Diode

A diode is a semiconductor device with the most basic structure. To understand diodes, it is necessary to understand P-type semiconductors and N-type semiconductors. A diode is a junction of these two semiconductors. This junction is called a "PN junction", hence the name "PN junction diode". Generally, a diode is a PN junction diode.

Fig.1 and Fig.2 It is the figure which represented the diode typically. The P-type semiconductor side is called the anode (symbol A) and the N-type semiconductor side is called the cathode (symbol K). There are two ways to use a diode, forward bias and reverse bias, depending on the direction of voltage application. (Pay attention to the orientation of the power supply in Figures 1 and 2)

Diode symbol The following symbols are used on the diode circuit diagram. Illustration of diode symbol Forward bias operation

Now, when a voltage is applied to the diode as in Fig. 3, holes (which carry positive charges) move from the P-type region to the N-type region (from left to right in Fig. 3), while electrons move from the N-type region. Move to the P-type area (from right to left in Figure 3). As a result, electrons and holes are combined at the PN junction and the charge disappears. Holes are continuously supplied from the positive electrode (P-type region), and electrons are similarly supplied from the negative electrode (N-type region), so current continues to flow.

FIG 3

Reverse bias operation

When a voltage is applied in the reverse direction of the forward bias as shown in FIG. 4, holes move to the positive electrode side (right to left in FIG. 4), electrons move to the negative electrode side, and holes and electrons move at the PN junction. No interaction occurs. As a result, almost no current flows.

Reverse Biased
Fig 4

Characteristics of diode

The "VI characteristic" that shows the relationship between the applied voltage V of a general diode and the flowing current is explained. This is a characteristic common to all diodes.

V-I Curve

Characteristic diagram of diode

Forward characteristic

The characteristics when a forward bias is applied to the diode.

When a voltage is applied to the diode in the forward direction, a current flows sharply when a voltage of 0.6 V or more is applied. Almost no current flows below 0.6V.

This voltage at which the current starts is called "forward voltage" and is indicated by the symbol VF in the data sheet. This VF depends on the diode and is about 0.5 to 1.5V. VF changes depending on the temperature and also the current flowing. For circuits where VF is a problem, check the characteristics with a data sheet.

Reverse direction characteristic 

Characteristics when reverse bias is applied to the diode. Almost no current flows when a voltage is applied to the diode in the opposite direction. The reverse current of a general diode is several nA to several μA. If the voltage in the opposite direction is further increased, the current rapidly flows at a certain voltage, and if a higher voltage is applied, the diode will be destroyed. This voltage is called the breakdown voltage and is indicated by the VR symbol. There is also a constant voltage diode (Zener diode) that positively applies this characteristic.