Take you to understand the characteristics of diodes and those parameters

The technical indicators used to indicate the performance and scope of the diode are called the parameters of the diode. Different types of diodes have different characteristic parameters.

Volt-ampere characteristics

A forward voltage is applied to the diode. When the voltage value is small, the current is extremely small; when the voltage exceeds 0.6V, the current starts to increase exponentially, which is usually called the diode's turn-on voltage; when the voltage reaches about 0.7V, The diode is in a fully conducting state, and this voltage is usually called the diode's conduction voltage, which is represented by the symbol UD. For germanium diodes, the turn-on voltage is 0.2V, and the turn-on voltage UD is about 0.3V. A reverse voltage is applied to the diode. When the voltage value is small, the current is extremely small, and its current value is the reverse saturation current IS. When the reverse voltage exceeds a certain value, the current begins to increase sharply, which is called reverse breakdown. This voltage is called the reverse breakdown voltage of the diode, which is represented by the symbol UBR. The breakdown voltage UBR values of different types of diodes vary greatly, ranging from tens of volts to several thousand volts.

Positive characteristics

When the forward voltage is applied, at the beginning of the forward characteristic, the forward voltage is very small, which is not enough to overcome the blocking effect of the electric field in the PN junction, and the forward current is almost zero. This section is called the dead zone. This forward voltage that cannot make the diode conduct is called the dead zone voltage.

When the forward voltage is greater than the dead zone voltage, the electric field in the PN junction is overcome, the diode is forwarded, and the current rises rapidly as the voltage increases. In the current range of normal use, the terminal voltage of the diode remains almost unchanged when it is turned on. This voltage is called the forward voltage of the diode.

When the forward voltage across the diode exceeds a certain value, the internal electric field is quickly weakened, the characteristic current increases rapidly, and the diode is forward-conducted. Called the threshold voltage or threshold voltage, the silicon tube is about 0.5V, and the germanium tube is about 0.1V. The forward voltage drop of a silicon diode is about 0.6~0.8V, and the forward voltage drop of a germanium diode is about 0.2~0.3V.

Reverse characteristics

When the applied reverse voltage does not exceed a certain range, the current through the diode is a reverse current formed by the drifting movement of minority carriers. Because the reverse current is very small, the diode is in an off state. This reverse current is also called reverse saturation current or leakage current. The reverse saturation current of the diode is greatly affected by temperature.  

Generally, the reverse current of a silicon tube is much smaller than that of a germanium tube. The reverse saturation current of a low-power silicon tube is on the order of nA, and that of a low-power germanium tube is on the order of μA. When the temperature rises, the semiconductor is excited by heat, the number of minority carriers increases, and the reverse saturation current also increases.

Breakdown characteristics

When the applied reverse voltage exceeds a certain value, the reverse current will suddenly increase. This phenomenon is called electrical breakdown. The critical voltage that causes electrical breakdown is called the diode reverse breakdown voltage. The diode loses unidirectional conductivity during electrical breakdown. If the diode does not overheat due to electrical breakdown, the unidirectional conductivity may not be permanently destroyed. After the voltage is removed, its performance can still be restored, otherwise the diode will be damaged. Therefore, avoid excessively high reverse voltage applied to the diode during use.  

Reverse breakdown is divided into Zener breakdown and avalanche breakdown according to the mechanism. In the case of high doping concentration, due to the small width of the barrier zone and the large reverse voltage, the covalent bond structure in the barrier zone is destroyed, and the valence electrons break away from the covalent bond, and electron-hole pairs are generated. , Resulting in a sharp increase in current, this breakdown is called Zener breakdown. If the doping concentration is low, the width of the barrier region is wider, and Zener breakdown is not easy to occur.

Another type of breakdown is avalanche breakdown. When the reverse voltage increases to a larger value, the external electric field accelerates the electron drift speed, which collides with the valence electrons in the covalent bond, knocking the valence electrons out of the covalent bond, and generating new electron-hole pairs. The newly generated electron-holes are accelerated by the electric field and knock out other valence electrons. The carriers increase in an avalanche manner, causing the current to increase sharply. This breakdown is called avalanche breakdown. No matter what kind of breakdown, if its current is not limited, it may cause permanent damage to the PN junction.

Reverse current

Reverse current refers to the reverse current flowing through the diode under normal temperature (25°C) and the highest reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current has a close relationship with temperature. About every 10°C increase in temperature, the reverse current doubles. For example, 2AP1 type germanium diode, if the reverse current is 250μA at 25℃, the temperature will rise to 35℃, the reverse current will rise to 500μA, and so on, at 75℃, its reverse current has reached 8mA, Not only loses the unidirectional conductivity, but also damages the tube due to overheating. For another example, the 2CP10 silicon diode has a reverse current of only 5μA at 25°C. When the temperature rises to 75°C, the reverse current is only 160μA. Therefore, silicon diodes have better stability at high temperatures than germanium diodes.

Dynamic resistance

The ratio of the change in voltage near the static operating point of the diode characteristic curve to the change in the corresponding current.

Voltage temperature coefficient

The voltage temperature coefficient refers to the relative change in the stable voltage for each degree of temperature increase.

Maximum operating frequency

The maximum operating frequency is the upper limit frequency of diode operation. Because the diode is the same as the PN junction, its junction capacitance is composed of barrier capacitance. So the value of the highest operating frequency mainly depends on the size of the PN junction capacitance. If it exceeds this value. Then the unidirectional conductivity will be affected.

Maximum rectified current

The maximum rectified current refers to the maximum forward average current value that the diode is allowed to pass through for a long time, and its value is related to the PN junction area and external heat dissipation conditions. Because the current passing through the tube will cause the die to heat up and the temperature rises. When the temperature exceeds the allowable limit (about 141°C for silicon tubes and about 90°C for germanium tubes), the die will overheat and be damaged. Therefore, under the specified heat dissipation conditions, do not exceed the maximum rectified current value of the diode during use.

Maximum reverse working voltage

When the reverse voltage applied to both ends of the diode reaches a certain value, the tube will break down and lose its unidirectional conductivity. In order to ensure safe use, the maximum reverse working voltage value is specified.