SEMICONDUCTOR SURFACE EFFECT DURING RECOMBINATION

In any real application, the semiconductor material isn't sufficiently large and therefore surfaces exist between the semiconductor and an adjacent medium. When a semiconductor is abruptly terminated, the perfect periodic nature of the idealized single-crystal  lattice ends abruptly at the surface. The disruption of the periodic potential function results in allowed electronic energy states within the energy bandgap. 

The Shockley-Read-Hall recombination theory shows that the excess minority carrier lifetime is inversely proportional to the density of trap states. So we may argue that since the density of trap states at the surface is larger than in the bulk, the excess minority carrier life time at the surface will be smaller than the corresponding life time in the bulk material. That means recombination process is faster in surface than bulk of the material.    

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PN Junction Breakdown Processes

We have seen that a PN junction allows a very small current to flow when reversed biased. This current is due to the movement of minority carriers. It is almost independent of the voltage applied. However if the reverse bias is made too high, the current through the PN junction increases abruptly. The voltage at which this phenomenon occurs is called breakdown voltage.

 

There are two processes which can cause junction breakdown.

1. Zener breakdown

2. Avalanche breakdown

 

Zener breakdown: 

When reverse bias is increased, the electric field(how electric field is created that explained above link1) at the junction also increases. High electric field causes covalent bonds to break. Thus a large number of carriers are generated. This causes a large current to flow. This mechanism of breakdown is called zener breakdown.

 

Avalanche breakdown: 

When reverse bias is increased, the electric field(how electric field is created that explained above link1) at the junction also increases. The increased electric field causes increase in the velocity of the minority carriers. That means minority carriers become more energetic. These high energy carrier heat an atom during their movement and break covalent bonds, thereby generating more carriers. Again these generated carriers are accelerated by the electric field. They break more covalent bonds during their travel. A chain reaction is thus established, creating a large number of carriers. This gives rise to a high reverse current. This mechanism of breakdown is called avalanche breakdown.

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Understanding the fields of Electrical and Electronics engineering

Electrical engineering involves analysis , design of devices which are constructed from conductors Like Transformers, Electric motor etc. That is why we called electric motor. 

Electronics engineering involves analysis and design of devices that are constructed form semiconductors like Op-amp, Diodes, Transistors etc. Since computers, printers etc. are made from these devices, we call them as electronic computers, electronic printers.

The above answer is correct but doesn't reflect the heart of the answer,  It appears that it is just the choice of material that we use. The real difference between Electrical and electronics is not just in the Material used but is due to the difference in properties of different types of materials.

Electrical is all about Generation (V), Transmission (I), Utilization (R), Measurement (V=IR & P=VI) and Manual Control (Switch /Regulator) whether Electronics is all about automatic control according to y = f(x).  To implement the automatic control, in the primary days Vacuum Tubes were used, after that Transistors were used. Now a days ICs are used in Electronics.

The relation between X (X1, X2, X3) and Y(Y1, Y2) is called Electronics. You could use various components between X and Y  but all the components must be controlling electrons by electrons. If you involve magnetic components, then this system is called Electro-magnetics. If you involve mechanical components it is called electro-mechanical. The input X is electrical input and the output Y is electrical output. If the input X and the output Y varies from 0V - nV continuously then this electronics is called Analog Electronics. If the input X and the output Y varies at some discrete steps then this electronics is called Digital Electronics.

So Electronics is all about controlling Electrons.

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