Semiconductor Basics
Some basic concepts in semiconductor physics
Before explaining its working principle, some basic concepts in semiconductor physics are briefly introduced.
semiconductor
A semiconductor is a medium between a conductor and an insulator, which can exhibit conductive or non-conductive properties under different conditions. Most of the materials used in electronic semiconductor devices are tetravalent elements such as silicon and germanium that are at the junction of metal and non-metal in the periodic table of elements.
Intrinsic semiconductor
Intrinsic semiconductor (intrinsic semiconductor) refers to a pure semiconductor completely free of impurities. Because it does not contain impurities, the carriers in it are only generated by intrinsic excitation, and its conductivity is very poor. Corresponding to it are extrinsic semiconductors, which are divided into N-type semiconductors and P-type semiconductors according to different doping.
N-type semiconductor
N-type semiconductors refer to semiconductors in which +5 valence elements (such as P, Sb, etc.) are doped into the intrinsic semiconductor. Due to the addition of an element with 5 electrons in the outermost layer, an extra electron will be added after forming a covalent bond, and this electron becomes a free electron. Because of doping, the semiconductor has more carriers as free electrons, so it is called N-type semiconductor. In N-type semiconductors, electrons are the majority carriers.
P-type semiconductor
P-type semiconductors refer to semiconductors in which +3 valence elements (such as B, AI, etc.) are doped into the intrinsic semiconductor. Due to the addition of an element with 3 electrons in the outermost layer, there will be one hole less after the formation of a covalent bond, and the surrounding electrons will fill this hole. It looks like the "vacancy" is moving, and we call this "vacancy" a hole. Because the carrier holes exhibit positive electrical characteristics, this semiconductor is called a P-type semiconductor. In P-type semiconductors, holes are the majority carriers.
Diode overview:
二极管分类：发光二极管，光电二极管.....
Light-emitting diodes are relatively common in our lives, for example: commercial walking 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 the diodes, and there are many diodes for different purposes: rectifier diodes, Zener diodes, photodiodes, switching diodes, etc. Rectifier diodes are common in our lives and are used in AC-to-DC circuits: mobile phone chargers, computer chargers, electric vehicle chargers, etc.
pn junction
All diodes have unidirectional conductivity, and the current can only enter from the positive pole and exit from the negative pole, because of the unique PN junction inside the diode
Diffusion and Drift
After the P-type semiconductor and the N-type semiconductor are combined, the holes in the P region and the free electrons in the N region are mostly called the majority, and the free electrons in the P region and the holes in the N region are almost zero called the minority. There is a difference in the concentration of free electrons and holes.
Since the hole concentration in the P area is higher than that in the N area, the holes diffuse to the N area, and the free electron concentration in the N area is higher than that in the P area, and the free electrons diffuse to the P area, just like a drop of ink in clear water, the ink itself When the concentration is high, it diffuses to the surroundings. This 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 the hole to a house, and people should live in the house. At this time, the free electrons are likened to people, and then they are combined into one body.
The impurity ions in the P region and the N region cannot move freely, why? Because the impurity ions are bound by the surrounding silicon atoms or germanium atoms. Near the interface of the P and N regions, a very thin space charge region is formed, and in this region, many particles have diffused to each other and recombined, or exhausted.
The impurity ions in the P region and the N region interact with each other. The impurity ions in the N region are positively charged, and the impurity ions in the P region are negatively charged. An internal electric field is formed in the space charge region. The diffusion movement makes the space charge region widen, and the internal electric field Also became stronger.
On the one hand, this internal electric field prevents the diffusion movement, and the diffusion is not easy to go on; on the other hand, it makes the holes (minority carriers) drift from the N region to the P region, and the free electrons drift from the P region to the N region. This drift is not The car drift is caused by the high potential of the N region and the low potential of the P region, which is called the minority carrier drift.
Slowly the space charge region stabilizes. To sum up, the multi-substance movement is called the diffusion movement, and the minority-substance movement is the drift movement. When the two kinds of movement reach a dynamic balance, a PN junction is formed. A semiconductor diode is formed by adding the corresponding electrode lead and the shell to the PN junction. The electrode drawn from the P region becomes the positive electrode, and the electrode drawn from the N region becomes the negative electrode.
3. On and off
When the forward conduction voltage is added to the PN junction, the pins in the P area are connected to the positive pole of the power supply, and the pins in the N area are connected to the negative pole of the power supply. The current direction 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 offset, which is conducive 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, a diffusion current will be formed at this time. The diode is also turned on, and the voltage at this time is called the conduction voltage.
On the contrary, connect the pins of the P area to the negative pole of the power supply, and the pins of the N area to the positive pole of the power supply. At this time, the direction of the current flow is the same as that of the internal electric field, and the internal electric field is enhanced to widen the space charge area, and the holes will be pulled to the P area. The direction of the electrons will be pulled to the direction of the N region, thereby preventing the diffusion movement and forming a reverse leakage current. Since the current is very small, this is the cut-off state.
When the reverse voltage increases to a certain extent, the reverse current will suddenly increase. If the external circuit cannot limit the current, the current will be so large that the PN junction will be burned, and the voltage at this time will become the breakdown voltage, and the diode will be useless at this time.
Diode plus forward bias voltage, dead zone OA area, because the forward voltage is relatively small, the diode does not conduct, almost no current, and is in a high-impedance 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 certain value, the current in the diode increases with the increase of the voltage, and the diode is turned on. The voltage at this time is called the conduction voltage, also called the threshold Voltage.
The turn-on voltage of the silicon tube is 0.6V (0.2v for the germanium tube), and the voltage across the diode remains unchanged when it is turned on, and the silicon tube is 0.7V (0.3v for the germanium tube), which is called the forward voltage drop at this time.
When electrons and holes recombine, visible light can be radiated, 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) and so on. Then add pins, encapsulate them with epoxy resin, and pass forward voltage to the light-emitting diodes to emit light.
Zener diodes use the characteristics of reverse breakdown of diodes. Zener diodes are connected in series in the circuit. When Zener diodes are broken down, although the current changes in a large range, the voltage across the diodes is basically stable. above and below the breakdown voltage.
Pay attention to the positive and negative poles when connecting the diode. Generally speaking, the long pin is the positive pole and the short pin is the negative pole. Some diodes have graphic symbols on the surface, which can also be measured with a multimeter. The black test leads are respectively connected to both ends of the diode. If the reading of the multimeter is less than 1 at this time, the red test lead is connected to the positive pole of the diode, and the black test lead is connected to the negative pole of the diode. If the reading is "1", then one end of the black test lead is positive.
