SCHOTTKY BARRIER (HOT-CARRIER) DIODES
A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode.
A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts. This lower voltage drop provides better system efficiency and higher switching speed.
In a Schottky diode, a semiconductor–metal junction is formed b etween a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the ano de of the diode. This Schottky barrier result s in both a low forward voltage drop and very fast swiitching.
1 Symbol and Construction
It can be seen from the circuit symbol that it is based on the nor mal diode one, but with additional elements to the bar a cross the triangle shape.
Its construction is quite different from the conventional p-n junction in that a metalsemiconductor j u n c t i o n is created such as shown in Figure 4.3 . The semiconductor is normally n-type silicon (although p-type silicon is sometimes used), w hiles a host of different metals, such as molybdenum, platinum, chrome, or tungsten, are used.
Different construction techniques will result in a different set of characteristics for the device, such as increased freq uency range, lower forward bias, and so on. Priorities do not permit an examination of each te chnique here, but information will usuall y be provided by the manufacturer. In general, how ever, Schottky diode construction results in a more uniform junction region and a high level of rug gedness.
In both materials, the electron is the majority carrier. In the met al, the level of minority carriers (holes) is insignifican t. When the materials are joined, the electr ons in the n-type silicon semiconductor material immediately flow into the adjoining metal, estab lishing a heavy flow of majority carriers. Since the injected carriers have a very high kinetic en ergy level compared to the electrons of the metal, the y are commonly called ―hot carriers.
The additional carrie rs in the metal establish a ―negative wall in the metal at the boundary between the two materials. The net result is a ―surface barrier betw een t h e two materials, preventing any further curr ent. That is, any electrons (negatively charged) in the silicon material face a carrier-free regio n and a ―negative wall at the surface of the metal.
The application of a forward bias as shown in the first quadrant o f Figure 4.2 will reduce the strength of the negative barrier through the attraction of the applied positive potential for electrons from this regi on. The result is a return to the heavy f low of electrons across the boundary, the magnitude of which is controlled by the level of the app lied bias potential.
The barrier at the junc tion for a Schottky diode is less than that of the p-n junction device in both the forward- and re verse-bias regions. The result is therefore a higher current at the same applied bias in the forward- and reverse-bias regions. This is a desirable effect in the forward-bias region but highl y undesirable in the reverse-bias region.
Figure 4.4 Comparison of characteristics of hot carrier and PN diode
Schottky diodes are used in many applications where other types of diode will not perform as well. They offer a number of advantages:
· Low turn on voltage: The turn on voltage for the diode is between 0.2 and 0.3 volts for a silicon diode against 0.6 to 0.7 volts for a standard silicon diode. This makes it have very much the same turn on voltage as a germanium diode.
· Fast recovery time: The fast recovery time because of the small amount of stored charge means that it can be used for high speed switching applications.
· Low junction capacitance: In view of the very small active area, often as a result of using a wire point contact onto the silicon, the capacitance levels are very small.
The advantages of the Schottky diode, mean that its performance can far exceed that of other diodes in many areas.
The Schottky barrier diodes are widely used in the electronics industry finding many uses as diode rectifier. Its unique properties enable it to be used in a number of applications where other diodes would not be able to provide the same level of performance. In particular it is used in areas including:
· RF mixer and detector diode: The Schottky diode has come into its own for radiofrequency applications because of its high switching speed and high frequency capability. In view of this Schottky barrier diodes are used in many high performance diode ring mixers. In addition to this their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors.
Power rectifier: Schottky barrier diodes are also used in high power applications, asrectifiers. Their high current density and low forward voltage drop mean that less power is wasted than if ordinary PN junction diodes were used. This increase in efficiency means that less heat has to be dissipated, and smaller heat sinks may be able to be incorporated in the design.
· Power OR circuits: Schottky diodes can be used in applications where a load is driven bytwo separate power supplies. One example may be a mains power supply and a battery supply. In these instances it is necessary that the power from one supply does not enter the other. This can be achieved using diodes. However it is important that any voltage drop across the diodes is minimised to ensure maximum efficiency. As in many other applications, this diode is ideal for this in view of its low forward voltage drop. Schottky diodes tend to have a high reverse leakage current. This can lead to problems with any sensing circuits that may be in use. Leakage paths into high impedance circuits can give rise to false readings. This must therefore be accommodated in the circuit design.
· Solar cell applications: Solar cells are typically connected to rechargeable batteries, oftenlead acid batteries because power may be required 24 hours a day and the Sun is not always available. Solar cells do not like the reverse charge applied and therefore a diode is required in series with the solar cells. Any voltage drop will result in a reduction in efficiency and therefore a low voltage drop diode is needed. As in other applications, the low voltage drop of the Schottky diode is particularly useful, and as a result they are the favoured form of diode in this application.
· Clamp diode - especially with its use in LS TTL: Schottky barrier diodes may also beused as a clamp diode in a transistor circuit to speed the operation when used as a switch. They were used in this role in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. In these chips the diodes are inserted between the collector and base of the driver transistor to act as a clamp. To produce a low or logic "0" output the transistor is driven hard on, and in this situation the base collector junction in the diode is forward biased. When the Schottky diode is present this takes most of the current and allows the turn off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.
Figure 4.5 An NPN transistors with Schottky diode clamp
In view of its properties, the Schottky diode finds uses in applications right through from power rectification to uses in clamp diodes in high speed logic devices and then on to high frequency RF applications as signal rectifiers and in mixers.
Their properties span many different types of circuit making them almost unique in the variety of areas and circuits in which they can be used.
A Zener diode is a t ype of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is lar ger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this el ectrical property.
Figure 4.6 Diode symbol
However, the Zener Diode or "Breakdown Diode" as they are sometimes called, are basically the same as the standard PN junction diode but are specially designed to have a low pre-determined Reverse Brea kdown Voltage that takes advantage of this high reverse voltage. The point at which a zener dio de breaks down or conducts is called the "Ze ner Voltage" (Vz).
The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction. When biased in the forward direction it behaves just like a nor mal signal diode passing the rated current, but when a reverse voltage is applied to it the reverse saturation current remains fairly constant over a wide range of voltages. The reverse vo ltage increases until the
diodes breakdown voltage V B is reached at which point a process called Avalanche Breakdown occurs in the depletion lay er and the current flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor). This breakdown voltage point is called the "zener voltage" for zener diodes.
Avalanche Breakdo wn: There is a limit for the reverse voltag e. Reverse voltage can increase until the diode brea kdown voltage reaches. This point is called Avalanche Breakdown region. At this stage maximu m current will flow through the zener diode. This breakdown point is referred as “Zener voltage”.
The point at which current flows can be very accurately cont rolled (to less than 1% tolerance) in the doping st age of the diodes construction giving the diode a specific zener breakdown voltage, (Vz) ra nging from a few volts up to a few hundred volts. This zener breakdown voltage on the I-V curve is almost a vertical straight line.
Zener diode characteristics
The Zener Diode is used in its "reverse bias" or reverse breakdo wn mode, i.e. the diodes anode connects to the negativ e supply. From the I-V characteristics curv e above, we can see that the zener diode has a regio n in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the di ode and remains nearly constant even with large cha nges in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max).
Figure 4.7 Zener diode characteristics
Applications of zener diode
1.The Zener Diode Regulator
Figu re 4.8 Zener diode act as voltage regulator
The constant reverse voltage of the zener diode makes it a valua ble component for the regulation of the output voltagge against both variations in the input volta ge from an unregulated power supply or variations in t he load resistance. The current through the ze ner will change to keep the voltage at within the limit s of the threshold of zener action and the maximum power it can dissipate.
2. Zener-Controlled Output Switching
This comparator application makes use of the properties of the zener diode to cause the output to switch between voltages determined by the zener diodes when the input voltage difference changes sign. The output circuit amounts to a zener regulator which switches from one zener voltage to the other on a transition.
3. Zener Limiter
A single Zener diode can limit one side of a sinusoidal waveform to the zener voltage while clamping the other side to near zero. With two opposing zeners, the waveform can be limited to the zener voltage on both polarities.
Figure 4.9 Zener limiter
4. Zener Role in Power Supplies
The zener diode is widely used as a voltage regulator because of its capacity to maintain a constant voltage over a sizeable range of currents. It can be used as a single component across the output of a rectifier or incorporated into one of the variety of one-chip regulators Basically there are two type of regulations such as:
a) Line Regulation
In this type of regulation, series resistance and load resistance are fixed, only input voltage is changing. Output voltage remains the same as long as the input voltage is maintained above a
Percentage of line regulation can be calculated by =
Where V0 is the output voltage and VIN is the input voltage and ΔV0 is the change in output voltage for a particular change in input voltage ΔVIN.
b) Load Regulation
In this type of regulation, input voltage is fixed and the load resistance is varying. Output volt remains same, as long as the load resistance is maintained above a minimum value.
Difference between Zener breakdown from avalanche breakdown
1.This occurs at junctions which being heavily doped have narrow depletion layers .
2.This breakdown voltage sets a very strong electric field across this narrow layer.
3.Here electric field is very strong to rupture the covalent bonds thereby generating electron-hole pairs. So even a small increase in reverse voltage is capable of producing large number of current carriers. i.e. why the junction has a very low resistance. This leads to Zener breakdown.
1.This occurs at junctions which being lightly doped have wide depletion layers.
2.Here electric field is not strong enough to produce Zener breakdown.
3.Her minority carriers collide with semi conductor atoms in the depletion region, which breaks the covalent bonds and electron-hole pairs are generated. Newly generated charge carriers are accelerated by the electric field which results in more collision and generates avalanche of charge Carriers. This results in avalanche breakdown.