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Diod Thyristor

Physical Processes in a Transistor | The Basic Circuit Configurations of Transistors | Models of Transistors | Bias Supply and Temperature Compensation for Transistors | Amplification by a Transistor | The Frequency Behaviour of Transistors | Transistors as Switches | Basic Types of Bipolar Transistor | L. Junction Field-Effect Transistors | Depletion-Type MOS-Transistor |


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  1. Triode Thyristor

The structure of a diode thyristor of the n-p-n-p type is shown in Fig. 6.1a. As is seen, it has three p-n junctions, two of them (Jl and J3) being forward-biased and the third (J2), which is sandwiched in between, being reverse-biased. The outer p -region is called the anode, and the outer n -region is called the cathode.

Fig. 6.1. Diode thyristor: (a) circuit, (b) structure and (c) its model as a combination of two transistors

 

Minority carriers are injected across the for­ward-biased junctions J1 and J3 into the regions adjacent to the junction J2 with the result that the resistance of J2 is brought down.

The current-voltage characteristic of a thyristor, shown in Fig. 6.2, illustrates the events taking place in a thyristor when the applied voltage is raised. At first the current is small and rises slowly-this corresponds to portion O A of the characteristic. In this condition the thyristor may be deemed in the OFF state.

Fig. 6.2. Current-voltage characteristic of a diode thyristor

Around point A, that is, at a certain definite voltage (tens or hundreds of volts), called the forward breakover voltage Ufbo, the two factors produce an equal but opposite effect, and ba­lance each other. Now even a minute increase in the applied voltage gives the second factor an edge over the first, and the resistance of J2 begins to decrease. Now the thyristor is driven to conduction (turned ON) in a cumulative fashion.

The current rises stepwise (portion AB of the characteristic) because an increase in voltage across J1 and J3 brings down the resistance of J2 and the voltage across it. As a result, the voltages across J1 and J3 rise still more, and this leads in turn to a further increase in the current, a reduction in the resistance of J2, and so on. This chain of events finally terminates in a condition which resembles the saturated state of a transistor-a heavy current at a low voltage (portion BC of the characteristic). The current in this state when the device is conducting (in the ON state) is mainly decided by the load re­sistance RA connected in series with the device. Owing to the resultant heavy current, nearly all of the supply voltage is dropped across RA.

Diode thyristors are specified in terms of the absolute maximum forward current If max (point C in Fig. 6.2) at which a small ON-state voltage, UON , still exists across the thyristor. If we reduce the current flowing through the device to below some value called the holding current, IH (point B), the current will drop suddenly and the voltage will rise as suddenly, indicating that the device has jumped back to the OFF state corresponding to the portion OA of the cha­racteristic (called the forward blocking region ).

 


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