A Complete Guide to Diodes

Ⅰ. Basic knowledge of semiconductors

1. P-type semiconductor

A P-type semiconductor refers to an intrinsic semiconductor doped with boron (B) or indium (In). Silicon in group IV has four valence electrons and boron in group III has three valence electrons. If a small amount of boron is doped into a silicon single crystal, there will not be enough valence electrons at a certain position for the silicon and boron to bond, thus creating electron-deficient holes. When a voltage is applied in this state, adjacent electrons move into the holes, making a new hole where the electron was, and the holes appear to move sequentially to the “–” electrode. This hole is the carrier of the P-type semiconductor.

2. N-type semiconductor

N-type semiconductors refer to intrinsic semiconductors doped with phosphorus (P), arsenic (As) or antimony (Sb) as impurities. Silicon in group IV has four valence electrons, and phosphorus in group V has five valence electrons. If a small amount of phosphorus is added to a pure silicon crystal, one valence electron of phosphorus can move freely as a remaining electron (free electron). When this free electron is attracted to the "+" electrode and moves, current flow occurs. This free electron is the carrier of the N-type semiconductor.

3. PN junction

The contact surface between P-type and N-type semiconductors is called a PN junction. When P-type and N-type semiconductors are bonded, holes and free electrons as carriers attract each other, bind and disappear near the boundary. Since there are no carriers in this region, it is called the depletion layer. It is the same state as an insulator.

In this state, we connect the "+" pole to the P-type region and the "-" pole to the n-type region, and apply a voltage so that electrons flow sequentially from the N-type region to the P-type region. The electrons will first combine with the holes and disappear, but the excess electrons will move to the "+" pole, thus creating the current flow.

Ⅱ. Introduction to diodes

A diode is an electronic device with two terminals (yin and yang dipole terminals, hence the name "two poles") with asymmetric conductance. The two poles make it possible in principle to only allow current to conduct in one direction. It is low resistance (ideally zero), high current in one direction, and high resistance in the other direction. Today, diodes (and ideally zeros) are made of semiconducting materials.

With this feature, diodes are often used as rectifiers in power engineering (to convert alternating current into direct current). In electronic engineering, they are often used as detectors (from AM waves to detect echoes). In computer hardware logic design, they are often used as logic gates for logic circuits .

Diodes are mainly composed of P-type semiconductors and N-type semiconductors. A PN junction is formed when P-type and N-type semiconductors are joined together by a specific process. This is the core of the diodes.

① The characteristics of P-type semiconductors: Inside it, there are a large number of positively charged holes and lack of electrons.

② N-type semiconductor features: Inside it, there are a large number of free electrons and lack of positively charged holes.

Ⅲ. The development of diodes

Around the early 1990s, thermal electron (vacuum tube) diodes and solid state (semiconductor) diodes were used as radio receiving detectors. Before the 1950s, vacuum diodes were widely used in radios due to the instability of early point-contact semiconductor diodes. In addition, most receivers use electronic diodes as amplifiers, so they can easily accommodate thermal diodes. And in some cases, both vacuum-tube rectifiers and gas-filled rectifiers will suffice.

In 1874, German physicist Karl Braun discovered the rectifying ability of crystals at the Karlsruhe Institute of Technology.

In 1880, Thomas Edison observed a unidirectional current flow between heated and unheated elements in a light bulb, later known as the Edison effect, and received a patent for applying the phenomenon to a DC voltmeter.

After 20 years, John Ambrose Fleming (a scientific advisor to the Marconi Corporation and a former Edison employee) realized that the Edison effect could be used as a radio detector. Fleming patented the first true thermionic diode - the Fleming valve - in England on 16 November 1904. Throughout the vacuum tube era, valve diodes were used in virtually all electronic products such as radios, televisions, sound systems, and instrumentation.

In 1946, Sylvania began offering 1N34 crystal diodes. Junction diodes were developed in the early 1950's.

In 2022, humans will realize the superconducting diode effect without external magnetic field for the first time.

Ⅳ. The characteristics and main parameters of the diodes

1. Characteristics

(1) Forward characteristics: If the forward voltage is connected to the diode, the voltage value is very small at the beginning, and the electric field in the PN junction cannot be suppressed, so the current value is close to 0. This period is called is the dead zone voltage. After the voltage gradually increases, the voltage and current rise rapidly. During normal use, the diode is in a conduction state, and the voltage across the diode remains constant.

(2) Reverse characteristics: When the applied reverse voltage does not exceed a certain range, the current passing through the diode is the reverse current formed by the drift movement of minority carriers. Since the reverse current is very small, the diode is in the cut-off state. This reverse current is also called reverse saturation current or leakage current. The reverse saturation current of a diode is greatly affected by temperature.

Generally, the reverse current of silicon tube is much smaller than that of germanium tube. The reverse saturation current of low-power silicon tubes is on the order of nA, and that of low-power germanium tubes 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.

(3) Breakdown characteristics: When a reverse voltage is applied to the diode, the diode is in a cut-off state. When the reverse voltage reaches a certain value, the reverse current will suddenly increase, and a breakdown phenomenon will appear at this time, which is called electrical breakdown. The critical voltage that causes electrical breakdown is called the diode reverse breakdown voltage. During electrical breakdown, the diode loses its unidirectional conductivity. If the diode does not overheat due to electrical breakdown, the unidirectional conductivity is not necessarily permanently destroyed. After removing the applied voltage, its performance can still be restored, otherwise the diode will be damaged. Therefore, when using it, we should avoid excessive reverse voltage applied to the diode.

Reverse breakdown is divided into Zener breakdown and avalanche breakdown according to the mechanism. In the case of high doping concentration, because the width of the barrier region is small, when the reverse voltage is large, the covalent bond structure in the barrier region is destroyed, and the valence electrons are detached from the covalent bond to generate electron-hole pairs, resulting in a sharp increase in current, and this breakdown is called Zener breakdown. If the doping concentration is low and the width of the barrier region is wide, Zener breakdown is not easy to occur.

Another kind of breakdown is avalanche breakdown. When the reverse voltage increases to a larger value, the external electric field accelerates the electron drift speed, thereby colliding 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 avalanche of carriers increases, resulting in a sharp increase in current, and this breakdown is called avalanche breakdown. No matter what kind of breakdown, if the current is not limited, it may cause permanent damage to the PN junction.

(4) Volt-ampere characteristics: Diodes can realize unidirectional wires. If the forward voltage is connected, the voltage flowing through the diode is proportional to the current, and the voltage value is small, and the current value will be relatively small. But if the voltage value is greater than 0.6 VA, then the diode turns on the voltage. If the voltage value is greater than 0.7 volts, the diode will be in a conducting state.

2. Main parameters

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

(1) Reverse current

The 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. For every 10°C increase in temperature, the reverse current doubles.

For example, if the reverse current of a 2AP1 type germanium diode is 250μA at 25°C, and the temperature rises to 35°C, the reverse current will rise to 500μA. By analogy, at 75°C, its reverse current has reached 8mA. This not only loses the unidirectional conductive properties, but also causes the tube to overheat and be damaged. As another example, the reverse current of 2CP10 type silicon diode is 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.

(2) Rated forward working current

It refers to the forward current value allowed to pass through the diode when it works 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 the silicon tube and about 90 for the germanium tube), it will overheat the die and damage it. Therefore, the diode should not exceed the rated forward operating current value of the diode. For example, the commonly used IN4001-4007 type germanium diode has a rated forward operating current of 1A.

(3) Dynamic resistance

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

(4) 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 the safety of use, we stipulate the reverse working voltage value. For example, the reverse withstand voltage of IN4001 diode is 50V, and the reverse withstand voltage of IN4007 is 1000V.

(5) Voltage temperature coefficient

The voltage temperature coefficient refers to the relative change of the stable voltage when the temperature rises by one degree Celsius.

(6) Maximum working frequency

It refers to the upper frequency limit of diode operation. Because the diode is the same as the PN junction, its junction capacitance is composed of barrier capacitance. Therefore, 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.

Ⅴ. Detection methods of diodes

In semiconductor circuits, diodes often suffer from various problems. Let's introduce some common detection methods of diodes.

1. Multiple detection methods

(1) Oscilloscope: We can use an oscilloscope to detect the continuity of the diode. We first connect the probes of the oscilloscope to the two ports of the diode, and then observe the waveform on the oscilloscope. In the case of forward bias, you will see a voltage waveform. Whereas in the case of reverse bias, the waveform will be flat.

(2) Multimeter: We can use a digital or analog multimeter to detect the continuity of the diode. We select the multimeter to the diode test mode (usually the diode symbol or diode test position), and then connect the test probes to the two ports of the diode. If the diode is normal, the digital multimeter will show a small resistance value, and the pointer of the analog multimeter will have a clear deflection. If the diode is bad, the multimeter will display infinite resistance or the pointer will not deflect.

(3) Bulb Test: This is an easy way to test diodes with a battery and light bulb. We connect the positive terminal of the battery to the positive terminal of the diode, one port of the bulb to the negative terminal of the diode, and another wire to connect the other port of the bulb to the negative terminal of the battery. If the diodes are OK, the bulb should light up. Note that this method can only detect the conductivity of the diode, and cannot judge its forward and reverse directionality.

(4) Diode Tester: This is a small hand-held device specially designed for testing diodes. By inserting the diode pins into the slots of the tester, the tester will display the type of diode (NPN or PNP), whether it is normal and other parameters.

2. Detection methods of various commonly used diodes

(1) Detection of ordinary diodes

The forward resistance of low-power germanium diodes is 300Ω to 500Ω, and that of silicon diodes is 1kΩ or greater. The reverse resistance of the germanium diode is tens of kiloohms, and the reverse resistance of the silicon diode is above 500kΩ (the value of the high power is smaller).

According to the characteristics of the diode's small forward resistance and large reverse resistance, we can judge the polarity of the diode. We set the multimeter to the ohm gear (generally use Rx100 or Rx1k gear. Do not use Rx1 gear or Rx10k gear. Because Rx1 gear uses too much current, it is easy to burn the tube; and Rx10k gear uses too high voltage, which may break down the tube), and connect the two polarities of the diode with the test leads respectively, and measure the two resistance values. When the measured resistance value is small, the end connected to the black test lead is the anode of the diode. In the same way, when the measured resistance value is larger, the end connected to the black test lead is the negative pole of the diode. If the measured reverse resistance is very small, it means that the internal short circuit of the diode; if the forward resistance is very large, it means that the internal circuit of the tube is broken. In both cases the diode needs to be scrapped.

Silicon diodes generally have a forward voltage drop of 0.6V to 0.7V, and germanium diodes have a forward voltage drop of 0.1V to 0.3V. Therefore, measuring the forward conduction voltage of the diode can determine whether the diode under test is a silicon tube or a germanium tube. The method is to string a resistor (1kΩ) at one end of the dry battery, and at the same time connect it to the diode according to the polarity, so that the diode is forward-conducting. At this time, we use a multimeter to measure the tube voltage drop across the diode. If it is 0.6V～0.7V, it is a silicon tube; if it is 0.1V～0.3v, it is a germanium tube.

(2) Measurement of light-emitting diodes

A light emitting diode is a semiconductor device that converts electrical energy into light energy. When it passes a certain current, it will emit light. It has the characteristics of small size, low working voltage and small working current, and is widely used in audio equipment and instruments. Currently commonly used light-emitting diodes have three colors: red, green, and yellow.

①The inside of the light-emitting diode is a PN junction, which has unidirectional conductivity. Therefore, its detection method is similar to the measurement of general diodes.

② Put the multimeter in the Rx1k or Rx10k block, and measure the forward and reverse resistance values. Generally, the forward resistance is less than 50kΩ, and the reverse resistance is greater than 200kΩ.

③ The operating current of the light-emitting diode is an important parameter. If the working current is too small, the light-emitting diodes will not light up; if the working current is too large, the light-emitting diodes will be easily damaged.

④ The forward turn-on voltage of light-emitting diodes is 1.2V ~ 2.5V (except for bright LEDs), and the reverse breakdown voltage is about 5V. 

(3) Measurement of photodiode

A photodiode is a semiconductor device that converts changes in light intensity into electrical signals.

① There is a window at the top of the photodiode that can emit light. Light is irradiated on the tube core through the window, and under the excitation of light, a large number of photoelectric particles are generated in the photosensitive diode. At this time, its conductivity is greatly enhanced, which reduces the internal resistance.

② Photosensitive diodes are similar to Zener diodes. It is also working in the reverse state, and a reverse voltage must be applied.

③ The forward resistance of the photodiode is about several thousand ohms, which does not change with the change of light. When there is no light, its reverse resistance value is larger, and when it is illuminated, its reverse resistance value becomes smaller. The stronger the light, the smaller the reverse resistance. When the light condition is removed, the reverse resistance immediately returns to the original resistance value.

④ According to the above principle, we use a multimeter to measure the reverse resistance of the photodiode, change the intensity of light while measuring, and observe the change of the reverse resistance of the photodiode. If there is no change or little change in the reverse resistance when there is light and no light, it means that the tube has failed.

(4) Measurement of Zener diode

We generally use the low blocking of the multimeter to measure the Zener diode. Since the battery in the meter is 1.5V, this voltage is not enough to cause the reverse breakdown of the Zener diode, so we use a low barrier to measure the forward and reverse resistance of the Zener diode. Its resistance should be the same as a normal diode.

The measurement of the voltage regulation value Vz of the Zener diode. When measuring, we must make the tube enter the reverse breakdown zone, so the power supply voltage must be greater than the stable voltage of the tube under test. In this way, we must use the high block (Rx10k block) of the multimeter. At this time, the battery in the watch is a laminated battery with a higher voltage. When the multimeter range is set to high resistance, we measure its reverse resistance. If the measured resistance value is Rx, then the voltage stabilization value of the Zener diode is:

Vz=E0×Rx/(Rx+nR0)

In the formula, n-the magnification number of the gear used, such as the highest electrical resistance of the multimeter used is Rx10k, then n=10000.

R0- is the central resistance of the multimeter.

E0- is the highest battery voltage value of the multimeter used.

Example: Measure a 2CW14 with MF50 multimeter. Ro=10Ω．The highest resistance is Rx10k block, Eo=15V, and the measured reverse resistance is 75kΩ, then its voltage stabilization value is:

vz=(15×75×1000)/(75×1000+10000×10)=6.4V

If the measured resistance value is very large (close to infinity), it means that the voltage regulation value vz of the tube under test is greater than E0, and it cannot be broken down. If the measured resistance value is very small (0 or only a few ohms), it means that the test leads are reversed, as long as the test leads are exchanged.

Ⅵ. The working principle of diodes

A crystal diode is a P-N junction formed by a P-type semiconductor and an N-type semiconductor. A space charge layer is formed on both sides of its interface, and a self-built electric field is built. When there is no external voltage, because the diffusion current caused by the carrier concentration difference on both sides of the P-N junction is equal to the drift current caused by the self-built electric field, it is in a state of electrical balance.

When a forward voltage bias is generated, 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 (this is the reason for conduction).

When a reverse voltage bias is generated, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current I0 that has nothing to do with the reverse bias voltage value in a certain reverse voltage range (that is the reason for non-conduction).

When the applied reverse voltage is high enough, the electric field strength in the P-N junction space charge layer will reach a critical value, thereby generating a large number of electron-hole pairs, resulting in a large reverse breakdown current, which is breakdown of diodes.

Ⅶ. What is a diode used for?

Diodes can be used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulators, and oscillators. The fundamental property of a diode is its tendency to conduct electric current in only one direction.

Ⅷ. Does diode convert AC to DC?

The diode allows the current to pass only in one direction. If the diodes are used in AC it will conduct only during half of the cycle. Thus they are used in the conversion of AC into DC. Hence, diodes are DC.