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Sep 18 2019

Typical Diode Characteristics and It’s V-A Curve

Introduction

A diode is a two-terminal device with unidirectional conduction. There are electronic diodes and crystal diodes. Most common and commonly used are crystal diodes. The unidirectional conduction characteristics of diodes are used in almost all electronic circuits, and it plays an important role in many circuits. It is one of the earliest semiconductor devices, and its application can be seen in a very wide range.

Introducing the diode as a circuit element. A diode conducts current in one direction but not the other.


Conductive Property

  • Positive

When the forward voltage is applied, the forward voltage is small at the beginning (Germanium tube is less than 0.1 V, silicon tube is less than 0.5 V), which is insufficient to overcome the blocking effect of the electric field in the PN junction. At this time, the forward current is close to zero, and this segment is called the deadband. This forward voltage that does not turn the diode on is called the deadband voltage. When the forward voltage is greater than the deadband voltage, the electric field in the PN junction block is overcome, in other words, the diode is forward-conducting, and the current rises rapidly as the voltage increases. In the normal use, the terminal voltage of the diode keeps constant during turn-on, and it is also called the forward voltage of the diode.

diode

  • Negative

When the applied reverse voltage does not exceed a certain range, the current through the diode is the reverse current formed by the minority carrier drift motion. Since the reverse current is small, the diode is in an off state. This reverse current is also called reverse saturation current or leakage current. Diodes made with different materials have different reverse currents. The silicon tube is about 1 mA to tens of mA, and the germanium tubes can be as high as hundreds of mA. In addition, the reverse current is greatly affected by the temperature. The stability of germanium tubes is poor than that of the silicon tubes.

 

  • Breakdown

When the applied reverse voltage exceeds a certain value, the reverse current suddenly increases. This phenomenon is called electric breakdown. The threshold voltage that causes it called the diode reverse breakdown voltage. In addition, the diode loses unidirectional conductivity during electrical breakdown. If the diode does not have overheating due to electrical breakdown, the unidirectional conductivity may not be permanently destroyed. After the voltage recoveries normally, the diode can operate well, otherwise the diode is damaged. Therefore, the reverse voltage applied to the diode should not exceed the rated value marked on the technique parameters table.

Reverse Breakdown

1)Avalanche breakdown

As the PN junction reverse voltage increases, the electric field in the space charge region enhanced. That is, through the electrons and holes in the space charge region, the energy obtained by the electric field increases, and the electrons and holes moving in the crystal will continuously collide with the crystal atoms. When the energy of the electrons and holes is large enough, by such collision, the electrons in the covalent bond can be excited to form a free electron-hole pair. This phenomenon is called impact ionization. The newly generated electrons and holes move in the opposite direction under the action of the electric field, regain energy, and can generate new electron-hole pairs by collision again. This is the multiplication effect of the current carrier.  When the reverse voltage increases to a certain value, the multiplication of carriers is like an avalanche on a steep snowy slope. The carriers increase much faster and faster, causing the reverse current to increase sharply, so avalanche breakdown occurs at the end.

Avalanche breakdown occurs mostly in diodes with low impurity concentrations, and a relatively high voltage is required, in addition, the breakdown voltage is inversely proportional to the concentration.

  

2)Zener breakdown

When a higher reverse voltage is applied, there is a strong electric field in the space charge region of the PN junction, which can destroy the covalent bond to separate the trapped electrons and cause electron-hole pairs to form a large reverse current. The electric field strength required for Zener breakdown is about 2*105V/cm, which can only be achieved in a PN junction with a particularly large impurity concentration. Because of the large impurity concentration, the charge density (ie, impurity ions) in the space charge region is also large. Therefore, the space charge region becomes narrow and the electric field strength can be high. So that Zener breakdown mostly occurs in diodes with higher impurity concentrations. If the doping concentration is low, and the barrier region is wide, the Zener breakdown will rarely happen.

The current directionality of most diodes is often referred to as “Rectifying". In a diode, current is only allowed to pass in a single direction (called forward bias) and cut off reversely (called reverse bias). Therefore, the diode can be thought of as an electronic check valve. In reality, however, diodes do not exhibit such perfect on-off directionality, but rather complex nonlinear electronic features, which are determined by the specific type of diodes.

The voltage and current of the diode are not linear, so the resistors should be connected when the different diodes are connected in parallel.

 

Frequency Characteristic

Due to the existence of junction capacitance, when the frequency is high to some extent, the capacitive reactance is so small to make PN junction short circuit. In this case, The diode will lose its unidirectional conductivity and cannot work. In addition, the larger the PN junction area, the larger the junction capacitance, and the more unable to work at high frequency.

 forward biased diode

Regional Working Characteristics

1) Forward workspace

The diode is forward-conducting, and the conduction current is determined by the external current, and the maximum current does not exceed the maximum forward operating current of the diode, and the forward voltage drop gradually increases with the current, but the variation is not large.

2) Deadband

The diode is in a positive bias state, and its forward bias voltage is smaller than its turn-on voltage, therefore, the diode cannot be turned on, and the forward current is zero.

3) Reverse workspace

When the diode is in the reverse working state, its reverse current is small. Generally, the silicon tube is several uA to tens of uA, and the diode is not conducted. Together with the forward workspace, this workspace reflects the unidirectional conductivity of the diode which can be used for rectification and other occasions.

4) Reverse breakdown zone

The diode is also in the reverse working state, but the reverse voltage is large. Although the reverse operating current of the diode increase rapidly, the reverse operating voltage remains substantially unchanged. This characteristic can be used for the Zener diode.

 

V-A Curve (Volt-ampere Characteristic Curve)

A metal conductor, when the temperature does not change significantly, its resistance is constant, thus its volt-ampere characteristic curve is a straight line passing through the origin of the coordinate. The electrical component having such volt-ampere characteristics is called a linear element, because their temperature can determine the resistance values.

Ohm’s law is an experimental law with metal conductors. Whether this conclusion is applicable to other conductors still requires experimental testing. Experiments have shown that, except metals, Ohm’s law is also applicable to electrolyte solutions, but not to gaseous conductors (such as fluorescent tubes, gases in neon tubes) and semiconductor components. That is to say, in these cases the current is not proportional to the voltage, and such electrical components are called nonlinear components.

The relationship between the voltage applied on the PN junction and the current flowing through the diode is called the volt-ampere characteristic curve, as the picture shows:

V-A curve diagram 

  • u > 0 is called positive property

  • u < 0 is called the reverse property

Note: The tube voltage drop of the diode: the silicon diode (non-illuminated type) has a forward voltage drop of 0.7V, and the neon tube is 0.3V. The forward tube voltage drop of LEDs vary with different illuminating colors. There are three reference values of main colors are as follows: the voltage drop of the red LED is 2.0-2.2V, the yellow LED is 1.8-2.0V, and the green LED is 3.0-3.2V. The rated current when emitting light is approximately 20 mA.

 diode symbol

  • Reverse breakdown

When the reverse voltage exceeds a certain value U(BR), the reverse current increases sharply, which is called reverse breakdown.

  • Barrier Capacitance

The capacitor equivalent to the width variation of the depletion layer is called the barrier capacitance Cb.

  • Varactor diode

When the PN junction is applied with a reverse voltage, Cb changes significantly with the change of u. According to this, various varactor diodes can be made.

  • Minority carrier

The minority of the PN junction in a stable state is called minority carrier.

  • Non-equilibrium minority carrier

When the PN junction is in a forward bias, the holes that diffuse from the P region to the N region and the free electrons that diffuse from the N region to the P region are called non-equilibrium minority carriers.

  • Diffusion Capacitance

The accumulation and release process of charge in the diffusion region is the same as the charging and discharging process of the capacitor. This effect is called diffusion capacitance.

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1 comment

    • Joey Chan on 2019-10-16 17:35:32

    You can add that a semiconductor diode is a nonlinear device that exhibits different equivalent resistances for DC and AC. The DC resistance of a diode is the ratio of the terminal voltage to its current when it operates at a point in the volt-ampere characteristic.

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