Who invented semiconductor?
Karl Braun, in Germany, 1874, discovered the galena chrstal rectified alternating current. This was the basis of the cat's whisker detector in crystal radio sets in the early 1900s. Perhaps this was the first semiconductor used for electrical purposes. On the other hand, carborundum is another 19th century semiconductor device but I don't know who invented that use or when. Selenium rectifiers were in common use in the 1950s Noyce is credited with a development of the integrated circuit in 1961---Kilby filed the patent for the first integrated circuit in 1959
Why a p-n junction diode conducts only in forward bias mode and not when reverse biased?
Forward biased condition:In forward biased condition, the positive terminal of the battery is connected to the p-type semiconductor and negative terminal of the battery is connected to the n-type semiconductor. We know that opposite charges attract and like charges repel. Hence, when we apply forward voltage, the large number of holes in the p-type semiconductor experience a repulsive force from the positive electric field or positive terminal and move towards the n-type semiconductor. Thus, the holes carry electric current. In the similar way free electrons experience a repulsive force from the negative electric field or negative terminal and moves towards the p-type semiconductor. Thus, the free electrons carry electric current.In this manner, both the free electrons and holes carry electric current in the forward biased condition.In forward biased biased condition, the width of depletion region is very narrow. Hence, the free electrons and holes easily overcomes the weak opposite electric field from the depletion region and cross the depletion region. Therefore, the electric current flows in the forward biased condition.Reverse biased condition:In reverse biased condition, the positive terminal of the battery is connected to the n-type semiconductor and negative terminal of the battery is connected to the p-type semiconductor. When we apply reverse voltage, the large number of free electrons in the n-type semiconductor experience a attractive force from the positive electric field or positive terminal and move towards the positive terminal of the battery. Hence, large number of atoms lose electrons and becomes positive ions. We know that depletion region is nothing but group of motionless charges or ions. Therefore, the width of depletion region increases at n-side. In the similar way, due to pulling holes away from the depletion region at p-side, the depletion width increases at p-side. Thus, the width of depletion region increases at p-n junction (p-side and n-side).In reverse biased condition, the the depletion width is very wide. Hence, it is very difficult for the free electrons and holes to overcome the strong opposite electric field from the depletion region and cross the depletion region. Therefore, no electric current flows in the reverse biased condition. About author.Asif Shaik, founder of physics and radio-electronicsSource: Forward biased p-n junction diode Reverse biased p-n junction diode