the object of the present invention consists in specifying a real - time method for the monitoring of the breakdown of pn - junctions in semiconductor components and also a semiconductor component constructed for the carrying out of the method , which semiconductor component allows an enlarged operating range of the semiconductor component . the object is achieved with the method and the semiconductor component according to the patent claims 1 and 7 . advantageous configurations of the method as well as of the semiconductor component are the subject of the subclaims or can be drawn from the following description as well as from the exemplary embodiment . in the case of the suggested method for monitoring the breakdown of pn - junctions in semiconductor components , optical radiation , which the pn - junction emits during a breakdown , is detected with a photosensitive electronic component integrated into the semiconductor component in the vicinity of the pn - junction . a voltage applied at the pn - junction or a current conducted across the pn - junction is then controlled in dependence on this radiation detected . the control can take place in such a manner that a complete breakdown of the pn - junction is prevented . in the event of the optical emission of the pn - junction increasing beyond a predeterminable threshold , the current or the voltage at the pn - junction is in this case reduced until the optical emission has once again fallen under the threshold value or does not increase any further . in this context , what is meant by a complete breakdown is a breakdown with a duration which leads to overheating or destruction of the semiconductor component . the suggested semiconductor component comprises at least the pn - junction to be monitored , a control unit for controlling a voltage applied at the pn - junction or a current conducted across the pn - junction and a photosensitive electronic component connected to the control unit . the photosensitive component is in this case integrated in the vicinity of the pn - junction in such a manner that it can detect optical radiation emitted during a breakdown at the pn - junction . the control unit is constructed such that it controls the voltage applied at the pn - junction or the current conducted across the pn - junction in dependence on the radiation detected . the present method enables the self - monitoring of the breakdown at a pn - junction in a semiconductor component without further limiting the operating range of the semiconductor component as a result . rather , the semiconductor component can thereby be operated with voltages or currents shortly before the onset of the breakdown , so that an increased power output can be achieved . the method is suitable here in particular for the self - monitoring of breakdown in fast ics and fast semiconductor components during their normal use . essential advantages of the method and of the semiconductor component therefore consist in an enlarged operating range and an increased power of the semiconductor component , in a larger temperature range for operation , a larger acceptance of component strays and protection against destruction and therefore failure of the semiconductor component . in the case of the present method and the semiconductor component configured therefor , use is made of the discovery that visible light is emitted during the breakdown of a pn - junction . the light emission increases as the breakdown progresses , so that it can be used as a signal for the starting breakdown and also as a measure for a continuing breakdown . in the case of the present method and the associated semiconductor component , a photosensitive electronic component , preferably a photodiode , is therefore integrated into the semiconductor component close to the generating location for the light emission , that is to say close to the pn - junction to be monitored . the location of this photosensitive component must in this case be selected to be sufficiently close to the pn - junction as the light emission is only weak and is additionally absorbed strongly in the semiconductor material . the signal measured by the photosensitive component , the photocurrent in the case of the photodiode , is used to control the supply ( voltage , current ) of the component or pn - junction to be protected . in the case of the method suggested , the weak light emission during the breakdown of a pn - junction , which was hitherto treated as useless , is thus used as a self test for the breakdown during the operation of the semiconductor component . the photosensitive component integrated to this end serves the detection of this light emission exclusively for the control of the supply of the monitored semiconductor component . it is self evident that a plurality of pn - junctions in a semiconductor component , for example a complex integrated circuit , can also be monitored with the method suggested . in this case , a corresponding photosensitive component is integrated at every pn - junction to be monitored and connected to a control unit which controls the supply of the semiconductor component of the pn - junction monitored . the component to be controlled can for example be a bipolar transistor , a heterobipolar transistor ( hbt ) or a field effect transistor ( fet ), particularly a mosfet . in the case of a field effect transistor , preferably the control signal of the control unit is used to control the source bulk or source drain or gate source voltage . no particular requirements are placed on the control unit itself . this control unit must merely be fast enough to be able to control the voltage or the current in order to prevent the breakdown . a pi or a pid control can be used here by way of example as control techniques . the control unit itself is in this case preferably likewise monolithically integrated into the semiconductor component , as is also the case for the photosensitive component . the suggested method and also the semiconductor component are explained briefly once more below with reference to an exemplary embodiment in connection with the drawings . here : fig1 shows an example for the integration of a photodiode into a heterobipolar transistor for carrying out the suggested method ; and fig2 schematically shows the principal construction of a semiconductor component according to the present invention . in the following example , the suggested method as well as the construction of a correspondingly configured semiconductor component is once more explained using the example of a heterobipolar transistor . the breakdown of the pn - junction in the heterobipolar transistor should in this case be monitored by the semiconductor component itself during the normal operation of this transistor , in order to prevent a breakdown during operation . in the case of the method , an in - situ detection takes place of the light which is emitted at the pn - junction by the semiconductor material in the avalanche breakdown . this illumination is converted to a photocurrent with a monolithically integrated photodetector which is placed close to the pn - junction . the photocurrent is used as a control signal for the supply of the heterobipolar transistor . the known construction of a heterobipolar transistor , with the emitter 1 , the base 2 , the collector 3 , the sic region 4 ( sic = selectively implanted collector ), the subcollector 5 located below that and the collector connection 6 , is hereby illustrated in fig1 , the upper region of this heterobipolar transistor is surrounded by a trench with an insulator 7 . for the detection of the light radiated during a breakdown by the pn - junction to be monitored between the base and collector , a photodiode 8 , the pn - junction of which can likewise be seen in the figure , is integrated in the vicinity of this junction , in the left part in fig1 . this photodiode 8 is connected to a control unit not shown in fig1 , which control unit controls the base current of the heterobipolar transistor in dependence on the photocurrent of the photodiode 8 . the control loop maintains the input parameter which is decisive for the semiconductor component to be monitored , that is to say the base current in the case of the heterobipolar transistor which is illustrated by way of example here , at a defined value in or shortly before breakdown . in the case of a mosfet as the component to be monitored , the gate voltage can for example be controlled as the decisive input parameter . the component can here be maintained both at the start of the breakdown or else in the breakdown . the determination of the working point for the control is left to the manufacturer or the user . the suggested method allows all possibilities . fig2 schematically shows the control unit 9 , the photodiode 8 as well as the supply 10 for the semiconductor component 11 to be monitored . the photocurrent generated in the photodiode 8 can , in the present example of the heterobipolar transistor , in the simplest case be passed for controlling to the base 2 as a second current portion ( with inverted polarity ). it is self evident however that a pid control of the base current is also possible , wherein the photocurrent of the photodiode 8 is used as control signal . in the case of the suggested method , the light radiated by the semiconductor component , which has already passed a quality inspection , during operation is detected by means of an integrated photodetector and the detected signal is used to control the working point of the semiconductor component . this control can be used in future oscillator circuits in order to generate more power with the transistors as the latter can be operated with the method without the danger of destruction defined in the breakdown .