Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to light-controllable
thyristors having a semiconductor body which, at the cathode
face, has at least one region which is intended to be exposed
to light during operation o~ the deYice, the bcdy having at
its cathode side a main emitter zone and an auxiliary emitter
zone which lies between the main emitter zone and the region
to be exposed to light, and a cathode base zone the doping of
which falls towards the anode side of the zone, the emit es
f~ zones being provided with respective electrodes, thar of the
au*~liary emitter zone also contacting the base zone.
Such a thyristor is already known. It consists
basically of two parts, one of which is in the form o~ a
light-controllable auxiliary thyristor and the other 2 main
thyristor. The light-controllable auxiliary thyr~stor serves
as a control current amplifier for the main thyristor whicn
carries the load current after ignition has been initiated.
If desired, a further auxiliary thyristor can be located
between the light-controllable auxiliary thyristor and the
ma~n thyristor.
When the surface region which is intended to be
expose~ to light ~n use, is in fact illuminated, the photo-
current of the light-controllable auxiliary thyristor is
arranged to flow into an electrode (known as a collector
electrode) which contacts this region, whence it flows in
t~e radial direction to the emitter of the auxiliary
thyristor and that of the main thyristor. If the emitter-
base voltage exceeds a specific value, for example, 0.5 V,
at a point on the p-n junction between the base zone and the
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emitter zone of the auxiliary thyristor or of the main
thyristor, the thyristor in question ignites.
In practice, it is desira~le to dri~e the light-
controllable auxiliary thyristor via light conductors.
Such light conductors normally have a diameter of 1 mm or
less. In order to achieve a good du/dt stability which is
dependent upon, inter alia, the conductivity of the base
zone, the diameter of the region intended to be exposed to
light should, on the one hand, be arranged to be only just
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sufficiently greater than the diameter of the light conductor
to enable it to be contacted. In order that the least
possible light power has to be used, the sensitivity of the
light-controllable auxiliary thyristor should, on the one
hand, be high. The light sensitivity can be increased, for
example, by providing the base zone with a high specific
res~stance-by means of low doping so that even with low
current values, the voltage drop required for ignition is
obtained.
i- It is an ob~ect of the present invention to pro~ide
a thyristor of the above_mentioned type in which whilst the
base zone has a high degree of doping where required, the
auxiliary thyristor has a high light sensiti~ity.
According to the invention, there is provided a light-
controllable thyristor having a semiconductor body with
cathode and anode faces, said body compricing a ~athode-s-de
:
base zone the doping of which is reduced in a region
adjacent the anode side thereof, a main emitter zone
located in said base zone at said cathode face, a region
at said cathode face intended to be exposed to light during
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operation of the thyristor, and an auxiliary emitter zone
located in the base zone at said cathode face between said
main emitter zone and said region, said main emitter and
auxiliary emi~ter zones being provided with main emitter
and auxiliary emitter electrodes respectively, said auxiliary
emitter electr~de also contacting said base zone; wherein
at least one trench is pro~ided on ~he cathode face of said
body extending into said base zone and having a depth at
^~ least equal to that of said auxiliary emitter zone, said
trench bounding the periphery of said auxiliary emitter
- zone facing said main emitter zone and serYing to separate
a part of said auxiliary emitter electrode which contacts
sa~d auxiliary emitter zone from a part thereof which contacts
said base zoneO
- The invention will now be further described with
reference to the drawings, in which :-
~igure 1 is a schematic plan view of the cathode s~de
. of a semiconductor body of a first exemplary
embodiment of the invention;
Figure 2 is a section taken along the line II-II o~
Figure 1;
F~gure 3 is a section taken along the line III-I}I of
Figure 1;
Figure 4 is a schematic side-sectional part of a second
exemplary embodiment of the invention;
Figure S is a schematic plan view of the cathode side
of a semiconductor body of a ~hird e~empl~ry
embodiment o~ the invention;
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P~gure 6 $s a section taken along the line VI-VI
of Figure 5;
Figure 7 ~s a schematic plan view of the cathode
side of a semiconductor body of a fourth
exemplary embodiment of the invention; and
Figure 8 is a section taken along the line VIII-VIII
of Figure 7.
In the various Figures, like elements (either in
f~ structure or in function) ha~e been indicated by the same
reference numeral. In the plan views, Figures l, 5 and 7,
the electrodes ha~e been shown shaded for the sake of
lmproved clarity.
The sem~conductor body shown in Figure l has a main
emitter zone l, an auxiliary emitter zone 2 and a base zone
lS 3 whi~h is common to the two emitter zones and the doping of
which decreases with d~stance from the cathode surface of ~he
~ body. The auxiliary emitter zone 2 forms a part of a light-
; ~ ~ controllable auxlliary thyristor, whilst the main emitter
zone ~ forms part of a main thyristor. The main emitter
zone 1~is contacted by means of an emitter electrode 5, and
the~auxiliary emitt zone 2 by means of an auxiliary emitter
electrode 4. The base zone 3 has a region 8 which is exposed
at the surface and is intended to be exposed to light in use
of the device. This region 8 is surrounded by an annular
collector ~electrode g, the outer periphery of which ha a
projecting portion 10 in the form of a segment of a circle.
The auxiliary emltter zone 2 in plan view is in the fonm of
a segment of an annulus. That part of the auxiliary emitter
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electrode 4 which contacts the auxiliary emitter zone 2
is separated from that part of the auxiliary em~tter
electrode 4 which contacts the base zone 3 by a trench 6,
which also has the shape of a segment of an annulus, along
a part of the periphery of the aux1liary emitter zone 2.
Between the region 8 o~ the surface of the base zone 3
which is intended to be exposed to light and the emitter
electrode 4, there is located a further trench 7 which also
has the shape o~ a segment o~ an annulus and the ends of
~0 which overlap with the respective ends of the trench 6 and
of the emitter zone 2. The part of the auxiliary emittPr
zone 2 carrying that part of the auxiliary emitter electrode
4 which also contacts the base zone 3 forms arms ~1 wnich
are bounded on their opposite ~ides by the trenches 6 and 7.
lS The inner periphery of the trench 7 lies along the
outer periphery of the part ~f the collector electrode 9
not provided with the pro~ection 10.
When the region 8 is expose~ to light, pairs of charge
- carriers are produced in the base zone 3, and, w~th the
indicated negati~e polarity of the emitter electrode 5, the
holes f~rst flow towards the surface of the region 8 and
then radi~lly outwards, whilst the electrons are dr~wn away
to the anode side of the device. The paths taken by the
charge carriers are schematically illustrated in Figure 2
by arrows. It can be seen that the current in passing the
trenches 6 and 7 is compelled to ma~e a detour which leads
it into the weakly doped, inner region of ~he base zone 3~
The trenches 6 and 7 are for this purpose made at least as
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deep as the emitter zone 2. They can be produced, for
example, by etching in combination with a known photo-
resist technique. The photo-current produced flows, by-
passing the trenches 6 and 7, to those parts of the ~mitter
electrode 4 which are remote from the region 8, and produces
a voltage drop beneath the emitter zone 2.
The photo-current of the light-controllable auxiliary
thyristor flows across the projection 10 of the collector
C~ electrode 9, which projection is arranged between the ends
of the trench 9 ~which is in the ~orm of an annular segment),
radlally outwards. This arrangement produces a concentration
of the photo-current for the auxiliary thyristor as soon as
~t.exceeds the magnitude necessary for the ignition of the
aux~liary thyristor. Then, as a result of the high control
current density obtalned by thls concentration, the auxiliary
thyristor ~s switched on with a relat~ely short ~gnit~on
delay t~me. The collector electrode 9 is not absolutely
necessary for the functioning of the auxiliary thyristor;
~ ~t ~s, however, expedient to provide an electrode corres-
ponding to the pro~ection 10 in order that the photo-current
can flow uniformly to the auxiliary emitter zone 2.
In the modif~cation shown in Figure 4, the base zone
belcw the re~ion 8 intended to be exposed to light is
arranged to be thinner ~han the remainder of the base zone 3.
In order to further increase the ignition sensitivity.of the
auxiliary thyristor, a further trench 17 is arranged
between the collector electrode 9 and the emitter zone 2
The fundamental difference between the embodiment
illustrated in Figures 5 and 6 and that illustrated in
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Figures 1 to 3 is that the main emitter zone 1 now serves
as a further auxiliary emitter zone and on that side of
the emitter zone 1 remote from the region 8 there is provided
a third emitter zone 13 which now forms part of a main
thyristor. The emitter zone 13 is provided with an
electrode 14. The outer periphery of the auxiliary emitter
electrode 4 has a segmental project~on 15 which is locaf ed
between the ends of the trench 6. By means of the segmental
( ~ projection 15, it is possible to obtain a concentration of
the control current for the second auxiliary thyristor (of
which the emitter zone 1 forms part) which is thus able to
ignite in the above-described manner with a short ignition
delay time.
The length of the segmental projections 10 and 15 can
be varied to set the effecti~e edge length of the emitter
zones ~ and 2, i.e. the length of their p-n ~unctions on the
side facing the region 8 t and thus to determine the switch-on
--j characteristics of the light-controllable auxiliary thyristor
and the second auxiliary thyristor~ If the ratio of the
effective edge lengths of the emitter zone 2 on the side
facing the region 8 to the ef ecti~e edge length of the
emitter zone 1 on the side facing the region 8 is made less
than 3:1 (for example, in the embodiment illustr~ted in
Figure 5, it is approximately 1:1) it is possible for the
light-controllable auxiliary thyristor to operate simply as
a current amplifier, i.e. not to ignite. This has the
advantage that no load current channel can form during the
-- swi~ching-on process in the light-controllable auxiliary
thyristor.
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Flgures 7 and 8 illustrate an embodiment in which
the region 8 intended to be exposed to light forms part
of the surface of the auxiliary emitter zone 2. In this
case, the trench 7 constricts the auxiliary emitter zone 2
to form a bridge-piece i6 which is adjoined by the two arms
11. The holes which are produced in this case~ as indicated
by the arrows in Figure 8, take a path a~ right-angles to
the thickness of the auxiliary emitter zone 2, beneath the
latter, and bypassing the trench 6 and 7, flow to the emitter
~-- 10 electrode 4 and thence to the emitter zone 1 which can form
part of an auxiliary thyristor, or a main thyristor.
In the exemplary embodiments illustrated, the trenches
have been shown as forming segments of an annulus~ This
form is advisable when the region 8 intended to ~e exposed
to light occupies a central position. However, the trenches
can also have other forms and can, for example, run in the
radial direction. }t is also possible to use trenches
having the shape of an annular segment together with radial
~-; trenches. The trenches can also be arranged with respect to
one another in such a way that one or more narrow arms of the
auxiliary emitter zone 2 remains between them. The trenches
can be filled with electrically insulating material or a
h~ghly-resistive material.
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