A finer question would be what is the purpose of the substrate in a semiconductor device? and the answer is that is the foundation on which you fabricate a chip.
Let’s understand in details on use of substrate in FET and MOSFET.
Without a substrate, you will not learn the circuit built on it and use a fet or MOSFET based device; because they can not exist without the substrate.
There are two types of substrate
To talk about transistor structure, we have to talk about what is made. the transistor has a part of the metal, which is conductive and where the gate terminal is connected, the part of the oxide which is the isolator, and the part of the semiconductor that can run both ways.
In fact, in the past the gate was connected to a metal layer, but is now used as a type of silicon, called silicon poly (in short, poly). the mos name, however, is stored. Recently, gates are made of metal again for performance reasons.
Move forward to use of substrate in FET and MOSFET
When looking at the periodic table, you will see that there is a metallic zone, a non-metallic zone, and a very narrow metal zone. from this narrow region, silicon (si) and germanium (ge) have 4 valence electrons and make good semiconductors.
When many atoms with four valence electrons join in the lattice, they make a covalent bond with a 4 share their electrons with 4 atoms are adjacent and opposite (that way, they can fill the valence band with 8 electrons). because they are all atoms, the charge in the lattice is zero. in this state, the electrons are bound together into atoms by a covalent bond. semiconductors are non-conductive and are considered pure or intrinsic.
Intrinsic semiconductor materials can be processed to create free electrons. when the valenta penta atom (with 5 electrons in the valence band, like phosphorus) is added to the semiconductor, we say that we are doping silicon into an n-type material, since negatively charged electrons are being made.
This happens because the phosphorus atoms join the silicon lattice with covalent bonds, but must release the extra electrons. lattice has more protons than electrons, which makes it positively charged. the number of free electrons is proportional to the number of dopant atoms.
We can also process silicon into p-type material by creating positively charged particles. what is this proton? no, they are actually the absence of electrons, as dark is the absence of light. let me explain: by adding a trivalent atom (with 3 electrons in a valence band, like boron), the electrons will disappear from the covalent bond between silicon and dopant atoms.
Let’s call this a hole. every time the free electrons pass through the hole, it jumps into it and fills the covalent band. however, under this lattice, the electron must have jumped from another covalent bond (since there are no free electrons), which leave a hole in another covalent bond. therefore, the illusion is created that the hole moves from the covalent bond to the covalent bond, and thus it is a positively charged hole. lattice has more electrons than protons, which makes it negatively charged.
Do not forget that for pure semiconductors type-n or p-type, global charge is still zero. free charge, either negative for electron or positive for hole, is being canceled by positive or negative charge in lattice.