Introduction Thyristor

structure on physical , electronic level, , thyristor symbol.

thyristors have 3 states:

function of gate terminal

the thyristor has 3 p-n junctions (serially named j1, j2, j3 anode).

layer diagram of thyristor.

when anode @ positive potential vak respect cathode no voltage applied @ gate, junctions j1 , j3 forward biased, while junction j2 reverse biased. j2 reverse biased, no conduction takes place (off state). if vak increased beyond breakdown voltage vbo of thyristor, avalanche breakdown of j2 takes place , thyristor starts conducting (on state).

if positive potential vg applied @ gate terminal respect cathode, breakdown of junction j2 occurs @ lower value of vak. selecting appropriate value of vg, thyristor can switched on state quickly.

once avalanche breakdown has occurred, thyristor continues conduct, irrespective of gate voltage, until: (a) potential vak removed or (b) current through device (anode−cathode) becomes less holding current specified manufacturer. hence vg can voltage pulse, such voltage output ujt relaxation oscillator.

the gate pulses characterized in terms of gate trigger voltage (vgt) , gate trigger current (igt). gate trigger current varies inversely gate pulse width in such way evident there minimum gate charge required trigger thyristor.

switching characteristics

v – characteristics.

in conventional thyristor, once has been switched on gate terminal, device remains latched in on-state (i.e. not need continuous supply of gate current remain in on state), providing anode current has exceeded latching current (il). long anode remains positively biased, cannot switched off until anode current falls below holding current (ih). in normal working condition latching current greater holding current. in above figure il has come above ih on y-axis since il>ih.

a thyristor can switched off if external circuit causes anode become negatively biased (a method known natural, or line, commutation). in applications done switching second thyristor discharge capacitor cathode of first thyristor. method called forced commutation.

after current in thyristor has extinguished, finite time delay must elapse before anode can again positively biased , retain thyristor in off-state. minimum delay called circuit commutated turn off time (tq). attempting positively bias anode within time causes thyristor self-triggered remaining charge carriers (holes , electrons) have not yet recombined.

for applications frequencies higher domestic ac mains supply (e.g. 50 hz or 60 hz), thyristors lower values of tq required. such fast thyristors can made diffusing heavy metal ions such gold or platinum act charge combination centers silicon. today, fast thyristors more made electron or proton irradiation of silicon, or ion implantation. irradiation more versatile heavy metal doping because permits dosage adjusted in fine steps, @ quite late stage in processing of silicon.