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basic working principle and performance of schottky diode

2016-11-15

the internal circuit structure of a typical schottky diode is an n-type semiconductor substrate on which an n-epitaxial layer with arsenic as dopant is formed. the anode is made of molybdenum or aluminum. silicon dioxide (sio2) is used to eliminate the electric field in the edge area and improve the withstand voltage value of the tube. the n-type substrate has very small on-state resistance, and its doping concentration is 100% higher than that of h-layer. an n cathode layer is formed under the substrate to reduce the contact resistance of the cathode. by adjusting the structure parameters, schottky barrier is formed between n-type substrate and anode metal. when the positive bias voltage is applied at both ends of the schottky barrier (the anode metal is connected to the positive power supply and the n-type substrate is connected to the negative electrode of the power supply), the schottky barrier layer becomes narrower and its internal resistance becomes smaller; on the contrary, if the reverse bias voltage is applied at both ends of the schottky barrier, the schottky barrier layer becomes wider and its internal resistance becomes larger. schottky diodes use formed schottky to block the reverse voltage on the contact surface of metals (such as lead) and semiconductors (n-type silicon).
 
there are fundamental differences between schottky diode and pn junction. its withstand voltage is only about 40v. its characteristics are: switching speed is very fast; reverse recovery time is particularly short. therefore, switching diode and low voltage high current rectifier diode can be made.

schottky diode is a metal semiconductor device made of noble metal (gold, silver, aluminum, platinum, etc.) a as positive electrode and n-type semiconductor b as negative electrode. the potential barrier formed on the contact surface of the two devices has rectification characteristics. because there are a lot of electrons in n-type semiconductors and only a few free electrons in noble metals, electrons diffuse from high concentration b to low concentration a. obviously, there is no hole in metal a, so there is no hole diffusion from a to b. with the diffusion of electrons from b to a, the electron concentration on the surface of b decreases gradually, and the surface electronegativity is destroyed, thus forming a potential barrier with the electric field direction of b → a. however, under the action of this electric field, the electrons in a will drift from a to b, thus weakening the electric field formed by diffusion. when a certain width of space charge region is established, the relative balance between the electron drift motion caused by electric field and the electron diffusion motion caused by different concentration will be relatively balanced, thus forming schottky barrier.
 
the internal circuit structure of a typical schottky rectifier is shown in figure 1. it is based on n-type semiconductor, on which an n-epitaxial layer with arsenic as dopant is formed. the metal material of anode (barrier layer) is molybdenum. silicon dioxide (sio2) is used to eliminate the electric field in the edge area and improve the withstand voltage value of the tube. the n-type substrate has very small on-state resistance, and its doping concentration is 100% higher than that of h-layer. an n cathode layer is formed under the substrate to reduce the contact resistance of the cathode. by adjusting the structure parameters, a suitable schottky barrier can be formed between the substrate and the anode metal. when the positive bias voltage e is added, metal a and n-type substrate b are respectively connected to the positive and negative electrodes of the power supply, and the barrier width wo becomes narrower. when negative bias - e is applied, the barrier width increases, as shown in figure 2.

the basic structure and principle of schottky rectifiers are quite different from pn junction rectifiers. pn junction rectifiers are usually called junction rectifiers, while metal half tube rectifiers are called schottky rectifiers. aluminum silicon schottky diodes manufactured by silicon plane technology have also been produced. this can not only save precious metals, greatly reduce costs, but also improve the consistency of parameters.
 
the function of schottky diode is mainly embodied in the switching circuit of bjt, which is clamped by connecting shockley diode on bjt, so that the transistor is close to the cut-off state in the on state, so as to improve the switching speed of the transistor. this method is used in ttl internal circuit of 74ls, 74als, 74as and other typical digital ic.