JP2008226896A - Semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bipolar transistor that can establish a high gain and a high speed when the transistor is subject to microfabrication. <P>SOLUTION: The semiconductor device has an external base area 7 with a larger band gap than a base area 5 on the side face of the base area. The base area 5 typically uses silicon-germanium. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a semiconductor device having a structure that prevents current from flowing in the lateral direction of a semiconductor layer. This structure is particularly useful when used for a bipolar transistor suitable for high-speed operation at a high gain, and is useful when applied to a bipolar transistor using, for example, silicon-germanium as a base layer.

  In order to increase the gain and speed of the bipolar transistor, it is necessary to allow the electrons that are the collector current to flow from the emitter through the base to the collector in one dimension as much as possible. However, in a bipolar transistor in which the current flow is perpendicular to the semiconductor substrate, a region for extracting each layer that is a component of the transistor is necessary. Therefore, an area necessary for pulling out terminals (hereinafter referred to as an external base) is provided around the base area. As a result, part of the electrons flows to the base terminal via the external base, and the gain is reduced. In addition, some of the electrons reaching the collector spread two-dimensionally around the emitter, causing a problem that the distance of the passing base becomes longer and the speed is lowered.

  In a bipolar transistor using silicon-germanium as a base layer, high gain and high speed are achieved depending on the composition ratio of germanium in the base layer. Examples include, for example, the 2003 IEDM Technical Digest p. 113 (Non-Patent Document 1). A silicon-germanium layer having a band gap smaller than that of silicon is used as a base, and due to the difference in the band gap, injection of electrons into the base is increased to achieve high gain. This also makes it possible to reduce the thickness and concentration of the base layer and to obtain good high-speed operation performance.

2003 IEDM Technical Digest p. 113

  An object of the present invention is to provide a semiconductor device having a structure that prevents a current from flowing in the lateral direction of a semiconductor layer. In particular, the present invention applies this structure to provide a bipolar transistor suitable for high speed and high speed operation. With respect to the bipolar transistor, in a more specific technical aspect, the present invention is to provide a bipolar transistor capable of realizing high gain and high speed when the transistor is miniaturized.

  In order to realize such an object, problems to be solved will be described below by taking a bipolar transistor using a silicon-germanium layer as a base layer as a representative example of the present invention as an example.

  FIG. 2 is a cross-sectional view of a conventional bipolar transistor model structure using silicon-germanium as a base layer. Such bipolar transistors have the following problems.

  3A and 3B are enlarged sectional views of main parts of the bipolar transistor. 3A relates to the present invention, and FIG. 3B relates to the previous example illustrated in FIG. Reference numeral 3 is a low concentration n-type silicon layer, 4 is a low concentration n-type silicon / germanium layer, 5 is a p-type silicon / germanium layer (base layer), 6 is a high concentration silicon layer (emitter layer), and 7 is a high concentration p. Type silicon layers (external base layers) 11 and 12 are silicon oxide films, and 50 and 51 are flows of electron current. In the main part of the bipolar transistor using silicon-germanium as the base layer illustrated in FIG. 2, the electron current flow 50 is transverse in the base layer 5 at the periphery of the emitter layer 6, as illustrated in FIG. 3B. It spreads greatly in the direction. For this reason, the distance that electrons pass through in the base layer in the vicinity of the emitter end becomes longer, and the speed is lowered. This reduction in operating speed becomes more prominent by making the emitter finer, the ratio of the periphery of the emitter becomes larger than that of the bottom surface of the emitter, and the ratio of the electron current flowing to the periphery increases.

  Further, a part of the electron current flows laterally from the peripheral portion of the emitter 6 through the base 5 (electron current 51) to become a base current, resulting in a decrease in gain. This reduction in gain becomes more prominent by miniaturizing the emitter for the same reason as the above-described reduction in operating speed.

  The present invention is intended to provide a semiconductor device, in particular, a bipolar transistor, in which high gain and high speed are hardly impaired even if the emitter is miniaturized.

  The present invention is particularly useful in a bipolar transistor using a single crystal silicon / germanium layer as a base layer.

  The present invention is configured such that the electron current does not spread laterally in the base in the periphery of the emitter, and further, the electron current does not flow from the emitter periphery to the base electrode. Therefore, even if the emitter is miniaturized and the ratio of the periphery of the emitter is larger than that of the bottom surface of the emitter, the operating speed and gain are not significantly reduced, and good characteristics can be obtained.

  The main aspects of the present invention are listed below.

  A representative semiconductor device of the present invention has a base band gap of an external base region 7 which is a second semiconductor layer of a second conductivity type, for example, a third semiconductor layer of the second conductivity type in FIG. It is characterized by being wider than the band gap of the region 5.

  A fifth semiconductor layer of a first conductivity type is provided between the first semiconductor layer and the second and third semiconductor layers, and a band gap of the fifth semiconductor layer is the first semiconductor layer. It is preferable to make it narrower than the band gap of the semiconductor layer.

  A sixth conductivity type first semiconductor layer having an impurity concentration lower than that of the first semiconductor layer may be provided between the first semiconductor layer and the fifth semiconductor layer.

  It is preferable that a seventh conductivity type first semiconductor layer having an impurity concentration lower than that of the fourth semiconductor layer is provided between the second semiconductor layer and the fourth semiconductor layer.

  It is preferable that a first insulating film is provided in a part between the first semiconductor layer and the third semiconductor layer.

  It is preferable that an eighth semiconductor layer of the first conductivity type having a higher impurity concentration than the surroundings is provided in a part below the second semiconductor layer.

  It is preferable that at least a part of the upper surfaces of the first, third and fourth semiconductor layers have a metal or a metal compound.

  It is preferable if the second semiconductor layer is a single crystal silicon / germanium layer and the band gap is controlled by the composition ratio of germanium.

  The semiconductor device is suitable for a bipolar transistor having the first semiconductor layer as a collector, the second and third semiconductor layers as a base, and the fourth semiconductor as an emitter.

  According to the first aspect, the present invention can provide a semiconductor device having a structure that prevents a current from flowing in the lateral direction of the semiconductor layer.

  According to another aspect of the present invention, a bipolar transistor suitable for high speed and high speed operation can be provided.

  Since a representative example of a preferred specific embodiment of the semiconductor device according to the present invention is a bipolar transistor, this embodiment will be described.

  The bipolar transistor according to the present invention has the following configuration. That is, the present invention relates to a first conductive type first semiconductor layer serving as a collector region, and a second conductive type second semiconductor layer serving as a base region provided on the surface of the first semiconductor layer; A third semiconductor layer of a second conductivity type provided on the surface of the first semiconductor layer and serving as an external base region in contact with the second semiconductor layer at a side surface; and an emitter provided on the second semiconductor layer A fourth semiconductor layer of a first conductivity type serving as a region is provided, and a band gap of the third semiconductor layer is wider than a band gap of the second semiconductor layer.

  In this embodiment, for example, when the third semiconductor layer is silicon and the second semiconductor layer is a silicon-germanium layer, the band gap of the third semiconductor layer serving as the external base region is the base region. Therefore, the electron current is blocked by the band gap difference and does not flow to the external base region. Therefore, the electron current flows almost one-dimensionally from the emitter to the collector, and good characteristics can be obtained without causing a decrease in gain or operating speed.

  The base, emitter, and collector thicknesses of the present invention are sufficient in accordance with a conventional bipolar transistor design. For example, the base layer is frequently used from about 5 nm to about 100 nm, and the emitter layer is used from about 10 nm to about 200 nm. Further, the thickness of the collector layer is not so greatly affected by the effect of the present invention, but in many cases, it is designed to be about 0.1 μm to 1 μm. The composition ratio of germanium in the base layer is usually about 5% to 25%.

  The present invention will be described using an example in which silicon-germanium is used as a base layer as a typical embodiment of the present invention. However, the present invention can be applied to various semiconductor materials, that is, semiconductor materials that can constitute a bipolar transistor. Is something.

  Needless to say, the present invention can be carried out by using, for example, a group III-V compound semiconductor material or a group II-VI compound semiconductor material as various semiconductor materials. As an example of the III-V group compound semiconductor material, a mixed crystal of (Al, Ga, In) and (As, P, Sb, N) can be given as a representative example. They are, for example, AlAs, AlP, AlSb, AlN, GaAs, GaP, GaSb, GaN, InAs, InP, InSb, InN, and the like. Furthermore, mixed crystals such as these three elements and four elements can also be used.

  Moreover, as a II-VI group compound semiconductor material, for example, a chalcogenide mixed crystal of Zn, Cd, and Hg is a typical example.

  Next, more specific embodiments of the semiconductor device according to the present invention will be described in detail with reference to the accompanying drawings.

<Example 1>
FIG. 1 is a sectional view of a bipolar transistor showing a first embodiment according to the present invention. The main points are as follows. On the side surface of the silicon-germanium base layer 5 (second semiconductor layer of the second conductivity type), a silicon or silicon-germanium external base layer 7 (second conductivity type second semiconductor layer having a wider band gap than the base layer 5). 3 semiconductor layers) are connected to each other. Here, when silicon-germanium is used for the external base layer 7, the band gap may be widened by making the composition ratio of germanium smaller than that of the base layer 5. Further, the distance from the peripheral edge of the emitter layer 6 (first conductive type fourth semiconductor layer) to the external base layer 7 is set to about 5 nm to 100 nm, depending on the thickness of the base layer 5. .

  Hereinafter, a method for manufacturing the bipolar transistor having the structure shown in FIG. 1 will be described with reference to FIGS. 1, 5A and 5B. 5A and 5B are cross-sectional views of the apparatus showing the manufacturing method.

  A high concentration n-type silicon collector layer 2 (first conductivity type first semiconductor layer) is formed on the entire surface or a part of the semiconductor substrate 1 by epitaxial growth or impurity diffusion. Next, a low-concentration n-type silicon layer 3, a low-concentration n-type silicon / germanium layer 4 and a high-concentration p-type silicon external base layer 7 (second semiconductor layer of the second conductivity type) are epitaxially grown sequentially. After forming the insulating film 11, an opening 66 of the insulating film is formed by patterning. Next, the high-concentration p-type silicon outer base layer 7 immediately below the opening 66 of the insulating film is etched by alkaline wet etching that can selectively etch silicon with respect to the silicon-germanium layer, and the opening 77 is formed. Form (FIG. 5A).

  Next, the p-type silicon-germanium base layer 5 (second semiconductor layer of the second conductivity type) is selectively epitaxially grown only in the opening 77 of the high-concentration p-type silicon outer base layer 7, An emitter layer 6 (first conductive type fourth semiconductor layer) of n-type silicon is formed and patterned (FIG. 5B).

  Next, the insulating film 11 is patterned, the insulating film 12 is deposited, openings are formed in the emitter, base, and collector regions, and the emitter electrode 101, the base electrode 102, and the collector electrode 103 are covered to cover the openings. The semiconductor device is formed. A cross-sectional view of this state is shown in FIG.

  In the examples, the high or low impurity concentration of the semiconductor layer is usually the impurity concentration used in the bipolar transistor field unless otherwise specified. The same applies to the following embodiments.

  Next, it will be described that the bipolar transistor shown in FIG. 1 can realize characteristics suitable for high-speed operation with high gain.

  FIG. 3A shows the flow of electron current corresponding to the example of FIG. In the bipolar transistor using silicon / germanium illustrated in FIG. 1 as the base layer 5 and silicon as the external base layer 6, electrons do not spread widely in the lateral direction at the periphery of the emitter 6 as shown in FIG. 3A. Flowing. This is because the band gap of the silicon or silicon-germanium external base layer 7 connected to the periphery of the silicon-germanium base layer 5 is wider than that of the base layer 5. For this reason, the distance through which electrons pass is short even in the base layer at the emitter end, and high-speed operation can be realized.

  Further, since the band gap of the external base layer 7 made of silicon connected to the periphery of the silicon-germanium base layer 5 is wider than that of the base layer 5, electrons flow from the peripheral portion of the emitter 6 into the external base layer 7. (The electron current 51 illustrated in FIG. 3B does not occur), and a high gain can be realized without causing a decrease in gain.

  In order to consider the gain when the germanium composition ratio of the external base layer 7 is changed, FIG. 4 shows the dependence of the gain ratio on the germanium composition ratio of the external base layer. The horizontal axis represents the Ge composition ratio of the external base, and the vertical axis represents the gain ratio. Here, the gain ratio when the germanium composition ratio of the external base layer is changed is 1 when the germanium composition ratio of the external base layer is 20%, which is the same as the germanium composition ratio of the base layer. As a result, when the germanium composition ratio of the external base is reduced by 5%, the gain ratio is approximately doubled, and when the germanium composition ratio of the external base is reduced by 20% and silicon is used, the gain ratio is increased by approximately 10 times. . That is, using the present invention, it can be seen that the gain can be greatly improved by reducing the germanium composition ratio of the external base. Although the band gap is used as the characteristic of the above-mentioned material, according to this example, it corresponds to the band gap of the material at the germanium composition ratio of silicon / germanium.

  The above feature of the present invention becomes more effective when the emitter is miniaturized. That is, even if the emitter is miniaturized and the ratio around the emitter becomes larger than the bottom surface of the emitter, the above-described behavior of the electron current does not cause a significant decrease in operating speed and gain, and good characteristics can be obtained. it can. That is, the present invention can provide a semiconductor device, in particular, a bipolar transistor, in which high gain and high speed are hardly impaired even if the emitter is miniaturized.

  According to this example, a bipolar transistor having a high gain and excellent high speed can be formed. As a result, by using this transistor, it is possible to realize low power, high speed, and high performance of the circuit.

<Example 2>
FIG. 6 is a cross-sectional view of a bipolar transistor showing a second embodiment of a semiconductor device according to the present invention. As in the first embodiment, a silicon or silicon germanium external base layer 7 having a wider band gap than the base layer 5 is connected to the side surface of the silicon / germanium base layer 5. In Example 2, a low-concentration n-type silicon layer 8 having an impurity concentration about two orders of magnitude lower than that of the emitter layer is provided between the emitter layer 6 and the base layer 5. Here, the thickness of the low concentration n-type silicon layer 8 is set to about 5 nm to 5 nm. As a result, the junction capacitance between the emitter and the base can be reduced, and high-speed operation performance can be realized even during low current operation. In this example, the low-concentration n-type silicon layer 8 is provided only in the opening of the insulating film 11, but it may be provided on the entire bottom surface of the emitter layer 6 or on the side surface in contact with the insulating film 11.

  In the cross-sectional view showing the embodiment, the same parts as those in FIG. 1 which is a representative example are denoted by the same reference numerals, and the description thereof is omitted unless particularly necessary. The same applies to the following embodiments.

<Example 3>
FIG. 7 is a sectional view of a bipolar transistor showing a third embodiment according to the present invention. Similar to the second embodiment, a low-concentration n-type silicon layer 8 is provided between the emitter layer 6 and the base layer 5 to reduce the junction capacitance between the emitter and the base. This point is the same as that of the second embodiment, but the positions to be provided are different. That is, in Example 3, the low concentration n-type silicon layer 8 is provided on the lower surface of the opening of the insulating film 11. Accordingly, even when the base layer 5 needs to be thinned for speeding up, or when the external base layer needs to be thickened to reduce the resistance of the external base layer, the gain can be increased. A high-speed bipolar transistor can be formed at high current operation.

<Example 4>
FIG. 8 is a cross-sectional view of a bipolar transistor showing a fourth embodiment of a semiconductor device according to the present invention. In this embodiment, an insulating film 13 is provided in a part between the collector layer 2 of high-concentration n-type silicon and the low-concentration n-type silicon layer 3. As a result, the junction capacitance between the collector and the base can be reduced, and high-speed operation performance can be realized. It should be noted that the insulating film 13 provided in this example may be provided in a part or the entire region at an appropriate position between the external base layer 7 and the high concentration collector layer 2.

<Example 5>
FIG. 9 is a sectional view of a bipolar transistor showing a fifth embodiment according to the present invention. In the fifth embodiment, an n-type layer 9 having an impurity concentration about one digit higher than that of the n-type silicon layer 3 and the n-type silicon / germanium layer 4 is provided under the base layer 5. Thereby, the operation speed performance of the transistor can be further improved. However, this increases the junction capacitance between the collector and base of the transistor and lowers the breakdown voltage. Therefore, the n-type layer 9 is placed at an appropriate position between the external base layer 7 and the high concentration collector layer 2 as necessary. What is necessary is just to provide. Note that the n-type layer 9 is formed by ion-implanting an n-type impurity after forming the opening 66 of the insulating film or the opening 77 of the high-concentration p-type silicon external base layer at the stage of FIG. 5A, for example. It can be formed in a self-aligned manner immediately below the layer 5.

<Example 6>
FIG. 10 is a sectional view of a bipolar transistor showing a sixth embodiment according to the present invention. In this embodiment, silicide films (reaction films of silicon and metal) or metal films 21, 22, 23 are provided on part or all of the upper surfaces of the emitter layer 6, the external base layer 7, and the collector layer 2. . As a result, the contact resistance between each polycrystalline silicon layer and the electrodes 101, 102, 103 can be reduced, and the series resistance can be reduced. The bipolar transistor according to the present invention can operate at high speed by reducing the parasitic resistance. Here, the metal films 21, 22, and 23 can be made of a highly heat-resistant metal such as Ti, Ni, or Co, or a metal compound of such a metal. Typical examples of metal compounds are silicides of these metals.

  Even when an insulating film having a different thickness is etched when forming a contact hole formed in a portion where the electrodes 101, 102, 103 are provided, the silicide film (reaction film of silicon and metal) or the metal films 21, 22, 23 is formed. Since it becomes an etching stopper, a transistor can be manufactured more stably. The silicide film (reaction film of silicon and metal) or the metal film is formed by exposing the respective layers and then removing the excess portion after depositing the metal film and reacting with silicon, or polycrystalline silicon. It can be easily formed by a method of selectively depositing on the layer.

  In each of the above embodiments, any some or all of the combinations can be used. Further, the apparatus of the present invention can be realized even if another semiconductor such as GaAs is used as the semiconductor. Of course, the p-type and n-type conductivity types in each embodiment can be used in reverse. The operation of the bipolar transistor of the embodiment can be performed by reversing the emitter and collector. Furthermore, each embodiment can coexist with an existing semiconductor device such as a MOS transistor. Further, although the preferred embodiments of the present invention have been described, the present invention is not limited to the above-described embodiments, and various design changes can be made without departing from the spirit of the present invention. .

  The present invention can provide a semiconductor device having characteristics in which high gain and high-speed operation hardly change even when the emitter is miniaturized and the ratio of the periphery of the emitter is larger than the bottom surface of the emitter.

  In the present invention, a bipolar transistor that uses a silicon-germanium layer as a base layer and has a high gain and a high speed operation even when the emitter is miniaturized can be provided.

  As described in the embodiments of the present invention, according to the present invention, a bipolar transistor suitable for high speed and high speed operation can be provided. It is also effective in improving low power. From the above, the transistor according to the present invention is used in a microwave communication system such as a cellular phone or a millimeter wave band such as an anti-collision radar, or an integrated circuit for large-capacity data transmission between cities or between devices. Probability is high.

Embodiments of the present invention are listed below.
(1) A semiconductor substrate, a first conductivity type first semiconductor layer on the semiconductor substrate, a second conductivity type second semiconductor layer provided on the first semiconductor layer, and the first conductivity type A second conductive type third semiconductor layer provided on the semiconductor layer and in contact with the second semiconductor layer at a side surface; and a first conductive type fourth semiconductor layer provided on the second semiconductor layer. And a band gap of the third semiconductor layer is wider than a band gap of the second semiconductor layer.
(2) A fifth semiconductor layer of a first conductivity type is provided between the first semiconductor layer and the second and third semiconductor layers, and a band gap of the fifth semiconductor layer is 2. The semiconductor device according to item (1), wherein the semiconductor device is narrower than a band gap of the first semiconductor layer.
(3) A sixth semiconductor layer of a first conductivity type having an impurity concentration lower than that of the first semiconductor layer is provided between the first semiconductor layer and the fifth semiconductor layer. The semiconductor device according to (2) above.
(4) A seventh semiconductor layer of a first conductivity type having an impurity concentration lower than that of the fourth semiconductor layer is provided between the second semiconductor layer and the fourth semiconductor layer. The semiconductor device according to any one of (1) to (3) above.
(4) The semiconductor device according to any one of (1) to (4) above, wherein a first insulating film is provided in a part between the first semiconductor layer and the third semiconductor layer. .
(6) The semiconductor according to any one of (1) to (5) above, wherein an eighth semiconductor layer of a first conductivity type having an impurity concentration higher than that of the surrounding is provided in a part below the second semiconductor layer. apparatus.
(7) The semiconductor device according to any one of (1) to (6) above, wherein a metal or a metal compound is included in at least a part of the upper surfaces of the first, third, and fourth semiconductor layers.
(8) The semiconductor device according to any one of (1) to (7), wherein the second semiconductor layer is a single crystal silicon / germanium layer, and the band gap is controlled by a composition ratio of germanium.
(9) The semiconductor device is a bipolar transistor in which the first semiconductor layer is a collector, the second and third semiconductor layers are bases, and the fourth semiconductor is an emitter. ) To (8).

FIG. 1 is a sectional view of a bipolar transistor showing a first embodiment according to the present invention. FIG. 2 is a cross-sectional view of a typical model of a conventional bipolar transistor. FIG. 3A is a cross-sectional view showing the flow of electron current by enlarging the main part of the bipolar transistor shown in FIG. FIG. 3B is an enlarged cross-sectional view showing the flow of an electron current in the main part of the bipolar transistor shown in FIG. FIG. 4 is a characteristic diagram showing the dependence of the gain ratio on the germanium composition ratio of the external base layer in the bipolar transistor according to the present invention. 5A is a cross-sectional view of the device shown in the order of manufacturing steps of the bipolar transistor shown in FIG. FIG. 5B is a cross-sectional view of the device shown in the order of manufacturing steps of the bipolar transistor shown in FIG. FIG. 6 is a cross-sectional view of a bipolar transistor showing a second embodiment according to the present invention. FIG. 7 is a sectional view of a bipolar transistor showing a third embodiment according to the present invention. FIG. 8 is a sectional view of a bipolar transistor showing a fourth embodiment according to the present invention. FIG. 9 is a sectional view of a bipolar transistor showing a fifth embodiment according to the present invention. FIG. 10 is a sectional view of a bipolar transistor showing a sixth embodiment according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Silicon substrate, 2 ... High concentration n-type silicon layer 3, 8 ... Low concentration n-type silicon layer, 4 ... Low concentration n-type silicon-germanium layer, 5 ... p-type silicon-germanium layer, 6 ... High concentration n Type silicon layer, 7 ... high-concentration p-type silicon layer, 9 ... n-type diffusion layer, 11, 12, 13 ... silicon oxide film, 21, 22, 23 ... metal or silicide film (reaction film of silicon and metal), 101 , 102, 103 ... electrodes

Claims (5)

  A semiconductor substrate; and a first conductive type first semiconductor layer on the semiconductor substrate; a second conductive type second semiconductor layer provided on the first semiconductor layer; and the first semiconductor layer A second conductivity type third semiconductor layer that is in contact with a side surface of the second semiconductor layer, and a first conductivity type fourth semiconductor layer provided on the second semiconductor layer, And a band gap of the third semiconductor layer is wider than a band gap of the second semiconductor layer.   A fifth semiconductor layer of a first conductivity type is provided between the first semiconductor layer and the second and third semiconductor layers, and a band gap of the fifth semiconductor layer is the first semiconductor layer. The semiconductor device according to claim 1, wherein the semiconductor device is narrower than a band gap of the semiconductor layer.   3. A sixth semiconductor layer of a first conductivity type having an impurity concentration lower than that of the first semiconductor layer is provided between the first semiconductor layer and the fifth semiconductor layer. The semiconductor device described.   2. A seventh semiconductor layer of a first conductivity type having an impurity concentration lower than that of the fourth semiconductor layer is provided between the second semiconductor layer and the fourth semiconductor layer. The semiconductor device according to -3.   2. The semiconductor device according to claim 1, wherein the semiconductor device is a bipolar transistor having the first semiconductor layer as a collector, the second and third semiconductor layers as a base, and the fourth semiconductor as an emitter. Semiconductor device.

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