The current-voltage characteristic of an LED or ordinary diode can be decomposed into four regions (see Figure 1), two in the reverse bias region and two in the forward bias region. In the case of an applied voltage, the structural shape of the curve is determined by the transport properties of electrons and holes in the diode depletion region.
If the diode applied voltage is in the same direction as the built-in electric field, and the depletion layer acts as an insulator, its width will increase to prevent any significant current flow through the pn junction, at which point the diode is said to be reverse biased: that is, the p-terminal applied potential lower than the applied potential of the n-terminal. When the applied reverse bias voltage is large, exceeding the reverse breakdown voltage VB, the current will increase sharply, which will generally permanently damage the diode. This process is called reverse breakdown. The second region of reverse bias (V is negative and the absolute value is less than VB) has only a small reverse saturation current. However, this current is temperature dependent, and at sufficiently high temperatures, large reverse current values can be measured.
On the other hand, if the direction of the applied voltage is opposite to the polarity of the built-in electric field, the diode is called forward biased, that is, the potential of the p terminal is higher than that of the n terminal. In this case, the applied voltage (or electric field) exceeds the built-in voltage (or electric field), as shown in Figure 1, where V is greater than VD, and electron-hole recombination may occur, resulting in a large amount of current flowing through the p-n junction. In the third region, the forward voltage is small: V<VD, and only a small forward current flows. VD is defined as the threshold voltage or diode forward turn-on voltage. When the voltage increases to be greater than VD, it enters the fourth region. The diode current is measured in this area, and the diode is also said to be on. After the voltage is greater than VD, the current increases exponentially with the voltage. The magnitude of the salty voltage is related to the properties of the semiconductor material. For a silicon diode, the turn-on voltage is about 0.7V. Different diodes have different turn-on voltages.