Thin film transistor

Abstract: A semiconductor device is an SOI type field effect transistor in which an active region is isolated and insulated by a transistor for isolation. A contact hole (13) for isolation is formed in a gate dielectric thin film (17) for isolation between a gate electrode (18) of the transistor for isolation and a channel region (14) below the gate electrode. In the semiconductor device thus structured, surplus carriers produced in a channel region (4) below a transfer gate electrode (8) are drawn through channel region (14) and isolation contact hole (13) into isolation gate electrode (18), thereby preventing such a disadvantageous phenomenon as a kink effect or the like due to a floating-substrate effect.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to semiconductor devices including MOS (Metal Oxide Semiconductor) field effect transistors (hereinafter referred to as "SOI (Silicon-On-Insulator)-MOSFET") formed in a semiconductor layer on an insulation layer and, more particularly, to a semiconductor device in which a degradation in breakdown voltage between sources/drains due to a so-called floating-substrate effect is prevented. Description of the Background Art Description will now be made on a conventional SOI-MOSFET with reference to Figs. 1 and 2. Referring to Figs. 1 and 2, the conventional SOI-MOSFET includes an insulator layer 2 formed on a silicon substrate 1 and a silicon layer 3 formed on insulator layer 2. A channel region 4 having a lower concentration of p type impurities (e.g., 10 16 -10 17 /cm 3 ) is formed in silicon layer 3. A source region 5 and a drain region 6 having a higher concentration of n type impurities (e.g., 10 19 -10 21 /cm 3 ) are formed on opposite sides of channel region 4 in silicon layer 3. A transfer gate electrode 8 is formed on channel region 4 with a gate dielectric thin film 7 interposed therebetween. Silicon layer 3 and transfer gate electrode 8 are covered with an interlayer insulation film 9. A contact hole 10 is made in interlayer insulation film 9. A conductive interconnection layer 11 is formed in contact hole 10. In the SOI-MOSFET thus structured, when a positive voltage is applied to transfer gate electrode 8, n conductivity type carriers (electrons) are induced into an upper layer of p type channel region 4. The upper layer of channel region 4 is inverted to be of the same n conductivity type as that of source and drain regions 5 and 6. This enables a current to flow between source and drain regions 5 and 6. Further, since the concentration of the n type carriers induced into the upper layer of channel region 4 varies dependently on a gate voltage to be applied to transfer gate electrode 8, the amount of the current flowing in channel region 4 can be controlled by the gate voltage. This is an operation principle of the MOSFET. When silicon layer 3 has a relatively large thickness such as of approximately 500nm (5000Å), if the SOI-MOSFET is rendered operative by application of a gate voltage, then carriers are accelerated at a high speed in channel region 4. The carriers accelerated in channel region 4 generate pairs of electrons and holes by impact ionization in the vicinity of drain region 6. The generated electrons flow into n + drain region 6, while the generated holes are left and stored in channel region 4. This causes an increase in potential of channel region 4 and an increase in channel current, thereby causing an unpreferable so-called kink effect on curves indicating the relationship between a drain voltage and a drain current. The kink effect is described in, for example, IEEE Electron Device Letter Vol. 9, No. 2, pp. 97-99, 1988. The kink effect will now be explained as follows by use of energy band diagrams shown in Figs. 3A-3C. The channel region and the source/drain regions on the opposite sides of the channel region form npn junction. An energy band thereof is as shown in Fig. 3A prior to junction of the respective regions. More specifically, a Fermi level E FN of the n type source and drain regions is higher than a Fermi level E FP of the p type channel region. If the respective regions are connected together, the energy band thereof is balanced when the respective Fermi levels are equal, so that a potential of the n type source/drain regions becomes higher than that of the p type channel region as shown in Fig. 3B. In this state, when the source region is grounded and a potential of +4V is applied to the drain region, holes are stored in the upper layer of the channel region due to the kink effect, resulting in an increase in potential of the channel region. Accordingly, such an energy band as shown in Fig. 3C is provided. This facilitates the flow of electrons from the source region to the drain region in a direction indicated by the arrow B, thereby causing a degradation in breakdown voltage developed across the source/drain regions. On the other hand, if silicon layer 3 has a very small thickness such as of approximately 500 to 1500Å, an SOI-MOSFET having such silicon layer 3 has the following excellent characteristics as compared with a normal SOI-MOSFET having relatively thick silicon layer 3 causing the above-described kink effect. For example, channel region 4 provided with thin silicon layer 3 is depleted overall by application of a voltage to transfer gate electrode 8, and a potential of channel region 4 is also controlled by the gate electrode. This results in reductions in a punch through phenomenon that a current flowing in channel region 4 cannot be controlled by transfer gate electrode 8 and in a short channel effect that a threshold voltage is abnormally decreased when the transfer gate electrode has a small length. However, if the overall channel region 4 is completely depleted, then the potential in channel region 4 becomes higher than that provided in the normal MOSFET. Accordingly, an electric barrier provided between source region 5 and channel region 4 is decreased. In addition, if the holes produced by the above-described impact ionization are temporarily stored in channel region 4, then the potential in channel region 4 further rises, whereby electrons are abruptly injected into channel region 4 from source region 5. That is, there is such a disadvantage that the breakdown voltage developed across the source/drain regions is liable to decrease in a thin-film SOI-MOSFET. A degradation in V D -I D characteristic of the MOSFET having relatively thick silicon layer 3 due to the kink effect appears in the form of distortion in circles indicated by the arrow A in the graph of Fig. 4. Further, a degradation in breakdown voltage developed across the source/drain regions in the MOSFET having relatively thin silicon layer 3 appears in the form of such a phenomenon that a drain current I D abruptly increases if a drain voltage V D exceeds a predetermined value, as shown in the graph of Fig. 5. A concern of the present invention is to provide a semiconductor device that realizes inhibition of a kink effect caused when an SOI layer is relatively thick and a degradation in breakdown voltage developed between source/drain regions caused when the SOI layer is relatively thin by preventing storage of surplus carriers into a channel region and that has no degradation in transistor characteristics due to a floating-substrate effect. European Patent Specifications Nos. EP-A-0,370,809 and EP-A-0,361,121 describe semiconductor devices which have points of similarity with the present invention. According to one aspect of the present invention there is provided a semiconductor device as set out in claim 1. In the foregoing structure, the channel region below the isolation gate electrode serves as a passage for surplus carriers such as generated holes. The surplus carriers that passed through the channel region are rapidly drawn out from a body contact. Thus, the surplus carriers are no longer stored in an upper layer of the channel region, thereby enabling suppression of a degradation in breakdown voltage developed across the source/drain regions due to a floating-substrate effect or occurrence of a kink effect. According to another aspect of the present invention there is provided a semiconductor device as set out in claim 9. In the foregoing structure, surplus carriers that passed through the channel region under the isolation gate electrode and introduced into the body region are drawn into the body interconnection layer through the body contact. Therefore, according to this aspect also, the surplus carriers are no longer stored in the channel region below the transfer gate electrode, thereby enabling prevention of a degradation in transistor characteristics due to a floating-substrate effect. As has been described above, according to the present invention, in the SOI-MOSFET, the surplus carriers produced in the channel region between the transfer gate electrodes are not stored but drawn out by electrically connecting the isolation gate electrode and the channel region below the gate electrode or by connecting the body region provided in contact with the channel region below the isolation gate electrode to the body interconnection layer through the body contact. Accordingly, an increase in potential in the channel region is suppressed, and the occurrence of the kink effect due to the floating-substrate effect and the degradation in the breakdown voltage developed across the source/drain regions are prevented. This makes it possible to obtain an SOI-MOSFET having an excellent V D -I D characteristic. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a conventional SOI-MOSFET. Fig. 2 is a plan view of the conventional SOI-MOSFET, and Fig. 1 corresponds to the cross-sectional view taken along the line X 3 -X 3 of Fig. 2. Figs. 3A, 3B and 3C are diagrams for use in explaining a kink effect of the SOI-MOSFET by using an energy band diagram of npn junction of an SOI layer below a transfer gate electrode. Fig. 4 is a diagram showing degradations in characteristics of a MOSFET due to the kink effect caused in the case with a relatively thick SOI layer, by using a graph of a V D -I D characteristic. Fig. 5 is a diagram showing degradations in characteristics of the MOSFET due to a decrease in breakdown voltage developed across source/drain regions caused in the case with a relatively thin SOI layer, by using a graph of the V D -I D characteristic. Fig. 6 is a cross-sectional view of an SOI-MOSFET according to a first embodiment of the present invention. Fig. 7 is a plan view of the SOI-MOSFET according to the above embodiment, and Fig. 6 corresponds to the cross-sectional view taken along the line X 1 -X 1 of Fig. 7. Fig. 8 is a cross-sectional view taken along the line Y 1 -Y 1 of Fig. 7. Fig. 9 is a diagram showing a graph of a V D -I D characteristic of the SOI-MOSFET according to the above embodiment. Fig. 10 is a cross-sectional view of an SOI-MOSFET according to a second embodiment of the present invention. Fig. 11 is a plan view of the SOI-MOSFET according to the second embodiment, and Fig. 10 corresponds to the cross-sectional view taken along the line X 2 -X 2 of Fig. 11. Fig. 12 is a cross-sectional view taken along the line Y 2 -Y 2 of Fig. 11. Fig. 13 is a cross-sectional view taken along the line Z 2 -Z 2 of Fig. 11. DESCRIPTION OF THE PREFERRED EMBODIMENTS Description will now be made on a first embodiment of the present invention with reference to Figs. 6-8. Referring to Figs. 6-8, in an SOI-MOSFET according to this embodiment, an insulator layer 2 is formed on a silicon substrate 1, and a silicon layer 3 as a semiconductor layer is formed on insulator layer 2. The thickness of silicon layer 3 can be selected in the range of approximately 50nm (500Å) to approximately 500nm (5000Å) as required. A channel region 4 having a low p type impurity concentration (e.g., 10 16 -10 17 /cm 3 ) is formed in silicon layer 3. A source region 5 and a drain region 6 both having a high n type impurity concentration (e.g., 10 19 -10 21 /cm 3 ) are formed to interpose channel region 4 therebetween in silicon layer 3. A transfer gate electrode 8 is formed on channel region 4 with a gate dielectric thin film 7 interposed therebetween. Transfer gate electrode 8 is formed of polycrystalline silicon doped with impurities such as of phosphorus. The above-described structure is the same as that of the conventional example; however, this embodiment is different from the conventional example in the following features. First, in the SOI-MOSFET according to this embodiment, isolation between transistors is made by employing a further isolating transistor. An isolation region in this embodiment is structured such that a gate electrode 18 for isolation is formed in the isolation region on silicon layer 3 with a gate dielectric thin film 17 for isolation interposed therebetween. A ground potential or a negative bias voltage is applied to isolation gate electrode 18 so as to prevent a surface of a channel region 14 formed in silicon layer 3 below gate electrode 18 from being inverted to be of an n type to cause the flow of a current, thereby retaining the insulation between SOI-MOSFETs in an active region. Further, an insulator film 12 for isolation is formed to cover gate electrode 18 in order to electrically insulate between an SOI-MOSFET for isolation that includes isolation gate electrode 18 as its principal constituent element and an SOI-MOSFET for driving that includes transfer gate electrode 8, source region 5 and drain region 6 as its principal constituent elements. A contact hole 13 for isolation is formed to penetrate a portion of gate dielectric thin film 17 for isolation. Isolation gate electrode 18 is electrically connected to the channel region under gate electrode 18 through contact hole 13. Source region 5, drain region 6, transfer gate electrode 8 and isolation gate electrode 18 are electrically connected to a conductive interconnection layer 11 passing through a contact hole 15 formed at a predetermined position in an interlayer insulation film 9 covering respective surfaces of the constituent elements 5, 6, 8 and 18. Conductive interconnection layer 11 is further connected to an external circuit. Description will now be made on an operation of this embodiment. Surplus carriers generated by impact ionization in a high-electric field region at the boundary between channel region 4 and drain region 6 pass through channel region 4 below gate electrode 8 and then flow into channel region 14 below isolation gate electrode 18. In this embodiment, the surplus carriers are holes since n channel MOSFETs are implemented. The surplus carriers that have flown into channel region 14 are further drawn into isolation gate electrode 18 from isolation contact hole 13 and then further introduced to the external circuit. Accordingly, the holes being the surplus carriers are no longer stored in channel region 4. This suppresses an increase in potential of channel region 4, resulting in a decrease in a so-called floating-substrate effect. Therefore, if an SOI film thickness of the SOI-MOSFET, i.e., the thickness of silicon layer 3 is relatively as large as approximately 500nm (5000Å), the kink effect that a distortion is produced in the V D -I D characteristic is prevented. If the SOI film has a relatively small thickness of approximately 50-150nm (500-1500Å), such an effect is provided that a decrease in breakdown voltage developed across source/drain regions is suppressed. As described above, according to this embodiment, an excellent V D -I D characteristic is obtained for both relatively thick silicon layer 3 and relatively thin silicon layer 3, thereby realizing substantial improvements in characteristic and reliability of the device. While the n channel MOSFET has been described in this embodiment, it is needless to say that the same effect is provided in a p channel MOSFET in which the respective conductivity types of channel regions 4 and 14 and source and drain regions 5 and 6 are opposite to the respective conductivity types of those in the n channel MOSFET. In addition, while silicon layer 3 is used as a semiconductor layer in this embodiment, the same effect can be obtained when other semiconductor materials such as gallium arsenide (GaAs) are used for the semiconductor layer. Fig. 9 shows a graph of the V D -I D characteristic of the SOI-MOSFET in this embodiment. A second embodiment of the present invention will now be described with reference to Figs. 10-13. Since the second embodiment is the same as the forgoing first embodiment except for the following points, like constituent elements are denoted with like reference numerals, and a description thereof will not be repeated here. This embodiment differs from the first embodiment in that a body regions 16 with impurities of the same conductivity type as channel regions 4 and 14 at a concentration of approximately 10 18 -10 20 /cm 3 is formed in silicon layer 3 in a predetermined region below isolation gate electrode 18 and that body region 16 is directly connected at 19 to an interconnection layer 20 which can be connected to an external body. This body region 16 serves to set or fix a lower part of channel region 4 in a predetermined potential such as a ground potential in order to achieve a stable transistor characteristic and passes through insulating layers 9 and 12. Description will now be made on an operation of the second embodiment. Surplus carriers (holes in this embodiment) produced by impact ionization in a high-electric field region at the boundary between channel region 4 and drain region 6 pass through channel region 4 below transfer gate electrode 8 and then flow into channel region 14 below isolation gate electrode 18. The surplus carriers that have passed through channel region 14 are introduced into body region 16, then introduced from body contact 19 to body interconnection layer 20 and drawn outside. Like the foregoing first embodiment, since the SOI-MOSFET of this embodiment is thus operated, the holes as the surplus carriers are no longer stored in channel region 4 below transfer gate electrode 8, thereby suppressing an increase in potential, leading to a decrease in the floating-substrate effect. Thus, according to this embodiment also, suppression is made for such a kink effect as can be seen when silicon layer 3 of the SOI-MOSFET has a relatively large film thickness and for such a decrease in breakdown voltage developed across the source/drain regions as can be seen when the SOI film has a relatively small thickness. The excellent V D -I D characteristic shown in the graph of Fig. 9 can be obtained. As related art to this embodiment, Japanese Patent laying-Open No. 62-109355 discloses an SOI-MOSFET that employs an element isolation method by a field insulation film and includes a region having the same conductivity type as that of a channel region and extending on the periphery of source/drain regions, and a body contact region for connecting these regions and an external interconnection layer. However, since the semiconductor device disclosed in the above laying-open does not employ a transistor for isolation, a region for electrically connecting the channel region and the body contact must separately be formed along the periphery of the source/drain regions. This causes a necessity to retain a larger area than a required area for an element isolation region, leading to a complication in manufacture process as well as a disadvantage in an increase of integration density. On the other hand, in this embodiment, the problems involved in the art disclosed in the laying-open are eliminated by positively utilizing the employment of the transistor for isolation. More specifically, channel region 14 below isolation gate electrode 18 is used as a region for electrically connecting body contact 16 and channel region 4. Channel region 14 is connected to channel region 4 in the cross-section taken along the line Y 2 -Y 2 of Fig. 11 as shown in Fig. 12. It is thus unnecessary to newly form the region for electrically connecting body contact 19 and channel region 4 other than a field shield region for isolation between transistors. This results in no increase in area required for body contact, exerting no adverse influence on an increase in integration density. Further, the field shield region extends over the entire device having the SOI-MOSFET formed therein, and channel region 14 below the field shield region extends likewise continuously over the entire device. Accordingly, body region 16 and body contact 19 may be provided in at least one predetermined place in the continuous field shield region. Like the first embodiment, in this embodiment also, it is needless to say that the same effect can be obtained in both the case with use of a p type channel MOSFET and the case with use of other semiconductors such as GaAs in place of silicon layer 3.

Claims: A semiconductor device, comprising: a semiconductor layer (3) formed on an insulator layer (2); a transfer gate electrode (8) formed on an active region of said semiconductor layer (3) with a gate dielectric thin film (7) interposed therebetween; an isolation gate electrode (18) formed on an isolation region of said semiconductor layer (3) with a gate dielectric thin film (17) for isolation interposed therebetween; respective first conductivity type channel regions (4,14) formed in said semiconductor (3) below said transfer gate electrode (8) and said isolation gate electrode (18); and second conductivity type source/drain regions (5,6) formed in said semiconductor layer (3) at opposite positions interposing said channel region (4) below said transfer gate electrode (8) therebetween, wherein said isolation gate dielectric thin film (17) has a contact hole (13) formed therein for electrically connecting said isolation gate electrode (18) and said channel region (14) through said contact hole (13) below said isolation gate electrode (18). A semiconductor device according to claim 1, wherein said semiconductor layer (3) comprises a silicon layer doped with p type impurities; said transfer gate electrode (8) and said isolation gate electrode (18) are formed of polycrystalline silicon doped with n type impurities; and said source/drain regions (5,6) are formed by doping n type impurities into said semiconductor layer (3). A semiconductor device according to claim 2, wherein said semiconductor layer (3) has a p type impurity concentration of 10 16 -10 17 /cm 3 ; and said source/drain regions (5,6) have an n type impurity concentration of 10 19 -10 21 /cm 3 . A semiconductor device according to claim 1, wherein the same material as that of said isolation gate electrode (18) is filled in said isolation contact hole (13). Use of a semiconductor device according to claim 1, wherein a predetermined negative bias voltage is applied to said channel region (14) below said isolation gate electrode (18) so as to prevent the surface thereof from being inverted to said second conductivity type. Use of a semiconductor device according to claim 2, wherein a ground potential is applied to said channel region (14) below said isolation gate electrode (18) so as to prevent the surface thereof from being inverted to be of an n conductivity type. Use of a semiconductor device according to claim 5, wherein a negative bias voltage is applied to said channel region (14) below said isolation gate electrode (18) so as to prevent the surface thereof from being inverted to n conductivity type. Use of a semiconductor device according to claim 1 wherein a ground potential is applied to said channel region (14) below said isolation gate electrode (18) so as to prevent the surface thereof from being inverted to said second conductivity type. A semiconductor device, comprising: a semiconductor layer (3) formed on an insulator layer (2); a transfer gate electrode (8) formed on said semiconductor layer (3) with a gate dielectric thin film (7) interposed therebetween; a gate electrode (18) for isolation formed on said semiconductor layer (3) with a dielectric thin film (17) for isolation interposed therebetween; respective first conductivity type channel regions (4,14) formed in said semiconductor layer (3) below said transfer gate electrode (8) and said isolation gate electrode (18); second conductivity type source/drain regions (5,6) formed in said semiconductor layer (3) at locations on either side of said channel region (4); and characterised in that the device further comprises: a first conductivity type body region (16) formed in said semiconductor layer (3) below said isolation gate electrode (18) and adjacent to said channel region (14) below said isolation gate electrode (18), said body region (16) having formed thereon an interconnection layer (20), the interconnection layer (20) extending between insulator films (9,12), said body region (16) and said layer (20) being in electrical contact through a body contact (19). A semiconductor device according to claim 9, wherein said semiconductor layer (3) comprises a silicon layer doped with p type impurities; said transfer gate electrode (8) and said isolation gate electrode (18) are formed of polycrystalline silicon doped with n type impurities; said source/drain regions (5,6) are formed by doping n type impurities into said semiconductor layer (3); and said body region (16) is formed by further doping p type impurities into said semiconductor layer (3). A semiconductor device according to claim 10, wherein said semiconductor layer (3) has a p type impurity concentration of 10 16 -10 17 /cm 3 ; said source/drain regions (5,6) have an n type impurity concentration of 10 19 -10 21 /cm 3 ; and said body region (16) has a p type impurity concentration of 10 18 -10 20 /cm 3 . A semiconductor device according to any one of claims 10 or 11, wherein the channel region (4) below said transfer gate electrode (8) and the channel region (14) below said isolation gate electrode (18) are electrically connected to each other. Use of a semiconductor device according to any of claims 9-12, wherein a ground potential is applied through said body contact (19) and said body interconnection layer (20) to said body region (16).

Inventors: AJIKA NATSUO C O MITSUBISHI DE, YAMAGUCHI YASUO C O MITSUBISHI, YAMANO TSUYOSHI C O MITSUBISHI, AJIKA, NATSUO, C/O MITSUBISHI DENKI K. K., YAMAGUCHI, YASUO, C/O MITSUBISHI DENKI K. K., YAMANO, TSUYOSHI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L29
IPC codes: H01L29, H01L27, H01L21