P type semiconductor and N type semiconductor are fabricated on the same semiconductor (usually silicon or germanium) substrate by epitaxy, dopant diffusion or ion implantation, etc. The boundary or interface between the two semiconductor materials is called PN junction[1]。The PN junction has a single conductivity (also known as rectification characteristics), and the semiconductor devices formed thereby also become diodes or rectifiers.PN junction is one of the basic components of various semiconductor devices.
For silicon crystal (or germanium crystal), pentavalent elements such as phosphorus and arsenic are called donor elements (donor impurities), and trivalent elements such as boron and indium are called acceptor elements (acceptor impurities).
N-type semiconductor(N is the first letter of Negative, so it is called because electrons are negatively charged): In semiconductor materials doped with donor elements (donor impurities), the number of conductive electrons is far more than the number of holes, which is called N-type semiconductor.In silicon crystal (or germanium crystal) doped with a small amount of phosphorus (or antimony), as semiconductor atoms (such as silicon atoms) are replaced by impurity atoms, four of the five outer electrons in the outer layer of phosphorus atoms form covalent bonds with the surrounding semiconductor atoms, and the extra electron is almost unbound, which is easier to becomefree electron。
P-type semiconductor(P is the first letter of Positive, so it is called because the hole is positively charged): In semiconductor materials doped with acceptor elements (acceptor impurities), the number of holes is far more than the number of conductive electrons, which is called P-type semiconductor.Silicon crystal (or germanium crystal) doped with a small amount of impurity boron (or indium) will produce a“hole”This hole may attract bound electrons to "fill", making boron atoms become negatively charged ions.In this way, such semiconductors become conductive materials due to their high concentration of "holes" ("equivalent to" positive charges).
Formation of PN junction
PN junction is composed of an N-type doped area and a P-type doped area in close contact, and its contact interface is called metallurgical junction interface.[1]
On a complete silicon chip, different doping processes are used to form N-type semiconductor on one side and P-type semiconductor on the other side. We call the area near the interface of the two semiconductors PN junction.
After the P-type semiconductor is combined with the N-type semiconductor, because the free electron in the N-type region is a multi carrier, and the hole is almost zero, it is called a minority carrier, while the hole in the P-type region is a multi carrier, and the free electron is a minority carrier, there is a concentration difference between the electron and the hole at their junction.Due to the difference in the concentration of free electrons and holes, many electrons in the N region diffuse to the P region, and many holes in the P region diffuse to the N region.As a result of their diffusion, holes are lost on the P side, leaving negatively charged impurity ions, while electrons are lost on the N side, leaving positively charged impurity ions.The ions in the semiconductor in the open circuit cannot move freely, so they do not participate in conduction.These immobile charged particles form a space charge region near the interface of P and N regions, also known as the depletion region. The thickness of the space charge region is related to the dopant concentration.
After the space charge area is formed, due to the interaction between positive and negative charges, a built-in electric field is formed in the space charge area, whose direction is from the positively charged N area to the negatively charged P area.Obviously, the direction of this electric field is opposite to the direction of carrier diffusion movement, which will prevent the multi carrier diffusion.
On the other hand, the built-in electric field will make the minority carrier holes in the N region drift to the P region, and the minority carrier electrons in the P region drift to the N region. The direction of the drift movement is just opposite to the direction of the diffusion movement.The holes drifting from N area to P area supplement the holes lost in P area on the original interface, and the electrons drifting from P area to N area supplement the electrons lost in N area on the original interface, which reduces the space charge and the internal electric field.Therefore, the result of drift movement is to narrow the space charge area and strengthen the diffusion movement.Finally, the diffusion of many carriers and the drift of few carriers reach dynamic equilibrium.On both sides of the junction surface of P-type semiconductor and N-type semiconductor, a thin layer of ions is left. The space charge area formed by this thin layer of ions is called PN junction.The internal electric field direction of PN junction is from N region to P region.
characteristic
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Feature Overview
It can be seen from the formation principle of the PN junction that the resistance of the internal electric field in the space charge area must be eliminated in order to make the PN junction conduct and form current.Therefore, a larger electric field opposite to the direction of the built-in electric field is applied to it, that is, area P is connected to the positive pole of the external power supply, and area N is connected to the negative pole, which can offset its built-in electric field, so that the carrier can continue to move, thus forming a linear forward current.The external reverse voltage is equivalent to enhancing the resistance of the built-in electric field to the multi carrier diffusion current. The PN junction cannot be connected, and only a very weak reverse current (formed by the drift movement of a few carriers, due to the limited number of minority carriers, the current is saturated).When the reverse voltage increases to a certain value, as the number and energy of minority carriers increase, the internal covalent bond will be destroyed by collision, so that the bound electrons and holes will be released, and the reverse current will sharply increase, which is called PN junction breakdown.When breakdown occurs, if the reverse current flowing through the PN junction is limited by external electricity, the PN junction can still work safely in the breakdown state, such as the voltage regulator.
PN node business card diagram
This is the characteristics of PN junction (unidirectional conduction, reverse saturation leakage or breakdown conductor), and is also the most basic and important physical principle of transistors and integrated circuits. It is indispensable for the analysis of all complex circuits based on transistors.For example, the diode works based on the one-way conduction principle of the PN junction;A PNP structure can form a triode, which contains two PN junctions.Diodes and triodes are the most basic components in electronic circuits.
Reverse breakdown
When the reverse voltage is applied to the PN junction, the space charge region becomes wider and the electric field in the region increases.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 large enough to burn the PN junction.The voltage when reverse current suddenly increases is called breakdown voltage.There are two basic breakdown mechanisms, namely tunnel breakdown (also called Zener breakdown) and avalanche breakdown. The former has a negative temperature coefficient when the breakdown voltage is less than 6V, and the latter has a positive temperature coefficient when the breakdown voltage is greater than 6V.
Avalanche breakdownWhen the carrier drift speed in the barrier layer increases to a certain extent with the increase of the internal electric field, its kinetic energy is enough to collide the valence electrons bound in the covalent bond, producing free electron hole pairs. Under the strong electric field, the newly generated carriers will collide with other neutral atoms, producing new free electron hole pairs, such a chain reaction,The number of carriers in the barrier layer increases sharply, like an avalanche.Avalanche breakdown occurs in the PN junction with low doping concentration. The barrier layer is wide, and there are more opportunities for collision ionization. The breakdown voltage of avalanche breakdown is high.[2]
Zener breakdownZener breakdown usually occurs in PN junction with high doping concentration.Because of the high doping concentration and narrow PN junction, the electric field in the junction layer is strong (up to 2.5 × 10) even if a small reverse voltage (below 5V) is appliedfiveV/m).Under the action of strong electric field, the valence electrons of atoms in the PN junction will be forced to pull out from the covalent bond, forming an "electron hole pair", thus generating a large number of carriers.Under the action of the reverse voltage, they form a large reverse current and appear breakdown.Obviously, the physical essence of Zener breakdown is field ionization.The avalanche breakdown voltage of silicon PN junction can be controlled within 8~1000V by adopting appropriate doping technology.The Zener breakdown voltage is lower than 5V.Two kinds of breakdown may occur at the same time between 5 and 8V.[3]
Thermoelectric breakdown: When the reverse voltage is applied to the pn junction, the reverse current flowing through the pn junction will cause heat loss.When the reverse voltage gradually increases, the power loss for a certain reverse current also increases, which will generate a lot of heat.If there is no good heat dissipation condition to transfer these heat energy in time, the junction temperature will rise.This breakdown caused by thermal instability is called thermoelectric breakdown.
Temperature characteristics of breakdown voltage: When the temperature rises, the lattice vibration intensifies, which shortens the average free path of carrier movement and reduces the kinetic energy before collision. The avalanche breakdown must increase the reverse voltage in order to have a positive temperature coefficient, but when the temperature rises, the valence electron energy state in the covalent bond is high, so the zener breakdown voltage decreases with the temperature rise, and has a negative temperature coefficient.
Unidirectional conductivity
The PN junction is connected when the forward voltage is applied
If the positive pole of the power supply is connected to the P area and the negative pole is connected to the N area, part of the applied forward voltage falls on the PN junction area, and the PN junction is in the forward bias.The current flows from the P side to the N side, and the holes and electrons move towards the interface, narrowing the space charge area, allowing the current to pass smoothly in the opposite direction to the electric field in the PN junction, weakening the internal electric field.As a result, the internal electric field weakens the barrier to the diffusion movement of many particles, and the diffusion current increases.The diffusion current is far greater than the drift current, so the influence of drift current can be ignored, and the PN junction shows low resistance.This process of unbalanced carrier entering into semiconductor due to the effect of external forward bias is called the electric injection of unbalanced carrier.[2]
Cut off when reverse voltage is applied to PN junction
(2) PN junctionApply reverse voltageBy
If the positive pole of the power supply is connected to the N area and the negative pole is connected to the P area, part of the applied reverse voltage falls on the PN junction area, and the PN junction is in reverse bias.The holes and electrons move away from the interface, which widens the space charge area and prevents the current from flowing. The direction is the same as that of the electric field in the PN junction, strengthening the internal electric field.The internal electric field increases the resistance to the diffusion movement of many carriers, and the diffusion current decreases greatly.At this time, the drift current formed by minority carriers in the PN junction area under the internal electric field is greater than the diffusion current, and the diffusion current can be ignoredHigh resistance。
Under certain temperature conditions, the minority carrier concentration determined by the intrinsic excitation is certain, so the drift current formed by minority carriers is constant and basically independent of the applied reverse voltage. This current is also called reverse saturation current.
When the forward voltage is applied to the PN junction, the resistance is low and the forward diffusion current is large;When the reverse voltage is applied to the PN junction, the resistance is high and the reverse drift current is small.It can be concluded that PN junction has single conductivity.
Volt ampere characteristic
PN junction volt ampere characteristic curve
The volt ampere characteristic (external characteristic) of the PN junction is shown in the figure, which visually represents the unidirectional conductivity of the PN junction.
The expression of the volt ampere characteristic is[4]：
WhereiDIs the current passing through the PN junction,vDIs the applied voltage at both ends of the PN junction,VTIs the voltage equivalent of temperature,
Where k is the Boltzmann constant (1.38 × 10-23J/K），Where k is the Boltzmann constant (1.38 × 10-23J/K), T is the thermodynamic temperature, that is, the absolute temperature (300K), q is the electronic charge (1.6 × 10-19C）。At room temperature, VT ≈ 26mV. Is is the reverse saturation current. For discrete devices, the typical value is 10-8~10-14A.The Is value of diode PN junction in integrated circuit is smaller.
WhenvD>>0, andvD>VTWhen,
；
WhenvD<0, and
When,iD≈–IS≈0。
Capacitance characteristic
PN junction additionReverse voltageThe positive and negative charges in the space charge area form a capacitive device.Its capacitance changes with the applied voltage, mainly including barrier capacitance (CT）And diffusion capacitance (CD）。Both barrier capacitance and diffusion capacitance are nonlinear.Barrier capacitance: The barrier capacitance is formed by the thin layer of ions in the space charge area.When the applied voltage changes the voltage drop on the PN junction, the thickness of the ion thin layer changes accordingly, which is equivalent to the change of the amount of charge stored in the PN junction.The barrier area is similar to a flat plate capacitor, and the two sides of its junction store ionic charges with equal values and opposite polarity. The amount of charge varies with the applied voltage, which is called barrier capacitanceTIndicates that the value is:
。The junction resistance is large when the PN junction is reversed,CTIts role cannot be ignored, especially at high frequencies, it has a greater impact on the circuit.CTIt is not a constant value, but varies with V. This characteristic can be used to make varactor diodes.
The PN junction has abrupt junction and slow junction. Considering the situation of abrupt junction, the PN junction is equivalent to a flat plate capacitor. Although the applied electric field will widen or narrow the barrier area, the change is relatively small and can be ignored
, the width L of the barrier layer under known dynamic balancezero, which can be obtained by substituting the above formula:
。
Schematic diagram of diffusion capacitance
Diffusion capacitance[2]When the PN junction is conducting electricity positively, after the multi carrier diffuses to the opposite region, it accumulates on the boundary of the PN junction and has a certain concentration distribution.The accumulated charge varies with the applied voltage. When the forward voltage of the PN junction increases, the forward current increases, which requires more carriers to accumulate to meet the requirements of the current increase;When the forward voltage decreases, the forward current decreases, and the electrons accumulated in the P region or the holes accumulated in the N region will be relatively reduced. In this way, when the applied voltage changes, there will be carriers "charging" and "discharging" to the PN junction.Diffusion capacitance C of PN junctionDThe capacitance effect of the electrons accumulated in the P region or the holes accumulated in the N region as a function of the applied voltage is described.
Because of the positive bias of the PN junction, the electrons diffused from the N region to the P region are combined with the holes provided by the external power supply to form a forward current.The newly diffused electrons accumulate near the PN junction in the P region, forming a certain multi carrier concentration gradient distribution curve.On the contrary, the holes diffused from P zone to N zone also form a similar concentration gradient distribution curve in N zone.The schematic diagram of diffusion capacitance is shown on the right.
CDIs nonlinear capacitance, when the PN junction is positively biased, CDThe number of carriers is small when reverse bias occurs, so the diffusion capacitance is small when reverse bias occurs.Generally, it can be ignored.
PN junction capacitance: Total capacitance of PN junction CjIs CTAnd CDSum of the two Cj= CT+CD, C when applying forward voltageDBig, CjMainly diffusion capacitance (tens of pF to thousands of pF), C when reverse voltage is appliedDNear zero, CjMainly based on barrier capacitance (several pF to dozens of pF).
application
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According to the different materials, doping distribution, geometric structure and bias conditions of PN junction, its basic characteristics can be used to manufacture a variety of functional crystal diodes.For example, the single conductivity of PN junction can be used to make rectifier diode, detector diode and switch diode, and the breakdown characteristics can be used to make voltage regulator diode and avalanche diode;Tunneling diode is fabricated by using highly doped PN junction tunneling effect;Varactor diode is made by the effect of junction capacitance changing with external voltage.Combining the photoelectric effect of semiconductor with PN junction can also make a variety of photoelectric devices.For example, semiconductor laser diodes and semiconductor light-emitting diodes can be manufactured by carrier injection and recombination of forward biased heterostructures;Photodetector can be made by modulating the reverse current of PN junction by light radiation;Solar cells can be made by using the photovoltaic effect.In addition, the interaction between the two PN junctions can generate a variety of electronic functions such as amplification and oscillation.PN junction is the core of bipolar transistor and field effect transistor, and is the basis of modern electronic technology.It is widely used in secondary pipes.
Zener diode
Volt ampere characteristic of zener diode
Once the PN junction is broken down, although the reverse current changes sharply, its terminal voltage is almost unchanged (approximately VBRAs long as its reverse current is limited, the PN junction will not burn out. This feature can be used to make a regulator diode. Its circuit symbol and volt ampere characteristics are shown in the figure above: its main parameters are: VZ、 Izmin、 Iz、 Izmax。
Varactor diode
When the PN junction is reverse biased, the reverse current is very small, approximately open circuit, so it is an ideal capacitive device mainly composed of barrier capacitance, and its incremental capacitance value changes with the applied voltage. Using this characteristic, a varactor diode can be made. Varactors are widely used in nonlinear circuits, such as voltage controlled oscillators, frequency modulation, etc.[6]
developing process
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After 1935, a group of scientists in Bell Laboratories turned to study Si materials. In 1940, polycrystalline Si rods were drawn by vacuum melting and mastered the technology of manufacturing P and N type polycrystalline Si by doping III and V impurities.The first Si PN junction was also fabricated by doping in the growth process, and the segregation of impurity elements in Si and the compensation of donor and acceptor impurities were found.
In 1948, William Shockley's paper "Theory of P-N Junction and P-N Junction Transistors in Semiconductors" was published in the internal journal of Bell Laboratories.Shockley's "Electrons and Holes in Semiconductors" published in 1950[5]The principle of junction transistor, which is different from the point contact transistor jointly invented by John Badin and Walter Bratton, is discussed in detail in.
manufacturing process
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PN junction is the basis of various semiconductor devices.Methods for manufacturing PN junction include:
(4) Ion implantation method: between abrupt junction and slow junction;
Epitaxial growth is usually used to manufacture heterostructures.
Breakdown mechanism
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PN structure has become the basis of almost all semiconductor power devices. The reverse blocking ability of common semiconductor power devices such as DMOS, IGBT, SCR, etc. directly depends on the breakdown voltage of PN junction. Therefore, the reverse blocking characteristics of PN junction directly determine the reliability and application range of semiconductor power devices.Under the condition that the doping concentration on both sides of the PN junction is a fixed value, it is generally believed that the breakdown voltage of the parallel plane junction except the super junction has the highest breakdown voltage in all plane junctions.In the actual manufacturing process of power semiconductor devices, spherical or cylindrical boundaries are generally introduced at the edge of PN junction. The breakdown voltage at this boundary is lower than that of parallel plane junction, which reduces the breakdown voltage of power semiconductor devices.Therefore, a series of junction termination techniques have been developed to eliminate or weaken the curvature effect of spherical or cylindrical junction, so that the breakdown voltage of the actually manufactured PN junction is close to or equal to the ideal breakdown voltage of the parallel plane junction.[7]
When the reverse bias voltage of PN junction is high, electric breakdown caused by collision ionization, namely avalanche breakdown, will occur.The energy of free carriers in semiconductor crystals is accelerated by the electric field built in the depletion region until they collide with the semiconductor lattice. The energy released in the collision process may break the valence bond and generate new electron hole pairs.The new electron hole pairs are accelerated to collide with the lattice respectively. If each electron (or hole) can produce more than one pair of electron hole pairs in the process of passing through the depletion region, the process can be continuously strengthened, and eventually the number of carriers in the depletion region increases, and the PN junction suffers avalanche breakdown.[7]
