So, here we go one by one:

Before start semiconductor we should have some basic knowledge about signal.

signal contain information about a veriety of things and activities in our physical world.like Information about the weather is contained in signals that contains the air temp. , pressure, wind speed etc. 

Now the question is  how to extract the information from that particular signal?

> extract the information from the signal one needs to process the signal(means signal will go through some process) in some  predetermined manner.

For this to be possible signal must be converted into electrical signal, e.g voltage and current

This process accomplished by a device known  as Transducer.

>> Don't confused with the term signal processing its just like, if you need oil from coconut then to extract the oil, coconut must go through some process . 

Types of signal

1. Analog

2. Digital

1. Analog signal: tt It derived from the fact that such signal is analogous to the physical signal that it represent.

It can take any value at any time within the given limit.

e.g, The amplitude of an analog signal exhibits a continous variation over its range of activity.

2. Digital signal: Its a representation of signal as sequence of numbers, each numbers represents the signal magnitude at an instant of time, the resulting signal is called as Digital signal.

ITHis 

contains veriety