What is a PN junction diode? Light-emitting diodes are relatively common in our lives, such as commercial traffic lights, traffic lights, LED screens, LED bulbs, and so on. Diodes are made of germanium or silicon semiconductor materials. The conductivity of semiconductor materials is between conductors and insulators at room temperature. This thing existed more than a hundred years ago, and it is the veteran of the semiconductor device family.
Light-emitting diodes are just one of them; there are many diodes for different purposes: rectifier diodes, Zener diodes, photodiodes, switching diodes, etc. Rectifier diodes are relatively common in our lives and are used in AC-to-DC circuits: mobile phone chargers, computer chargers, electric vehicle chargers, etc.
Unidirectional Conductivity of Diodes
The diodes mentioned above all have a typical performance, unidirectional conductivity; that is to say, the current can only go in from the anode of the diode and go out from the negative electrode, and vice versa. Why? There is something called a PN junction in a diode, which prevents the current from flowing backward. Next, let’s talk about PN knots to my friends.
What is a PN junction?
These substances are divided into conductors, semiconductors, and insulators according to their different electrical conductivity. The electrical conductivity of semiconductor materials is between conductors and insulators. Commonly used semiconductor materials are tetravalent silicon and germanium (zhě). What is tetravalent? That is, there are four electrons in the outermost shell. Pure semiconductors, also known as intrinsic semiconductors, have poor electrical conductivity and cannot be directly used to manufacture semiconductor devices. Trivalent elements (boron) are doped into the intrinsic semiconductor by diffusion process on one side, and pentavalent elements ( Phosphorus) replace the original small amount of silicon or germanium atoms.
The trivalent element (boron) has only three electrons in the outermost layer, but silicon and germanium have four electrons in the outer layer. What should I do if one is missing? Then a hole is formed, which is a P-type semiconductor. As a result, the P-type semiconductor becomes a semiconductor with a higher concentration of holes.
The pentavalent element (phosphorus) has five electrons; what if there is one more? The extra electron is almost unbound; it is free; it is called a free electron, an N-type semiconductor. Thus, the N-type semiconductor becomes a semiconductor with a higher concentration of electrons.
After combining the P-type semiconductor and the N-type semiconductor, the holes in the P region and the free electrons in the N region are mostly called multiples. The free electrons in the P region and the holes in the N region are almost zero, which is called the minority. There is a difference in the concentration of free electrons and holes. Since the hole concentration in the P region is higher than that in the N region, the spots diffuse to the N region. The free electron concentration in the N region is higher than that in the P region, and the free electrons diffuse to the P region, just like a drop of ink in clean water, the ink itself. When the concentration is high, it diffuses to the surroundings. It is the diffusion movement. The holes in the P region and the free electrons in the N region may meet and then recombine. What is recombination? Compare a fix to a house, and a home needs to live in it. At this time, free electrons compare to people, then combined into one.
The impurity ions in the P and N regions cannot move arbitrarily; why?
Because the impurity ions are bound by surrounding silicon or germanium atoms, near the interface of the P and N regions, a thin space charge region is formed; in this region, the many sons have diffused to each other and recombined or depleted. The impurity ions in the P region interact with the impurity ions in the N region; the impurity ions in the N region. And the impurity ions in the P region are negatively charged, forming an internal electric field in the space charge region. It also became more robust.
The internal electric field of the diode
On the one hand, this internal electric field prevents the diffusion movement, and the diffusion is not easy to proceed with; on the other hand, holes (minority carriers) drift from the N region to the P region, and free electrons drift from the P region to the N region. This drift is not Vehicle drift is caused by the influence of the high potential of the N region and the low potential of the P region to generate drift, which is called minority carrier drift.
Slowly the space charge region stabilizes. To sum up, the multi-sub-motion motion is called diffusion motion, and the minority-sub-motion motion is drift motion. When the two motions reach a dynamic balance, a PN junction generates. The PN junction with the corresponding electrical lead and the casing constitutes a semiconductor diode. The electrode drawn from the P region becomes the positive electrode, and the electrode illustrated from the N region becomes the negative electrode.
When the PN junction is added with a forward voltage, the P area pin is connected to the positive pole of the power supply, and the N area pin is connected to the negative bar of the power supply. The direction of current flows from the P region to the N region, and the internal electric field inside the PN junction is opposite. When the voltage is greater than the internal electric field voltage, the external power supply cancels the internal electric field.
The internal electric field is canceled, which is beneficial to the diffusion movement. The space charge region gradually becomes the P region and the N region. When the space charge region becomes thinner and thinner, it will form a diffusion current at this time. The diode is also turned on, and the voltage is called the turn-on voltage. On the contrary, connect the P region pin to the power supply’s negative pole and the plug of the N region to the lively bar of the power supply. At this time, the direction of the current flow is the same as the direction of the internal electric field, and the internal electric field is enhanced to widen the space charge region and pull the holes to the P region. In the direction of, the electrons will be drawn to the order of the N region, thus preventing the diffusion movement and forming a reverse leakage current. Since the wind is minimal, this is the cut-off state.
The reverse current will suddenly increase when the reverse voltage rises to a certain extent. If the external circuit cannot limit the current, the current will be so large that the PN junction will be burned, the voltage will become the breakdown voltage, and the diode will be useless at this time.
The effect of voltage
The forward bias voltage is applied to the diode, the dead zone OA area because the forward voltage is relatively small, the diode is not conducting, there is almost no current, and it is in a high resistance state. At this time, the voltage across the diode is the dead zone voltage, and the silicon diode is 0.5V (The germanium tube is 0.1v); when the forward voltage is higher than a specific value, the current in the diode increases as the voltage increases, and the diode is turned on. The voltage at this time is called the conduction voltage, also called the threshold Voltage.
The conduction voltage of the silicon tube is 0.6V (germanium tube is 0.2v), the voltage across the diode remains unchanged when it is turned on. The silicon tube is 0.7V (germanium tube is 0.3v), called forward voltage drop.
When electrons and holes recombine, they can emit visible light, and the light emitted by different compounds doped in the PN junction is also different, such as gallium (Ga), arsenic (As), phosphorus (P), nitrogen (N), etc. Then add the pins, encapsulate it with epoxy resin, turn on the forward voltage, and the light-emitting diode emits light like this.
Zener diodes take advantage of the reverse breakdown characteristics of diodes. Zener diodes are connected in series in the circuit. When the Zener diode is broken down, the voltage across the diode is stable even though the current varies in a wide range. Above and below the breakdown voltage.
When connecting diodes, pay attention to the positive and negative poles. Generally speaking, the long pin is the positive pole, and the short pin is the negative pole. Some diodes will have graphic symbols on the surface, which can also be measured with a multimeter. Adjust the multimeter to the diode gear and the red. The black test leads are respectively connected to the two ends of the diode. If the multimeter reading is less than one at this time, the red test lead is connected to the positive electrode of the diode, and the black test lead is connected to the negative electrode of the diode. If the reading is “1”, the end of the black test lead is positive.