Field-Effect Transistor Device

The present invention relates to a field-effect transistor device, and more particularly to a field-effect transistor device having a plurality of MIS transistors forming a sense amplifier.

Most memory devices of a large capacity have a latch-type sense amplifier. FIG. 1 shows a latch-type sense amplifier having N-channel MOS transistors Q1 and Q2. Bit lines BL and BL are set at the same potential before data is transferred through them. Their potential varies when the data is transferred through them. Enable signal SAE is at the V.sub.DD level, e.g., 5 V. It falls to the V.sub.SS level, e.g., 0 V, in order to sense a potential difference corresponding to the data supplied to bit lines BL and BL. When signal SAE falls to the V.sub.SS level, MOS transistors Q1 and Q2 are activated thereby amplifying the potential difference to a predetermined level. More specifically, the sense amplifier sets one of the bit lines BL and BL at the V.sub.DD level, and sets the other bit line at the V.sub.SS level.

FIG. 2 is a plan view of the sense amplifier shown in FIG. 1, which is formed on a semiconductor memory chip. Wiring layers 10A and 10B are, for example, formed of aluminum, electrically insulated from semiconductor substrate 12, and extend in the same direction. Gate electrode 14A of MOS transistor Q2 and gate electrode 14B of MOS transistor Q1 are also insulated from substrate 12, and extend in the same direction as wiring layers 10A and 10B. Gate electrodes 14A and 14B are made, for example, of polycrystalline silicon. Both end portions of gate electrode 14A are overlapped by wiring layers 10A. Similarly, both end portions of gate electrode 14B are overlapped by wiring layers 10B. Aluminum wiring layers 10A are connected to the ends of gate electrode 14A by means of contact holes 16A, thereby forming bit line BL. Aluminum wiring layers 10B are coupled to the ends of gate electrode 14B by means of contact holes 16B, thereby forming bit line BL.

Drain region 18A and source region 18B of MOS transistor Q1 are formed within substrate 12, on the left and right sides of gate electrode 14B, respectively. Source region 20A and drain region 20B of MOS transistor Q2 are formed within substrate 12, on the left and right sides of gate electrode 14A, respectively. One of wiring layers 10A is connected to drain region 18A by means of contact hole 22A, and one of wiring layers 10B is connected to drain region 20B by means of contact hole 22B. Conductor layers 24A and 24B are formed in contact with source regions 18B and 20A. A wiring layer 26 is connected to conductor layer 24A by means of contact hole 28A and to conductor layer 24B by means of contact hole 28B. Wiring layers 10A, 10B and 26 and gate electrodes 14A and 14B are electrically insulated from one another by insulation layers (not shown).

Source region 18B and 20A and drain regions 18A and 20B have been simultaneously formed and self-aligned by ion-implantation of impurities, using gate electrodes 14A and 14B as masks. Ion implantation of this type is usually performed, as is illustrated in FIG. 3, in order to suppress the channeling effect. More specifically, the impurity ions are implanted into substrate 10 in the direction inclined to the surface of substrate 10 at a predetermined angle. Gate electrode 30 traps the impurity ions moving toward portion 31 of substrate 10. As a result, the implanted impurity ions are distributed in the substrate 10 asymmetrically with respect to the gate electrode 30.

As shown in FIG. 2, drain and source regions 18A and 18B of transistor Q1 are located on the right and left sides of gate electrode 14B, respectively, and, in contrast, drain and source regions 20B and 20A of transistor Q2 are positioned on the left and right sides of gate electrode 14A, respectively. Hence, transistors Q1 and Q2 inevitably have different characteristics. This difference is prominent, particularly when transistors Q1 and Q2 have a lightly-doped drain structure. For example, the driving ability (or, drain current) of transistor Q1 differs from that of transistor Q2 by about 30% at most.

Due to this great difference in driving ability, the sensitivity of the sense amplifier falls below the design value. If the sense amplifier is to function as desired, a greater potential difference must be provided between bit lines BL and BL. Also due to the difference in driving ability, the time required for providing a sufficient potential difference between bit lines BL and BL can become longer than the maximum tolerance value. This time depends on the sensed difference in potential, i.e., the data being transferred through bit lines BL and BL. It also depends on whether lines BL and BL are set at the V.sub.DD level and the V.sub.SS level, or vice versa. If this sense amplifier, which operates slowly, is used in a dynamic RAM whose memory cells must be refreshed frequently, it will cause the RAM to mulfunction.

FIG. 4 shows a sense amplifier of CMOS structure generally used. The MOS transistors forming this CMOS sense amplifier (or a MOS type flip-flop) usually have different driving abilities due to the ion implantation. Hence, the operation of the CMOS sense amplifier cannot always meet the design specifications.

It is an object of the present invention to provide a field-effect transistor device comprising field-effect transistors which have the same characteristics as the design specifications.

According to the present invention, there is provided a field-effect transistor device comprising a semiconductor substrate; a plurality of field-effect transistors, each having a gate electrode formed over the semiconductor substrate and source and drain regions self-aligned in the semiconductor substrate on two opposing sides of the gate electrode, respectively, the source-to-drain paths of the field-effect transistors being oriented in the same direction; and wiring means for connecting the plurality of the field-effect transistors.

The field-effect transistors forming the field-effect transistor device can have the same characteristics for the following reasons. In the case where impurity ions are implanted into the semiconductor substrate in a direction inclined to the surface of the substrate at a predetermined angle, the distribution of the impurity ions in the source and drain regions of either field-effect transistor are asymmetrical with respect to the gate electrode. However, the field-effect transistors can have source regions with the same distribution of impurity ions and drain regions with the same distribution of impurity ions, since the source-to-drain paths of the field-effect transistors are oriented in the same direction.

Furthermore, when the impurity concentration is low in that portion of drain region of each field-effect transistor which is close to the gate electrode, the field-effect transistor device can have a higher driving ability than the case where the source-to-drain paths of the field-effect transistors are oriented in the opposite directions.

FIG. 1 is a circuit diagram of a conventional latch-type sense amplifier incorporated in a semiconductor memory;

FIG. 2 is a plan view of the sense amplifier shown in FIG. 1;

FIG. 3 is a cross-sectional view of the sense amplifier circuit, illustrating how impurity ions are implanted during the manufacture of the circuit;

FIG. 4 is a circuit diagram of a conventional sense amplifier of CMOS structure;

FIG. 5 is a plan view of a sense amplifier unit according to a first embodiment of this invention;

FIG. 6 is a plan view of a CMOS sense amplifier unit according to a second embodiment of this invention; and

FIGS. 7A and 7B are circuit diagrams of two CMOS flip-flops to which the present invention is applied.

A first embodiment of the invention, i.e., a latch-type sense amplifier unit, will be described with reference to FIG. 5 which is a plan view. The sense amplifier unit is combined with memory cells (not shown) to form a semiconductor memory such as a dynamic RAM. This sense amplifier unit comprises two sense amplifiers 50A and 50B which each are equivalent to the circuit of the sense amplifier illustrated in FIG. 1.

Sense amplifier 50A has conductor layers 52A and 52B provided on an insulation layer (not shown) formed on p-type silicon substrate 54. Conductor layers 52A and 52B each serve as a gate electrode and a conductor. Similarly, sense amplifier 50B has conductor layers 52C and 52D provided on the insulation layer. Conductor layers 52C and 52D each serve as a gate electrode and a conductor. Conductor layers 52A to 52D are formed, for instance, of polycrystalline silicon. As shown in FIG. 5, those portions (or electrode portions) of layers 52A to 52D which function as gate electrodes extend in the same direction.

Sense amplifiers 50A and 50B also have n-type regions 56A to 56F. Regions 56A to 56F have been formed in self alignment in substrate 54 by implanting impurity ions into substrate 54, using the electrode portion of conductor layers 52A to 52D as masks.

As is shown in FIG. 5, conductor layers 58A to 58H are provided above semiconductor regions 56A to 56F and substrate 54, and are electrically insulated therefrom. Layers 58A, 58C, 58E, 58F, 58G, and 58H extend in parallel to the electrode portions of conductor layers 52A to 52D. Layers 58B and 58D extend parallel to conductor layers 52A to 52D except in the space between regions 56A to 56C and regions 56D to 56F. Layers 58A, 58B, 58C and 58D are connected to regions 56A, 56C, 56D and 56F, by means of contact holes 60A, 60B, 60C and 60D. Conductor layer 52A is connected at one end to conductor layer 58E by means of contact hole 62A, and at the other end to conductor layer 58B by means of contact hole 62B. Conductor layer 52B is coupled at one end to conductor layer 58A by means of contact hole 62C, and at the other end to conductor layer 58F by means of contact hole 62D. Conductor layer 52C is coupled at one end to conductor layer 58G by means of contact hole 62E, and at the other end to conductor layer 58D by means of contact hole 62F. Conductor layer 52D is coupled at one end to conductor layer 58C by means of contact hole 62G, and at the other end to conductor layer 58H by means of contact hole 62H. Electrode layer 64A and 64B are formed in contact with regions 56B and 56E, respectively. Conductor layer 68 is formed above substrate 54, regions 56A to 56F, conductor layers 52A to 52D and conductor layers 58A to 58H, and are electrically insulated therefrom. Layer 68 extends at right angles to the electrode portions of conductor layers 52A to 52D. Conductor layer 68 is made, for example, of aluminum and is connected to electrode layers 64A and 64B by means of contact holes 70A and 70B.

In sense amplifier 50A, conductor layer 52A and regions 56A and 56B form n-channel MOS transistor Q1, and conductor layer 52B and regions 56D and 56E form n-channel MOS transistor Q2. Conductor layers 58A, 52B and 58F form bit line BL. Conductor layers 58E, 52A and 58B form bit line BL. Enable signal SAE is supplied to conductor layer 68.

Bit lines BL and BL intersect with each other in the space between MOS transistors Q1 and Q2. They extend parallel in other locations. Hence, regions 56A and 56B function as the drain and source of MOS transistor Q1, and regions 56D and 56E function as the drain and source of MOS transistor Q2. Therefore, in substrate 54, the source-to-drain path of MOS transistor Q1 is oriented in the same direction as the source-to-drain path of MOS transistor Q2.

With the latch-type sense amplifier unit shown in FIG. 5, the ion implantation for forming regions 56A to 56F can be performed under the same conditions on both MOS transistors Q1 and Q2. Both MOS transistors can, therefore, have the identical driving ability, and such problems that would result from a difference in the driving ability can be avoided.

FIG. 6 is a partially sectional, plan view of a CMOS sense amplifier unit according to a second embodiment of the present invention. This embodiment has two CMOS sense amplifiers 72A and 72B which each are equivalent to the circuit of the CMOS sense amplifier shown in FIG. 4. In FIG. 6, the same numerals are used to designate the same elements as used in the unit of FIG. 5, and primed numerals denote the elements not provided in the unit of FIG. 5.

As shown in FIG. 6, the CMOS sense amplifier comprises n-type well 74 formed in substrate 54, and p-type regions 76A to 76F formed in n-type well 74. Regions 76A to 76F are formed and self-aligned by implanting impurity ions into well 74, using those portions of conductor layers 56A' to 52D' which function as gate electrodes. Enable signal SAE, which is an inverse to enable signal SAE, is supplied to conductor layer 68'.

In sense amplifier 72A, conductor layer 52A' and regions 76A and 76B form p-channel MOS transistor Q4, and conductor layer 52B' and regions 76D and 76E form p-channel MOS transistor Q3. Conductor layers 58A, 52B, 58F, 58E', 52A' and 58B' form bit line BL. Conductor layers 58E, 52A, 58B, 58A', 52B' and 58F' form bit line BL.

In the second embodiment, bit lines BL and BL intersect with each other in the space between n-channel MOS transistors Q1 and Q2, and also in the space between p-channel MOS transistors Q3 and Q4. They extend in parallel in other locations. Hence, regions 56A and 56B function as the drain and source of MOS transistor Q1, regions 56D and 56E function as the drain and source of MOS transistor Q2, regions 76D and 76E function as the drain and source of MOS transistor Q3, and region 76A and 76B function as the drain and source of MOS transistor Q4. Therefore, in substrate 54, the source-to-drain paths of all MOS transistors Q1 to Q4 are oriented in the same direction. Therefore, the second embodiment can accomplish the same advantages as the first embodiment.

In the sense amplifier unit of FIG. 5, those portions of bit lines BL and BL which are made of polycrystalline silicon have different lengths. Bit lines BL and BL therefore have different capacitances. On the other hand, in the embodiment of FIG. 6, those portions of bit lines BL and BL which are made of polycrystalline silicon have the same length, and bit lines BL and BL have the same capacitance. Hence, the CMOS sense amplifier unit of FIG. 6 is free from a decrease in sensitivity.

The present invention is not limited to the above embodiments. It can be applied to CMOS flip-flops of the types illustrated in FIG. 7A and 7B.

Furthermore, each gate electrode can be of the Schottky barrier type which can be formed directly on a semiconductor substrate.