Semiconductors and the PN junction: why current flows one way
Diodes and transistors all start from the PN junction. The principles behind forward voltage, reverse blocking and temperature behaviour.
Add impurities to silicon and you get N-type, with spare electrons, or P-type, short of electrons (many holes). The boundary where the two meet is a PN junction.
Forward and reverse
- Forward (+ on P, − on N): above about 0.6 to 0.7 V the current rises steeply. This is the forward voltage Vf of a silicon diode.
- Reverse: the depletion layer at the boundary widens and almost no current flows. Apply too large a reverse voltage and breakdown occurs.
Temperature behaviour
Vf falls by about 2 mV for every 1 °C rise in temperature. Because of this a diode can be used as a temperature sensor; on the other hand, with high-current diodes you have to watch for thermal runaway, where more current crowds in as the part gets hotter.
Connection to later steps
| Device | PN junctions |
|---|---|
| Diode | One junction |
| BJT | Two junctions (NPN, PNP) |
| MOSFET | The gate voltage forms a channel; the structure contains a body diode |
The body diode hidden inside a MOSFET is the path that carries the current during dead time in an inverter.