A charge transfer device

Abstract: In a charge transfer device, a low impurity density region (6a) is provided in its portion forming a floating capacitor (8). It becomes possible thereby to reduce the capacitance of the floating capacitor (8) and thus to ensure a larger output voltage relative to a signal charge.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a charge transfer device, particularly to a charge transfer device for converting a signal electric charge to a signal electric potential by means of a floating capacitor. Description of the prior art A prior art charge transfer device is illustrated in Fig. 1 by way of a longitudinal section. This device is an example of a buried channel type two-phase driven type device and is constituted by a p-type silicon substrate 1, an n-type impurity layer 2 formed thereon, first layer charge transfer electrodes 4a, 4b and 4c consisting of polycrystal silicon, second layer charge transfer electrodes 5a and 5b consisting of polycrystal silicon, an insulating film 3 formed between the silicon substrate and the first and the second layer charge transfer electrodes and also between the first layer charge transfer electrodes and the second layer charge transfer electrodes, n⁻-type impurity regions 6 formed in surface regions of the n-type impurity layer 2, an output gate 7, a floating capacitor 8 for converting a transferred signal charge to a potential variation, a reset gate 9, an n⁺-type impurity region 10 formed in a surface region of the n-type impurity layer 2 and an output transistor 11 for detecting the potential variation in the floating capacitor 8. A similar charge transfer device which corresponds to the preamble of claim 1 is known from EP-A-0 192 142. The first layer charge transfer electrodes 4a, 4b and so on are coupled with the left side neighboring second layer charge transfer electrodes 5a, 5b and so on, and to these couples, transfer clock signals ⊘₁ and ⊘₂ of reverse phases to each other are applied alternately. further, a fixed output gate 7, a rest pulse ⊘ R is applied to the reset gate 9 and a source voltage V₂ is applied to the n⁺-type impurity region 10 and a drain terminal of the output transistor 11. Next, operation of the prior art device shown in Fig. 1 will be explained with reference to a potential graphs illustrated in Fig. 2. When the rest pulse ⊘ R goes to a high level, a potential of a portion of the n-type impurity layer 2 located between the output gate 7 and the reset gate 9 is set at the source voltage V₂. Then, when the reset pulse ⊘ R goes to a low level, said portion of the n-type impurity layer 2, which forms one of the electrodes of the floating capacitor 8, is electrically isolated from other portions. At this time, the transfer clock signal ⊘₁ and ⊘₂ are set at a high level and a low level, respectively and thus signal charge is stored under the first layer charge transfer electrodes 4a, 4c and so on to which the transfer clock signal ⊘₁ is applied [ Fig. 2(a)]. Next, when the transfer clock signals ⊘₁ and ⊘₂ go to a low level and a high level, respectively, the signal charge stored under the charge transfer electrodes to which the trnsfer clock signal ⊘₁ is applied is transferred to portions under right-side neighboring charge transfer electrodes to which the transfer clock signal ⊘₂ is applied. At this time, the signal charge stored under the final charge transfer electrode 4a flows into the floating capacitor 8 through a channel under the output gate 7 [Fig. 2(b)]. When the transferred signal charge quantity is represented by Q and the capacitance of the floating capacitor is represented by C, potential variation amount ΔV of the floating capacitor 8 can be represented as follows: ΔV = Q/C This potential variation amount ΔV is taken out through the output transistor 11. In such a prior art charge transfer device, however, the output voltage value corresponding to the signal charge quantity is decided by the capacitance of the floating capacitor. Therefore, the level of the output voltage corresponding to a fixed signal charge quantity is elevated as the capacitance of the floating diffusion layer is made smaller. For this reason, in order to raise the sensitivity of the charge transfer device, it has been usually employed to reduce the surface area of the floating capacitor, that is, to narrow the distance between the output gate 7 and the reset gate 9 thereby to narrow the distance between channel stoppers existent at both ends of the n-type impurity layer 2. In these days, however, reduction of the capacitance by such means has been achieved to an almost maximum extent and so there is a demand for new means for capacitance reduction in place thereof. An example for such a prior art is FR-A-2 606 553. SUMMARY OF THE INVENTION An object of the present invention therefore is to realize a charge transfer device having new means for reducing capacitance of its floating capacitor. The present invention thus provides a charge transfer device as defined in claim 1. The above charge transfer device may be formed on a silicon substrate for example of a p-type having an impurity density of 2 x 10¹⁵/cm³. The semiconductor material that forms the region (a) may be of an n-type impurity layer having an impurity density for example of 4 x 10¹⁶ - 5 x 10¹⁶/cm³, such as 4.5 x 10¹⁶/cm³. When the semiconductor-material that forms the region (a) is of an n-type impurity layer having an impurity density of 4.5 x 10¹⁶/cm³, the low impurity density region (e) and the high impurity density region (f) may be of an n⁻-type impurity layer having an impurity density for example of 1 x 10¹⁶ - 2 x 10¹⁶/cm³, such as 2 x 10¹⁶/cm³ and an n⁺-type impurity layer having an impurity density for example of 3 x 10²⁰/cm³, respectively. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a longitudinal section of a prior art charge transfer device. Fig. 2 shows potential graphs relative to the prior art charge transfer device shown in Fig. 1. Fig. 3 shows a longitudinal section of an embodiment of the charge transfer device according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below further in detail with respect to its embodiment shown in Fig. 3. In Fig. 3, a charge transfer device as an embodiment of the present invention is illustrated by way of a longitudinal section and portions identified by reference numbers same as those shown in Figs. 1 and 2 are portions same as those explained with reference the prior art device shown in Figs. 1 and 2. In this embodiment, the p-type silicon substrate 1 may have an impurity density for example of 2 x 10¹⁵/cm³, the n-type impurity layer 2 may have an impurity density for example of 4 x 10¹⁶ - 5 x 10¹⁶/cm³, such as 4.5 x 10¹⁶/cm³. When the impurity density of the n-type impurity layer 2 is 4.5 x 10¹⁶/cm³, the n-type impurity regions 6 may have an impurity density for example of 3 x 10¹⁶ - 4 x 10¹⁶/cm³. The n⁺-type impurity region 10 may have an impurity density for example 3 x 10²⁰/cm³. The device of the present invention shown in Fig. 3 is different from the prior art device shown in Figs. 1 and 2 in that an n⁻-type impurity region 6a is formed in the surface region of the n-type impurity layer 2 between the output gate 7 and reset gate 9, in the former device. This region 6a may be formed together with the n⁻-type impurity regions 6 when the letter is formed by introducing a p-type impurity into the surface region of the n-type impurity layer 2 between the first layer transfer electrodes, after the first layer transfer electrodes 4a , 4b and so on and the reset gate 9 are formed on the silicon substrate. The operation itself of the device of the present invention shown in Fig. 3 is similar to that of the prior art shown in Figs. 1 and 2, and so when the signal charge quantity transferred to the floating capacitor 8 is represented by Q and the capacitance of the floating capacitor 8 is represented by C, the potential variation amount ΔV of the floating capacitor may similarly be represented as follows: ΔV = Q/C wherein however the capacitance of the floating capacitor in the present invention is made smaller than that in the prior art, owing to the fact that an n⁻-type impurity region 6a is formed in the portion which forms one of the electrodes of the floating capacitor, according to the present invention. For example, when the impurity density in the n-type impurity layer 2 is 4.0 x 10¹⁶/cm³ and the impurity density in the n⁻-type impurity region 6a is 3.0 x 10¹⁶/cm³, it is possible to reduce the capacitance of the floating capacitor 8 for about 20%. As explained in detail as above, a low impurity density region is provided in a portion of an n-type impurity layer forming a floating capacitor. It becomes possible thereby to reduce the capacitance of the floating capacitor and thus to ensure a larger output voltage relative to a signal charge.

Claims: A charge transfer device comprising: (a) a semiconductor material region (2) of a first conductivity type of intermediate impurity density, (b) plural charge transfer electrodes (4a,4b,4c;5a,5b) formed on the semiconductor material region with an interposed insulating film (3), (c) an output gate (7) formed on the semiconductor material region with an interposed insulating film, adjacent to a terminal (7) of one of said transfer electrodes, (d) a reset gate (9) formed on the semiconductor material region with an interposed insulating film (3), at a predetermined distance apart from the output gate, (e) a first region (6a) of said first conductivity type formed in the semiconductor material region between the output gate and the reset gate, and (f) a second, high impurity density region (10) of said first conductivity type formed in the semiconductor material region at a side opposite to the first region relative to the reset gate, characterized in that said first region has a low impurity density. The charge transfer device according to claim 1, which is formed on a p-type silicon substrate and wherein the region (a) is an n-type impurity layer having an impurity density of 4 x 10¹⁶ - 5 x 10¹⁶/cm³, the region (e) is an n⁻-type impurity region having an impurity density of 2 x 10¹⁶ - 4 x 10¹⁶/cm³ and the region (f) is an n⁺-type impurity region having an impurity density of about 3 x 10²⁰/cm³.

Inventors: YAMAMOTO JUNICHI C O NEC CORPO, YAMAMOTO, JUNICHI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H01L29
IPC codes: H01L21, H01L29