Breakdown voltage is the voltage at which a reverse-biased diode suddenly starts to conduct a very large current. Below that limit the diode blocks current in the reverse direction. Once the applied voltage reaches the breakdown voltage, the field accelerates electrons and holes in the semiconductor so strongly that the reverse current climbs by orders of magnitude within a few hundred millivolts (Wikipedia, Z-Diode). A 1N4007 rectifier diode is rated for 1,000 V reverse voltage and passes no more than 5 µA at that level (Vishay, 1N4001 to 1N4007 datasheet). Zener diodes are designed to operate in breakdown, the BZX55 series from 2.4 to 75 V (Vishay, BZX55 datasheet). Below roughly 5 V the junction breaks down through the Zener effect, above it through the avalanche effect. Read on to see how to tell the two mechanisms apart on the datasheet, which symbol the breakdown voltage carries for diodes, TVS diodes, MOSFETs and IGBTs, when breakdown destroys the device, and why the insulation under a power semiconductor has to hold many times its blocking voltage. We make such parts as insulation and molded parts from polyimide, polyester and aramid paper.
Breakdown voltage in semiconductors and insulators: the same avalanche, different outcomes
German keeps two words apart: Durchbruchspannung for the semiconductor junction, Durchschlagspannung for insulating materials and air. English calls both breakdown voltage, and the physics is related. In both cases a strong field accelerates free charge carriers until they knock out further carriers on impact, and their number grows like an avalanche (Wikipedia, Lawinendurchbruch). The field strengths are also closer than most people expect: silicon carbide withstands roughly 2.2 MV/cm before breakdown, which is 220 kV/mm and about ten times the value for silicon (IEEE, Silicon carbide). ROHM gives 2.8 MV/cm for silicon carbide and 0.3 MV/cm, or roughly 30 kV/mm, for silicon (ROHM, High Power Devices tutorial). A 25 µm Kapton HN polyimide film reaches 303 kV/mm (Qnity, Kapton HN data sheet).
| Property | Diode junction | Insulating film | Air gap |
|---|---|---|---|
| Typical limit voltage | 2.4 V (Zener diode) to 1,200 V (module) | 7.7 kV for 25 µm Kapton HN | around 30 kV at 1 cm |
| Field strength at breakdown | SiC 220 to 280 kV/mm, Si around 30 kV/mm | 303 kV/mm | around 3 kV/mm |
| Mechanism | Zener tunnelling or avalanche | electrical, thermal, erosion | avalanche, streamer from 1 cm |
| After the event | recovers as long as power dissipation stays limited | permanent damage, carbonised channel | recovers completely |
| Datasheet parameter | V_BR, V_Z, V_RRM, V(BR)DSS | dielectric strength in kV/mm | minimum clearance in mm |
The last row is the practical difference. A Zener diode goes through breakdown millions of times, a film exactly once. How an insulating material breaks down and what voltage air holds at which gap is covered in the article on voltage breakdown. The material value in kV/mm is the subject of the article on electric strength, while air, oil and Paschen's law are covered under dielectric strength.
The Zener and avalanche effects share the breakdown range at around 5 V
The Zener effect needs a heavily doped junction. The depletion region is then so thin that electrons tunnel from the valence band into the conduction band without gaining energy. In silicon diodes the Zener voltage lies roughly between 2 and 5.5 V (Wikipedia, Zener-Effekt). The avalanche effect takes over in more lightly doped, wider junctions: an accelerated carrier knocks an electron out of the lattice through impact ionisation, both knock out more, and the current rises exponentially and much more steeply than in Zener breakdown. One rule follows, and some texts get it backwards: the higher the doping, the lower the breakdown voltage (fmh-studios, Durchbruchspannung).
Reference works disagree on where exactly the boundary lies: 5 V, 5.5 V, 4.5 to 5 V or 5 to 7 V. The datasheet gives a clearer answer, because the two effects move in opposite directions with temperature. The Zener effect lowers the breakdown voltage as the junction warms up, the avalanche effect raises it. The temperature coefficient of the BZX55 series therefore changes sign exactly where one effect hands over to the other (Vishay, BZX55 datasheet).
| Zener diode | Zener voltage at test current | Temperature coefficient | Dominant effect |
|---|---|---|---|
| BZX55C2V4 | 2.28 to 2.56 V | −0.09 to −0.06 %/K | Zener |
| BZX55C4V7 | 4.4 to 5.0 V | −0.05 to +0.02 %/K | both |
| BZX55C5V1 | 4.8 to 5.4 V | −0.02 to +0.02 %/K | both, almost cancelled out |
| BZX55C6V2 | 5.8 to 6.6 V | +0.03 to +0.06 %/K | avalanche |
| BZX55C12 | 11.4 to 12.7 V | +0.03 to +0.11 %/K | avalanche |
| BZX55C75 | 70 to 79 V | +0.04 to +0.12 %/K | avalanche |
Test current 5 mA, 2.5 mA for the 75 V type. In the circuit this means a 5.1 V diode works as a simple reference, because the two effects almost cancel. A 12 V diode can rise by 11 % in the worst case when it warms up by 100 K, which is about 1.3 V. If you need a stable voltage above 5 V, Wikipedia describes a 6.2 V Zener diode at plus 2 mV/K in series with a forward-biased silicon diode at minus 2 mV/K. Production tolerance comes on top: a BZX55C5V1 may sit anywhere between 4.8 and 5.4 V.
On the datasheet, breakdown voltage goes by five names
Searching a datasheet for "breakdown voltage" often finds nothing, because each device class uses its own symbols and test conditions. The table puts them side by side, each with a real datasheet value.
| Device | Symbol | Example value | Condition | Source |
|---|---|---|---|---|
| 1N4007 rectifier diode | V_RRM, repetitive peak reverse voltage | 1,000 V | reverse current max. 5 µA at 25 °C, 50 µA at 125 °C | Vishay |
| BZX55C5V1 Zener diode | V_Z | 4.8 to 5.4 V | at 5 mA test current | Vishay |
| SMBJ12A TVS diode | V_R, V_BR, V_C | 12 V, 13.3 to 14.7 V, 19.9 V | V_BR at 1 mA, V_C at peak pulse | Littelfuse |
| IRL540N MOSFET, TO-220 | V(BR)DSS | at least 100 V | V_GS 0 V, I_D 250 µA, plus 0.11 V/°C | Infineon |
| IGBT in the FS450R12OE4 module | V_CES, collector-emitter voltage | 1,200 V | at 25 °C junction temperature, the inverter diode is rated V_RRM 1,200 V | Infineon |
The table holds two ways of reading the number. For the rectifier diode, the MOSFET and the IGBT, the value is a limit: the manufacturer guarantees the device blocks up to that point. Where it actually breaks down is not stated, and you should not rely on it. For Zener and TVS diodes the value is an operating point with a tolerance, measured at a defined test current. The TVS diode shows the whole chain: it stays silent up to 12 V, conducts from 13.3 V and clamps a transient at 19.9 V. Bipolar transistors carry three limits at once, V_CEO typically between 20 and 700 V, while the base-emitter junction V_EBO already breaks down at around 5 V (Wikipedia, Breakdown voltage).
Temperature shifts both readings. The guaranteed maximum reverse current of the 1N4007 rises tenfold between 25 and 125 °C. The breakdown voltage of the IRL540N typically grows by 0.11 V per kelvin, so at a cold start at minus 40 °C it sits about 7 V below its room-temperature value.
When breakdown destroys a diode
Whether a diode survives breakdown depends on the power dissipation. The avalanche effect is reversible as long as the device's permitted total power dissipation is not exceeded (Wikipedia, Lawinendurchbruch). Semiconductor engineering distinguishes three types:
- Zener breakdown occurs in heavily doped junctions below about 5 V. The characteristic bends softly, and the diode recovers completely.
- Avalanche breakdown needs two things, the critical field strength and free electrons from the leakage current. If the source supplies more than about 1 mA, the avalanche sustains itself. At 100 µA it fires and dies out in turn, which causes the typical noise of avalanche diodes (Vishay, Technical Note 86133).
- Thermal breakdown, also called second breakdown, builds on itself: the reverse current heats the junction, and the heat raises the reverse current. Because no junction is perfectly uniform, it burns through at one spot (fmh-studios, Durchbruch der Diode).
For design work the device class is what counts. Zener and TVS diodes are built for breakdown, a rectifier diode is not. A 1N4007 must not exceed its permitted reverse voltage even for a few microseconds, because the current constricts into a thin channel and destroys the junction (Wikipedia, Diode). Power MOSFETs sit in between. The IRL540N is specified as avalanche rated and tolerates a single avalanche pulse of 310 mJ (Infineon). That is an energy budget for switch-off spikes on inductive loads. It does not cover continuous operation in breakdown.
Under the power semiconductor, insulation takes over
A power semiconductor usually sheds its heat through a metal surface into a heat sink, and in common packages such as the TO-220 that surface is electrically connected to the chip. If the heat sink sits at a different potential, an insulation goes in between, and its breakdown voltage has to lie far above the blocking voltage of the semiconductor. In the FS450R12OE4 IGBT module this is an aluminium oxide ceramic acting as basic insulation. The diodes block 1,200 V, the insulation has to hold 2.5 kV at 50 Hz for one minute. The creepage distance from terminal to heat sink is 18.5 mm, the clearance 16 mm, and the housing has a CTI above 200 (Infineon, FS450R12OE4_B81 datasheet).
For discrete transistors in TO-220 or TO-247 packages, an insulating washer does this job. It has to do two things at once: block kilovolts and conduct heat. Distributor data for three common versions show how differently materials handle that.
| Insulating washer | Thickness | Breakdown voltage | Heat transfer | Source |
|---|---|---|---|---|
| Polyimide with phase-change coating, TO-220 | 0.077 mm, carrier 0.05 mm | 7.8 kV | 0.15 K/W over 6.45 cm² | DAU Components, KAP 220 G |
| Kapton MT with acrylic adhesive | 0.078 mm, carrier 0.050 mm | at least 7,000 V rms to IEC 60243-1 | 0.46 W/(m·K), thermal class F | CMC, type 70791 |
| Silicone washer, TO-220 | 0.3 mm | 6 kV | 0.4 K/W, washer 18 x 13 mm | electrokit |
The MOSFET from the table above blocks 100 V, the polyimide washer underneath holds 7.8 kV, a factor of almost 80. Where space is tight and temperatures run high, we recommend polyimide: a quarter of the thickness of the silicone washer and more breakdown voltage. On thermal resistance the two are closer than the table suggests. The 0.15 K/W of the polyimide washer applies to 6.45 cm²; scaled to the area of an 18 x 13 mm washer it comes to about 0.4 K/W, the same as the silicone washer. Always read distributor figures like these together with area, thickness and test standard. Mica washers are listed at "5 kV/mm" in one place and "5.00 kV" in another for the same 0.05 mm part, and without those details neither figure is usable. Kapton MT is the thermally conductive grade made for semiconductor cooling, and the other grades are compared in the article on Kapton.
The most common objection from development engineers goes like this: the washer holds 7.8 kV, the DC link runs at 800 V, so there is a tenfold margin. That calculation ignores where insulating washers actually fail. The datasheet value comes from a flat sample between electrodes in a short-time test. In the device, the washer fails at the screw hole, at a burr on the punched edge, at a metal chip under the tab, or at the rim, where the path across the surface to the heat sink is shorter than the creepage distance IEC 60664-1 requires. Our recommendation is therefore to draw an overhang of the washer beyond the tab, to punch the hole burr-free and to release the insulation with the test voltage of the equipment standard, as listed in the article on dielectric withstand voltage.
Designing blocking voltage and insulation together
Semiconductor and insulating part are often chosen by different people, one looking at V_RRM, the other at kV/mm. This sequence brings the two together:
- Choose the blocking voltage of the semiconductor with a margin above the highest operating voltage, switching spikes included. For MOSFETs, allow for a cold start, because V(BR)DSS falls as the temperature drops.
- Read the reverse current at operating temperature. For the 1N4007 it grows tenfold between 25 and 125 °C.
- Take the insulation test voltage from the standard of the end product. If the heat sink is earthed or accessible, it also applies to the insulation between semiconductor and heat sink.
- Size clearances and creepage distances on the circuit board and at the heat sink to IEC 60664-1. Creepage depends on the CTI of the insulating material, and a material with a CTI of 600 or more manages with shorter paths than one below 175 (Würth Elektronik, ANP151). How the creepage path is measured is shown in the article on creepage distance.
- Draw the insulating part with material, thickness, overhang and hole pattern, and define the edge as an inspection characteristic.
For power electronics and inverters, for example in electromobility, we supply polyimide and polyester insulating films as washers and separators. In stamping and forming we process thicknesses from 0.023 to 3.0 mm, as rolls, sheets, cut blanks, die-cut parts or formed parts, from a single piece and within two weeks at most when the material is in stock. Send us your drawing, quantity and thickness, and for rolls also width, length and core diameter. Our technical consulting team will propose material and thickness to match the voltage of your semiconductor.
