We have already learned something about circuit analysis, while to really build our circuits into practical ones, there’s still much more to do.
A simple question is that, if we want to amplify our signals, like turning up the radio or enlarging an image, what kind of circuits should we build? Apparently simple resistors, capacitors an|d inductors cannot manage tasks like that.
Therefore, we need the theory of p–n junction and some semiconductor devices.
1. P-type & N-type semiconductors
When we are talking about “computers” and “chemical elements”, one term always comes into our minds: “Silicon“ or Si. It’s a perfect tetravalent metalloid, with 4 outermost electrons, hence it can be changed to be conductor or nonconductor easily. It is, actually, a kind of intrinsic semiconductor: It is a pure semiconductor without any significant dopant species present. In intrinsic semiconductors the number of excited electrons and the number of holes are equal.
What we are going to do is doping, which means to combine our silicon with some other elements like phosphorus or Boron. This kind of doping will change the intrinsic semiconductors into doped ones. Their electrons will outnumber holes, which means they are ultimately N-type semiconductor, or their holes will outnumber electrons, which means they are ultimately P-type semiconductor. After the doping processes, we can assemble our semiconductors and make them into semiconductor devices.
2. Diode & P-N Junction
The simplest application of P-type & N-type semiconductors should be the production of diodes. We always utilize this kind of components as lights or to indicate the directions of currents in the circuits for us. Actually the structure of a diode is exactly a p–n junction.
We can imagine the electrons and holes as grains of sand with negative and positive charges. We already know from our high school physics lessons that something with a higher concentration will always diffuse themselves, and something with a lower concentration will always drift themselves, these are exactly what our “sands” will do. When we put a piece of P-type semiconductor and a piece of N-type semiconductor together, the majority carriers in these semiconductors will diffuse, and the minority carriers will drift themselves. While they meet each other on the boundary, departure of an electron from the N-side to the P-side leaves a positive donor ion behind on the N-side, and likewise the hole leaves a negative acceptor ion on the P-side. This creates an electric field that provides a force opposing the continued exchange of charge carriers. Just like the sands pile up and finally become a dam. This sand dam stop the sands, which are electrons and holes, from flowing. The sand dam is a p-n junction, or called depletion region.
One of the distinct characteristics of a p-n junction is that it has forward bias and reverse bias.
In forward bias, the p-type is connected with the positive terminal of a battery and the n-type is connected with the negative terminal. The external electric field will push the majority carriers into the p-n junction. This makes the depletion region, or p-n junction, narrower. Just like we lower our sand dam so that the sands can flow smoothly. Similarly, under reverse bias, the electric field in the p-n junction become stronger, just like we make a higher dam to stop the sand currents.
- The current formula in a p-n junction:$$\begin{aligned} i=I_{S}(e^{\frac{qu}{kT}}-1)=I_{S}(e^{\frac{u}{U_{T}}}-1) \end{aligned}$$ Where $I_{S}$ is the reverse saturation current, $q$ is the charge of an electron, $k$ is Boltzmann constant, $T$ is thermodynamic temperature. We can replace $\frac{kT}{q}$ with $U_{T}$. When $T=300K$, $U_{T}\approx 26mV$.
- Important features of a diode including the maximum forward rectified current ( $I_{FM}$ ), the maximum reverse operating voltage ( $U_{RM}$ ), cut-in voltage ( $U_{on}$ ), etc. The average $U_{on}$ of a silicon diode is about 0.7V while for a germanium diode it’s about 0.5V.
- There are different types of diodes, like LED, Schottky diode, Zener diode, etc.