1 basic concepts of diode
The diode is composed of a tube core, a tube shell and two electrodes. The tube core is a PN junction, the junction ends each leads to a lead, and made of plastic, glass or metal material as a package, in order to form a crystal diode, as shown below. The lead out of the P region is called a positive electrode or an anode, and the lead out of the N region is called a negative electrode or a cathode.

1.1 volt ampere characteristic of diode
The volt ampere characteristic of diode is the relationship between the voltage of two ends of the diode and the current flowing through the diode. It is used to describe the curve of the relationship between the two curves called the volt ampere characteristic curve. The volt ampere characteristics of the silicon diode are shown in the following figure by the process of the transistor.

1.2 positive features
1) the positive voltage is smaller, the resistance of the diode is larger, the forward current is almost zero, and the curve OA segment is called the non conducting zone or the dead zone. As usual, the voltage of the silicon tube is about 0.5 volts, and the dead zone voltage is about 0.2 volts, which is called the threshold voltage or the threshold voltage.
2) when the forward voltage across the dead voltage, the electric field within the PN junction was nearly cancellation, the resistor diode forward current is very small, start growth, into the forward conduction zone, but the voltage and current are not comparable for AB. With the voltage increase of forward current rapid growth, such as BC characteristic curve steep, similar to the linear relationship is sufficient voltage, conduction condition.
(3) the forward voltage at two ends of the diode is called forward voltage drop (or tube pressure drop), and almost constant. The pressure drop of silicon tube is about 0.7V, and the pressure drop of germanium tube is about 0.3V.
1.3 reverse features
1) suffer diode reverse voltage, enhance the electric field in the PN junction diode, great resistance, while only very small reverse current. If the curve OD segment is called the reverse stop area, the current is called the reverse saturation current. In reality, the reverse current is used to illustrate that the larger the reverse resistance of diode, the better the reverse stop function. The reverse common silicon diode saturation current are as follows in the tens of hundreds of a germanium diode is a high power diode is slightly larger.
(2) when the reverse voltage increases to a certain value (Fig. D), the reverse current increases sharply and enters the reverse breakdown region. The voltage of the D point is called the reverse breakdown voltage. Diode breakdown after the current will be destroyed by the tube, in addition to the voltage regulator tube, the diode reverse voltage can not cross the breakdown voltage.

2 diode rectifier circuit
2.1 one way half wave rectifier circuit
As an active switch diode, U2 positive half cycle, the active power supply and load switch, U2 negative half cycle, the initiative will cut off the power supply and the load. So, by the following figure shows, load bias stability, size change obtain pulsating DC voltage uo as shown below. Because the circuit input in the positive half week to U2, is called half wave rectifier circuit. If the polarity on the rectifier diode, can obtain pulsating DC voltage of negative polarity.