JPS60198778A - Switching element - Google Patents

Abstract

PURPOSE:To reduce Qrr by softening the reverse recovery characteristic by a method wherein the lifetime is made ununiform in the region with lower impurity concentration of two regions of different conductivity which form P-N junctions where most of reverse voltage is impressed at the time of turn-off operation. CONSTITUTION:An N type Si is put into the P-N-P three layer structure with a P-emitter 12, an N-base 11, and a P-base 13 by diffusing a P type impurity from both surfaces. Next, an N-emitter 14 is formed on the base 13 by diffusion of POCl3, etc. At the time of diffusing a lifetime killer, the lifetime killer is enabled to reach only part of the base by controlling the time and the diffusion temperature; then, only a region in the base 11 close to the emitter 12 is provided with a region where the lifetime decreases. Such a manner causes less accumulation of carriers at the time of on-operation, and they are swiftly exhausted. As a result, the reverse recovery characteristic is softened without deteriorating the ON-characteristic; further, Qrr can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to improvements in switching elements such as thyristors and diodes.

[Prior art and its problems]

High-voltage thyristors and diodes are used in various power conversion devices. However, the higher the voltage resistance of these elements, the larger the protection circuit for those elements within the device becomes, leading to an increase in the size and cost of the device. In addition, there are cases where it is necessary to use a higher voltage element than necessary for safety against back electromotive force caused by actance in the circuit.

To avoid the above, K is improvement of the turn-off characteristics of the element, 4! ! F K (13, Softening of reverse recovery characteristics, (
2).

Two points are important: reducing the dispersion of the reverse recovery charge Qrr. We believe that reducing the variation in Qrr in (2) can be achieved by reducing the value of Qrr itself. These characteristics are often determined by the internal structure of the element. It is known that the reverse recovery characteristics tend to become slightly softer by making the low impurity layer higher in resistance and increasing its thickness.

However, this causes a large increase in the on-voltage, and therefore, in high voltage devices, there is no margin in the design of the impurity concentration and thickness of the low resistance layer, making it difficult to use the above method in reality.

[Purpose of the invention]

The present invention was made in view of the above-mentioned drawbacks, and it is an object of the present invention to soften the reverse recovery characteristic and further reduce Qrr without impairing other characteristics.

[Summary of the invention]

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

First, the conventional thyristor has the circuit shown in Figure 1 (one company SCa,
2 is L and 3 is the power supply VB), the simulation results are shown. A one-dimensional jxact model was used inside the thyristor. It is assumed that the thyristor used in the calculation has an impurity profile as shown in FIG. The thyristor shown in FIG. 2 has an impurity concentration of 2.
A p-type impurity is diffused by about 110 μm from both sides of an n-8i substrate of 4×10 18 /cd and 1000 μm thick to form a p emitter (b) with a surface impurity concentration of lXlO19 and a p base of 0 $. After this, p base (2
), the p-layer surface is etched by about 55 μm, and an n-type impurity is diffused about 20 μm from the surface of the p base (Ll is 45 μm) to form an n-emitter α◆ with a surface impurity concentration of about lXl0”/m. .The minority carrier lifetime of this thyristor is 6 on n base (b).
In the 0 μSec region, it is set to 1.5 μsec. The size of the element was 4°d, a reverse voltage of 500V was applied to the element in the on state of 100 A/m, and dI/d t = 10 A/
Turn off in μsec. The current voltage and hole distribution inside the device at that time are shown in Figure 3+a), lb
). In FIG. 3(b), H] is t =, O,
(o) is t = 425μsec, (c) is t = 43
This is the time of 5.5μSee. There is a large amount of carrier accumulation near the n-base and p-emitter junctions, and the depletion layer is wide, so it takes a long time for the reverse current to reach its maximum value. Therefore, the reverse recovery characteristics are not soft and the Qrr is also large.

Next, the results of calculations for the above-mentioned model will be shown. Turn-off was calculated under the same conditions as above for a thyristor having the same impurity profile as the conventional thyristor and having minority carrier lifetime distribution as shown in FIG. 4. The calculation results at that time are shown in Figure 5 1a)
, shown in (b).

Needless to compare with Fig. 3(a), Fig. 5(at) has a soft reverse recovery characteristic and a small Qrr, showing good off-characteristics. It is easy to understand by comparing b). In Fig. 5(b), ge) is t = o, 10] is t, = 406 μsec, and (c) is t.
=413.5 μsec. What most influences the reverse recovery characteristics of the thyristor is the accumulation of overridden carriers in the on-state near the n-base/p-emitter junction. Even if the current density is the same, if the accumulation of overriding energy in this part is small, a depletion layer in the above junction (this is the junction to which most of the reverse voltage is applied) is formed quickly, and carriers are discharged in that part. is also done in a short period of time. As a result, the time required for the reverse current to reach its maximum value is shortened, and the reverse recovery characteristics become softer, and KQrr also becomes smaller.

Figure 6 shows the value of life time in the low lifetime layer and the softness of the reverse recovery characteristic S (tb/la...a)
The relationship between Qrr (b) and on-voltage VON (c) is shown. If the value of 2 ear time of the low lifetime layer becomes 174 or less of the lifetime value of other n base regions.

The softness was approximately 1.25 times greater, which means that the reverse recovery characteristics have improved considerably. Furthermore, if the lifetime is less than 1150, the on-voltage increases and the power loss during energization increases, making it unsuitable for practical use.

Therefore, the lifetime value of the low lifetime layer is 1/4 to 1150 times better than the lifetime of other N-pace layers.

Furthermore, what influences the reverse recovery characteristics is mostly the carrier accumulation state in the region where the depletion layer expands when a reverse voltage is applied near the N-base P emitter junction.

It is effective for the low lifetime layer to exist in the middle or more of the depletion layer formed by applying a reverse voltage. Therefore, the low lifetime layer requires at least the width of the depletion layer formed by a predetermined reverse voltage. Furthermore, since the back electromotive force due to the reactance must be suppressed to H, which is the rated withstand voltage, it is considered that a reverse withstand voltage of % or more of the rated withstand 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 greatly affect the characteristics in the region outside the width of the depletion layer caused by the reverse breakdown voltage of the rated breakdown voltage H, so the low lifetime layer is wide even if the rated breakdown voltage H It suffices if it is located inside the end of the depletion layer caused by the reverse voltage.

〔Effect of the invention〕

By using the present invention, as shown in FIG. 5(b), it is possible to obtain a device in which there is less accumulation of carriers when on, and the accumulated carriers are quickly discharged. As a result, it is possible to realize an element with softer reverse recovery characteristics and a smaller Qrrt without deteriorating the on-characteristics compared to the conventional example.

[Embodiments of the invention]

An embodiment of the present invention will be described below using the drawings. 7th
The figure is a sectional view of a thyristor according to an embodiment of the present invention. The manufacturing method is to first diffuse p-type impurities such as Ga into n-type Si from both sides to form a p-emitter @.

It has a pnp three-layer structure of n base (6) and p base (2).

Next, by diffusing POCl3 etc. on the p base (to), n
Form an emitter (b). In the conventional example, after forming these four layers, a lifetime killer such as Au is diffused from the p emitter (6) side to make the lifetime within the n base@ almost uniform. However, in the present invention, for example, when diffusing a lifetime killer such as Au.

By controlling the time and diffusion temperature, the lifetime killer can reach only a part of the n-pace (b), and only the area near the p emitter (6) within the n-pace (b) is shown in Figure 4. A region 4 is provided where the lifetime is low. These lifetime controlsff
1P Emitta (6) and N Emitor (b) form an anodel electrode axis and cathode electrode αη [Other embodiment of invention] In the above -described embodiment, the Sirista is described as an example, but one or more is 1 or more. The same applies to diodes. Furthermore, lifetime control can be achieved not only by Au but also by diffusion of Pt or the like or electron beam irradiation.

[Brief explanation of the drawing]

Figure 1 shows the circuit used for simulation when the thyristor is turned on once, Figure 2 shows the impurity profile of the thyristor, and Figure 3 shows the current voltage and the distribution inside the element when the thyristor is turned off in the conventional manner. figure,
Fig. 4 is a diagram showing the lifetime distribution of the present invention, Fig. 5 is a diagram showing the turn-on current voltage and hole distribution inside the device of the present invention, and Fig. 6 is a diagram showing the lifetime of the low lifetime layer. FIG. 7 is a cross-sectional view of a thyristor for explaining an embodiment of the present invention. 11... N base 12... P emitter 13...
・P base 14...-n emitter 15... Region where lifetime is low 16... Anode electrode 1
7... Cathode electrode Fig. 1 Fig. 4 Fig. 8 (α) (mu) D "〃倃 χI, mm) Fig. 5 (OL) EMI77FRAN# χ(,/LLm1 Fig. 6 h-1← Figure 7

Claims (3)

[Claims] (1) In a switching element having at least one pn junction, in the region with lower impurity content among the two regions of different conductivity types forming the pn junction to which most of the inverse 4 voltage is applied during turn-off operation. A switching element characterized in that the lifetime is made non-uniform, and the value of the lifetime in a portion close to the pn junction is set to 174 to 1150 compared to other portions. (2) Of the two regions of different conductivity types that form the pn junction to which most of the reverse voltage is applied during turn-off operation,
The switching device according to claim 1, wherein the region with a low lifetime in the region with low impurity is located inside the termination of a depletion layer generated from the pn junction by 1% of the rated withstand voltage. element. (3) The switching element according to claim 1, wherein the low lifetime region starts from the pn junction and is located outside the end of a depletion layer generated by a predetermined reverse voltage.