Working principle of PN junction semiconductor diode
In this article, I will explain the working principle of PN junction semiconductor diode. As you know that a diode operates both in forward bias and reverse bias so explaining the both in detail with necessary V-I characteristics curves.
The basis portion is a general description of the p-and n-type semiconductor , and the pn junction to be found. A semiconductor diode is a bipolar device, and composed of a p-and n-type semiconductor crystal. The joint face constituting the pn junction. Between differently doped semiconductor substrate, a charge transfer takes place. Some electrons from the n-type region migrate into the p-type semiconductor. These are known from the positive charge carriers, holes or defect electrons are attracted.
In the contact region both semiconductor crystals are not electrically neutral. The n-type semiconductor region forming a positive charge. In the adjacent p-type semiconductor creates a negative charge region. When de-energized, the diffusion process comes naturally to a standstill. In the contact area of both types of semiconductors, the boundary layer, thereby a space charge zone is based. The amount of the diffusion voltage of the semiconductor material and the strength of the impurity depends. Through the diffusion process, the boundary layer depleted of free charge carriers. It is high impedance and the barrier layer.
If an external voltage is applied to this pn junction, the polarity so determined, whether a current can flow. Is on the negative terminal of the p-type semiconductor and the positive terminal at the n-type semiconductor, then the barrier layer is broadened. The measurable current values remain in the uA range. With reverse polarity with the positive pole to the negative terminal of the p-zone, and on the n-type region, the barrier layer is degraded. It is low and allows the flow of current. The current values are depending on the diode type and design from a few mA to kA. The semiconductor diode acts as a mechanical check valve. The current flow can only take place in one direction and is locked in the opposite direction.The diode in the forward direction
The circuit symbol of a simple semiconductor diode is a closed arrowhead on a vertical line. The two electrodes are referred to as anode and cathode. The direction of the arrow from the top to the bar indicates the direction of current of the diode conducting. The following diagram shows the characteristics of various diodes in the passband.
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Sufficiently far below the threshold voltage, the passage of current (1 to 100) uA is almost immeasurable. The barrier layer has not been removed, and the pn junction is a high resistance.
Near the threshold voltage increases nonlinearly with him depleting the current blocking layer.
Above the threshold voltage, the barrier layer is degraded and a very low resistance of the semiconductor. The forward current increases very rapidly strong. It may not exceed a maximum value and must be limited by a series resistor.
The resulting heat loss when conducting semiconductor improves its conductivity. Each 10 degree increase in temperature doubles by pairing germanium diodes in the number of free charge carriers in the crystal, while even tripled in silicon diodes. Without current limit, the component is destroyed ultimately.
The need to break down the barrier voltage is called threshold or threshold voltage. It corresponds to the diffusion voltage of the pn junction energized. Above the threshold voltage of the semiconductor diode is low and conductive. The diode is operated then in a forward direction or in the passband.
Semiconductor diodes do not have a constant resistance value. He is the chosen working point. The steep curve area above the threshold voltage, a constant DC resistance value can be calculated to a good approximation according to Ohm's law. The resistor value is calculated as the ratio of voltage and current at the operating point.
For more accurate calculations or the use of the diode in specific circuits of the differential resistance, also called AC resistance to use. It can be determined graphically by applying the characteristic of the tangent to the operating point using the slope triangle.The diode in stop band
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The diode operates in the reverse direction, when the anode potential is more negative compared with the cathode. The current flow is reduced to a minimal residual current is up to 10 7-fold smaller in comparison to the flow direction. It is never as more deliver null in the semiconductor crystal a few impurities carriers. The p-type region provides electrons and electron holes, the n-type region and holes as minority carriers. Both can pass through the barrier freely and cause the reverse current. An increase in temperature an appreciable increase in the reverse current is measurable since semiconductors are among the hot conductors.
Compared with silicon germanium diodes diodes have higher leakage currents. The maximum blocking voltage of the germanium diode is lower. Power diodes have higher leakage current values as their pn junction has a larger cross-sectional area. The diagram shows the basic complete characteristic curves of the two types of diodes.Diodes with high reverse voltage
Rectifier in the power engineering to guide high currents and high voltages can lock securely at the same time. However, to the necessary high degree of doping reduces the maximum reverse voltage, since these semiconductors have only a narrow barrier. This disadvantage is eliminated by the installation of a further semiconductor layer. Between the heavily doped p-and n-type semiconductors is a lightly doped semiconductor region. This range is low p-or n-doped, highly resistive and widened by the barrier layer. In the on mode, this intermediate layer is then flooded with charge carriers of both sides and thus low. These diodes are called psn semiconductors.
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High-voltage diodes for a few kilovolts, reverse voltage condition a wide undoped semiconductor region between the p-and n-type crystals. It is intrinsic, an intrinsic layer, called the i-zone. At high reverse blocking voltage, the depletion region extends across the entire width of the i-region. In the forward direction this area is flooded on both sides like the psn diodes of electrons and holes and low impedance.Important diode boundary data
The limit values given by manufacturers in the data sheets are maintained for each is essential. Be in operation some values ??are not reached, this has no effect on the other limits. Failure to observe the component is destroyed.The reverse voltage UR
Up to this maximum DC voltage, the diode in the reverse or reverse operation remains high impedance. Silicon diodes have reverse voltages to up to 4 kV. For germanium diodes, the maximum value is around. 100 V. The selenium diodes almost no longer used only reach (25 ... 40) V.The peak reverse voltage URM
The value represents the maximum periodic peak value of an alternating voltage in the reverse direction at an operating frequency greater than 20 HzThe forward current IF and I0
The value specifies the maximum allowed direct or effective current through the diode is not destroyed in the passband. The target current I0 is the arithmetic average of the forward current and is slightly lower.The maximum peak current IFM
It is expressed in the forward direction for an operating frequency of about 20 Hz with sinusoidal loads. This value applies to rectangular signals with a duty cycle of 0.5.The power dissipation Ptot
Therefore, the maximum continuous power is referred to, that does not destroy the semiconductor. It is calculated from the product of the voltage across the diode and current through the diode. Silicon diodes tolerate the higher power dissipation compared to germanium diodes.The junction temperature Tj
The semiconductor crystal is allowed to warm up without permanent damage to this maximum temperature. The highest temperature of the component body is low, since the heat of the crystal has to be transported only to the outside. Crystal silicon diodes tolerate temperatures to 190 ° C. In germanium semiconductors, the limit is 100 ° C. Selenium semiconductor tolerate up to 80 ° C.The ambient temperature TU
All limits are for an ambient temperature of 25 ° C, unless the value is specified separately.Some diode characteristics
The following characteristics are not followed, the circuit can exhibit unexpected properties. Some important parameters are:The forward voltage VF
The value must for a given forward current IF. The forward voltage generally corresponds to the diffusion voltage of the pn junction in de-energized state.Reverse current IR
The data sheets for the reverse current for a certain reverse voltage UR is given. Silicon diodes have the lowest reverse currents. An order of magnitude worse are germanium diodes. Even higher reverse currents have the selenium cells.The diode capacitance CD
The diode, the depletion region of the barrier layer to the electrical potential of a charged plate capacitor can be compared. Diodes with a large cross-sectional area at the pn junction have high capacitance values. The values range from a few pF to 500 pF diode peak in specific capacitance diodes.
There are diodes with very short switching times, the switching diodes. The reverse recovery time determines the upper frequency at which the diode is still operating as expected. The following diagram shows the behavior of a 1N4005 Universal diode operating at too high a frequency. The input signal is a sine wave voltage once, and the other time, a square wave voltage having a peak value of 15 V at a frequency of 50 kHz. The diode is connected as a half-wave rectifier and the output voltage is measured at the load resistor kO first
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Of (0 to 10) is the anode potential is more positive than that of the cathode. The diode operates in the transmission mode and the output voltage equal to the input voltage. Of (10 to 20) is the diode should be locked and the output voltage V 0. This state is reached after 5 microseconds. Within this the charge carriers from the pn junction area flow from. Thereafter, based on the exclusion zone, and the diode is not conducting.
The yellow highlighted area, the permissible power dissipation of the diode can be exceeded very quickly. It is calculated as P tot = R · U I R. At high reverse voltage UR a large reverse current I R, which corresponds to the first microseconds of the value of the forward current is flowing.
your way of explanation along with the diagram is excellent. keep it up and post more on BJT and FET.