need help...confusion in some questions

helllo...
hope you fine...
can you reply me for that questions with good reasons?
q#1 why do we need input impedence of a transistor high for amplifiers?
q#2 why do we need input impedence of a transistor some times low in various applications?
q#3 how doping level effects our devices such as transistors?why we use different doping levels such as high,,low,,medium in transistors??

i will be very happy if you answer my questions...
thanks...