MOS TRANSISTOR

Abstract: A MOS transistor is formed in a silicon layer (3) applied on an insulating base (1). The channel region (24,23) of the transistor has a weakly doped region (24) located nearest the surface of the layer and a more heavily doped region (23) nearest the base. The latter region extends in under the source region (22) of the transistor and is connected to the source contact of the transistor via a highly doped re­ gion (21) of the same conduction type. The more heavily doped region (23) is doped with a doping dose of at least 2 x 10¹² cm⁻².

Description: The present invention relates to a MOS transistor according to the precharacterising part of Claim 1. Such a MOS transistor is substantially known from the PAT.ABSTR. OF Japan 51-147 186. A prior art transistor of the above-mentioned kind is shown in Figures 1a-1d. The transistor is made in so-called SOS technique ("Silicon-On-Sapphire" technique). It is usually included in an integrated circuit with a large number of transistors, the different transistors being formed in separate silicon layers, arranged on a base of sapphire. Transistors of opposite types - P-transistors and N-transistors - usually cooperate in pairs in so-called CMOS circuits (complementary MOS circuits). Such a circuit is shown in Figure 1. Figure 1a shows a section through a circuit of the above-mentioned kind. On the sapphire base 1 two rectangular silicon layers 4,9,6 and 20,24,22 are applied. The layer 4,9,6 consists of a P-transistor whose source 4 and drain 6 consist of highly doped P-conducting layers. The channel region 9 consist of a slightly N-doped layer. Above this and separated from the silicon by a thin insulating silicon dioxide layer 10, the control contact 11 of the transistor is arranged, which control contact consists of a layer of N-conducting polycrystalline silicon. The sapphire base and the transistor, with the exception of the contacted parts of the source and drain regions, are covered by a protective layer 12 of silicon dioxide. The source region 4 is contacted by a metal layer 13 and the drain region 6 by a metal layer 14. The N-transistor 3 has heavily N-doped source and drain regions 22 and 20 and a channel region 24 which is slightly P-conducting. Above the channel region the control oxide layer 26 and the control electrode 27 are arranged. The transistor has a source contact 28, and its drain contact consists of the metal layer 14 which also constitutes a drain contact for the P-transistor 2. Figure 1b shows a section through the transistor 3, which section is perpendicular to the section in Figure 1a and passes through the channel region 24. As will be clear from the figure, the control oxide layer 26 and the control electrode 27 cover both the upper surface of the silicon layer 24 and the two edge surfaces thereof. Figure 1c shows the transistor 3 with source and drain regions 22 and 20 and the channel region 24. The contact surfaces of the drain contact 14 and the source contact 28 with drain and source regions are shown in dashed lines. The control contact 27 is also shown in dashed lines. Figure 1d shows the circuit diagram of the CMOS circuit. The drain contacts of the transistors are electrically connected together since they consist of one and the same metal layer 14. The source contact 13 of the transistor 2 is connected to a positive supply voltage, and the source contact 28 of the transistor 3 is connected to a negative supply voltage. The control electrodes 11 and 27 of the transistors are connected together and constitute the input of the circuit, to which an input signal Ui is supplied. The common drain contact 14 constitutes the output of the circuit, where the output signal Uu of the circuit is obtained. In the known type of CMOS circuits described above, a space charge region forms at the N-transistor at the junction between the drain and channel region of the transistor. At sufficiently high supply voltage, electron hole pairs are formed in the space charge region. The holes travel to the channel region and cause this to become charged, which acts in the same way as a positive control voltage. The non-linearity caused thereby is a disadvantage in the case of analog operation of the CMOS circuit. It is known that this disadvantage may be reduced if the channel region is connected to the source region (is "grounded"). However, it has proved to be difficult to bring about an effective improvement in this way, primarily because the resistance between the source contact and the central parts of the channel region is high as a result of the weak doping of the region. For the same reason, known transistors of this kind possess less satisfactory high-frequency properties; further, they are impaired by noise caused by the state of the surface at the junction between the sapphire and the channel region. The MOS transistor described in Patent Abstracts of Japan, abstract of JP 51-147 186 (Fujitsu K.K.), December 17, 1976, is provided with a contact part of P-type between the source and the sapphire substrate and this contact part is shorted with the N-type source by the source electrode. The superficial part of the channel region must have low doping - a doping dose of the order of magnitude of at most 10¹¹ cm⁻² - for the threshold voltage of the transistor to receive a useful value. A transistor of a similar kind is described in the IEEE Journal of Solid-State Circuits, Vol. 23, No. 1, pages 198-201, which was published as late as in February 1988. From Patent Abstract of Japan, abstract of JP 58-98 969 (Nippon Denki K.K.), June 13, 1983, a transistor is known which has a metal silicide layer nearest the substrate and partly below the channel region, the metal silicide layer being connected to the source contact of the transistor. The silicide layer is stated to provide elimination of certain parasitic effects. However, for reasons of manufacturing technique, it is difficult and complicated to manufacture a silicide layer of this kind, and such a layer involves a clear risk of leakage currents. In known MOS transistors of the kind referred to here, it has proved that the highly doped source and drain regions tend to creep towards each other, during the manufacture, within that part of the silicon layer which is located at a greater depth below the surface. In these transistors, therefore, the length of the channel region must be kept greater than what would otherwise be necessary. Furthermore, it has been found that these known transistors are relatively sensitive to radiation. Radioactive radiation, for example gamma rays, produces a positive charge of the sapphire, whereby an N-channel is induced in the surface of the slightly doped channel region, which surface faces the sapphire. As will be clear from Figure 1b, a transistor of the above kind consists of a main portion on the plane surface of the silicon layer and two "corner transistors" 31 and 32. Since the control electrode at the corners 31 and 32 influences the silicon layer from two directions, the effective threshold voltage will be lower (and thus the leakage currents higher) at the corners than on the plane surface of the transistor. It has also been found that the silicon oxide layer 26 becomes thicker at the edges below the corners 31 and 32. This causes the threshold voltage there to become more radiation-sensitive than for the main portion. Taken together, these effects at the corners and edges lead to a deterioration of the properties of the transistor as a whole. The requirements for accurate control of the manufacturing process will therefore be high to avoid that this deterioration becomes too great. The invention aims to provide a transistor of the above-mentioned kind, which exhibits a high constant gain, improved high frequency properties, low noise, lower leakage currents (and hence lower losses), and a low radiation sensitivity, which permits the use of narrower channel regions and hence lower operating voltages, and which places lower demands on the control of the manufacturing process in view of the properties of the transistors at the edges of the transistors. To achieve this aim the invention suggests a transistor according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figures 1a to 1d a known CMOS circuit having been dealt with above, Figure 2a a section through a CMOS circuit, designed for effective grounding of the channel region, Figure 2b a plan view of the N-transistor included in the circuit of Figure 2a, Figure 3a a plan view of an alternative N-transistor, Figure 3b a section through the transistor, Figure 4a an N-transistor according to the invention, which is formed for elimination of the problems at the edge of the transistor, Figure 4b a section through the transistor in Figure 4a, Figure 5 a further N-transistor, which lacks the connection of the channel region to the source region, Figure 6 a section through an N-transistor according to the invention, in which the more heavily doped part of the channel region does not extend all the way to the drain region, Figure 7a -7c a so-called closed transistor. Figure 2a shows a section through a CMOS circuit with the transistor pair 2,3. As with the known circuit shown in Figure 1a, the two transistors are formed in two silicon layers arranged on a sapphire base 1. Instead of sapphire, an arbitrarily insulating material with suitable properties may be used, for example aluminium oxide or a silicon oxide layer arranged on a silicon base. The thickness of the silicon layer may be 0.3 to 0.6 µm. The source, drain and control contacts 13,28,14,11,27 and the insulating and protective layers 10,26,12 have the same design as in the case of the circuit shown in Figure 1a. The P-transistor 2 has the source region 4 and the drain region 6,7. The latter has a more highly doped part 6 arranged nearest the silicon surface, and a more weakly doped part 7 arranged nearest the sapphire base. The channel region has a more weakly doped part 9 and a more heavily doped part 8 located at a lower level. The latter doped part 8 extends laterally in under the source region 4 and is connected to a heavily doped part 5 arranged at the surfaces OF the silicon layer The source contact 13 also contacts the region 5. In similar manner, the drain region 20, 25 of the N-transistor 3 has a superficial highly doped part 20 and a part 25, located at a lower level, with a weaker doping. The channel region has a more weakly doped part 24 located nearest the surface, and a more heavily doped part 23 located nearer to the sapphire base, the latter part extending laterally in under the source region 22 and being connected, via a highly doped region 21, to the source contact 28. The different regions have been generated by ion implantation of dopants, for example phosphorous for the N-conducting regions and boron for the P-conducting regions. The doping dose is, for example, P-/N- 10¹¹ cm⁻² P/N 10¹³ cm⁻² P+/N+ 10¹⁵ - 10¹⁶ cm⁻² The doping dose of the more heavily doped parts 8 and 23 of the channel regions should exceed 2-3 x 10¹² cm⁻² in order to obtain the favourable properties of a device according to the invention. Figure 2b shows a plan view of the N-transistor 3 with the source and drain contacts 28,14 shown in dashed lines. The transistor has the cross-section shown in Figure 2a along its entire length. The P-transistor 2 is constructed in the same way. With the aid of the lower, more heavily doped layers 8 and 23, respectively, of the channel region, a low-resistive connection between the source contact and all parts of the channel region is obtained and hence an efficient grounding of the channel region to the source contact. In this way, the non-linearities of prior art transistors, mentioned in the introductory part of the description, are eliminated and better high frequency properties are obtained. Further, the noise properties of the transistor are considerable obtained. Finally, the above-mentioned charge effect is eliminated, which makes the transistor considerably less sensitive to radioactive radiation. Further, the higher doping of the lower part of the channel region reduces or eliminates the risk of the source and drain regions creeping towards each other, as mentioned above. In this way, shorter channels and higher operating voltages than previously can be used. The weaker doping of the lower parts 7,25 of the drain regions also contributes to the reduced risk of the source and drain regions creeping towards each other. Figure 3a shows a plan view of an N-transistor, and Figure 3b shows a section at A-A through this transistor. Instead of the P+ layer 21, as shown in Figure 2, extending along the length of the entire transistor, a plurality of separate layers 21a to 21c are arranged on top of the P layer 23, according to Figure 3, and distributed along the transistor. In this way, the source region 22 is divided into a plurality of separate regions 22a to 22d. The number of P+ layers may be both greater and smaller than in Figure 3 (three). Also, only one single P+ layer with a similar embodiment as the layer 21b in Figure 3 may be arranged. The P+ layers 21 need not, as in Figure 3, extend all the way to the channel region 24 but may terminate at a distance therefrom, the source region 22 becoming thereby one single cam-shaped, coherent region. Figure 4a shows a plan view of and Figure 4b, the latter exhibiting a section B-B through an N-transistor according to the invention. It differs from that shown in Figure 2 by the P+ layer 21 at both ends of the transistor having portions extending to the channel region 24. This eliminates the "corner transistor parts" 31 and 32 in Figure 1b. This gives, inter alia, the following considerable advantages, lower leakage current and hence lower losses, the possibility to operate at lower voltages, improved noise properties, reduced sensitivity to radiation, and reduced demands for exact process control. The "edge stop" according to Figure 4 can be used, as shown in Figure 4, together with a grounding of the channel region. Alternatively, as shown in Figure 5, it can be used in such a transistor in which the channel region is not grounded to the source contact. The width of the "edge stop", i.e. the extension along the control electrode counting from the edge, should be at least as large as the thickness of the silicon layer. Figure 6 shows a further N-transistor. It corresponds to the N-transistor shown in Figure 2 with the exception that the lower, more heavily doped part 23 of the channel region does not extend to the drain region 20,25. With reference to Figure 4, the invention has been described above as applied to a so-called edge transistor, i.e. a transistor in which the channel region extends out to the edge of the silicon layer. Figure 7 shows a so-called closed transistor, i.e. a transistor in which the channel region is surrounded by the source or drain region (in the case shown in Figure 7 by the source region) of the transistor. Figure 7a shows a plan view of a closed N-transistor, Figure 7b a section taken along A-A in Figure 7a, and Figure 7c a section taken along B-B in Figure 7a. The reference numerals correspond to those used in Figure 2. The drain region 20,25 of the transistor is located in the centre thereof. The drain region is surrounded by the channel region 24 with its more heavily doped part 23, located at a lower level, which extends in under the source region 22, which in turn surrounds the channel region. In each one of the corners of the transistor, P+ layers 21a to 21d are arranged between the layer 23 and the surface of the silicon layer. In this way the source region 22 is divided into four partial regions 22a to 22d. The drain contact 14 and the annular source contact 28 are shown in dashed lines in Figure 7a. The transistor according to Figure 7 exhibits the same advantages as the transistor shown in Figures 2 and 3, and since it is a closed transistor no problems with "corner transistors" arise. To render the grounding even more effective, the P+ region can be formed as in Figures 2 and 3. In the foregoing, only that case has been described in which those semiconductor layers in which the transistors are formed consist of silicon, and in which the insulating layers consist of silicon oxide. The invention can, of course, be applied to other semiconductor materials than silicon and to other forms of protective and insulating layers than silicon oxide.

Claims: A MOS transistor (3) formed in a semiconductor layer, and arranged on an insulating base (1), comprising: a channel region (24) of a first conduction type (P) formed in said semiconductor layer; source (22) and drain (20) regions of a second conduction type (N) formed in said semiconductor layer and separated by the channel region (24); a source contact (28) connected to the source region (22); a drain contact (14) Connected to the drain region (20); a control electrode (27) arranged on top of the channel region (24) and separated therefrom by an insulating layer (26), the channel region (24) and the control electrode (27) extending to opposite edges of the semiconductor layer; a bottom region (23) of the first conductivity type (P) formed in said semiconductor layer underlying the source region (22), the bottom region (23) being contiguous to the channel layer (24) and being more heavily doped than the channel layer (24); characterized in that an edge stop region (21) of the first conduction type (P) is formed in said semiconductor layer and is arranged contiguous to the source region (22), the bottom region (23), and the surface of the semiconductor layer, and that the edge stop region (21) and the bottom region (23) completely separate the source region (22) from the base (1) and from the edges of the semiconductor layer. A MOS transistor according to claim 1, characterized in that the edge stop region (21) is more heavily doped than the bottom region (23). A MOS transistor according to claim 1 or 2, characterized in that the source contact (28) is connected to the edge stop region (21). A MOS transistor according to any of the preceding claims, characterized in that the bottom region (23) is doped with a doping dose of at least 2·10¹² cm⁻².

Inventors: SVEDBERG, PER
Applicants: ASEA BROWN BOVERI AB
Application year: 1988
Main classification: H01L29
IPC codes: H01L29, H01L27