And welcome to a new post series. The posts in this series will be computer-based (as you can understand with the title). I will try to teach you the functioning of your computer components for example from scratch. Of course, it will take time and will not be easy for everyone to understand, that's why I will try to image as much as possible so that they are understandable to the greatest number. Of course, it will take time and will not be easy for everyone to understand, that's why I will try to image as much as possible so that they are understandable to the greatest number.
Electrons
To start talking about transistors, I first have to explain what semiconductor materials are and for that we will have to talk a little about quantum physics (do not be afraid, we will not go into the details with complicated explanations). In quantum physics, the representation of the atom and its electrons is different from what you have seen in classical physics. Indeed in quantum physics, the electrons are not positioned on "orbits" around the nucleus, in fact they do not really have a definite place either. In reality, they have a probability of being at a place around the nucleus, this probability is called an atomic orbital. This probability is described by 3 quantum values and a 4th value called the spin, it is the quantum state of the electron, for each quantum state there exists a given energy. In an atom, there can only be one electron per quantum state.
Conductivity
Now that you know a little more, let's talk about the energy of an electron. In solids, electrons can take energy values only between certain intervals, these intervals are called "allowed band", between these bands there are other bands, which they are called "forbidden bands", the difference between two permitted bands is called gap. To study the conductivity of a material we put the latter to its ground state. This is the state of the material when its temperature tends towards 0 ° K is -273,15 ° C. In this state, we can observe 3 different cases. To do this one studies the last permissible band full of electrons - called the valence band - and the one just above the latter - called the conductivity band. The first case (actually, there is no order) is that of an insulating material, when a material is insulating, its conductivity band is empty. And the gap between the valence band and the conductivity band is of the order of 10eV the gap is so large that no electron is able to go in the conduction band. The second case is that of a conductive material, in which case the conductivity band contains electrons in small quantities even at the absolute 0 temperature. A weak electric field can pass an electron to the higher energy level, so we can conclude that this material is conductive. And finally, let's talk about semiconductor materials. In the ground state, the conductivity band of these materials is empty, but the gap between the latter and the valence band is small, of the order of eV, then a simple excitation, or a temperature change allows some electrons to cross the gap and reach the conductivity band, making the material conductive.
Now you know what a semiconductor material is, but you are wondering what elements of our good old periodic table are conductive elements. The semiconductor elements are in column IV of the classification, but be careful, all the elements of column IV are not necessarily semiconductors, it depends on the "size" of the gap of these elements. For the rest of this article we will only use silicon which is a semiconductor element. All semiconductor elements have 4 valence electrons (electrons on their outer layer)
Doping
Now let's talk about doping (no it's not the doping that you have in mind with cyclists who inject not very licit substances to improve their performance, but the result is not very different). There are two types of doping in semiconductor elements: P - positive doping - and N - negative doping.
P-type doping consists in adding atoms of column III to semiconductor materials, this will have the effect of creating materials having 7 electrons on their outer layer, these materials will therefore be positively charged and will have an electron defect, this is why P type doped semiconductors are called "holes". The atoms of these materials are therefore very conductive, because under certain conditions, an electron can be transmitted from one atom to another.
N-type doping, unlike P type doping in N-type doping, has introduced atoms belonging to column V of the classification (atoms having 5 electrons on their outer layer), when the dopant comes into contact with the silicon (or any other semiconductor element), they share pairs of electrons, and they therefore contain 8 electrons on their outer layer. Except that there were 9 electrons present on the outer layer of these atoms, an electron of the dopant is "solitary", it became a free electron capable of conducting the current.
Now that you know what's going on at the atom level, we're going to talk about more concrete things. I told you that semiconductor materials are boundary materials between insulator and conductor. In reality, these materials are conductive only if the current passing through them exceeds a certain threshold, this threshold varies according to the materials. It is 0.6V for silicon. Doping can reduce this threshold and give some properties to the materials but we will see this in a future post which will be focused on transistors and diodes.
