The present invention relates to a semiconductor device comprising a lateral super junction field effect transistor, JFET, that can be implemented by placing a plurality of alternating n- 5 and p-type layers on top of each other and connecting them in parallel.

Such devices have previously been described in many patent documents, e.g. in US 11,031,480 B2, US 2019/0198609 Al, US 2017/0222043 Al, and US 2011/0127606 Al. w The stack of alternating n- and p-layers will, if they are matched in charge, completely deplete each other and a uniform electric field can be formed in the material with almost optimal use of the material in terms of breakdown voltage. The stack of alternating n- and p- layers needs to be terminated at the bottom and the field needs to turn 90 degrees since the substrate is grounded and have constant potential along the whole drift region. This will 15 increase the electric field locally and this increase in field will induce an electric breakdown lower than the breakdown in the stack. Several different attempts to shape the electric field have been demonstrated.

US 2011/0127606 Al suggests an n-buffer layer (160) that is located under the bottom0 channel all the way between the source and drain, or partially (160-1) between the source and drain. Another suggestion is to place a floating n+region (661) in the substrate under the drain to shield the drain from the high field.

US 2017/0222043 Al suggests a diffused p-region (253) and/or a diffused n-region (252)5 under the source and drain respectively, also to shape the electric field and decrease the maximum electric field.

US 2019/0198609 Al suggests a region (202) with linear or non-linear increasing thickness going from the source to the drain. This is also with the intention to shape the electric field. The stack of n- and p-layers is previously demonstrated in several places in the above cited documents. Normally the bottom channel is abruptly ended at the drain and under the channel the substrate is acting as a bottom gate and also a substrate that should support the breakdown voltage of the device. In the channels the electric field is perfectly lateral but under the drain it has to turn to a vertical field since the backside is grounded. This means that the field has to turn 90 degrees under the bottom channel in the substrate. If nothing is done, the electric field profile from source to drain will form a U-shape with highest field near the drain. The U-shape means that the field is not uniformly distributed, and the breakdown voltage will be lower than is possible.

The object of the present invention is to reduce the above drawbacks and to obtaining a higher breakdown voltage.

This object is obtained by the device according to the present invention, where a lowermost layer of the first conductivity type is arranged in the form of consecutive dots with different lengths and distances between deep polycrystalline trenches of the second conductivity type in the bottom part of the JFET.

Further improvements can be obtained through the devices defined in the dependent claims.

The invention will now be explained with the help of a couple of non-limiting embodiments of a semiconductor device, focusing on the JFET part as shown on the accompanying drawings, in which Fig. 1 shows a first embodiment of the invention, and Fig. 2 shows a second embodiment of a further development of the invention.

Fig. 1. shows the invention, starting with a highly doped substrate 1 of first conductivity type which is connected to a grounded back contact. On the substrate is a thick, low-doped layer 2 of first conductivity type epitaxially grown. The thickness of the epitaxial layer should be large enough to support the breakdown voltage of the device. On the low-doped layer 2 of first conductivity type is a layer nl of second conductivity type epitaxially grown. On the epitaxial layer nl is an implantation mask placed and ion implantation is done forming a masked layer pl of first conductivity type. By partitioning the layer and varying the lengths and distances of the resulting p-regions in the masked layer of first conductivity type pl, the effective amount of charges of first conductivity type will decrease towards to the drain side D of the structure. This will decrease the electric field near drain and thus a more uniform electric field is achieved and a higher breakdown voltage. On top of the structure is now an epitaxial layer of second conductivity type n2 placed and a first gate of the first conductivity type is either implanted or epitaxially grown on the channel n2. These two layers are then repeated upwards as demonstrated in several earlier publications, e.g. in the above cited patent documents.

From the surface are deep trenches etched and then filled with highly doped silicon. In the figure is shown two filled trenches 3 that are of second conductivity type and connecting the channels of second conductivity type n2-n6. The filled trench 4 of first conductivity type is used to connect the layers of first conductivity type pl-p5. The gates p2-p5 are connected to ground in the third dimension, for example by making interruptions in the source trench as demonstrated in US 11,031,480 B2, or by making filled trench pillars of first conductivity type as shown in US 2019/0198609 Al and US 2017/0222043 Al. K semiconductor device according to the invention can be combined with a further isolated region X arranged over the substrate, comprising logics and analogue control functions, isolated with deep polycrystalline trenches of the first conductivity type 4, on both sides thereof, to the left of the parts shown in the figure. Such a semiconductor device is e.g. described in US 11,031,480 B2.

Preferably the consecutive dots 5 of different lengths have decreasing lengths in direction towards the drain side D of the structure, and the distances 6 between the dots 5 increase in the direction towards the drain side D of the structure. The whole structure is mirrored around the line of symmetry L which allows for high voltage on the drain trench. Fig. 2 shows a modified device where the gate layers p2-p5 are made with ion implantation through a photoresist mask creating interruptions 7 in the layers p2-p5 of first conductivity type. In the interruptions the doping is the same as in the adjacent channel regions n2-n5 5 of second conductivity type. The interruptions 7 are evenly distributed along the drift region. The interruptions 7 in the layer will divide the region in several shorter regions 8 of first conductivity type. The leftmost region of the shorter regions is connected to the ground as described earlier and the other regions are floating. For small drain voltages the floating regions of first conductivity type will deplete the channel region less than a long p-gate w which is grounded along the whole drift region. This will increase the current through the device for low drain voltages. For higher voltages a current will flow through the shorter regions and connecting them together.

For this to happen the length of the interruptions 7 should not be too large. E.g. the distance 15 7 between the regions 8 can be about 0.3pm, and the length of the regions 8 can be about

5pm.

In the drawings the device according to the invention has been described when the first conductivity type is p-type, and the second conductivity type is n-type. However, the device0 according to the invention can also be implemented so that the first conductivity type is n- type, and the second conductivity type is p-type.