Semiconductor integrated circuit with differential circuit

Abstract: Capacitors (C1) and (C2) as insulation layer separated elements forming element pair of a differential circuit has a common independent N well (12) formed independently of N well (1) connected to a power source VDD. Since the independent N well is in floating condition, even when high level spike noise to be generated upon switching of CMOS or so forth, is superimposed on the power source VDD, the high level spike noise is not directly transmitted to the independent N well to make fluctuation of bias potential of the independent N well small and moderate. Therefore, a potential difference in the independent N well due to impedance resistance in the independent N well becomes significantly small.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor integrated circuit with a differential circuit. Description of the Related Art Conventionally, since a differential circuit can be fabricated with pairs of elements which has the same characteristics, it can provide temperature compensation with respect to a differential input of the differential circuit. Therefore, such semiconductor integrated circuit can operate in wide temperature range and thus frequently used in amplifier circuits and so forth. For example, Fig. 1A is a circuit diagram showing a differential amplifier circuit included in the conventional semiconductor integrated circuit, and Fig. 1B is a waveform diagram showing the operation thereof. With reference to Figs. 1A and 1B, the differential amplifier circuit is constructed with respective pairs of elements of N-type transistors N1 to N2, P-type transistors P1 to P2, N-type transistors N4 to N5, P-type transistors P4 to P5, capacitors C1 to C2 and N-type transistor N3. P-type transistors P1 to P2 and P-type transistors P4 to P5 are junction separated elements having PN junction surface with N well 2 connected to a power source VDD. Also, the capacitors C1 and C2 are insulation layer separated elements having PN junction surface formed above the same N well 2 via the insulation layer. The capacitors C1 and C2 is input differential signals IN and their inverted signals IN at respective one terminals and transmit differential signals to gates of respective N-type transistors N1 and N2 by connecting the other terminals C12 and C22 of the capacitors C1 and C2 to N-type transistors N1 and N2. The N-type transistors N1 and N2 amplifies the differential signals by operating two transistors in mutually different phases corresponding to the differential signals input to the gate terminals. Then, an output OUT of the differential amplifier circuit is output through a drain terminal of the N-type transistor N2. The P-type transistors P1 and P2 operate as load elements of the element pair constituted with the N-type transistors N1 and N2. The N-type transistor N3 is input a reference voltage VR at a gate terminal. A drain terminal of the N-type transistor N3 is connected to a common junction of element pair constituted with the N-type transistors N1 and N2 to supply a constant current. The N-type transistors N4 and N5 and the P-type transistors P4 and P5 are input a control signal Φ or its inverted signal Φ at gate terminals to be turned ON when the control signal Φ is high level. A bias voltage VB is supplied to the other terminals C12 and C22 of the capacitors C1 and C2. Thus, a bias circuit is constituted by the N-type transistors N4 and N5 and the P-type transistors P4 and P5. Next, referring to Fig. 1B, the operation of the conventional differential amplifier circuit will be explained. The differential amplifier circuit are input two input signals in time division corresponding to high level period and low level period of the control signal Φ, respectively as the differential input signals IN or IN and output OUT as result of comparison of both input signals before transition of the control signal Φ from low level to high level. At first, when the control signal Φ is high level, transistors N4 and N5 and P4 and P5 are turned ON to supply a bias voltage VB to the other terminals C12 and C22 of the capacitors C1 and C2. In the capacitors C1 and C2, charge proportional to the differential input signal IN or IN is accumulated. Next, when the control signal Φ becomes low level, the transistors N4 and N5 and P4 and P5 are turned off. Thus, the charge accumulated in the capacitors C1 and C2 becomes holding condition. Then, another input signal is input in time division as the differential input signal IN or IN . In this example, there is shown a case where the potential of the input signal to be compared is lowered as shown by waveform of the differential input signal IN of Fig. 1B. Since the charge accumulated in the capacitor C1 is maintained, the potential of the other terminal of the capacitor C1 is lowered from the bias voltage VB as shown by the solid line associating with the differential input signal IN. On the other hand, since the potential of the differential input signal IN is not varied and the potential is stored in the capacitor C2, the potential at the other terminal C22 of the capacitor C2 is maintained at the bias voltage VB as shown by the solid line. Accordingly, the differential amplifier circuit outputs low level as the output OUT as a result of comparison of two input signals input in time division respectively corresponding to the high level period and the low level period of the control signal Φ, before transition from the low level to the high level of the control signal Φ. In this case, the capacitor C1 and C2 of the differential amplifier circuits is formed above an N well 2 the same as N well 2 where the P-type transistors P1 to P5 are formed, via an insulation layer. Since the N well 2 is biased by the power source VDD, a parasitic capacity between other element can be lowered. On the other hand, since the noise applied to the electrodes of the capacitors C1 and C2 becomes the same phase, the noise is eliminated by the differential circuit to reduce malfunction of the differential amplifier circuit. However, in the conventional semiconductor integrated circuit with the differential circuit, high level spike noise generated during switching may superimposed on a power source line associating with increasing of package side, such as installed CMOS logic circuit or so forth. The high level spike noise superimposed on the power source line is superimposed on the potential of the N well connected to the power source line, as shown in Fig. 1B, and is superimposed on a signal potential of the other terminals C12 and C22 of the capacitors C1 and C2 via the parasitic capacity between the N well and the capacitors C1 and C2. In this case, for impedance resistance in the N well, phase difference of the noises to be superimposed on the signal potential of the other terminals C12 and C22 of the capacitors C1 and C2 is caused. Also, there is a limit in a common-mode rejection ratio CMRR of the differential circuit to make it difficult to completely remove the noise to cause malfunction of the differential circuit. Patent abstracts of Japan, vol. 18, no.22 & JP 5 259 390 A discloses an operational amplifier and wells US 4 264 872,A discloses a differential amplifier. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor integrated circuit having a differential circuit which can improve a common-mode rejection'ratio (CMRR) of the differential circuit with respect to a high level spike noise superimposed on a power source line. A semiconductor integrated circuit with a differential circuit, according to the present invention is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to be limitative to the present invention, but are for explanation and understanding only. In the drawings: Fig. 1A is a circuit diagram showing the conventional semiconductor integrated circuit, Fig. 1B is a waveform chart showing operation thereof; Fig. 2A is a circuit diagram showing a first embodiment of a semiconductor integrated circuit helpful for understanding the invention, Fig. 2B is a waveform chart showing operation thereof; Fig. 3A is a plan view showing detailed structure of capacitors C1 and C2 of Fig. 2A, Fig. 3B is a section thereof; Fig. 4A is a plan view showing detailed structure of capacitors C1 and C2 in a second embodiment of the semiconductor integrated circuit according to the present invention, and Fig. 4B is a section thereof; and Fig. 5A is a plan view showing detailed structure of capacitors C1 and C2 in a third embodiment not covered by the present invention, and Fig. 5B is a section thereof. DESCRIPTION OF THE PREFERRED EMBODIMENT Next, the present invention will be discussed with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to those skilled in the art that the present invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to unnecessary obscure the present invention. Figs. 2A is a circuit diagram showing the first embodiment of a semiconductor integrated circuit helpful for understanding the present invention, and showing a differential circuit portion of the shown embodiment. Fig. 2B is a waveform chart showing signal waveforms at various positions of the first embodiment of the differential circuit in the first embodiment of the semiconductor integrated circuit. With reference to Figs. 2A and 2B, the shown embodiment of the differential circuit is constructed with respective pairs of elements of N-type transistors N1 and N2, P-type transistors P1 and P2, N-type transistors N4 and N5, P-type transistors P4 and P5, capacitors C1 and C2, and N-type transistor N3, similarly to the conventional differential circuit of the semiconductor integrated circuit of Figs. 1A and 1B. A difference between the shown embodiment of the differential circuit and the conventional circuit of Fig. 1A is in that an independent N well 12 which is independent of an N well 1 and not connected to a power source VDD, and in that the capacitors C1 and C2 serving as insulation layer separated elements forming the element pair of the differential circuit have the independent N well 12 in common via a lower insulation layer. Therefore, an operation of the circuit including a parasitic capacitance between the capacitors C1 and C2 and the N well 1 is different. It should be noted that portion of the shown embodiment of the differential circuit where the structure and operation overlap with the conventional circuit of Fig. 1A are neglected from discussion given hereinafter for keeping disclosure simple enough to facilitate clear understanding of the present invention. Figs. 3A and 3B are plan view and section respectively showing detailed constriction of the element pair constituted of the capacitors C1 to C2 in the circuit of Fig. 2A. With reference to Figs. 3A and 3B, the independent N well 12 is formed on the P type semiconductor substrate 11. The element pair constituted of the capacitors C1 and C2 are formed above the independent N well 12 via an insulation layer 13. The independent N well 12 is lowered the impedance therein by expending an N + diffusion layer 17 toward inside and is not supplied the power to be held in floating condition. Explaining the sectional structure with taking the capacitor C2 as example, the capacitor C2 is constructed with an upper electrode 21 and a lower electrode 22, each of them being formed of a polycrystalline silicon, an insulation layer 23 formed between the upper electrode 21 and the lower electrode 22, and a metal wiring 16 connecting the upper electrode 21 and the lower electrode 22 to the terminals C21 and C22 via contact holes 15 opened through an insulation layer 14. The structure of the capacitor C1 is the same as that set forth above. Next, operation of the shown embodiment of the semiconductor integrated circuit will be discussed with reference to Figs. 2A, 2B, 3A and 3B. The independent N well 12 in the shown embodiment is in floating condition. Therefore, as shown in Fig. 2B, when the high level spike-like noise generated upon switching of CMOS or so forth is superposed on the power source VDD, the high level spike-like noise is not directly transmitted to the independent N well 12 to permit to maintain constant potential. Therefore, even when a parasitic capacity CS is present between the lower electrode 22 forming the capacitors C1 and C2 and the independent N well 12, the noise is never transmitted to the other terminals C12 and C22, and thus malfunction of the differential circuit will never be caused. It should be noted that since the independent N well 12 is in the floating condition in the shown embodiment, the bias potential of the independent N well 12 becomes unstable. However, since the capacitors C1 and C2 are formed on the independent N well 12 via the insulation layer 13, the bias potential of the independent N well 12 will never influence for the terminal signals of the capacitors C1 and C2. Also, since the independent N well 12 is in the floating condition, even if the bias potential of the independent N well 12 is fluctuated by indirectly influenced by circumferential noise, different from direct influence, fluctuation of the bias potential of the independent N well 12 becomes small and moderate. Therefore, a potential difference in the independent N well 12 by the impedance resistance in the independent N well 12 becomes significantly small to make the fluctuation to be superimposed on the signal potential of the other terminals C12 and C22 of the capacitors C1 and C2 small and moderate to have only in-phase components. Thus, the differential circuit having common-mode rejection ratio performance will never cause malfunction. Furthermore, in the shown embodiment, by internally extending the N + diffusion layer 17, the impedance resistance in the N well can be lowered. Therefore, the common-mode rejection ratio performance of the differential circuit can be further improved. Figs. 4A and 4B are plan view and section respectively showing the element pair constituted of the capacitors C1 and C2 in the second embodiment of the semiconductor integrated circuit according to the present invention. The difference of the shown embodiment to the first embodiment shown in Figs. 3A and 3B is in that the N + diffusion layer 17 in the independent N well 12 is backed with a metal wiring 16 with quite small resistance at a plurality of positions via the contact holes 15 opened through insulation layer 14. Other construction is similar to the first embodiment of the invention. Therefore, the elements common to the first embodiment will be represented by the same reference numerals and detailed description thereof will be neglected from discussion. By backing with the metal wiring 16, the impedance resistance in the N + diffusion layer 17 and the independent N well 12 is lowered to further reduce phase difference due to fluctuation superimposed on the signal potential of the other terminals C12 and C22. Therefore, the shown embodiment is effective in improving the common-mode rejection ratio performance of the differential circuit. Figs. 5A and 5B are plan view and section showing the detailed constriction of the element pair constituted of capacitors C1 and C2 in the third embodiment of the semiconductor integrated circuit not covered by the present invention. The difference of the shown embodiment to the first embodiment shown in Fig. 2 is in that the N + diffusion layer 17 of the independent N well 12 is connected to the metal wiring 16 via the contact holes 15 opened through the insulation layer 14. Also, the metal wiring 16 is connected to the power source VDD via a resistor element R to bias the independent N well 12 at the voltage of the power source VDD. Other construction is similar to the first embodiment of the invention. Therefore, the elements common to the first embodiment will be represented by the same reference numerals and detailed description thereof will be neglected from discussion. The shown embodiment of the independent N well 12 is biased to the voltage of the power source VDD via the resistor element R. Therefore, even when high level spike noise is superimposed on the power source VDD upon switching of the CMOS, the noise is attenuated by the resistor element R and is not transmitted to the independent N well 12 to maintain at the constant potential. As set forth, the noise is not transmitted to the terminals C11 and C22 of the capacitors C1 and C2 and thus malfunction will never be caused. Furthermore, since the independent N well 12 is biased at the voltage of the power source VDD, it is advantageous in that the element, such as P-type transistor, which may suffered the influence of the bias of the independent N well 12 on the performance thereof can be formed within the independent N well 12. Of course, it is obvious that the similar effect is obtainable even when the independent N well 12 is biased at a potential of (1/2)VDD, GND or so forth via the resistor element R. By combining the detailed structures of the element pairs constituted with the capacitors C1 and C2 in the first to third embodiments of the semiconductor integrated circuit according to the present invention, The potential difference to be caused in the independent N well 12 becomes smaller to achieve synergistic effect, such as enhancement of the anti-noise effect. On the other hand, in the first to third embodiments of the semiconductor integrated circuit, examples where the capacitor elements are taken as element pair has been discussed. Similar effect can be obtained with respect to other examples where other circuit elements, such as the resistor elements are taken as element pair. Furthermore, in the first to third embodiments of the semiconductor integrated circuit, discussion has been given for the case where the capacitor elements forming the element pair have the independent N well 12 formed independently of the N well 1 in common via the lower insulation layer. However, it is possible that each of the capacitor elements forming the element pair has independent well via lower insulation layer, and the independent wells are mutually connected by the metal wiring to achieve the similar effect. Although the invention has been illustrated and described with respect to exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the scope of the present invention. Therefore, the present invention should not be understood as limited to the second embodiment set out above but to include all possible embodiments which can be embodies within a scope encompassed and equivalents thereof with respect to the feature set out in the appended claims.

Claims: A semiconductor device comprising: a substrate of a first conductivity type having a first well (1) of an opposite conductivity type and a second well (12) of said opposite conductivity type formed independently from said first well (1) and free from a connection for a power supply; a first input terminal (IN); a second input terminal ( IN ): an output terminal (OUT) ; a differential amplifier circuit connected to said output terminal (OUT) and having a first input node and a second input node, and said differential amplifier circuit having an element formed in said first well (1); a first capacitor (C1) connected between said first input terminal (IN) and said first input node and formed above said second well (12) on an insulating layer (13); and a second capacitor (C2) connected between said second input terminal ( IN ) and said second input node and formed above said second well (12) on said insulating layer (13), wherein said second well (12) is connected to a low resistance wiring (16) at a plurality of portions. The device as claimed in claim 1, wherein said element includes a load element formed in said first well (1). The device as claimed in claim 1, further comprising a bias circuit connected to said first and second nodes for biasing said first and second nodes at a predetermined bias voltage, respectively. The device as claimed in claim 3, wherein said bias circuit has a transistor (P4) formed in said first well (1). The device as claimed in claim 1, further comprising a low resistance region of said opposite conductivity type connected to said second well (12).

Inventors: KOBATAKE HIROYUKI, KOBATAKE, HIROYUKI, KOBATAKE, HIROYUKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: H01L27
IPC codes: H01L27, H03F3, H01L21