Field-effect transistor devices

Abstract: A MOSFET formed in a semiconductor substrate (1) comprising a buried channel layer (7) formed between a source region (3) and a drain region (4). The device further comprises a gate electrode (6) made of a material such as a high melting point metal or a silicide thereof, the Fermi level of which lies between the conduction and the valence bands of the semiconductor. The consequence is that the impurity concentration in the substrate (1) can be relatively high and that the buried channel layer can be made shallow in order to prevent the occurrence of the short-channel effect.

Description: BACKGROUND OF THE INVENTION This invention relates to a field-effect transistor (FET) and, more particularly, to a metal-oxide semiconductor FET (MOSFET). Nowadays a MOSFET is widely used and required to have more highly integrated construction. A MOSFET, in general, comprises a semiconductor substrate, a source electrode and a drain electrode formed therein which have a conductivity opposite to that of the substrate, a gate electrode, and a gate insulation layer formed between the substrate and the gate electrode. Hereinafter such a structure is referred to as a "typical structure". There is another structure providing a buried channel layer formed between the source and the drain electrodes having an opposite conductivity to that of the substrate. Hereinafter this structure is referred to as a "buried channel structure". The most common material used for a gate electrode of a conventional MOSFET is polysilicon. Usually, phosphor or boron is diffused, at a high density, into the polysilicon so as to form a gate electrode made of N⁺ polysilicon or P⁺ polysilicon in order to lower the resistance of the gate electrode and make stable the work function thereof. Metals having a high melting point such as tungsten , molybdenum or silicides thereof are also well known materials used for a gate electrode. However, there are several drawbacks in the conventional MOSFET's using above described materials. The drawbacks in a MOSFET using N⁺ polysilicon for a gate electrode will firstly be described. Since it is desired to set a threshold voltage at about 0.8V in an N-channel FET and at about -0.8V in a P-channel FET generally, the former adopts the typical structure and the latter adopts the buried channel structure. However, in the former, that is, in an N-channel FET, there is a defect of decreasing carrier mobility when circuit integration is conducted. The circuit integration makes an electric field in a channel increase and causes carriers to tend to move through a surface region of a substrate. Thus carrier mobility is decreased, so that operation speed and drive capability of the device are reduced. Further, many hot carriers are produced in the surface region of the substrate and these hot carriers are trapped in an insulation layer. The trapped hot carriers cause a change in electric characteristics of the device. This change results in the decrease of the reliability of the device. On the other hand, a P-channel FET has lesser defect described above because of its buried channel structure. However, it has the other defect of decreasing a threshold voltage when circuit integration is made, because its channel length becomes short with integration (hereinafter this effect is referred to as "short channel effect". Second, the drawbacks in a MOSFET using P⁺ polysilicon for a gate electrode are described. When P⁺ polysilicon is used for a gate electrode, buried channel structure is adopted to an N-channel FET and typical structure is adopted to a P-channel FET in order to set a threshold voltage at a designed value. Therefore, as described above, the defect caused by the short channel effect arises in the former and the defect of low carrier mobility and low reliability arises in the latter. In MOSFET's where metals having a high melting point such as tungsten, molybdenum or silicide thereof are used for a gate electrode, the following drawbacks still exist. In these MOSFET's, the density of impurities must be decreased in order to develop drive capability. Therefore, typical structure must be adopted to both N-channel FET's and P-channel FET's and it was found that drive capability and device reliability are lowered when circuit integration is made. A device known from JP-A-56-87368 shows a MOSFET with a p-type silicon substrate, and having a gate electrode made of molybdenum. The short channel is formed by injection of impurities. Solid State Tech. vol.27, no.8. Aug. 1984, P145-149, shows the use of tungsten and Molybdenum Silicide gate electrodes. IEDM Tech. Digest 9-11, Sept. 1974, p550-553, shows that for short channel MOSFETS, the threshold voltage varies with channel length (Fig.3). This document also shows that the impurity concentration profile may affect the threshold voltage, but emphasised that device optimization in the presence of short channel effects is a complex problem involving many interdependent parameters. SUMMARY OF THE INVENTION With the above in view, the present invention has been made and its object is to provide a novel MOSFET which has a high operation speed, a high carrier mobility, a high current drive capability, and a high reliability, even when circuit integration is made. In addition to that, the object of this invention is to prevent the threshold voltage of the MOSFET from decreasing when its channel length is shortened, that is, by the short channel effect. According to the invention, there is provided a short channel semiconductor device comprising a silicon substrate of first conductivity type, a source electrode and a drain electrode of second conductivity type and formed on the substrate, a buried channel of the second conductivity type and formed between the source electrode and the drain electrode, and a gate electrode formed above the channel through the intermediary of an insulation layer, wherein said gate electrode is made of material having a Fermi level located between the conduction band and the valence band of the semiconductor, and characterized in that an impurity concentration in the substrate is more than 1x10¹⁷ cm⁻³. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a classifying chart of MOSFET's based on materials constituting a substrate and a gate; Fig. 2 is a cross sectional view showing an embodiment of a MOSFET device according to this invention; Figs. 3 through 7 are graphs showing characteristics of the MOSFET's according to this invention and prior art. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a classification chart of MOSFET's, where the abscissa indicates an impurity density in a substrate and the ordinate indicates a work function or a Fermi level of a material constituting a gate electrode. In this chart, a region A contains conventional MOSFET's each having an N⁺ polysilicon gate electrode, where an N-channel one adopts the typical structure and a P-channel one adopts the buried channel structure as described before. A region B contains conventional MOSFET's each having a P⁺ polysilicon gate electrode, where an N-channel one adopts the buried channel structure and a P-channel one adopts the typical structure as also described before. A region C contains conventional MOSFET's each having a gate electrode which is constituted by metal having a high melting point such as wolfram, molybdenum or silisides thereof, where the Fermi level of these materials is located between those of an N⁺ polysilicon and a P⁺ polysilicon. In conventional MOSFET's belonging to the region C, an impurity density in a substrate tends to decrease so as to improve current drive capability. Therefore, both N-channel and P-channel MOSFET's in region C adopt the typical structure as described before. The inventor discovered the fact that MOSFET's belonging to a region D are very useful for solving above described drawbacks residing in conventional devices, though such MOSFET's were regarded as useless before. The present invention was derived from this discovery by the inventor. Thus a device according to the invention has the same kind of gate electrode as that of conventional MOSFET's in the region C, but an impurity density in the device is higher than that in the conventional MOSFET's. The device adopts the buried channel structure because of a high impurity density in a substrate. Therefore, the device has a higher mobility and a higher reliability than a device having a typical structure. Besides that, since the Fermi level of a gate material is located between those of N⁺ polysilicon and P⁺ polysilicon, a channel layer can be thinned so as to prevent short channel effect, that is, to prevent the threshold voltage of the device from decreasing. Now, referring to the drawings, a preferred embodiment of the invention will be described in detail. Figure 2 shows a cross sectional view of a MOSFET device according to the invention. On a silicon substrate 1, a field oxide 2 is formed which surrounds a device region. In the device region, a source electrode 3 and a drain electrode 4 having a conductivity opposite to the substrate 1 are formed, and therebetween a channel 7, which also has a conductivity opposite to the substrate 1, is formed. Above the substrate 1, a gate electrode 6 is formed with an insulation layer 5 interposed therebetween. The first characterizing feature of the device is that the device adopts the buried channel structure, and the second characterizing feature is that the gate electrode 6 is made of metals having a high melting point such as tungsten , molybdenum or silicide thereof, these materials having a Fermi level located between those of N⁺ polysilicon and P⁺ polysilicon. Owing to the first feature, that is, use of the buried channel structure, a mobility is improved over a device having typical structure. Owing to the second feature, that is, use of a specific material for the gate electrode, it is possible to obtain a desired threshold value even if the channel 7 is formed to be shallow. When the superficial channel can be formed the mobility is improved. Therefore, an operation speed and current drive capability of the device can be improved. Besides, the shallow channel prevents the short channel effect. For a gate material, Mo, W, MoSi₂, WSi₂, etc. are used. Polysilicon containing no impurities is also used. When MoSi₂ is used for the gate electrode of a CMOSFET having a threshold voltage of ±0.8V, it is preferred to make an impurity density in the substrate higher than 1x10¹⁷ cm⁻³ at a peak value. In this condition, buried channel structure can be adopted by both N-channel and P-channel transistors. Since the threshold value slightly depends on the thickness of an insulation layer, in the buried channel structure device, the thickness of the layer is not critical. Figure 3 shows the distribution of impurities in a substrate from the surface toward inside measured for a buried channel NMOSFET according to the prior art which has a P⁺ polysilicon gate electrode (shown by a solid line), and to this invention which has a MoSi₂ gate electrode (shown by a broken line). As shown in the graph, a PN junction point Q is located in a shallower position in the device of the invention than that of the prior art. This means that the device according to the invention has a shallow channel. Figures 4 and 5 are graphs showing relations between an effective channel length and a drain current in an NMOSFET and a PMOSFET, respectively. In the graphs, a curve M shows a characteristic of a device having a MoSi₂ gate electrode according to the invention, and curves P and N show characteristics of devices having a conventional P⁺ polysilicon gate electrode and a conventional N⁺ polysilicon gate electrode, respectively, according to the prior art. These graphs clearly show that the device according to the invention has larger current drive capability than the devices of the prior art. Figure 6 shows a fluctuation of a drain current caused by a substrate current. Increase in the fluctuation of the drain current reduces reliability of the device. This graph indicates that the device according to the invention has higher reliability than the prior art device having an N⁺ polysilicon gate electrode though it has less reliability than the prior art device having a P⁺ polysilicon gate electrode. Figure 7 shows a relation between an effective channel length and a threshold voltage. A threshold voltage keeps a high level until an effective channel length rapidly decreases. As shown in the graph, the knee point of the curve M, which corresponds to the device according to the invention, is located to the left of that of curve N, which corresponds to the prior device. Thus, the invention prevents the short channel effect. In conclusion, since a MOSFET according to the invention adopts the buried channel structure and has a gate electrode made of materials having an intermediate Fermi level, the following improvement can be attained. (i) A high mobility can be obtained even if a high integration is carried out so as to improve the operation speed and current drive capability. (ii) A threshold voltage can be kept at a high value by preventing the short channel effect.

Claims: A short channel semiconductor device comprising a silicon substrate (1) of first conductivity type, a source electrode (3) and a drain electrode (4) of second conductivity type and formed on the substrate (1), a buried channel (7) of the second conductivity type and formed between the source electrode (3) and the drain electrode (4), and a gate electrode (6) formed above the channel (7) through the intermediary of an insulation layer (5), wherein said gate electrode (6) is made of material having a Fermi level located between the conduction band and the valence band of the semiconductor, and characterized in that an impurity concentration in the substrate (1) is more than 1x10¹⁷ cm⁻³. A short channel semiconductor device according to claim 1, wherein the gate electrode is made of tungsten, molybdenum or silicides thereof.

Inventors: NOGUCHI TATSUO, NOGUCHI, TATSUO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: H01L29
IPC codes: H01L29