Whereas a transistor contains a third terminal called the base through which voltage has to be applied throughout for the transistor to continue in conduction state. It acts like a valve, and allows current flow in only one direction, but not the reverse. It is used to convert a. The central one is called the base in a bipolar or current biased transistor and is called the Gate in a field effect transistor.
A small current or potential on the base or Gate causes a large current across the transistor. Thus the transistor acts like a switch to on and off current and also to regulate the potential.
It is used to switch a. Diode is uncontrolled switch, thyristor is semi-controlled switch and transistor is fully controlled switched. Simple words Transistor is a fully controllable switching device. Thyristor is a semi-controlled switching device. Diode is a uncontrollable switching device. All are semiconductor devices with difference in PN-Junction layers. Diode is a two layer device while transistor and thyristor are three and four layer devices respectively.
Diode is consist of onlye one PN junction which allow current to flow only in one direction while thyristor is 4 layers 3 junction semiconductor device which can also allow current in both direction. Which is current control device. Basic function of the transistor are switching amplifying the signal.
Products By Bayt. As we have already discussed that the operation of a thyristor majorly depends on the applied external potential at the gate terminal. So, let us understand the case when no any external potential is provided at the gate terminal but forward voltage is applied between anode and cathode.
Hence, as we can see in the figure shown above that forward voltage is applied between anode and cathode that causes the junction J 1 and J 3 to be forward biased. But as the same time junction J 2 will be reverse biased. This will lead to generation of depletion region around J 2.
Hence no forward current will flow through the device and only negligibly small leakage current will flow through it. This state is said to be practically off state of the thyristor SCR.
Now, suppose if no any external gate potential is applied but a reverse potential is applied between anode and cathode. This biasing arrangement reverse biases the junction J 1 and J 3 but forward biases the junction J 2. Still only the leakage current will flow through the device.
Hence we can say without any gate potential, SCR will not conduct in either forward or reverse biased condition. Let us now consider the case when gate terminal is triggered with a forward potential. Also a forward voltage is provided between cathode and anode. So in this case, the electrons present in the n region experiences repulsion from the negative terminal of the battery. This movement generates gate current through the device.
Also the holes in the p region gets repelled by the positive terminal of the battery and drift across the junction J 2 thereby giving rise to anode current.
This regenerative action allows the SCR to conduct heavily. However, it is to be noteworthy here that once the SCR starts conducting, then the gate potential plays no any further role in conduction. And the device continues to be in ON state. So, from the above discussion we can say that though both diode and thyristor are semiconductor devices. But the operation of the two are quite different thus find applications in different fields.
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