How semiconductor diode get damaged under high current?

The current flow through the resistance in the given circuit is : (Cut-off voltage for both diodes is 0.7 V)

A n-p-n transistor can be used as

The output (X) of the logic circuit shown in figure will be

 

In the above diagram, balance length for E' is ℓ₀. If cross sectional area of wire AB is doubled then what will be new balancing length ?

The figure shows a logic circuit with two inputs A and B and an output C. The voltage wave forms of A, B and C are as given, then logic circuit will be equivalent to :-