JPH06132520A - Gate turn off thyriter - Google Patents

Abstract

PURPOSE:To lower the resistance of a cathode base layer without lowering injection efficiency of electrons from a cathode emitter layer, by making a forbidden band width of the cathode base layer narrower than an anode layer. CONSTITUTION:A pnpn structure, with an anode emitter layer 1 of p-type, an anode base layer 2 of n-type a cathode base layer 3 of p-type and a cathode emitter layer 4 of n-type, is provided. The anode emitter layer 1 and the anode base layer 2 are Si (forbidden band width 1.1eV). The cathode base layer is divided into two layers, and a cathode base layer 31 on the anode base layer side is Si, equal to the forbidden band width of the anode base layer, and further, a cathode base layer 32 on the cathode emitter layer side is SiGe (forbidden band width, 0.65eV), narrower than the forbidden band width of the anode base layer. With the cathode emitter layer 4 being the same SiGe as the cathode base 32 whose forbidden band width is narrow, the forbidden band width of the cathode base layer is made to be narrower, for smaller resistivity and improved voltage-resistance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved structure of a gate turn-off thyristor (hereinafter abbreviated as GTO).

[0002]

2. Description of the Related Art In general, GTO is at least pnpn.
It is configured to include four semiconductor layers. In GTO, a control signal is applied to the gate electrode provided on the cathode base layer to turn on and off. The most important item is the turn-off characteristic. The turn-off is to restore the forward blocking characteristic by applying a reverse bias to the gate electrode and discharging the internal charge accumulated in the element to the outside when the element is in the ON state. The lateral resistance of the cathode base layer below the cathode emitter layer is the factor that determines the turn-off characteristics, and the impurity concentration of the cathode base layer and the thickness of the cathode base layer determine the resistivity. In order to speed up the turn-off operation, it is necessary that the resistance of the cathode base layer is small. However, if the resistivity of the cathode base layer is reduced, the injection efficiency of electrons from the cathode emitter layer is reduced, and the current amplification factor of the npn transistor composed of the cathode emitter layer, the cathode base layer and the anode base layer is reduced, There is a limit to reducing the resistance of the cathode base layer because the turn-off characteristic deteriorates. As a structure for solving this, JP-A-1-223
A heterojunction between the cathode emitter layer and the cathode base layer as shown in Japanese Patent No. 767, in which the forbidden band width of the cathode emitter layer is made wider than that of the cathode base layer and the impurity concentration of the cathode base layer is further increased. There is.
This enhances the efficiency of electron injection from the cathode emitter layer and reduces the lateral resistivity of the cathode base layer.

[0003]

In the above prior art, by increasing the impurity concentration of the cathode base layer, the width of the depletion layer spreading in the cathode base layer in the blocked state is narrowed, and the electric field formed there is extremely reduced. There is a problem that there is a limit to the impurity concentration of the cathode base layer because it becomes higher and the junction breaks down or the leak current due to the tunnel effect increases.

The present invention is to propose a structure capable of reducing the resistivity of the cathode base layer and increasing the breakdown voltage without lowering the efficiency of electron injection from the cathode emitter layer.

[0005]

GTO according to the present invention
Has four semiconductor layers of pnpn in which the cathode emitter layer, the cathode base layer, the anode base layer, and the anode emitter layer are sequentially different in conductivity type, and the cathode emitter layer is divided into a plurality of parts. In the GTO in which the cathode electrode is arranged in contact with each cathode emitter layer, the anode electrode is connected to the anode emitter layer, and the gate electrode is connected to the cathode base layer with low resistance, at least in the vicinity of the junction with the cathode emitter layer. Basically, the band gap of the cathode base layer is narrower than the band gap of the anode base layer.

[0006]

[Function] Since the narrower the forbidden band, the smaller the resistivity,
Even if the impurity concentration of the cathode base layer is set to the conventional level, the resistivity can be reduced by narrowing the forbidden band width of the cathode base layer, so that the internal stored charge can be quickly extracted at turn-off, and the turn-off characteristic can be improved. Also,
If the resistivity of the cathode base layer is set to a conventional level, the impurity concentration can be reduced, so that the efficiency of injecting electrons from the cathode emitter layer can be increased and the breakdown voltage of the junction can be reduced. In particular, the resistivity in the vicinity of the junction with the anode base layer does not strongly participate in the extraction of the internal charge, so the resistivity in the vicinity of the junction with the cathode emitter layer is a problem. In the present invention, in particular, the forbidden band width near the junction with the cathode emitter layer is narrowed and the anode base layer side is the same, so that the injection of electrons from the cathode emitter layer is increased without lowering the breakdown voltage of the junction. It is possible to improve the turn-off operation. To further improve the injection efficiency, the heterojunction structure is used so that the forbidden band width of the cathode emitter layer is wider than that of the cathode base layer.

[0007]

EXAMPLE FIG. 1 is a GTO sectional structure of an example. In this GTO, the anode emitter layer 1 is p-type, the anode base layer 2 is n-type, and the cathode base layer is p-type. The cathode emitter layer 4 has an n-type pnpn structure. The anode emitter layer 1 and the anode base layer 2 are Si (forbidden band width is 1.1 eV). The cathode base layer is divided into two layers, the cathode base layer 31 on the side of the anode base layer is made to have Si equal to the band gap of the anode base layer, and the cathode base layer 32 on the side of the cathode emitter layer is made wider than the band gap of the anode base layer. Narrow SiGe (Forbidden band width is 0.65
eV). The cathode emitter layer 4 was made of the same SiGe as the cathode base layer 32 having a narrow band gap. Specifically, the manufacturing process on the cathode side will be described. First, a cathode base layer 31 having the same forbidden band width as that of the anode base layer is formed by impurity diffusion using a Si substrate which will be the anode base layer 2. Next, the cathode base layer 32 and the cathode emitter layer 4 having a narrow band gap are formed on the cathode base layer 31 by an epitaxial growth method such as a molecular beam epitaxial method or a chemical vapor deposition method. The cathode emitter layer 4 may be made n-type by changing the impurities introduced during the epitaxial growth, or may be made n-type by performing the impurity diffusion after being epitaxially grown with the same conductivity type as the cathode base layer.

FIG. 2 shows the band structure of AA 'in FIG. There is a heterojunction in the cathode base layer. G
By setting the position of the heterojunction within the range where the depletion layer does not reach when TO is in the blocking state, it is possible to prevent the breakdown voltage from being lowered due to the defect at the interface of the epitaxial growth layer. Although the energy level discontinuity occurs because the forbidden band width Eg changes in the cathode base layer, the holes that have diffused through the anode base layer 2 have a narrow forbidden band width and a high energy level Ev in the valence band. It becomes easy for the cathode base layer 32 to flow.
Since the gate electrode is in contact with the cathode base layer 32 with a low resistance, the discharge of the internally accumulated charges is accelerated and the turn-off is accelerated during the turn-off operation. Further, since holes easily flow into the cathode base layer 32, the efficiency of injecting electrons from the cathode emitter layer 4 is also increased, and the npn composed of the cathode emitter layer 4, the cathode base layers 31, 32 and the anode base layer 2 is formed. Since the current amplification factor of the transistor is increased, the turn-off characteristic is improved.

In FIG. 3, the entire cathode base layer 3 is made of SiGe which is narrower than the band gap of the anode base layer. The cathode emitter layer 4 was made of SiGe as in the embodiment of FIG. Specifically, the manufacturing process on the cathode side will be described. First, an epitaxial growth method such as a molecular beam epitaxy method or a chemical vapor deposition method is used on the anode base layer 2 by using a Si substrate as the anode base layer 2. The cathode base layer 3 and the cathode emitter layer 4 having a narrow band gap are formed by using the above. The cathode emitter layer 4 may be made n-type by impurity diffusion after epitaxial growth. FIG. 4 shows the band structure of BB ′ in FIG. The junction interface between the cathode base layer 3 and the anode base layer 2 becomes a heterojunction. Since there is no heterojunction in the cathode base layer 3, the energy level Ec of the conduction band is continuous and there is no energy barrier. Therefore, the diffusion of the electrons injected from the cathode emitter layer 4 to the cathode base layer 3 is not blocked, and the electrons are efficiently sent to the anode base layer 2 to improve the ON characteristics. FIG. 5 shows the same cross-sectional structure as in FIG. 3, in which the cathode base layer 3 is made of SiGe and the mixed crystal ratio of silicon and germanium is continuously changed to make the forbidden band width Eg continuously narrow as it approaches the cathode emitter layer 4. It is a structure. For example, only silicon is formed near the interface with the anode base layer 2, and epitaxial growth is performed while changing the mixed crystal ratio of silicon and germanium so that the ratio of silicon and germanium is 1: 1 near the interface with the cathode emitter layer 4. . After the epitaxial growth corresponding to the cathode base layer 3, the cathode emitter layer 4 is epitaxially grown with the mixed crystal ratio of silicon and germanium unchanged. Since the energy level Ev of the conduction band can be inclined, the holes that have reached the cathode base layer easily move to the vicinity of the junction with the cathode emitter layer 4 having a high energy level. Cathode emitter layer 4
Since the hole concentration in the vicinity of the junction is increased, the turn-off operation can be speeded up and the efficiency of injecting electrons from the cathode emitter layer 4 can be improved as in the case shown in FIG.

In FIG. 6, the cathode base layer 3 is made of SiGe which is narrower than the forbidden band width of the anode base layer, and the cathode emitter layer 4 is made of SGe which is wider than the forbidden band width of the cathode base layer 3.
In this example, the interface between the cathode base layer 3 and the cathode emitter layer 4 is i. Specifically, the manufacturing process on the cathode side will be described. First, an epitaxial growth method such as a molecular beam epitaxy method or a chemical vapor deposition method is used on the anode base layer 2 by using a Si substrate as the anode base layer 2. The cathode base layer 3 having a narrow band gap is formed by using the above. A cathode emitter layer 4 having a wider forbidden band width than the cathode base layer 3 is formed thereon by an epitaxial growth method. Although the cathode emitter layer 4 is made of Si in this embodiment, SiC or Ga having a wide band gap is used.
It may be As or the like.

FIG. 7 shows the band structure at CC 'in FIG. Cathode emitter layer 4 and cathode base layer 3
By making the interface of the heterojunction into a heterojunction, the energy barrier of the conduction band in this junction becomes large, and holes in the cathode base layer 3 are prevented from reaching the cathode emitter layer 4, so that the Electron injection efficiency is improved.

Although four types of embodiments have been described above, the present invention can be constructed by using these embodiments together. In addition, a so-called pnipn structure in which a high impurity concentration layer of the same conductivity type as the anode base layer is formed between the anode base layer and the anode emitter layer, or an anode short circuit structure in which the anode base layer is in low resistance contact with the anode electrode The present invention is effective.

[0013]

As described above, according to the present invention, the avalanche voltage is reduced by making the forbidden band width of the cathode base layer at least near the junction with the cathode emitter layer narrower than the forbidden band width of the anode base layer. It is possible to increase the injection of electrons from the cathode emitter layer without lowering the voltage, and further improve the turn-off operation.

[Brief description of drawings]

FIG. 1 is a sectional structural view of the present invention.

FIG. 2 is a diagram showing a band structure in the case of FIG.

FIG. 3 is a cross-sectional structure diagram in the case where a cathode base layer and an anode base layer have a heterojunction.

FIG. 4 is a diagram showing a band structure in the case of FIG.

FIG. 5 is a diagram showing a band structure when the forbidden band width of the cathode base layer is continuously changed.

FIG. 6 is a cross-sectional structure diagram in the case where there are two heterojunctions.

FIG. 7 is a diagram showing a band structure in the case of FIG. 6;

[Explanation of symbols]

1 ... Anode emitter layer, 2 ... Anode base layer, 3,
31, 32 ... Cathode base layer, 4 ... Cathode emitter layer, 5 ... Semiconductor substrate, 6 ... Anode electrode, 7 ... Cathode electrode, 8 ... Gate electrode, 9, 91, 92 ... Heterojunction.

Claims (2)

[Claims] 1. A semiconductor substrate having a cathode emitter layer, a cathode base layer, an anode base layer and an anode emitter layer whose conductivity types are sequentially different from each other, and the cathode emitter layer is divided into a plurality of parts. In a gate turn-off thyristor arranged on one main surface, a cathode electrode is in contact with each cathode emitter layer, an anode electrode is in contact with the anode emitter layer, and a gate electrode is in contact with the cathode base layer at low resistance. A gate turn-off thyristor characterized in that the band gap of the cathode base layer near the junction with is narrower than the band gap of the anode base layer. 2. The gate turn-off thyristor according to claim 1, wherein the forbidden band width of the cathode base layer is equal to the forbidden band width of the anode base layer on the anode base layer side and narrows on the cathode emitter layer side. Gate turn-off thyristor.