1. Field of the Invention
The present invention relates to an insulated gate field effect transistor (herein after refer to as an insulated gate FET or an FET) and its manufacturing method.
2. Description of the Prior Art
Heretofore there has been proposed an insulated gate FET of the type that it has a high resistivity semiconductor layer formed on a substrate having an insulating surface, a gate electrode formed on the semiconductor layer with a gate insulating layer sandwiched therebetween in a manner to divide the semiconductor into two as viewed from above, and N or P conductivity type source and drain regions formed in the semiconductor layer in a manner to leave a channel forming region between first and second regions on both sides of the gate electrode as viewed from above, the source and drain regions being lower in resistivity than the channel region.
The insulated gate FET of such a construction is called an N-channel type or P-channel type insulated gate FET depending upon whether the source and drain regions are the N or P conductivity type, and it operates in such a manner as follows:
When supplied with a control voltage across the source region and the gate electrode with a DC power source connected across the source and drain regions via a load, the insulated gate FET remains in the OFF state if the control voltage is lower than a certain threshold voltage when the FET is the N-channel type, or if the control voltage is higher than the threshold voltage when the FET is the P-channel type. In this case, substantially no current flow (drain current) is caused in the drain region, supplying no current to the load. In the case where the control voltage is higher than the threshold voltage when the FET is the N-channel type, or where the control voltage is lower than the threshold voltage when the FET is the P-channel type, however, a channel region of the same conductivity type as that of the source and drain regions is formed in the channel forming region to extend between the source and drain regions on the side of the gate insulating layer, and the FET is turned ON to cause the drain current to flow, feeding current to the load.
As a modification of the above insulated gate FET has been proposed such a structure that the entire region of the semiconductor layer is formed of a single-crystal semiconductor, and accordingly, the channel forming region, the first and second regions and the source and drain regions formed therein, respectively, are all formed of the single-crystal semiconductor.
With such an insulated gate FET, however, the semiconductor layer cannot be formed on the substrate unless the substrate is made of an insulating or semi-insulating single-crystal semiconductor.
When the semiconductor layer is formed of the single-crystal semiconductor layer, especially when the channel forming region is former of the single-crystal semiconductor, it has a smaller optical energy gap than does it when formed of a non-single-crystal semiconductor. For example, when the semiconductor layer is made of the single-crystal silicon, the optical energy gap of the channel forming region is 1.1 eV. On account of this, when the FET is in the OFF state, the train current is small but larger than in the case where the channel forming region is formed of the non-single-crystal semiconductor.
For this reason, the abovesaid insulated gate FET is poorer in the OFF characteristic than in the case where the channel forming region is made of the non-single-crystal semiconductor.
Another modified form of the above insulated gate FET heretofore proposed has such a structure that the entire region of the semiconductor layer is formed of a non-single-crystal semiconductor doped with a recombination center neutralizer.
In the case of such an insulated gate FET, even if the substrate is not made of the insulating or semi-insulating single-crystal semiconductor, and even if the substrate is a metallic substrate which has an insulated surface, or such as a glass, ceramic, organic synthetic resin or like insulating material substrate, the semiconductor layer can be formed on the substrate. Further, since the channel forming region is made of the non-single-crystal semiconductor doped with a recombination center neutralizer, it has a larger optical energy gap than in the case where it is formed of the single-crystal semiconductor, so long as it is sufficiently doped with the recombination center neutralizer. For instance, when the semiconductor layer is formed of non-single-crystal silicon well doped with the recombination center neutralizer, the channel forming region has an optical energy gap in the range of 1.7 to 1.8 eV. In consequence, when the insulated gate FET is in the OFF state, the drain current will be markedly small, negligible as compared with that when the channel forming region is formed of the single-crystal semiconductor. Accordingly, so long as the semiconductor layer is sufficiently doped with the recombination center neutralizer, the FET will exhibit a more excellent OFF characteristic than does it when the channel forming region is made of the single-crystal semiconductor.
In the case of such an insulate gate FET having the semiconductor layer formed of the non-single-crystal semiconductor, impurity-doped regions are formed in the first and second regions, for example, by ion implantation of an N- or P-type impurity, and then the source and drain regions are formed by heat treatment for the activation of the impurity doped in the impurity-doped regions. During the heat treatment, however, the recombination center neutralizer doped in the channel forming region is diffused therefrom to the outside by the heat. Therefore, the channel forming region contains no required and sufficient amount of recombination center neutralizer, and hence has a smaller optical energy gap than the predetermined.
Accordingly, the conventional insulated gate FET with the semiconductor layer formed of the non-single-crystal semiconductor possesses an excellent OFF characteristic as compared with the case where the channel forming region is made of the single-crystal semiconductor, but the OFF characteristic is not fully satisfactory.
Moreover, in the case of the above prior art insulated gate FET of the type having the semiconductor layer formed of the non-single-crystal semiconductor, since the source and drain regions are also obtained by heat treatment, the recombination center neutralizer doped therein is diffused to the outside during the heat treatment. Thus, since the source and drain regions have the same optical energy gap as that of the channel forming region, there is set up between each of the source and drain regions and the channel forming region substantially no or very small potential barrier against carriers flowing from the source or drain regions toward the channel forming region.
This is another cause of the unsatisfactory OFF characteristic of the conventional insulated gate FET which has the semiconductor layer formed of the non-single-crystal semiconductor.
Besides, when the semiconductor layer, and accordingly the source and drain regions are formed of the non-single-crystal semiconductor, they has the same degree of crystallization as that of the channel forming region and a far higher resistance than in the case where they are made of the single-crystal semiconductor. On account of this, in the conventional insulated gate FET of the type having the semiconductor layer formed of the non-single-crystal semiconductor, the speed of switching between the ON and the OFF state is lower than in the case where the source and drain regions are formed of the single-crystal semiconductor. Accordingly, this FET has the defect that its ON-OFF operation cannot be achieved at high speed.
It is therefore an object of the present invention to provide a novel insulated gate FET which is free from the abovesaid defects of the prior art.
Another object of the present invention is to provide a novel method for the manufacture of such a novel insulated gate FET.
The insulated gate FET of the present invention has also the same structure as the above-described conventional insulated gate FET. That is, it has a high resistivity semiconductor layer formed on a substrate having an insulating surface, a gate electrode formed on the semiconductor layer with a gate insulating layer sandwiched therebetween so that it separates the semiconductor layer into two as viewed from above, and N or P conductivity type source and drain regions formed in the semiconductor layer so that they define a channel forming region between first and second regions on both sides of the gate electrode as viewed from above and extend vertically from the upper surface of the first and second regions toward the substrate, the source and drain regions having a lower resistivity than that of the channel forming region.
In the insulated gate FET of the present invention, however, the semiconductor layer is formed of a non-single-crystal semiconductor doped with a required and sufficient amount of recombination center neutralizer, and accordingly, the channel forming region is also formed of such a non-single-crystal semiconductor. In the first and second regions which constitute the source and drain regions in the semiconductor layer, there are provided on the sides of the source and drain regions, respectively, crystallized regions which have a higher degree of crystallization than the channel forming region and are doped with the recombination center neutralizer.
The insulated gate FET of the present invention is identical in construction with the aforesaid conventional insulated gage FET which has the semiconductor layer formed of the non-single-crystal semiconductor, except the inclusion of the abovesaid crystallized regions in the semiconductor layer.
Accordingly, the insulated gate FET of the present invention also operates in the same manner as the aforementioned conventional FET. That is, when supplied with a control voltage across the source region and the gate electrode with the power source connected across the source and drain regions via a load, it remains in the OFF state and causes no current flow to the load if the control voltage is lower (or higher) than a certain threshold voltage, and if the control voltage is higher (or lower) than the threshold voltage, it is turned ON to cause drain current to flow, supplying current to the load.
The insulated gate FET of the present invention has also the semiconductor layer formed of the non-single-crystal semiconductor, and hence it is free from the requirement that the substrate be an insulating or semi-insulating single-crystal semiconductor, as is the case with the conventional FET of this kind.
Further, since the semiconductor layer, and consequently the channel forming region is constituted of the non-single-crystal semiconductor doped with the recombination center neutralizer, the insulated gate FET of the present invention exhibits an excellent OFF characteristic over the FET in which the channel forming region is made of the single-crystal semiconductor.
In the insulated gate FET of the present invention, however, the channel forming region is doped with a required and sufficient amount of recombination center neutralizer, as will be evident from the manufacturing method of the present invention described later. Accordingly, the channel forming region has a predetermined optical energy gap, ensuring to provide an excellent OFF characteristic as compared with that of the conventional FET which has the semiconductor layer formed of the non-single-crystal semiconductor.
Moreover, in the insulated gate FET of the present invention, the crystallized regions, which have a higher degree of crystallization than the channel forming region and are doped with the recombination center neutralizer, are formed in the first and second regions which constitute the source and drain regions, respectively, and the crystallized regions form the effective regions of the source and drain regions. On the other hand, the crystallized regions have a smaller optical energy gap than does the channel forming region. Accordingly, there is established between each of the source and drain regions and the channel forming region a potential barrier against carriers which flow from the source or drain region toward the channel forming region.
This ensures that the FET of the present invention exhibits an excellent OFF characteristic over the conventional PET which has the semiconductor layer formed of the non-single-crystal semiconductor.
Besides, in the insulated gate FET of the present invention, the crystallized regions, which constitute the effective regions of the source and drain regions, are formed in the first and second regions, as mentioned above, and the crystallized regions are far lower in resistance than in the case where the first and second regions are not crystallized. On account of this, the speed at which the FET of the present invention is switched between the ON and OFF state is higher than in the case of the prior art FET which has the semiconductor layer formed of the non-single-crystal semiconductor. In other word, the ON-OFF operation of the FET of the present invention is higher in speed than the ON-OFF operation of the conventional FET.
The insulated gate FET manufacturing method of the present invention includes the following steps.
The manufacture starts with the formation of a non-single-crystal semiconductor layer doped with the recombination center neutralizer on a substrate having an insulating surface.
Next, a gate electrode is formed on the non-single-crystal semiconductor layer with a gate insulating layer sandwiched therebetween in such a manner that the non-single-crystal semiconductor layer is separated into two as viewed from above.
Next, source and drain regions doped with N- or P-type impurity and the recombination center neutralizer are formed in first and second regions of the non-single-crystal semiconductor layer on both sides of the gate electrode, as viewed from above, in such a manner to leave therebetween a channel forming region doped with the recombination center neutralizer.
Next, the first and second regions of the non-single-crystal semiconductor layer are exposed to irradiation by light for annealing at a temperature at which the recombination center neutralizer doped in the non-single-crystal semiconductor layer does not substantially diffuse to the outside. By this, the first and second regions of the non-single-crystal semiconductor layer are crystallized to form crystallized regions on the sides of the source and drain regions. And the N-type or P-type impurity in the source and drain regions is activated. The crystallized regions have a higher degree of crystallization than the channel forming region, are doped with the recombination center neutralizer and extend vertically from the upper surface of the first and second regions toward the substrate. In this instance, it is preferable that the gate insulating layer be formed on the semiconductor layer to cover the entire area of the surface of each of first and second regions before the exposure to the light irradiation for annealing so as to prevent that the recombination center neutralizer diffuse to the outside from the source and drain regions and the crystallized regions. Further, it is preferable that the light irradiation for annealing be performed intermittently so as to prevent that the high-temperature heating of the crystallized regions by the light irradiation will cause unnecessary diffusion from the source and drain regions and the crystallized regions of the recombination center neutralizer to the outside.
With such a manufacturing method of the present invention, it is possible to easily fabricate the insulated gate FET of the present invention which possesses the aforesaid advantages.