JPH0640581B2 - Switching element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement of a switching element having an npnp four-layer structure such as a thyristor.

[Prior art and its problems]

High breakdown voltage thyristors and the like are used in various power conversion devices. However, the higher the breakdown voltage of these elements, the larger the protection circuit for those elements in the device, which leads to an increase in the size of the device or a cost increase.
In some cases, it is required to use an unnecessarily high withstand voltage element for safety against back electromotive force due to reactance in the circuit.

In order to avoid the above, it is important to improve the turn-off characteristics of the device, especially (1), soften the reverse recovery characteristic, (2), and reduce the variation of the reverse recovery charge Qrr. (2)
It is considered that reducing the variation of Qrr can be achieved by reducing the value of Qrr itself. These characteristics often depend on the internal structure of the device. It is known that the reverse recovery characteristics tend to be slightly softened by increasing the resistance of the low impurity layer and increasing its thickness.
However, this causes a large increase in the on-voltage, and therefore, in the high breakdown voltage element, there is no design margin in the impurity concentration and thickness of the low resistance layer, and it is difficult to actually use the above method.

[Object of the Invention]

The present invention has been made in view of the above-described drawbacks, and is to soften the reverse recovery characteristic and further reduce Qrr without impairing other characteristics.

[Outline of Invention]

According to the present invention, in a switching element such as a thyristor having a four-layer structure of at least npnp, the impurity concentration is low among two regions of different conductivity types forming a pn junction to which most of the reverse voltage is applied during turn-off operation. The switching element is characterized in that, in the vicinity of the pn junction in one region, a region having a lifetime of 1/4 to 1/50 as compared with other portions is continuously formed from the pn junction.

The present invention is based on the following numerical calculations and results.

First, the conventional thyristor is the circuit of FIG. 1 (1 is SCR,
2 L, 3 denotes a Shimiyureshiyon results when turned off in a power supply V _R). Inside the thyristor is one-dimensional
The exact model was used. Second thyristor used for calculation
It shall have an impurity profile as shown. The thyristor shown in FIG. 2 has an impurity concentration of 2.4 × 1.
0 ¹³ / cm ³ and 1000 μm thick n-Si substrate with p from both sides
Type impurity diffused about 110μm, surface impurity concentration 1 ×
10 ¹⁹ / cm ³ p emitter (12) and p base (13) p
Form the layers. Then, the surface of the p-layer serving as the p-base (13) is etched by about 55 μm, the p-base (13) is set at 45 μm, and the n-type impurities are diffused by about 20 μm from the surface, and the surface impurity concentration is about 1 × 10 ²¹ / cm ^3. N emitter (14) is formed. The minority carrier lifetime of this thyristor is 60 μsec for n-base (11), P emitter
The area (12) is set to 1.5 μsec. Element size is 4
0 cm ³ and 100 A / cm ³ on-state element with reverse voltage 5
00V is applied and turned off at dI / dt = 10 A / μsec. The current voltage and the hole distribution inside the device at that time are shown in FIGS. 3 (a) and 3 (b). In FIG. 3 (b), (a) is t
= 0, (b) t = 425 μsec, (c) t = 435.5 μs
It's ec time. A large amount of carriers accumulate near the n-base and p-emitter junctions, and it takes a long time for the depletion layer to spread and reach the maximum reverse current value. Therefore, the reverse recovery characteristic is not soft and Qrr is also large.

Next, the calculation results of the above model will be shown. For a thyristor having the same impurity profile as the conventional thyristor and having a minority carrier lifetime distribution as shown in FIG. 4, turn-off calculation was performed under the same conditions as above. The calculation results at that time are shown in Fig. 5 (a) and (b).
Shown in.

Needless to compare with FIG. 3 (a), FIG. 5 (a) shows a good reverse characteristic with a soft reverse recovery characteristic and a small Qrr. This is easy to understand by comparing FIG. 5 (b) and FIG. 3 (b). In FIG. 5 (b), (a) is t = 0, and (b) is t = 4.
06 μsec, when (c) is t = 413.5 μsec. N-base has the greatest effect on the reverse recovery characteristics of thyristors.
It is a state of accumulation of excess carriers in the ON state near the p-emitter junction. Even if the current density is the same, if the accumulation of excess carriers in this part is small, the depletion layer in the above junction (which is the junction to which most of the reverse voltage is applied) is formed in a short time, and the carrier discharge in that part is also short. Done in time.
As a result, the time required to reach the maximum reverse current is short, the reverse recovery characteristic becomes soft, and Qrr becomes small.

FIG. 6 shows the relationship between the lifetime value in the low lifetime layer, the softness S (tb / ta ... A) Qrr (b) of the reverse recovery characteristic, and the on-voltage V _ON (c). When the lifetime value of the low lifetime layer becomes 1/4 or less of the lifetime value of the other n base region, the softness becomes about 1.25 times, and it can be said that the reverse recovery characteristic is improved considerably. If the lifetime is 1/50 or less, the on-voltage will increase and the power loss during energization will increase, making it unsuitable for practical use. Therefore, the lifetime value of the low lifetime layer is preferably 1/4 to 1/50 of the other N base layer lifetimes. Further, since the carrier accumulation state in the region where the depletion layer spreads at the time of applying a reverse voltage near the N-base P emitter junction almost affects the reverse recovery characteristic, the low lifetime layer is a depletion layer formed by the reverse voltage application. It is effective that the width is more than the width.
Therefore, the low lifetime layer needs at least the width of the depletion layer formed by the predetermined reverse voltage. Further, since the counter electromotive force due to the reactance must be suppressed to 1/2 of the rated breakdown voltage, it is considered that the reverse breakdown voltage of 1/2 or more of the rated breakdown voltage is not applied to the element (thyristor). Therefore, it is considered that the carrier accumulation state that determines the reverse recovery characteristics does not significantly affect the characteristics in the region outside the width of the depletion layer caused by the reverse breakdown voltage of 1/2 of the rated breakdown voltage. It suffices if it is inside the end of the depletion layer generated by the reverse voltage of half the breakdown voltage.

〔The invention's effect〕

According to the present invention, as shown in FIG. 5 (b), there is little carrier accumulation at the time of turning on, and the stored carrier can be discharged quickly. As a result, compared to the conventional example, the reverse recovery characteristic is softened without deteriorating the ON characteristic, and Qrr
It is possible to realize an element having a small value.

Example of Invention

An embodiment of the present invention will be described below with reference to the drawings. 7th
The drawing is a cross-sectional view of a thyristor according to an embodiment of the present invention. As a manufacturing method, first, p-type impurities such as Ga are diffused from both sides into n-type Si to form a p-emitter (12), an n-base (11) and a p-base (1
3) pnp three-layer structure. Next, on the p base (13), POCl
_An n emitter (14) is formed by diffusion of ₃ or the like. In the conventional example, after the four layers are formed, the lifetime killer such as Au is diffused from the p-emitter (11) side to make the lifetime in the n-base (12) substantially uniform. However, in the present invention, when a lifetime killer such as Au is diffused, the lifetime killer can reach only a part of the n base (11) by controlling the time and diffusion temperature, as shown in FIG. P emitter (1) in the n base (11)
The area (15) where the lifetime is reduced only in the area close to 2) is provided. After these lifetime controls p.
An anode electrode (16) and a cathode electrode (17) are formed on the surfaces of (12) and the n emitter (14).

The lifetime control can be achieved not only by Au but also by diffusion of Pt or the like or electron beam irradiation.

[Brief description of drawings]

FIG. 1 is a circuit used for the simulation when the thyristor is turned off, FIG. 2 is a diagram showing an impurity profile of the thyristor, and FIG. 3 is a conventional current voltage when turned off and hole distribution inside the device. Fig. 4 is a diagram showing the lifetime distribution of the present invention, Fig. 5 is a diagram showing the current-voltage and hole distribution inside the device when the present invention is turned off, and Fig. 6 is a low lifetime layer. FIG. 7 is a diagram showing the relationship among the lifetime value of S, Qrr and the on-voltage V _ON, and FIG. 7 is a thyristor sectional view for explaining an embodiment of the present invention. 11 …… n base, 12 …… p emitter 13 …… p base, 14 …… n emitter 15 …… area where life time is low 16 …… anode electrode, 17 …… cathode electrode

Claims (3)

[Claims] 1. A switching element having a four-layer structure of at least npnp, which has a lower impurity concentration of two regions of different conductivity types forming a pn junction to which most of the reverse voltage is applied during turn-off operation. The lifetime is 1 / near the pn junction in the region compared to other parts.
A switching element having a region of 4 to 1/50 formed continuously from the pn junction. 2. Of the two regions of different conductivity types forming the pn junction to which most of the reverse voltage is applied at the time of turn-off operation, the region having the lower lifetime in the region having the lower impurity concentration is the pn junction. 3. The switching element according to claim 1, wherein the switching element is located inside a termination of a depletion layer generated by a voltage of 1/2 of the rated breakdown voltage. 3. The switching element according to claim 1, wherein the low lifetime region starts from the pn junction and extends to the outside of the end of the depletion layer generated by a predetermined reverse breakdown voltage.