In late 1957, SCR was first manufactured.
Semiconductor devices are being developed as per the requirement that promote new technology and superiority in modern age based on their constructional arrangements.
Power Semiconductor Devices are of two types fundamentally.
Double Diffused Diode
Thyristors for Phase Control
The power diodes (di-electrode) consist of only two semiconductor layers viz. one P-type and one N-type semiconductor.
Fig.1 Schematic Diagram of Diode with symbol (right).
The most simple construction of diode is made to change the direction of flow of current in a certain direction i.e. it provides unidirectional current to flow through it at any part of an electrical as well as electronic circuit.
The diode only allows the anode current to flow whereas it rejects the cathode current i.e. the positive terminal of any battery (DC source) must have to be connected to the anode side, more specifically to the P-type semiconductor terminal. This Condition is called the biasing of diodes. If current is flowing through the diode from the source, the diode will be in forward biased condition, and, if not, the diode will be in reverse biased condition.
Major carriers of electricity for P-type semiconductors are holes, and free electrons are minor carriers; whereas in N-type semiconductors the electrons are major carriers and holes are minor carriers. In forward biased condition the anode is connected with the positive terminal of the source and immediately after closing the switch the PN junction depletion zone starts thinning and due to the increasing order of holes and free electrons the current from anode to cathode starts flowing.
To enhance the densities of holes in P-type semiconductor and free electrons in N-type semiconductor the energy level of the valence shell electrons must be greater than the energy of conduction band. This can be achieved by applying a certain level of anode-cathode voltage. The conduction will not start below this voltage level i.e. the anode current will remain zero even after applying voltage below that certain value in closed circuit condition. This certain level of voltage is called Threshold Voltage. The amount of this voltage for Silicon semiconductor is 0.7 Volt and for Germanium it is 0.3 Volt. The forward biased Volt-Ampere characteristics of the power diode has a sharp increment of current from zero just after the threshold voltage.
In reverse biased condition when the switch is closed the depletion zone between two semiconductor layers becomes thicker and the densities of major carriers of each semiconductor layer start decreasing; that is why no current flows through the diode in reverse biased condition by major carriers. But after injection of a reverse voltage the minor carriers i.e. free electrons for P-type semiconductor layer and holes for N-type semiconductor layer allows a very little amount of current to flow through it. If the rating of the anode current in forward biased condition is in few milliAmpere, the anode current in reverse biased condition will be in few microAmpere rating. After injecting a certain amount of reverse voltage a huge negative current starts flowing through the diode. This phenomenon is called reverse breakdown of diodes. The negative voltage applied across the diode is called reverse breakdown voltage (VRB). Before the reverse breakdown voltage, upto a certain level of negative diode voltage (negative anode-cathode voltage) the breakdown can be recovered by reducing the reverse voltage. This voltage is called reverse recovery voltage (VRR).
But in the case of an ideal diode, it does not allow any reverse current to flow through it. Also it has zero internal resistance. So after threshold voltage current rises perpendicularly at a very high level theoretically upto infinity i.e. the diode becomes short circuited at forward biased condition with the applied voltage greater than the threshold voltage.
Bipolar Junction Transistors
The semiconductor device with three semiconductor layers are the transistors. As the transistors consist of three semiconductor layers, it has two P-N junctions. The P-type and N-type semiconductor layers are connected one after another forming PNP or NPN Bipolar Junction Thyristors, in short BJT. Obviously in the BJTs one P-N junction will be in forward biased condition and other will be in reverse biased condition. The reverse biased junction opposes the anode current to flow through the BJT. So that another source is provided to increase the potential difference between two layers involved in the reverse biased condition.
Three semiconductor layers are known as Collector, Base, and Emitter as per their functions. The Base is injected with the external voltage to overcome the current rejection situation in BJT. That is why a minimum of two different supplies are required to operate BJTs.
There are three configurations for BJT to operate as a switch; Common-Base Configuration, Common-Collector Configuration, and Common-Emitter Configuration to tie up the two negative terminals of two voltage sources at a common ground.
Symbols of PNP Transistors and NPN Transistors are shown respectively
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