BREAKDOWN DEVICES $" + 0 ) 2 6 - 4 Learning Objectives Ç The Silicon Controlled Rectifier, usually referred to as an SCR, is one of the family of semiconductors that includes transistors and diodes ➣ What are Breakdown Devices ? ➣ Unijunction Transistor ➣ UJT Relaxation Oscillator ➣ Programmable UJT(PUT) ➣ Silicon Controlled Rectifier ➣ Comparison between Tran- sistors and Thyristors ➣ Transient Effects in an SCR ➣ Phase Control ➣ Theft Alarm ➣ Emergency Lighting System ➣ Light Activated SCR (LASCR) ➣ The Shockley Diode ➣ Triac ➣ Diac ➣ Silicon Controlled Switch (SCS) ANODE GATE = CATHODE = =
2390 Electrical Technology 64.1. What are Breakdown Devices ? These are solid-state devices whose working depends on the phenomenon of avalanche breakdown . They are sometimes referred to by the generic name of thyristor which is a semiconductor switch whose bistable action depends on P-N-P-N regenerative feedback. We will discuss the following devices : 1. Unijunction Transistor (UJT). 2. Silicon Controlled Rectifier (SCR). 3. Light Activated SCR (LASCR). 4. Triac (short for ‘triode ac’). 5. Diac (short for ‘diode ac’). 6. Silicon Controlled Switch (SCS). These devices have two or more junctions and can be switched ON or OFF at an extremely fast rate . They are also referred to as latching devices. A latch is a kind of switch which initially once closed, remains closed until someone opens it. 64.2. Unijunction Transistor Basically, it is a three-terminal silicon diode. As its name indicates, it has only one P-N junction. It differs from an ordinary diode in that it has three leads and it differs from a FET in that it has no ability to amplify . However, it has the ability to control a large ac power with a small signal . It also exhibits a negative resistance characteristic which makes it useful as an oscillator. ( a ) Construction It consists of a lightly-doped silicon bar with a heavily-doped P -type material alloyed to its one side (closer to B 2 ) for producing single P-N junction. As shown in Fig. 64.1 ( a ), there are three terminals : one emitter, E and two bases B 2 and B 1 at the top and bottom of the silicon bar. The emitter leg is drawn at an angle to the vertical and arrow points in the direction of conventional current when UJT is in the conducting state. ( b ) Interbase Resistance ( R BB ) It is the resistance between B 2 and B 1 i.e . it is the total resistance of the silicon bar from one end to the other with emitter terminal open [Fig. 64.2 ( a )]. From the equivalent circuit of Fig. 64.2 ( b ), it is seen that R BB = R B 2 + R B1 It should also be noted that point A is such that R B 1 > R B 2 . Usually, R B 1 = 60% of R B 1 . The resistance R B 1 has been shown as a variable resistor because its value varies inversely as I E . Fig. 64.1 Cat-Transistor Fig. 64.2
2391 Breakdown Devices ( c ) Intrinsic Stand-off Ratio As seen from Fig. 64.3 ( a ), when a battery of 30 V is applied across B 2 B 1 , there is a progres- sive fall of voltage over R BB provided E is open. It is obvious from Fig. 64.3 ( b ) that emitter acts as a voltage-divider tap on fixed resistance R BB .
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