Note: Descriptions are shown in the official language in which they were submitted.
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sACKGRoUND OF THE INVENTION
Field of the Invention
The present invention relates generally to a thyristor
which can be controlled by light and having at least four zones
of alternatiny opposite conductivity types.
Descriptio_ of the Prior Art:
In the past, thyristors have been designed with at least
four zones of alternating opposite conductivity types, in which
the first and fourth zones are contacted by the main electrodes
and act as emitter zones, the second zone adjoins the first, and
the third, least-doped zone is located between the second and
fourth zones. Both the second and the third zone, together with
the forward-blocking PN-junction formed between them, extend to
the surface of the thyristor semiconductor chip on the side with
the first zone. The part of the forward-blocking PN-junction
which emerges at the surface can be acted on by light for the
purpose of triggering the thyristor. There is provided in the
said surface a highly doped zone of the conductivity type of the
first zone, the latter being connected with the second zone by an
electrical contact on the surface.
Such a thyristor is known for instance from German patent
applisation DT-OS 24 08 079 of Sittig, filed Feb. 20, 1975 and
laid open July 24, 1975. Although such thyristors have since
been utilized with success, further optimization is possible.
SUMMARY OF THE INVENTION
.
Accordingly, an object of this invention is to provide
an improved thyristor of the type mentioned, which is
triggerable at still lower light intensities without adverse
effect on other important properties, e.g. the resistance
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against beinc~ trigaered by a fast voltage risc dlJ/dt (~ICr~ C<
voltage s-teepness").
The invention broadly comprehends a light-controlled
thyristor. The thyristor has a pair of main electrodes comprised
of a cathode electrode and an anode electrode. The thyristor
includes a semiconductor chip having at leas-t four zones of
alternating opposite conductivity types, the first zone contacted
by the cathode electrode and the fourth zone contacted by the
anode elec-trode. The first and fourth zones act as emitter
zones. The second zone adjoins the first zone, and the third
zone which has the lowest do~ing lies betweenthe second and
fourth zones. A forward blocking PN-junction is formed between
the second and -third zones, both the second and the third zones
together with the forward blocking PN-junction formed between
them extend to a surface of the semiconductor chip on the side
with the first zone. The semiconductor chip also has current
concentration means com~rised of at least a part of the thi~
zone extending to the surface forming a curve. The curve at
least substantially circumscribes a part of the second zone ex-
tending to the surface. The thyristor triggers when the portionof the forward blocking PN-junction reaching the surface is ir-
radiated with light. The thyristor further includes a highly
doped zone of the conductivity -type of -the first æone provided in
the surface o the semiconductor chlp. The part of the third zone
that extends to the suxface at least partially circumscribes a
part of the highly doped zone,thus irradiation of the forward
blocking PN-junction produces a current in the second zone. This
current is concentrated by the curve towards the highly do~ed
zone. The thyristor has an electxical contact connectirlg the
highly doped zone with the second zone outside the part of the
second zone surrounded by the third zone ~t the sul-face.
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BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following ;s
detailed description when considered in connection with the
accompanying drawings, wherein:
; FIGURE 1 is a section through a thyristor of the invention
in which the highly doped zone of the conductivity type of the
first zone lies in an opening of the part of the third zone reach-
; 10 ing the surface;
FIGURE 2 iS a top view of the thyristor of FIG. l;
FIGURE 3 is a top view of a thyristor in which the part of
the third zone at the surface is in the form of a spiral curve;
`~ FIGURE ~ is a section through a thyristor of the invention
in which the part of the third zone at the surface is closed, and
FIGURE 5 is a top view of the thyristor of FIG. 4.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like reference numel-als designate
identical or correspondiny p~rts throughout the several views, and more par-
ticularly to FIGURE 1 thereoF, ~he thyristor depicted has a first zone 1
which is heavily N-doped and thus acts as N-elnitter zone. It is contacted by
the cathode C. The second zone 2 is P-doped and would in a conventional
thyristor triggered by way of a control electrode be contacted by the cathode.
It contacts cathode C through emitter short circuits 13. The third, N-doped,
zone 3 exhibits the lowest doping of all the zones and serves in particular
for incorporation of the blocking layer, which in the forward direction forms
at the forward blocking junc~ion 5 and in the reverse direction at the reverse
blocking junction 6. The four~h, P-doped zone 4 has on the outside a highly
doped region 4' which is contacted by the anode A and acts as P-emitter zone.
In the surface 7 of the semiconductor chip of the thyristor is the highly
N-doped zone 9 which, with the highly P-doped region 11, forms a P N - junc-
tion which is bridged over by the electrical contact 10. The zone 9 thus
~Jorks in a kno~n manner (cf. e.g. Textbook entitled "Semiconductor
Power Devices" by Sorab K. Ghandi, P. 220, published by
John Wylie & Sons, 1977~ as the N-emitter of an auxiliary
thyristor, which after.being triggered furnishes the control
current for triggering the main thyristor.
To this extent the structure of the thyristor correspon~s
to the state of the art.
In the present invention however, the part of the third
zone 3 extending to the surface 7 of the thyristor semiconductor
chip forms, on the one hand, a near.y closed curve 3' with
an opening 8, and, on the other hand, parallel strips 3"
situated inside the curve 3'. The N doped zone 9 is
located in the opening 8 and connects with a part
3~ of region 11 of zone 2 making a closed ring
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of highly P-doped material surrounding the curve 3', the P N -junction
bridged over by the electrical contact 10.
By the described special structure of the N-zone 3 at the surface 7 the
hole current produced by the light irradiation L at the surface portion of the
PN-junction 5, made optimally long by the strip configuration, is caused to
flow off at high concentration through the opening 8 to the cathode C. By
way of zone 9 situated in the opening 8 and serving as emitter zone of the
auxiliary thyristor, the hole current is caused to trigger the auxiliary
thyristor which in turn triggers the main thyristor. The effect of the P -
ring is to distribute the control current produced by the firing of the auxil- ,iary thyristor immediately over the entire surroundings of main emitter zone 1 !and thereby produce rapicl firing of the thyristor over the entire cross section.
It is of great significance here that the easier triggering of the thyris-
tor structure of the invention is not achieved at the expense of a decrease in
the value of the critical voltage rise rate du/dt, which was to be feared on
general principles. ~his is the result of the fact that the capacitance per
unit area in a thyristor structure of the invention is, unexpectedly, distinc-
tly smaller in the light-sensitive region than at the unexcited PN-junction,
l and in addition the du/dt sensitivity decreases at high forward blocking volt-l ages. Only on this account does the local current density produced by the
configuration of the invention not lead to a du/dt triggering.
Another particularly advantageous feature of the thyristor structure of
the invention is that between the strips 3" on the surface 7 of the semicon-
ductor chip there are corresponding P~ strips 12 which all join to a like-
doped strip 12' lying crosswise to them between them and zone 9.
The ~ s rips 12 correspond;ng to the W-]trips 3" cause a lowering of the
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ohmic resistance for the hole current produced at the surface PN-junction 5
and conduct it to the crosswise P strip 12', which then-like a bus bar as
it were- acts over the whole extent of the adjacent part of zone 9 of the
auxiliary thyristor being triggered.
5` The curve 3' can, in principle, have more than one opening, each with a
zone 9 situated in it, but a single opening is best on account of the conse-
¦ quent possibly very high hole concentration. A circular form for the curve
¦ 3' and the ring 11, and a central arrangement of this configuration in a cir-cular hole in the cathode C is advantageous, since the thyristor can then be
dimensioned more simply. The enlargement of the zone 9 inside the curve 3'
and the corresponding long form and facing arrangement of the transverse N-
strip 12' produce an especially strong interaction between these two parts.
The fabrication of a thyristor conforming to the invention is not diffi-
¦ cult. For instance, in a first diffusion process into an N-doped silicon sub-
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strate of specific resistance 200 ohm-cm and thickness 800 ~m, there are pro- '
duced the zones 2 and 4 of, for example, 90 ~um depth, during which process the !
regions of the strips 3" and the curve 3', the zone 9 and zone 1 are protected¦
¦¦ against in diffusion by a diffusion mask. Then, with continued masking of
the mentioned regions plus the surface region of zone 2, the regions 11 and 4'
are produced. Finally the N zones 1 and 9 are diffused with appropriate
masking. The P regions 4' and 11 and the N+ zones 1 and 9 have a penetration
depth of perhaps 15 jum. The width of the strips 3" may be 50/um, the diameter
I of the c1rcular curve 3', 3mm, that of the ring llg 4mm, the width of the
opening 3, lmm, the diameter of the circular hole in the emitter zone 1, 5.5mm.,The diameter o-f the entire thyristor is, for example, 14mm. The emitter short
circuits 13 may have an average separation of 1-2 mm and are distributed in a
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known manner over the entire cathode, C.
An element thus dimensioned blocks voltages of 4.5 K ~, as an example,
and at a lKV forward blocking voltage can be triggered, for instance, by
I means of a GaAs light-elllitting diode (950 nm) with a ligllt level of 5 mll.
The critical voltage rise rate du/dt at room temperature is bet-ter than 3000
. V/lusec. . I
The special advantages of a thyristor conforming to the invention, lie
not only in the low light level required for triggering, but also in the fact
that the quantum efficiency over a spectral band of about 540 nm - lO00 nm is
nearly 1 so that a variety of light sources can be used. The light level re-
quired for triggering is largely independent of the bloc~ing voltage at ~hicl
triggering occurs. A good du/dt strength is assured and, in addition, the
thyristor is also protected in large measure against damage from overvoltages,
I or has a high "breakover strength", for the curvature of the forward blocking
PN-junction 5 in the light-sensitive region produces there a reduction in the
j avalanche breakdown voltage so that overvoltages fire the thyristor from this
¦I region out over its whole volume and consequently no damage can occur.
The light-sensitive surface of another embodilnent of the thyristor o~ the
invention is drawn to scale in FIG. 3 (the scale of 1 mm is indicated at the
lower right of the figure). In this thyristor the reference numbers corres-
pond to those in the thyristor of FIGS. l and 2. It differs from the latter,
, however, in that the curve 3' is spiral in form and now has a long narrow
¦~ channel as its opening, and in that the highly-doped zone 9 is now entirely
¦ contained in the region enclosed by the spiral curve 3'. In a thyristor struc-
I¦ ture such as this, the electrical contact lO must be brought across the N-
¦1 region 3' but be insulated from it. This insulation is preferably an appro~-
imately l ~nl thick oxide film, not shown. This film is oxidized onto the in
ll i
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terior of the closed ring ll. Over the reg1on of the N -zone 9 which is to
be contacted, a window is then etched in the oxide film. This film serves
not only as insulation but simultaneously as passivating layer for the light-
sensitive structure and in addition it reduces the light losses occasioned by
~ reflection at the silicon surface.
This thyristor works as follows. If light in incident on the region
enclosed by the spiral curve 3' then the hole current must flow to the
! cathode G through the relatively high resistance of the long narrow opening
8 (about 2Knby sultable dimensioning in FIG. 3 with a length of ~ 4 mm and
a width of ~ 0.2 mm for the opening 8). Consequently the entire region of
the P-doped zone 2 inside the curve 3' is raised to a higher potential rela-
; tive to its surroundings. This holds particularly with respect to the N
region 9 which, through the metallization lO, is at the potential of the P+
ring ll. At a potential difference of about 0.6 V the N region 9 injects
electrons and the thyristor fires.
Such a thyristor very conveniently can be triggered at minimal light
levels since the ohmic resistance between the regions 12 or 12' and ll is
greatly increased by choosing a very long and narrow opening 8 for curve 3'.
This, of course, can be achieved in principle also by making the dimensions
of the opening 8 of the thyristor in FIGS l and 2 very small, but thereby the
diffusion and metallization lO of the N region 9 in such a narrow opening be-
comes extremely difficult. Along with a spiral form of the curve 3', any other
form of curve 3' is also usable to advantage, if need be, in which there is a
long narrow opening 8.
A third configuration of the thyristor of the invention is drawn to scale
in FIGS. 4 and 5. Again the reference numbers are the same as those for the
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¦ thyristors in FIGS. 1, 2 and 3. However, this thyristor differs from the
~¦ others in that the part of the third zone extending to the surface forms a
¦ closed curve 3' which does not enclose the first zone, and in that this curvel encircles the N doped zone 9 as well as a P doped zone 14. The P doped
zone 14 is connected with the cathode metallization by the metallization 15.
In such a thyristor structure the two metallizations 10 and 15 pass over the
N region 3', but are insulated from it. This insulation, as in the thyristor
of FIG. 3, is preferably a ,vl ~um thick oxidation film 16 which is oxidized
onto the inside of region 11. Over the regions of the N and P zones 9 and
14 which are to be contacted, windows are then etched in the oxide film.
This thyristor works in the following way, as schematically indicated in ¦
FIG. 4. Incident light produces a current i, which flows to the cathode
through the resistance R, corresponding to the opening 8 of the thyristors of
FIG. 1 and 2 or 3. This current raises the potential of the entire region of
the P-doped zone 2 lying inside the zone 3' with respect to its surroundings.
This is especially true of the N~ region 9 in the above embodiments, region 9
being tied by the metallization 10 to the potential of the P+ region 11. At
¦ a potential difference of about 0.6V the N region 9 injects electrons.
A very special advantage of this thyristor is that with the choice of a
very high triggering sensitivity, premature triggering by an all-surface cur-
rent, as by the displacement current produced by a voltage rise rate du jdt
or by its inverse current, is prevented. Such an all-surface current i2 flows
to the cathode through the series resistances R2 and R3 represented schematic-
ally in FIG. 4. In this way, however, the potential of the P region 11 and
the N+ region 9 connected to it, is raised, so that with suitable dimensioning
between the region 9 and the surrounding P-region of the second zone no poten-
¦ tial difference is produced by the surface current i2 and an injection with
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consequent premature and undesired triggering of the thyristor is avoided.
Suitable dimensioning is achieved on the one halld by ma~illg the area Fl
of the part of the second zone enclosed by the curve 3' ~s small ~s possible
to compensate the all-surface current and on the other hand by increasing the
resistance Rl of this zone by forming a comb structure on the inner side of
curve 3'. Beyond this, it is necessary to choose the ra-tio F2/Fl of the ~rea
F2 f the second zone bounded by the inner edge of the cathode C and the curve
3' to the area Fl of the part of the second zone enclosed by the curve 3' so
that i~ equals the reciprocal ratio Rl/(R2 + R3) of the resistances R2 + R3
and Rl of the regions of the second zone with areas F2 and Fl (F2/Fl = Rl/
(R2 + R3)). Thus every current from the area Fl flows to the cathode C
through the P region 14 and the metallization 15. The highest potential
arises in the P-region under the N~ region 9. If a current from the are~ F2
simultaneously flows to the cathode, then the potential of the P region 11
and therefore of the metallization 10 and the N region 9 also, is raised and
injection of electrons is prevented.
D. Silber and M. Fuellmann (International Electron Devices
Meeting, 1975, Washington) ha~e already proposed a compensating
arrangement for preventing such undesired triggering in which
the potential of the N region of the auxiliary thyristor as
well as that of the surrounding P region is raised. In this
arrangement, however, the areas Fl and F2 are not adjacent,
rather a connection to the outer edge o the thyristor is
required.
Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings.
It is therefore to be understood that within the scope of the
appended claims, the invention may be practiced otherwise than
as specifically described herein.
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