An ultra-fast breakover diode has a turn on time TON that is less than 0.3 microseconds, where the forward breakover voltage is greater than +400 volts and varies less than one percent per ten degrees Celsius change. In another aspect of the invention, a breakover diode has a reverse breakdown voltage that is greater, in absolute magnitude, than the forward breakover voltage, where the forward breakover voltage is greater than +400 volts. Yet another aspect of the invention involves a string of series-connected breakover diode dice, along with a resistor string, in a packaged circuit. The packaged circuit acts like a single breakover diode having a large forward breakover voltage and a comparably large reverse breakdown voltage, even though the packaged circuit includes no discrete high voltage reverse breakdown diode. The packaged circuit is usable to supply a triggering current to a thyristor in a voltage protection circuit.