What is a diode

The full English name of diode: diode

The diode has two terminals, the positive terminal is called the anode and the negative terminal is called the cathode. The current can only flow from the anode to the cathode. Some beginners tend to have such a wrong understanding: "a 'half' of a semiconductor is a 'half' of a half; a diode also has only 'half' current flowing (this is wrong), so all diodes are semiconductors." In fact, diodes and semiconductors are completely different things. We can only say that diodes are devices composed of semiconductors. Semiconductors can flow current in either direction.

Working principle of crystal diode

A crystal diode is a pn junction formed by a p-type semiconductor and an n-type semiconductor. Space charge layers are formed on both sides of the interface, and a self-built electric field is built. When there is no external voltage, the diffusion current caused by the difference in carrier concentration on both sides of the pn junction and the drift current caused by the self-built electric field are equal and are in an electrical equilibrium state. When there is a forward voltage bias from the outside, the mutual suppression of the external electric field and the self-built electric field increases the diffusion current of the carriers and causes a forward current. When there is a reverse voltage bias from the outside, the external electric field and the self-built electric field are further strengthened, forming a reverse saturation current I0 that is independent of the reverse bias voltage value within a certain reverse voltage range. When the external reverse voltage is high to a certain extent, the electric field strength in the space charge layer of the pn junction reaches a critical value, resulting in a carrier multiplication process, generating a large number of electron-hole pairs, and generating a large reverse breakdown current, which is called the breakdown phenomenon of the diode.

Types of diodes

Semiconductors can be divided into intrinsic semiconductors, P-type semiconductors, and N-type semiconductors.

Intrinsic semiconductor: Silicon and germanium are both semiconductors, and pure silicon and germanium (purity of 11 9s) crystals are called intrinsic semiconductors. Silicon and germanium are tetravalent elements, and their crystal structures are stable.

P-type semiconductor: P-type semiconductor is a crystal synthesized by mixing trivalent atoms, such as a very small amount (one in ten million) of indium, into a tetravalent intrinsic semiconductor. Since the trivalent atoms enter the tetravalent atoms, a part with one less electron is generated in the crystal structure. Since there is one less electron, it is positively charged. The "P" in P-type is taken from the first letter of the word "Positve".

N-type semiconductor: If a 5-valent atom, such as arsenic, is mixed into a 4-valent intrinsic semiconductor, a crystal with one extra electron will be produced, showing negative charge. The N is the first letter of "Negative".

Conductive properties of a diode

The most important characteristic of a diode is its unidirectional conductivity. In a circuit, current can only flow into the diode from the positive electrode and out of the negative electrode. The following simple experiment will illustrate the forward and reverse characteristics of a diode.

1. Forward characteristics.

In electronic circuits, if the positive pole of a diode is connected to the high potential end and the negative pole is connected to the low potential end, the diode will conduct. This connection method is called forward bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode still cannot conduct, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (this value is called the "threshold voltage", which is about 0.2V for germanium tubes and about 0.6V for silicon tubes) can the diode conduct directly. After conduction, the voltage across the diode remains basically unchanged (about 0.3V for germanium tubes and about 0.7V for silicon tubes), which is called the "forward voltage drop" of the diode.

2. Reverse characteristics.

In electronic circuits, the positive pole of a diode is connected to the low potential end, and the negative pole is connected to the high potential end. At this time, almost no current flows through the diode, and the diode is in a cut-off state. This connection method is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, which is called leakage current. When the reverse voltage across the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose its unidirectional conductive properties. This state is called diode breakdown.

The main parameters of the diode

The technical indicators used to indicate the performance and application range of diodes are called diode parameters. Different types of diodes have different characteristic parameters. For beginners, it is necessary to understand the following main parameters:

1. Rated forward operating current

Refers to the maximum forward current value allowed to pass through the diode when it is working continuously for a long time. Because when the current passes through the tube, the tube core will heat up and the temperature will rise. When the temperature exceeds the allowable limit (about 140 for silicon tubes and about 90 for germanium tubes), the tube core will overheat and be damaged. Therefore, do not exceed the rated forward working current value of the diode when using it. For example, the rated forward working current of the commonly used IN4001-4007 germanium diode is 1A.

2. Maximum reverse operating voltage

When the reverse voltage applied to both ends of the diode reaches a certain value, the diode will be broken down and lose its unidirectional conductivity. In order to ensure safe use, the maximum reverse working voltage value is specified. For example, the reverse withstand voltage of IN4001 diode is 50V, and the reverse withstand voltage of IN4007 is 1000V.

3. Reverse current

Reverse current refers to the reverse current flowing through the diode under the specified temperature and the maximum reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current is closely related to temperature. The reverse current doubles for every 10% increase in temperature. For example, for a 2AP1 germanium diode, if the reverse current is 250uA at 25°C, the reverse current will rise to 500uA when the temperature rises to 35°C. Similarly, at 75°C, its reverse current has reached 8mA, which not only loses its unidirectional conductivity, but also causes the tube to overheat and be damaged. For another example, a 2CP10 silicon diode has a reverse current of only 5uA at 25°C, and when the temperature rises to 75°C, the reverse current is only 160uA. Therefore, silicon diodes have better stability at high temperatures than germanium diodes.

Identification of diodes

The N pole (negative pole) of a low-power diode is mostly marked with a color circle on the surface of the diode. Some diodes also use special diode symbols to indicate the P pole (positive pole) or N pole (negative pole). There are also symbols marked as "P" and "N" to determine the polarity of the diode. The positive and negative poles of the light-emitting diode can be identified by the length of the pins, the long pin is positive and the short pin is negative. When using a digital multimeter to measure the diode, the red test pen is connected to the positive pole of the diode and the black test pen is connected to the negative pole of the diode. The resistance value measured at this time is the forward conduction resistance value of the diode, which is exactly the opposite of the test pen connection method of the pointer multimeter.

Application of diode

Semiconductor diodes are used in almost all electronic circuits. They play an important role in many circuits. They are one of the earliest semiconductor devices and are widely used.