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Power Electronic Riddle No.8 - Two thyristor from same manufacturer
Why two thyristor from same manufacturer may not behave in equal manner? Explain the reasons.
Author : Abhishek Kumar - From: India
 
#1
Tue, April 26th, 2011 - 10:56
Figure below shows a conceptual view of a typical thyristor with the three p–n junctions and the external electrodes labeled.



A high-resistivity region, n-base, is present in all thyristors. It is this region, the n-base and associated junction, J2 of above Figure, which must support the large applied forward voltages that occur when the switch is in its off- or forward-blocking state (non-conducting). The n-base is typically doped with impurity phosphorous atoms at a concentration of 1013 to 1014 cm−3. The n-base can be tens to hundreds of micrometer thick to support large voltages. High-voltage thyristors are generally made by diffusing aluminum or gallium into both surfaces to create p-doped regions forming deep junctions with the n-base. The doping profile of the p-regions ranges from about 1015 to 1017 cm−3. These p-regions can be up to tens of micrometer thick. The cathode region (typically only a few micrometer thick) is formed by using phosphorous atoms at a doping density of 1017 to 1018 cm−3.
The higher the forward-blocking voltage rating of the thyristor, the thicker the n-base region must be. However, increasing the thickness of this high-resistivity region results in slower turn-on and turn-off (i.e. longer switching times and/or lower frequency of switching cycles because of more stored charge during conduction). For example, a device rated for a forward blocking voltage of 1 kV will, by its physical construction, switch much more slowly than one rated for 100 V. In addition, the thicker high-resistivity region of the 1 kV device will cause a larger forward voltage drop during conduction than the 100V device carrying the same current. Impurity atoms, such as platinum or gold, or electron irradiation are used to create charge-carrier recombination sites in the thyristor. The large number of recombination sites reduces the mean carrier lifetime (average time that an electron or hole moves through the Si before recombining with its opposite charge-carrier type). A reduced carrier lifetime shortens the switching times (in particular the turn-off or recovery time) at the expense of increasing the forward-conduction drop. There are other effects associated with the relative thickness and layout of the various regions that make up modern thyristors, but the major tradeoff between forward-blocking voltage rating and switching times, and between forward-blocking voltage rating and forward-voltage drop during conduction should be kept in mind. (In signal-level electronics an analogous tradeoff appears as a lowering of amplification (gain) to achieve higher operating frequencies, and is often referred to as the gain-bandwidth product.). Therefore as you see some important characteristics of semiconductor devices such as thyristors are function of material ( cost concerning) and accuracy of manufacturing procedure especially in automated procedure systems. 
Author : Hamid - From: Iran
 
#2
Fri, April 29th, 2011 - 23:56
As a conclusion can we say that it is due to the difference in level of concentration or doping! 
Author : Abhishek - From: Indian
 
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