Diode

A diode is an electrical component that can pass almost unhindered current in one direction and in the other direction almost isolated. Therefore, it is spoken by the forward direction and the reverse direction. Wherein AC can be due to this property with a diode rectifier, that is to achieve a conversion to DC power.

Such behavior was discovered in 1874 by Ferdinand Braun in point contacts to lead sulfide ( galena ) (similar to image germanium diode peak ). A year earlier, the Edison - Richardson - effect was discovered, which was applied in the diode vacuum tube.

The term diode is now mostly used for semiconductor diodes with a pn junction or a rectifying metal -semiconductor junction ( Schottky contact ) work. Colloquially refers to diode even only on silicon diodes with a pn junction, while other variants are characterized by additions to names (example: germanium diode, thermionic diode ).

In the mid- 20th century, people spoke well of valve cells instead of diodes.

In addition to the effect of the direct direction, a semiconductor junction is more useful properties are exploited, for example, Zener, photographic, light-emitting diodes and solid state detectors for radiation.

• 3.3.1 junction capacitance
• 3.3.2 diffusing capacity

• 4.1 JEDEC
• 4.2 per Electron

Structure and physics of a semiconductor diode

The basis of the semiconductor diode is either a PN -doped semiconductor crystal (usually made of silicon, but also germanium, see germanium diode, gallium arsenide ) or a metal -semiconductor junction (see Schottky diode).

The conductivity of such a transition depends on the polarity of the operating voltage to the anode (p- doped) and a cathode ( n-doped), or by the direction of current flow. The pn junction (gray area) is an area which is free of mobile charge carriers, as positive charge carriers (so-called defect electrons or holes ) of the p-type crystal and negative charge carriers ( free electrons ) of the n-type crystal on the other diffuses side of the pn junction and there disappeared by recombination (see Article pn junction ). The original sources of the charge carriers, the doping atoms are fixed and then form an ion space-charge, the electrostatic field keeps the two types from each other, and thus the charge prevents the further recombination. About the whole space charge region is formed across the diffusion voltage. This can be achieved by externally applied voltage - depending on the polarity - are compensated, then the pn junction is conductive, or amplified, then he remains locked.

Pn - diode

Schottky diode

In the Schottky diode on the other hand, a metal -semiconductor contact is used.

Mechanical equivalent model of the diode

The function of a rectifier diode in the circuit can be thought of something like a check valve in the water circuit: When a pressure (voltage) on this valve (diode) is carried in the reverse direction, so the water (current ) flow is blocked. In the forward direction, the pressure must (voltage) to be large enough to overcome the spring force of the valve ( = locks or threshold voltage of the diode ) can. Thereby, the valve opens (the LED ) and the current can flow. This pressure, which is required in the mechanical model to overcome the spring force corresponds to, with a diode, the so-called threshold voltage of () or minimum forward voltage (English forward voltage drop) which has to be applied in the forward direction to the diode, so that in conducting a Condition passes. In ordinary silicon diodes it is about 0.7 V.

Open ball check valve

Bicycle valve; the internal pressure acts as a restoring force

The check valve behaves in turn analogous to the Shockley formula that was developed to describe the semiconductor diode (see below for Ideal Diode ), which by using the formula, among other things for the approximate calculation of valves.

Electrical behavior

The analysis of electronic circuits requires a mathematical description of the diode. For this purpose there is the graphic current-voltage characteristic, exact equations and simplified models.

Symbols

The detailed analysis of a diode requires a differentiation of symbols. The following table this facilitates the overview.

In addition, the following constants of nature are important:

Static behavior

The static behavior describes a diode for DC voltage and applies approximately for AC voltages with low frequency, about 50 Hz mains voltage, but depending on the version up to the MHz range. By altering the voltage -induced redistribution processes in the pn junction are ignored.

Characteristic

Most graphically the current-voltage characteristic curve describes the static behavior of a diode. The characteristic curve is divided into three sections: the passband, the stopband and the breakdown region.

If one considers the characteristic, no significant current flows in the passband at the beginning despite the applied voltage through the diode (referred to in the art normal flow ). Only at a voltage of about 0.4 V the current starts at Si diodes to rise noticeably. From about 0.6 V to 0.7 V then the current increases sharply, and one therefore speaks of the threshold voltage. Schottky - diode - germanium and a substantial current flows even at about 0.2 V and the threshold voltage is about 0.3 V to 0.4 V.

In the stop band, a very small current, called leakage current flows. In this case, Ge and Schottky diodes have considerably higher values ​​than silicon diodes.

Depending on the doping starts at Si diodes at or from about -50 V to -1000 V to the breakdown region and the diode becomes conductive in the reverse direction. The same is true for a Schottky diode at about -10 V to -200 V. This is known as the breakdown voltage, which is given with a positive sign. By special allocations can be reached in Si diodes also breakthroughs to below -5 V, which is used especially in Zener diodes.

Ideal Diode / Shockley equation

The Shockley equation (named after William Bradford Shockley ) describes the characteristic of the diode in the passband and is the special case of an Arrhenius equation.

• Reverse saturation current (short: reverse current )
• Emission coefficient
• Temperature stress at room temperature
• Absolute temperature
• Boltzmann constant
• Elementary charge

Since these non-linear characteristic is difficult to handle, one tries it depending on the application with different degrees of simplification in some areas linear display (see below).

At 120 mV to Do a relative error of <1%, if you omit the second term in the bracket. Then we have

On closer inspection, the diode current from the diffusion current, taking into account the high-current effect, the leakage current and the breakdown current is composed:

Temperature dependence

The diode characteristic curve varies with the temperature. In the Shockley equation two temperature-dependent terms are included:

Here, the band gap voltage ( gap voltage) of silicon.

This voltage can actually be recognized in practice for many rough calculations as the value of the forward voltage of silicon diodes and pn junctions. It is used (often temperature-compensated ) for generating reference voltages.

In addition, one must also take into account the temperature dependence of the voltage:

This value is the relevant temperature range 300 K to constant enough to use them to do temperature measurements based on the forward voltage can.

Diffusion current

The diffusion current occurs in the central pass-band where he dominates the other effects. The equation is derived from the ideal diode with:

In Schottky diodes can be described using the same formula the emission current.

High-current effect

The high-current effect causes an increase in the average currents in on for infinity. Here, the knee current describes the border to the high current range. It thus less current flows, and the characteristic curve has a flatter, but still exponential.

Leakage current ( recombination )

Upon application of a reverse voltage, the electrons and holes are discharged to the respective contacts, thereby increases the space charge region, and the diode should not conduct current. In practice, however, continue to measure a low current, the so-called leakage current ( reverse current ). It results from the diffusion of charge carriers by space charge region in the oppositely doped region, where they are discharged due to the applied voltage. This provides the p-type region and the n- zone electron holes as minority charge carriers that lead to the reverse current.

For the mathematical calculation applies:

With

• - Leak - reverse saturation current
• - Emission coefficient in the reverse direction
• - Diffusion voltage
• - Capacity coefficient

The reverse current is strongly voltage-and temperature -dependent and depends on the production technology as well as purity and Störstellenarmut.

Breakthrough

The reverse current of a pn diode in blocking polarization in general is low. If you increase the voltage further, however, in the reverse direction, the reverse current increases above a certain reverse voltage slowly at first and then suddenly to. This increase in the reverse current (reverse current) is called generally "breakthrough", and the corresponding voltage is called breakdown voltage. The breakdown voltage of a diode is generally dependent on the semiconductor material and the dopant and can be of the rectifier diodes in the range between 50 and 1000 volts.

With, the breakdown knee current, and the break - emission coefficient.

For most semiconductor diode of this condition is undesirable since it destroys the device with ordinary diodes because of the high power dissipation and the thin constricted current flow channel. Cause of the break are very high electric field strengths. We can distinguish three different mechanisms: the avalanche, the Zener and the thermal breakthrough.

The avalanche breakdown (including avalanche breakdown or avalanche effect called ) is characterized by a charge carrier multiplication by impact ionization from. It is (also known as Zener diodes ), for example in the IMPATT and suppressor diode, the avalanche photo-diode as well as zener diodes higher voltage used (see also avalanche diode ). The avalanche is also in some rectifier diodes types ( avalanche rectifier diode, avalanche Type) permitted and specified so that they are not destroyed in one-off or periodic overvoltage events up to certain energies.

In Zener breakdown, however, the energy bands are strongly shifted by a special doping. When exceeding the breakdown voltage - in this case, we speak mostly of the zener voltage - occurs a tunnel effect, which allows valence band without changing energy intake in the conduction band from the valence band. The Zener breakdown is used in zener diodes to about 5 volts and is used, among other things, the provision of reference voltages.

The thermal break describes the breakdown of the reverse bias voltage due to the high temperature and the associated charge carrier generation. In general, it leads to the destruction of the diode by diffusion processes.

Differential resistance

The differential resistance is obtained from the tangent passes through the operating point of the diode. It is also referred to as a dynamic resistance. By the use of a line rather than the actual exponential function of the required processing steps are substantially simplified.

Operating point: A

For large currents is very small, and the bulk resistance occurs increasingly in evidence. This is a real resistance and stirred much from the conductivity of the base material of the diode chip. He's in the equivalent circuit with series.

The equivalent circuit and is depending on the type of diode only up to frequencies of 10 to 100 kHz. At higher frequencies, as they also occur when switching on and off, one must also take into account the capacitive properties as well as the Sperrerholzeit the diode.

Bulk resistance

The bulk resistance is caused by the electrical resistance of the semiconductor material as well as the resistance of the connection to the semiconductor. The bulk resistance is taken into account by the following formula:

Static small-signal model

The static small-signal model is used for dimensioning the working point of simple circuits. Here corresponds to the above-mentioned differential resistance (see above). In addition, if desired, the bulk resistance.

For the operation to the reverse breakdown, that is, as the Zener diode, of the parameters used to model the behavior.

Dynamic small-signal model

For AC applications have to take into account the capacity of the diode, which produces mainly occur at high frequencies. A distinction is made between the junction capacitance and the material for switching applications diffusing capacity.

The dynamic small-signal model in addition to the static small-signal model takes into account the capacitance of the diode. Thus one can be dimensioned also simple (low frequency ) circuits with capacitance diodes.

Junction capacitance

The pn-junction diode having a capacitance which is dependent on the width of the space charge zone. With increasing reverse bias increases the width of the charge-free zone, whereby the capacitance decreases.

The zero capacity is directly proportional to the area of the pn junction. The diffusion voltage is also dependent on the doping. Take and with increasing doping. The diffusion voltage is usually in the range between 0.5 and 1 volt.

The capacity coefficient represents the doping profile of the pn junction dar. Direct transitions from the p- to the n- layers lead to a value of, while transitions with linear behavior of the p- to the n- layers lead to a value of.

The above formula is approximately valid for only up to a value of. The formula can thus - as shown dotted in the diagram - not reflect the actual course of in this area. About this value increases only slightly. For a value of the further course of is replaced by the tangent at the point, which is very close to the actual course:

Substituting we obtain the equation

Here is.

Diffusion capacity

When a forward voltage occurs in the neutral regions ( ie outside the space charge region ) to minority carrier surpluses, which form the so-called diffusion charges. These spatially separated charges must for changes in forward voltage be on or broken down and thus influence the dynamic behavior of the diode.

IDD is called the diffusion current, and the so-called transit time:

Approximation, one can also assume that this also applies to the diffusion region and. This gives the approximate equation:

• Is the case of Si diodes.
• In Schottky diodes is, therefore, in Schottky diodes, the diffusion capacity usually be neglected.

The diffusion capacitance or the Sperrerholzeit caused losses in fast switching applications (switching power supplies ), so we used here - if Schottky diodes can not be used due to their limited blocking voltage - very fast silicon diodes. Switch for diode, however, a large diffusion capacity is desired in order to achieve a low impedance at high frequencies.

Switching behavior

The switching behavior can be described only very limited with the small-signal model, because here the nonlinear behavior of the diode is important. The description by the Diffusing Capacity are for the " off " Although a high fitting image, but is due to the nonlinearity next quantitatively.

The change from the current line in the forward direction to the locking behavior is not immediately at a PN diode. First additional minority carriers must be removed. If not wait for the recombination, the minority charge carriers to flow as reverse recovery charge ( ) from a short current pulse in the reverse direction. Only then, the voltage is negative and the diode is more or less abruptly in the blocking state. The time for the diode may block is called Sperrerholzeit () and is of the order of the transit time.

This delay time allows for a sufficiently high frequency, the construction of high speed switches and variable attenuators using pin diodes as a DC -controlled AC resistance with a short reaction time. Very fast switchable phase shifter with pin diodes are also required in phased array antennas.

Often a faster transition to the blocking state is in demand and there are (ns scale 5-200 for silicon pn diodes) offered in accordance with fast diodes with short transit time. In Schottky diodes, the minority carriers play a significant role and accordingly there is a very quick transition to the blocking state.

The transition from the blocking state to the conducting state is not instantaneous, though quite fast. Especially when PIN diode for high blocking voltages, it takes a certain time until the minority carriers have flooded the barrier layer and the intrinsic region. In a very fast rise of the current, the voltage in the forward direction may be initially significantly higher than in the stationary case. With "normal " diodes ( no PIN) the delay fairly short, and the overshoot of the voltage is low and rarely relevant.

Switch Model

The relationships shown in the foregoing sections rely on non-linear equations. Thus, the circuit analysis considerably more difficult. However, such precise models are often not required, and the recourse to the switch model simplifies the calculation. Variant b ) goes into the conductive state when the positive voltage values ​​of the diode bear and in variant c ) of the voltage-dependent switch closes as soon as the voltage exceeds the forward voltage UF. The voltage source is drawn due to the current direction (P = U * I ) no power to the circuit from, but only receives power.

Marking and labeling

The cathode unipolar diodes is usually marked with a ring or color point. The cathode terminal of light-emitting diode is indicated by a dot, a shorter leg connector and / or a Gehäuseabflachung. In laser diodes, the anode of which is usually connected to the housing.

The diode type can be characterized by two standards. According to JEDEC standard or according to per - Electron, each with a color code or label. In the designation of the color code is printed first ring wider, and at the same time referred to the connection of the cathode. At the label, the cathode is characterized by a ring. Some manufacturers conduct their own naming schemes.

Also worth mentioning is the marking on bridge rectifiers with two connections for the AC voltage to be applied "AC" and the removable DC " " and "-". In the type designations are often the maximum reverse voltage and current ratings contained, with about " E40 C30 " for 40 V voltage (E ) and 30 mA (C ) stands.

JEDEC

The caption for Diodes JEDEC consists of a number and a letter and a further four digit number (eg, " 1N4148 "). The four-digit number may here be given in the following color coding:

Pro Electron

The labeling of the diodes by Pro Electron is made up of two or three letters and one two -to three- digit number.

Examples: B A 159 B A T 20

The letter-number sequence can be alternatively stated as a color code:

Parameters

Semiconductor diodes (signal diodes, rectifier diodes, but also laser, protective and light-emitting diodes ) have certain characteristics for the specification. They are mentioned in the data sheets and are important for the application and the design their circuits with other components.

The most important characteristics and limits of diodes are:

• Maximum reverse voltage (rectifier and signal diodes, LEDs and laser diodes )
• Maximum continuous and peak current in the forward direction (rectifier and signal diodes, LEDs and laser diodes )
• The forward voltage or threshold voltage at a certain current ( ⅟ 10 rated current for rectifier diodes)
• Zener diodes at the maximum continuous power dissipation and the zener voltage
• With rectifier and signal diodes, the switching time ( also reverse recovery time or recovery time lock, Eng. reverse recovery time, trr briefly mentioned )
• At Suppressors ( TVS ), the response time, the energy and the peak power that can be absorbed during avalanche breakdown in the reverse direction, the breakdown voltage and the maximum voltage without breakdown guaranteed in the reverse direction
• Especially in the case of Schottky diodes strongly temperature-dependent leakage current ( reverse current )

Diode types

There are a number of diodes used for different purposes:

In addition to the diode types mentioned above, there are a number of other types that are used less frequently can be any specific category or assign.

The avalanche effect is utilized in the avalanche diodes. More diodes, the field -effect diode ( Curristor ), the Gunn diode, the tunnel diode, the Sirutor, the IMPATT diode or avalanche time diode (abbreviated LLDPE ) and the recovery diode (English step recovery diode), a special type of charge storage diode.

The switching diode is a small-signal diode with very fast switching behavior. The switch diode, however, is a particularly slow diode, low junction capacitance and is used for switching of high-frequency, by being acted upon either by a reverse direct voltage or with a direct current in the direction of flow.

Word origin

The word comes from Ancient Greek diode δίοδος DIODOS " pass", " pass ", " path "; the feminine noun is made up of the preposition διά diá " by", " round " and the word ὁδός Hodos "way".