First-generation computers were characterized by the use of vacuum tubes as their principal electronic component. vacuum tubes are bulky and produce a lot of heat,so first generation computers were large and required extensive air conditioning to keep them cool.In addition,because vacuum tubes do not operate very fast,there computers were relatively slow.
In the second generation of computers, transistors replaced vacuum tubes.Although invented in 1948,the first all-transistor computer did not become available until 1959.Transistors are smaller and less expensive than vacuum tubes,and they operate faster and produce less heat.Hence,with second-generation computers,the size and cost of computers decreased,their speed increased,and their air conditioning needs were reduced.
The technical development that marks the third generation of computers is the use of integrated circuits or ICS in computers.An integrated circuit is a piece of silicon(a chip)containing numerous transistors.One IC replaces many transistors in a computer;result in a continuation of the trends begun in the second generation.These trends include reduced size,reduced cost,increased speed,and reduced need for air conditioning.
The fourth generation of computers is more difficult to define than the other three generations.This generation is characterized by more and more transistors being contained on a silicon chip.First there was Large Scale Integration(LSI),with hundreds and thousands of transistors per chip,then came Very Large Scale Integration(VLSI),with tens of thousands and hundreds of thousands of transistors.The trend continues today.