and Tr 3 . It is noted that the floating diode FD is periodically restored to a specified reset potential V RD when a clock r (to be applied synchronously with the two-phase clocks  1 ,  2 ) is applied to a reset gate RG from external of the device. As is known, to enhance the charge-to-voltage conversion factor of the floating diode FD, it is desirable to reduce the area of the floating diode FD to thereby reduce its electrostatic capacity. In the conventional device, the width of the transfer channel X-X' is narrowed below the output gate OG connecting the transfer section 101 and the pickup section 102, and the area of the floating diode FD is reduced accordingly. However, the conventional charge transfer device has the following drawback. That is, since the width of the linear transfer channel X-X' is merely narrowed, there arises a potential barrier 20 slanted in such a direction as will obstruct the transfer, to the width-narrowed point (i.e., below the output gate OG), as shown in Fig. 2 (a) during the charge transfer. This potential barrier 20 causes some transfer fault and, especially when the charge transfer device is used in a solid-state image pickup device, adversely affects its characteristics for low illuminance. EP-A-0 456 241 discloses a charge transfer device according to the preamble of claim 1. SUMMARY OF THE INVENTION The object of the present invention is to provide a charge transfer device capable of not only enhancing the charge-to-voltage conversion factor by reducing the area of the pickup section but also eliminating the possibility of occurrence of transfer faults. In accordance with the present invention, there is provided a charge transfer device having a transfer channel which extends between a transfer section and a pickup section, wherein a first portion of the transfer channel at the pickup section lies at an angle relative to a second portion of the transfer channel at the transfer section and wherein said first portion of the transfer channel has a reduced width compared to said second portion of the transfer channel, the reduction in said width of the transfer channel occuring at a third portion thereof between said first and second portions, characterized in that said angle is substantially a right angle and is contained in said third portion. Due to the fact that the transfer channel is bent generally at a right angle between the transfer section and the pickup section, even though the width of the transfer channel is narrowed at the point where the transfer channel is bent, there will arise no potential barrier that prevents the charge from moving toward the pickup section. Possible slanting of the potential as expected from the narrowed width of the transfer channel will occur rather in such a direction as to accelerate the transfer. Accordingly, there will no longer arise a potential barrier to the transfer channel, so that the possibility of transfer faults occurring is eliminated. Yet the width of the transfer channel can be set narrow at the point where the transfer channel is bent, independently of the width of the transfer channel on the transfer section side. As a result, the area of the pickup section is reduced more than ever, allowing the charge-to-voltage conversion factor to be further enhanced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the construction of a charge transfer device of an embodiment according to the present invention; Fig. 2 shows potential distributions which the charge transfer device of Fig. 1 and the conventional charge transfer device of Fig. 3 respectively have when the charge transfer is effected; Fig. 3 shows the construction of the conventional charge transfer device; Fig. 4 shows a variant of the charge transfer device of Fig. 1; and Fig. 5 shows another variant of the charge transfer device of Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The charge transfer device of the present invention is described below in more detail with reference to an embodiment thereof as illustrated in Fig. 1. As shown in Fig. 1, the charge transfer device has a transfer section 11, a pickup section 12, and an amplification section 13, all provided on a substrate 10. The transfer section 11 has transfer gates G 1 - G 4 . The transfer gates G 1 - G 4 at the stages of the transfer section 11 are driven by the two-phase clocks  1 ,  2 applied thereto alternately from external to transfer a signal charge along a transfer channel Y-Y' to the pickup section 12. As illustrated, the transfer channel Y-Y' is bent perpendicularly between the transfer section 11 and the pickup section 12, and more specifically at the final stage G 4 of the transfer section 11, in this example. The width of the transfer channel Y-Y' is narrowed also at this point G 4 , so that the width of a part of the transfer channel on the pickup section 12 side is narrower than that of a part thereof on the transfersection 11 side. The hatched portions in the figure represent barrier regions which function to determine the direction in which the signal charge is transferred. The signal charge transferred from the transfer section 11 passes under an output gate OG, to which a DC voltage Vog is being applied, and enters a floating diode FD. In this embodiment, the output gate OG and the floating diode FD constitute the pickup section 12. Then the floating diode FD generates a voltage corresponding to an amount of the signal charge. Based on the fact that the width of the bent transfer channel Y-Y' is narrowed at, or more precisely, below the final stage G 4 of the transfer section 11, the floating diode FD can be reduced to a small area so that the charge-to-voltage conversion factor can be set high. The floating diode FD is periodically restored to the reset potential V RD when the clock r (to be applied synchronously with the two-phase clocks 1, 2) is applied to a reset gate RG from external of the device, as in the conventional device. In this case, the output gate OG and the reset gate RG are arranged generally perpendicularly to the transfer gates G 1 - G 4 . An electric potential change of the floating diode FD is transferred to the amplification section 13. The amplification section 13, made up of MOS transistors Tr 1 , Tr 2 , Tr 3 , and Tr 4 provided between the power supply OD and ground, amplifies the voltage signal received from the floating diode FD and provides its output signal OS to a connecting point of the MOS transistors Tr 2 and Tr 3 , as in the prior art device. As described above, this charge transfer device has the transfer channel Y-Y' which is bent generally at a right angle between the transfer section 11 and the pickup section 12. Therefore, even if the width of the transfer channel Y-Y' is narrowed at the bending or turning point thereof, any potential barrier can be prevented from arising to the transfer channel Y-Y' as shown in Fig. 2 (b), thus eliminating the possibility of the occurrence of transfer faults. Rather, the potential change (slanting 30 of the potential) as expected from the narrowed width of the transfer channel will arise in such a direction as to accelerate the transfer, as illustrated in Fig. 2 (b). Also, since the transfer channel Y-Y' is bent between the transfer section 11 and the pickup section 12, the width of the transfer channel Y-Y' in the pickup section 12 can be set independently of the width of up to the final stage of the transfer section. Therefore, the area of the floating diode FD can be further reduced, allowing the charge-to-voltage conversion factor to be further increased. In the embodiment of Fig. 1, the transfer gates G 1 -G 3 of the transfer section 11 are double-layered and the transfer gate G 4 , at which the transfer channel Y-Y' is turned, is single-layered. However, the transfer gate G 4 may be also double-layered as shown in Fig. 4. Moreover, in the embodiment of Fig. 1, the direction in which the output gate OG extends is perpendicular to the direction in which the transfer gates G 1 -G 4 extend, and the transfer channel Y-Y' is bent below the final transfer gate G 4 . However, this structure can be changed as shown in Fig. 5, where the direction in which the output gate OG extends is parallel with the direction in which the transfer gates extend, and the transfer channel Y-Y' is bent at the output gate OG. In the variant of Fig. 5, the output gate OG is included in the transfer section 11. The invention may be embodied in other specific forms without departure from the scope thereof as defined by the claims.

Claims: A charge transfer device having a transfer channel (Y-Y') which extends between a transfer section (11) and a pickup section (12), wherein a first portion of the transfer channel at the pickup section (12) lies at an angle relative to a second portion of the transfer channel at the transfer section (11) and wherein said first portion of the transfer channel has a reduced width compared to said second portion of the transfer channel, the reduction in said width of the transfer channel occuring at a third portion thereof between said first and second portions, characterized in that said angle is substantially a right angle and is contained in said third portion. A charger transfer device according to claim 1, wherein the transfer section (11) has a plurality of transfer gates (G 1 -G 4 ) for transferring signal charge along the transfer channel (Y-Y') in a specified direction, and the pickup section (12) is operatively connected to the transfer gate (G 4 ) of a final stage of the transfer section (11) for converting the signal charge received from the transfer section to a voltage signal. A charge transfer device according to claim 2, wherein an output gate (OG) is included in the pickup section (12) and the transfer channel (Y-Y') includes said angle at the transfer gate (G 4 ) of the final stage. A charge transfer device according to claim 3, wherein the transfer gate (G 4 ) of the final stage of the transfer section (11) is single-layered. A charge transfer device according to claim 3, wherein the transfer gate (G 4 ) of the final stage of the transfer section (11) is double-layered. A charge transfer device according to claim 2, wherein an output gate (OG) is included in the transfer section (11) and the transfer channel (Y-Y') includes said angle at the output gate (OG).

Inventors: HASHIGUCHI KAZUO, NAGAKAWA TADASHI, HASHIGUCHI, KAZUO, NAGAKAWA, TADASHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, H01L21