Charge transfer device and its driving method

Abstract: A charge transfer device and its driving method are disclosed such that transfer pulses each having an amplitude substantially equal to that of transfer pulses applied to transfer electrodes at transfer stages before a plurality of successively-arranged transfer stages including a final transfer stage and DC offset levels so decreased gradually as to gradually make shallow the depth of potential wells formed under the transfer electrodes toward the final transfer stage are applied to transfer electrodes (16 n-2 to 16 n ; 17 n-2 to 17 n ) at successively-arranged plural transfer stages including the final stage. Further, a charge transfer device and its driving method are disclosed such that a transfer pulse having an amplitude substantially equal to that of transfer pulses applied to transfer electrodes at transfer stages before the final transfer stage and a DC offset voltage so biased as to allow the depth of potential well formed under the transfer electrodes at the final transfer stage to be shallower than that of potential wells formed under the transfer electrodes at the transfer stages before the final transfer stage is applied to the transfer electrode at the final transfer stage.

Description: BACKGROUND OF THE INVENTION The present invention relates to a charge transfer device and its driving method, and more particularly to a charge transfer device driven by a low voltage. Conventional charge transfer devices now on the market are driven by a DC supply voltage from 12 to 15V and the transfer pulse voltage lies from 5 to 12V in general. However, when taking into account the overall system configuration using charge transfer devices, since almost all the semiconductor devices are usually driven by 5V, it is preferable to realize a charge transfer device (CTD) whose DC supply voltage and transfer pulse voltage for charge coupled device (CCD) registers are both 5V (referred to as complete 5V CTDs). With respect to the realization of 5V transfer pulse CTDs, since a potential difference (barrier height) between the barrier portion and the storage portion to both of which an in-phase transfer pulse is applied, can be optimized within a conventional controllable range on the basis the two-phase driving method now widely adopted as the charge transfer device driving technique, it is possible to realize satisfactory 5V transfer pulse CTDs even if the potential difference margin is reduced during transfer operation. With respect to the realization of 5V DC supply voltage CTDs, the problem is how to minimize the voltage applied to the reset drains for detecting and discharging charges transferred to the output portion. Various techniques have been so far proposed with respect to the above-mentioned minimization of the reset drain voltage. The first method is to obtain a high reset drain voltage on the basis of an internal voltage boosting circuit. That is, a high voltage is generated internally by use of a 5V supply voltage and a 5V pulse voltage both supplied from outside and the generated high voltage is applied to the reset drains. In this method, however, there exist problems in that a relatively-large area is required to form such a voltage generating circuit as described above, and additionally the signal S/N ratio is reduced because the generated high voltage is subjected to the influence of noise and therefore noise is superposed upon the output signal. The second method is to directly supply an external DC supply voltage to the reset drains from outside. The typical methods are disclosed in U.S. Patent No. 4603426 or in Japanese Patent Application No. 63-77676 (Japanese Patent Laid-open (Kokai) No. 1-248664) proposed by the same inventor. In more detail, U.S. Patent No. 4603426 discloses such a method that the final stage is driven by a voltage from -4 to 5V and the other stages are driven by a transfer clock pulse changing from 0 to 5V. Japanese Patent Appli. No, 63-77676 discloses such a method where the final stage is driven by a clock pulse changing between a -3V or less low-voltage level and a 5V or more high-voltage level and the other stages are driven by a transfer clock pulse changing from 0 to 5V. Further, a 5V supply voltage is directly supplied to the reset drains in both the above-mentioned second conventional methods. In the prior art CCD registers as described above, however, since the final transfer stage is driven by a clock pulse whose amplitude is wider than that of the externally-applied clock pulse, there exists a problem in that induced noise is easily superposed upon the CCD output signal and therefore the S/N ratio is deteriorated. Further, there exists another problem in that an additional circuit for increasing the clock pulse amplitude requires as large an area as possible for the high voltage generating circuit explained in the first conventional method. EP-A-0192 142 shows a charge transfer device transferring charges in potential wells, all the wells but the final one having similar depths. The charges are transferred by transfer pulses of equal amplitude. However, there is no disclosure of gradually decreasing DC offset levels. According to the present invention there is provided a charge device for transferring signal charges in sequence toward an output gate by controlling a depth of potential wells formed under transfer electrodes in response to transfer pulses applied to the transfer electrodes at transfer stages, which comprises means for applying transfer pulses each having an amplitude substantially equal to that of transfer pulses applied to transfer electrodes at transfer stages before a plurality of successively-arranged transfer stages including a final transfer stage before the output gate and DC offset levels so decreased gradually as to gradually make shallow the depth of potential wells formed under the transfer electrodes toward the final transfer stage, to transfer electrodes at the successively arranged plural transfer stages including the final transfer stage before the output gate. In the charge transfer device according to the present invention, since transfer pulses whose DC offset levels gradually decrease are applied to the transfer electrodes in such a way that depth of potential wells formed under the transfer electrodes decreases gradually towards the final transfer stage, it is unnecessary to apply a large amplitude transfer pulse to the transfer electrode at the final stage. Therefore, it is possible to minimize the generation of induced noise, thus realizing a high-reliable charge transfer device that has a high occupied area efficiency and a driving method that has a low device driving voltage. BRIEF DESCRIPTION OF THE DRAWINGS In the attached drawings: Fig. 1 is a cross-sectional view showing elements of the first embodiment of the charge transfer device according to the present invention; Fig. 2 is a potential diagram for assistance in explaining the operation of the charge transfer device shown in Fig. 1; Fig. 3 is a circuit diagram showing a circuit for generating voltages to gradually change the depth of the potential wells; Fig. 4 is a cross-sectional view showing elements of the second embodiment according to the present invention; Fig. 5 is a potential diagram for the second embodiment; Fig. 6 is a cross-sectional view showing elements of the third embodiment according to the present invention; Fig. 7 is a potential diagram for the third embodiment; Fig. 8 is a cross-sectional view showing elements of the fourth embodiment according to the present invention; Fig. 9 is a potential diagram for the fourth embodiment; Fig. 10 is a cross-sectional view showing elements of the fifth embodiment according to the present invention; Fig. 11 is a potential diagram for the fifth embodiment; Fig. 12 is a circuit diagram showing a circuit for generating a voltage applied to the final stage; Fig. 13 is a circuit diagram showing another circuit for generating a pulse applied to the final stage; and Fig. 14 is a circuit diagram showing the other circuit for generating a pulse applied to the final stage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the charge transfer device according to the present invention will be described hereinbelow with reference to Figs. 1 to 3. In Fig. 1, on the upper surface of a p-type silicon semiconductor substrate, for instance, a semiconductor injection region 12 (a buried channel in which barrier and storage regions are formed) is formed by injecting n-type impurity ions of the conductivity type opposite to that of the substrate 11. Further, in the injection region 12, additional injection regions 13 l ... 13 n (at which storage regions are formed) are formed by further injecting n-type impurity ions of the conductivity type opposite to that of the substrate 11 at high impurity atom concentration. Further, in the injection region 12 on the output side, a floating diffusion layer 14 and a reset drain 15 are respectively formed so as to include n-type impurity ions of the conductivity type opposite to that of the substrate 11 of high impurity atom concentration. Further, transfer electrodes 16 l ... 16 n at the first layer (only electrodes 16 n-5 to 16 n are shown in Fig. 1) are arranged between the two adjacent additional injection regions 13 and above the injection region 12; transfer electrodes 17 l ... 17 n at the second layer (only electrodes 17 n-5 to 17 n are shown in Fig. 1) are arranged over each additional injection region 13; and an output gate 18 and a reset gate 19 are formed and arranged between the additional injection region 13 n at the final transfer stage and the floating diffusion layer 14 and between the floating diffusion layer 14 and the reset drain 15, respectively above the injection region 12. A voltage supply wire 21 is connected to the first layer transfer electrodes 16 n-2i and the second layer transfer electrodes 17 n-2i to apply a first phase clock pulse; a voltage supply wire 22 is connected to the first layer transfer electrodes 16 n-(2i-1) and the second layer transfer electrodes 17 n-(2i-1) to apply a second phase clock pulse; and a plurality of pulse generators (three generators in Fig. 1) 31, 32 and 33 are connected to a plurality (three) of successively-arranged first-layer transfer electrodes 16 n-2 to 16 n and to a plurality (three) of successively-arranged second layer transfer electrodes 17 n-2 to 17 n both including the transfer electrode at the final transfer stage. These pulse generators 31, 32 and 33 can be formed on the same substrate 11 or as external circuits. The amplitude of each of pulses outputted from these pulse generators is substantially equal to that of each of clock pulses supplied via the common pulse supply wires 21 and 22; however, the high levels of the pulses from these pulse generators 31, 32 and 33 decrease gradually toward the final transfer stage. For example, when the clock pulses supplied via the common pulse supply wires 21 and 22 are at 0V in low level and at 5.0V at high level, the three pulses generated by the pulse generators 31, 32 and 33 are determined so as to be at -1.0V, -2.0V and -3.0V, respectively in low level and at 4.0V, 3.0V and 2.0V, respectively in high level. Fig. 3 shows a circuit for generating pulses whose voltage levels change in sequence as described above, by way of example. In the circuit shown, a fixed potential VB of 5V is divided by a plurality of resistors R; a cathode of each of diodes D is connected to each junction point between two adjoining single or plural resistors; and a transfer pulse 1 is applied to an anode of each of the diodes D via a capacitor C to synthesize plural divided voltages, so that the pulse 1 whose voltage changes at each transfer stage can be obtained. In the same circuit configuration, it is also possible to obtain a pulse 2 whose voltage changes at each transfer stage. The circuit as shown in Fig. 3 can be formed on the same charge transfer device or externally. Fig. 2 shows the depth of potential wells formed under the transfer electrodes 16 and 17 when the above-mentioned pulses are applied to the transfer electrodes 16 and 17 formed in the charge transfer device shown in Fig. 1. Fig. 2 indicates that the depth of the potential wells formed under the transfer electrodes 16 and 17 connected to the pulse generators 31, 32 and 33 decreases gradually toward the final transfer stage. As described above, in the present embodiment, it is possible to reduce the reset drain voltage without applying pulses with a large amplitude. As a result, it is possible to minimize the generation of induced noise and therefore provide a high-reliable charge transfer device driven by a low voltage. Fig. 4 is a cross-sectional view showing elements of a second embodiment of the present invention. The same reference numerals shown in Fig. 1 have been retained for similar elements shown in Fig. 4 and having the same functions. The difference between the first and second embodiments is that a clock generator 41 is connected to only the final transfer stage in Fig. 4. In this second embodiment, the barrier difference (in potential between the barrier portion and the storage portion) is determined on the basis of a difference in potential well depth between the buried channel region 12 under the electrode 16 and the additional injection region 13 under the electrode 17. In other words, it is possible to control the barrier height on the basis of the impurity atom concentration at the additional injection region 13. The final stage clock pulse generator 41 generates a pulse in such a way that the amplitude thereof is substantially equal to that of the transfer pulses applied to the transfer electrodes at transfer stages other than the final stage and further the depth of potential wells under the electrodes 16 n and 17 n at the final stage are determined to be shallower than that under the electrodes at stages other than the final stage. For example, in this embodiment, the pulse applied to the final stage electrodes 16 n and 17 n changes from -2.5V to 2.5V, while the pulse applied to the transfer electrodes 16 l to 16 n-1 and 17 l to 17 n other than the final stage transfer electrodes changes from 0 to 5V. In general, it is preferable to determine the high level of the transfer pulse applied to the final transfer stage electrodes as (VL+VH)/2 and the low level thereof as (VL-VH)/2 and to apply a dc voltage of about VL to the output gate, where VH denotes a high voltage level of the transfer pulse applied to the transfer stages before the final transfer stage and VL denotes a low voltage level thereof. Fig. 5 shows the depth of potential wells obtained when the charge transfer device shown in Fig. 4 is driven by the above-mentioned 2-phase clock signals, in which the reset drain 15 is determined at 5V; the barrier height between the barrier portion 16 and the storage portion 13 is 1.0V; and the modulation factor representing a proportion of the potential well depth to the voltage applied to the gate electrode is 0.8. Fig. 5 indicates that the potential difference generated when signal charge is transferred from the charge storage portion at the transfer stage which is one stage before the final transfer stage is 1.0V. The potential difference generated when signal charge is transferred from the charge storage portion at the final transfer stage to the output gate is also 1.0V. These potential differences are sufficient for normal charge transfer operation. Figs. 6 and 7 show a third embodiment according to the present invention, which is similar to the second embodiment shown in Fig. 4 in that the impurity atom concentration at the additional injection regions 13 formed on the surface of the buried channel range 12 is the same at the final stage and the stage one stage before the final stage. However, a different point between the second and this third embodiment is that the barrier difference is determined to be larger at the three stages before the stage one before the final stage. Further, a clock pulse is applied from the final stage clock pulse generator 41 to the final stage transfer electrodes 16 n and 17 n in the same way as in the second embodiment. Fig. 7 shows the depth of potential wells obtained when the charge transfer device shown in Fig. 6 is driven by the 2-phase clock signals. Fig. 7 indicates that the barrier height is small at the final stage and the stage one before the final stage. Where the device is formed with small barrier heights all over the device as in the second embodiment shown in Fig. 4, a relatively large CCD register width is required to transfer a sufficient quantity of signal charges. However, since a sufficient potential difference can be secured at the transfer stages, except the final stage and the stage one before the final stage, even if a large barrier difference is set, it is not necessarily to reduce the barrier difference all over the device. As described above, it is possible to reduce the register width by determining large barrier heights at the transfer stages except the final stage and the stage one before the final stage, so that parasitic capacitance can be reduced. Further, it is also possible to reduce the register width by increasing the length L of the charge storage portion. In the charge transfer device formed as described above, since the allowable range of variance in the barrier difference at the final stage and the stage one before the final stage is narrower than that at the other transfer stages, it is preferable to form both stages separately, without forming all the transfer stages simultaneously. Figs. 8 and 9 show a fourth embodiment according to the present invention. In this embodiment, the electrodes 17 l to 17 n , the output gate 18 and the reset gate 19 are all formed as a second layer polysilicon over first barrier layers 60 l to 60 n+2 formed by injecting impurity ions of the conductivity type the same as that of the substrate 11 on the surface of the buried channel layer 12. Further, a floating diffusion region 14 is formed between the two barrier layers 60 n+1 and 60 n+2 , and a reset drain 15 is formed inside the barrier layer 60 n+2 , respectively. Therefore, the functions of the first polysilicon layer and those of the second polysilicon layer are intercharged as compared with the second and third embodiments shown in Figs. 4 and 6. Further, in this fourth embodiment, a clock pulse is applied from a final stage clock pulse generator 41 to the final stage transfer electrodes 16 n and 17 n in the same way as with the second embodiment. Fig. 9 shows a potential diagram of this fourth embodiment, in which the reset drain is set to 5V; the barrier difference at the barrier layer 60 is 1.0V; and the modulation factor of the potential well depth to the voltage applied to the gate electrode is determined as 0.8. Therefore, the potential well depth diagram shown in Fig. 9 during charge transfer of this fourth embodiment shown in Fig. 8 is quite the same as that shown in Fig. 5 of the second embodiment shown in Fig. 4. Figs. 10 and 11 show a fifth embodiment according to the present invention. Although being similar to the fourth embodiment, a different point between the fourth and fifth embodiment is that the first barrier layers 61 l to 61 n-1 are formed, under the transfer stages except the final stage of the second polysilicon layer, by injecting impurity ions of the conductivity type the same as that of the substrate 11 in such a way that the barrier differences at the transfer stages other than the final stage are determined larger than that at the final stage. In this embodiment, the functions of the first polysilicon layer and those of the second polysilicon layer are interchanged as similar manner as the fourth embodiment. Therefore, as shown in Fig. 11, the barrier differences at the transfer stages before the final stage are determined larger than that at the final stage as 2V, it is possible to reduce the transfer register width and the parasitic capacitance as in the third embodiment. Figs. 12 to 14 are circuit diagrams showing some examples of the final transfer stage clock pulse generators incorporated with the second to fifth embodiments of the present invention, in which a reference numeral 100 denotes a semiconductor chip. In the example shown in Fig. 12, a resistor 101 whose one end is grounded is connected in series to an offset-biasing capacitor to which a pulse changing between 0 and 5V is applied. An intermediate junction point 104 between the resistor 101 and the capacitor 103 is connected to a final stage gate 102 as the final stage pulse inputting terminal. Since the pulse voltage from 0 to 5V applied to the capacitor 103 can be reduced by a voltage of 2.5V, for instance due to the resistor clamping operation, a pulse ranging between -2.5V and 2.5V is applied to the final stage gate 102. In the example shown in Fig. 13, two resistors 107 and 108 are connected in series between a supply voltage 106 and the ground, and an offset-biasing capacitor 103 is connected to a junction point between the two resistors 107 and 108. Therefore, the upper and lower limit voltage values applied to the final stage gate 102 change according to the values of the dividing resistors 106 and 107. However, the voltage range between the upper and lower limit values will not change. In the example shown in Fig. 14, a diode 109 is connected between the junction point between the two resistors 107 and 108 and the intermediate junction point 104 to realize a so-called diode-clam type circuit. The clamping operation of this circuit is substantially the same as that shown in Fig. 13. As described above, in the charge transfer device and its driving method according to the present invention, since the potential well depth at a plurality of transfer stages including the final stage is so determined as to become shallower gradually toward the final transfer stage, it is unnecessary to apply a large-amplitude pulse at the final transfer stage, so that it is possible to drive the charge transfer device at a high reliability by a low voltage. In addition, it is possible to realize a charge transfer device occupying a small area. Further, in the charge transfer device and its driving method according to the present invention, since the biased transfer pulse is applied in such a way that the potential well depth becomes shallow only at the final transfer stage and the amplitude is the same as at the other transfer stages, it is also possible to drive the charge transfer device of a small occupied area by a low voltage and with a high reliability. It is to be noted that for the charge transfer device shown in Fig. 1, the functions of the first polysilicon layer and those of the second polysilicon layer may be interchanged as shown in the embodiments according to Figs. 8 and 10.

Claims: A charge transfer device for transferring signal charges in sequence toward an output gate (18) by controlling the depth of potential wells formed under transfer electrodes (16-16n, 17-17n) in response to transfer pulses applied to the transfer electrodes at transfer stages, which comprises means (21, 22) for applying the transfer pulses each having an amplitude substantially equal to that of transfer pulses applied to transfer electrodes at transfer stages before a plurality of successively-arranged transfer stages including a final transfer stage (16n, 17n) before the output gate and characterised by means for applying DC offset levels so decreased gradually as to gradually make shallow the depth of potential wells formed under the transfer electrodes toward the final transfer stage, to the transfer electrodes at the successively arranged plural transfer stages including the final transfer stage before the output gate. The charge transfer device of claim 1, wherein a potential difference between barrier portions and charge storage portions at transfer stages before a plurality of transfer stages including at least the final transfer stage is determined larger than that between barrier portions and charge storage portions at the plural transfer stages including at least the final transfer stage. The charge transfer device of claim 2, wherein impurity injection regions forming barrier portions at transfer stages are the same as an impurity injection region under the output gate in conductivity type.

Inventors: GOTO HIROSHIGE, GOTO, HIROSHIGE
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G11C27
IPC codes: H01L27, H01L21, G11C27, G11C19, H01L29