One cannot understand the working of electronic devices especially diode and transistor without knowing the mechanism of current flow in electronic materials. Silicon (Si) is the most used electronic material in making of electronic devices. Its crystalline structure is given in fig-1 which will help us in understanding the working principle of electronic devices especially diode and transistor.
Atomic number of Si is 14. Every Si atom has four valence electrons. In intrinsic (Pure) form one Si atom makes four covalent bonds with four other Si atoms surrounding it as shown in Fig-1
At room temperature Si atom has enough energy for some of its valence electrons so they can jump to conduction band. When an electron jumps to conduction band it leaves an empty space behind it called “hole”. This is also known as electron-hole pair (See fig-2).
At any instant piece of Si material has some free electrons moving randomly between atoms or moving in conduction band. When we apply the voltage or potential difference across the two ends. The randomly moving electrons in conduction band are attracted by the positive terminal of the battery. Hence they start moving towards positive end. This is called “electronic current”.
When electrons move in one direction. It seems that holes are moving in the opposite direction. This is named as “hole current”. All types of semiconductor materials have electron current and hole current.