Abstract:
A break-over photodiode, designed as a light-sensitive thyristor, can be stacked using a series connection with a plurality of break-over photodiodes, such stacking representing a high-voltage break-over diode. The break-over photodiode can be triggered by lateral illumination in an edge zone, and includes a gate-layer resistivity under the emitter which is greater in an edge zone of the break-over photodiode than in the central zone of the break-over photodiode. The light sensitivity of the laterally illuminatable break-over photodiode is increased by a greater gate-layer resistivity in the edge zone as compared to the central zone.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a break-over photodiode. 
     BACKGROUND INFORMATION 
     German Patent No. 44 17 164 describes a break-over photodiode that can be stacked with a plurality of additional break-over photodiodes. This stack represents a light-sensitive high-voltage switch. Each break-over photodiode of this stack has a light-sensitive region illuminated by light-emitting elements so that at a predefined time, for example, in an ignition voltage distributor of a car at the time of the ignition of the cylinder pertaining to the light-sensitive high-voltage switch, the break-over photodiode switches through. With the disclosed design of the individual break-over photodiodes, light sensitivity to lateral illumination is a critical parameter, as is the minimum required light flux for triggering the break-over photodiode, which should be reduced. Furthermore, it is desirable to ensure that the break-over photodiode, in particular a stack of break-over photodiodes, will not switch through in an undesirable manner as a result of parasitic or displacement currents. 
     SUMMARY OF THE INVENTION 
     The arrangement according to the present invention has the advantage of an increased light sensitivity in an edge region that can be laterally illuminated. Thus, the break-over photodiode can be triggered through lateral illumination even when voltages that are low in comparison to the break-over voltage are applied. 
     By selecting the thickness of an n -  layer in an appropriate manner, a higher current amplification can be achieved within the break-over photodiode than at its edges. This has proven to be advantageous for a homogeneous current distribution when the break-over photodiode switches through. Thus, a concentration of higher currents in the edge region of the break-over photodiode due to the high field intensities prevailing there can be counteracted. 
     It is further advantageous to dimension an edge-gate-cathode resistivity formed by a p region located between the n -  region and the edge emitter so that a break-over of the break-over photodiode occurs without the central region being illuminated. The break-over photodiode has a high break-over current in such case, which protects the diode from unintentional switching due to parasitic currents and/or cut-off currents. Such advantage is particularly important in the case of small chip surfaces or when no other methods for protecting the break-over photodiode from undesired switching are available. 
     Designing a rand emitter located in the edge region with a greater thickness than that of an emitter located in the central area and/or selecting the doping profile of the edge emitters and internal emitter in an appropriate manner is a simple way of making an edge-gate-cathode resistivity greater than a center-gate-cathode resistivity located in the central area. 
     In order to design the edge-gate-cathode resistivity to be greater than the center-gate-cathode resistivity, the gate can be dimensioned appropriately in the edge region or the central region. This provides another advantage of a larger effective base width of an npn partial transistor in the edge region, and prevents the current from being excessively amplified in the edge-region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a conventional break-over photodiode for lateral illumination. 
     FIG. 2 shows a break-over photodiode with an avalanche geometry. 
     FIG. 3 shows a break-over photodiode with a punch-through geometry. 
     FIG. 4 shows a first embodiment according to the present invention. 
     FIG. 5 shows a second embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a conventional break-over photodiode 100, provided for lateral illumination 9 in an edge region 110. Henceforward, the symbols for a break-over photodiode designed as a pnpn thyristor will be used for all figures. Replacing &#34;p&#34; with &#34;n&#34; and &#34;anode&#34; with &#34;cathode,&#34; the corresponding description of an npnp thyristor design is obtained. 
     A p region 2 connected to an anode metal plating 1 is followed by an n -  region 3, into which a gate 4, configured as a p region, is embedded. An edge emitter 5a is in turn embedded into the edge region 110 of gate 4 as an n region. An internal emitter 5b, consisting of a plurality of adjacent n regions, is embedded into central region 120 of gate 4. Edge emitter 5a, internal emitter 5b, and gate 4 are short-circuited via a cathode metal plating 6. An n +  region 8 is embedded on the edges facing away from p region 2 of n -  region 3. The area of break-over photodiode 100 that is not covered by cathode metal plating 6 is sealed with a silicon oxide layer 7. p region 2, n -  region 3, and gate 4 form a pnp partial transistor, and emitter regions 5a,b, gate 4, and n region 3 form an npn partial transistor. This is the conventional design of a pnpn thyristor, which switches through from a break-over voltage applied between anode metal plating 1 and cathode metal plating 6 in the direction of flow. Alternatively, break-over photodiode 100 can also be triggered under this break-over voltage by lateral light incidence 9. n +  region 8 serves as a &#34;channel stop.&#34; It limits a space charge region formed before break-over photodiode 100 switches through and thus suppresses surface effects. Silicon oxide layer 7 serves as a protection and electrical insulation. The short circuit between edge emitter 5a, internal emitter 5b, and gate 4 through cathode metal plating 6 reduces the light sensitivity of break-over photodiode 100 to lateral illumination 9 (the minimum required light flux for triggering break-over photodiode 100 below the break-over voltage is set high), but it is required, among other things, that the high sturdiness requirements for break-over photodiode 100 be satisfied with quick voltage changes. 
     The triggering mechanism of thyristors, configured as break-over photodiodes are discussed in detail below. When a voltage is applied between anode and cathode, the pn junction between gate 4 and n region 3 is blocked. This blockage can be overcome by a build-up of a voltage differential greater than 0.6 V (for silicon at room temperature) between emitter regions 5a,b and gate 4, since then the npn transistor formed by emitter regions 5a,b, gate 4, and n region 3 becomes conducting. Since emitter region 5a,b and gate 4 are short-circuited via the cathode metal plating 6, a sufficient voltage differential can build up between the emitter regions 5a,b and the gate 4 only if a sufficiently high current flows through the p region that forms gate 4. The intensity of the sufficient current depends on the layer resistivity of gate 4 under the emitter region. In edge region 110, an edge-gate-cathode resistivity R --  rand is formed basically by the p region located under edge emitter 5a. In the central region 120, a center-gate-cathode resistivity R --  mitte is basically formed by the p region located under the internal emitter. The magnitude of these resistivities R --  rand and R --  mitte define the current required for triggering the break-over photodiode. Since a photoelectric current is generated by illuminating the edge region 110, the resistivity R --  rand is critical for triggering the break-over photodiode by illuminating i. The greater the resistivity R --  rand, the easier it is to trigger the break-over photodiode by illuminating it. The break-over photodiode can, however, also be triggered without illuminating it if the parasitic currents and/or cut-off currents exceed a certain value. Normally parasitic currents are currents generated as a result of dynamic effects, for example, when a voltage is applied between anode and cathode due to parasitic capacitances or pn junction capacitances. In order to trigger the diode with such currents (&#34;parasitic currents,&#34; short for &#34;parasitic and/or cut-off currents&#34;), it is important to determine whether the diode has an avalanche geometry or a punch-through geometry. 
     FIG. 2 shows a break-over photodiode with an avalanche geometry. This geometry is defined by the fact that break-over of the diode occurs through the avalanche effect, i.e., due to high field intensities. Such high field intensities arise at the high curvatures of space charge region 20 located in edge zone 110 for the geometry illustrated in FIG. 2. Therefore, the parasitic currents flow predominantly in edge zone 110 as illustrated in FIG. 2 through current paths 21. Therefore, in order to trigger this diode by parasitic currents, resistivity R --  rand is also critical, since an increase in R --  rand (which is required for high light sensitivity) also results in an increase in the sensitivity to triggering by parasitic currents. Thus, a high sensitivity to light coupled with a high degree of safety against undesired switching due to parasitic currents is difficult to achieve in such break-over photodiodes. 
     FIG. 3 shows the case of a break-over photodiode with punch-through geometry, where, contrary to the avalanche geometry, the difference in current amplification of the pnp partial transistor between the edge and central zones is significant. A space charge region 30 approaches p region 2 shortly before switching so that an effective base width, given by the distance of space charge region 30 from p region 2, becomes extremely small. For this to occur, n region 3 must be sufficiently thin. This results in high current amplification even of the currents flowing prior to switching in the central zone of the break-over photodiode. Therefore, despite the high field intensities in the curved area of space charge region 30, most of the parasitic currents flow along current paths 31, which are concentrated in central zone 120 of the break-over photodiode. If the voltage applied between anode and cathode metal plating 1 and 6, respectively (without illumination) is increased until it reaches the &#34;break-over voltage,&#34; the voltage that drops at the layer resistivity of gate 4 (described with the explanation to FIG. 1) reaches a certain value (for silicon approximately 0.6 V at room temperature), which results in the break-over photodiode switching through. In the punch-through geometry described above, this voltage drop is first achieved through the currents flowing before switching-through takes place in central area 120 of break-over photodiode 100. Triggering in punch-through geometry through parasitic currents is therefore determined by R --  mitte, as long as the difference between R --  rand and R --  mitte is not so great that the low intensity of the parasitic currents on the edge is overcompensated for by a very large resistivity R --  rand. 
     Lateral illumination 9 produces charge carriers, which become insulated in the space charge region of edge zone 110. In such case, holes flow to gate 4 and form a current that causes a voltage drop at the layer resistivity of gate 4 in edge zone 110. The punch-through geometry allows reaching a certain voltage drop at gate 4 to be set separately as a condition for the break-over photodiode to switch through, triggered by illumination below the break-over voltage, as well as for switching through to occur as a result of the break-over voltage being reached (without illumination). Also measures other than the use of punch-through geometry are conceivable, of course, which prevent intense parasitic currents from flowing in the edge zone. 
     R --  rand is critical for triggering as a result of lateral illumination 9 below the break-over voltage, since the light-induced current, which is to deliver the voltage drop required for break-over at the R --  rand (e.g., 0.6 V for silicon at room temperature) only flows in the edge zone (110). The edge zone (110) can have a symmetrical design, so that the break-over photodiode can be illuminated laterally from either side. 
     On the other hand, current paths 31 of the currents that flow even without illumination prior to reaching the break-over voltage are concentrated in central area 120. They are responsible for the voltage drop required for break-over at R --  mitte. If this voltage drop reaches 0.6 V (for silicon at room temperature), the voltage applied between cathode metal plating (1) and anode metal plating (6) has reached the break-over voltage value and the break-over photodiode switches through. 
     FIG. 4 shows a first embodiment 210 of a break-over photodiode 100 according to the present invention with thick edge emitter 5a&#39;. Edge emitter 5a&#39; in edge zone 110, which is thick compared to the internal emitter 5b in the central area 120, ensures high layer resistivity of gate 4, and thus a high R --  rand, in the edge zone where charge carriers are produced through lateral illumination 9, due to the small cross section in the lateral direction under edge emitter 5a&#39;. Thus, the minimum light flux required for reaching the triggering condition (a voltage drop of, for example, 0.6 V for silicon at room temperature) is reduced. 
     FIG. 5 shows a second embodiment 310 of break-over photodiode 100 according to the present invention with a gate 50, which is thinner in edge zone 110 than in central zone 120. As in the case of first embodiment 210, here as well the layer resistivity of the gate in the edge zone and R --  rand is greater than in the central area before the break-over photodiode switches through. Also in this case, increased light sensitivity is obtained to lateral illumination. If a break-over of break-over photodiode 210 or 310 occurs without illumination in the central area 120, in order to ensure a high break-over current, R --  rand must also be selected so that the triggering condition (voltage drop of approx. 0.6 V for silicon at room temperature) is not attained in edge zone 110 of gate 4 first due to the currents flowing before break-over photodiode 210 or 310 switches through, despite the greater current amplification of the pnp partial transistor in central area 120. Thus, the diodes shown in FIGS. 4 and 5, which otherwise have punch-through geometry or other means limiting a considerable portion of the parasitic currents to the central zone, can be triggered with a low light flux and are well-protected from unintended triggering, since parasitic currents flow mainly in the central area. 
     In embodiments of the photodiodes 210 and 310 according to the present invention, an increased edge-gate-cathode resistivity R--rand can optionally be achieved by a suitable doping profile of gate 4 in the edge and central zones. A combination of a suitable choice of emitter thickness, gate thickness, and doping profile in the edge and central zones is also conceivable.