Impressions from the third-year labs

My first task in the labs, right after I returned from the last winter holidays, was to experiment with the PN junctions. PN junction is an electronic component consisting of two different types of semiconductor materials. It is formed by connecting the N- and P-type semiconductors. Semiconductors have a low bandgap compared to insulators. The bandgap is the amount of energy that electrons are required to overcome to be excited from the valence band to the conduction band. Then the electrons can move freely, leading to the flow of electrical current in a circuit. The amount of electrons is too low to enable a significant amount of current to flow in intrinsic semiconductors connected to the power supply. Therefore, chemical impurities containing atoms with a suitable electron energy structure are doped into the semiconductor. N-type semiconductors are doped by atoms that contain electrons with energy close to the conduction band. When the N-type semiconductor is connected to a power supply with applied potential difference, i.e. the power supply does work to move electrons in a circuit, the doped electrons are excited to the conduction band. Conversely, the P-type semiconductors are doped by atoms which contain electrons with energy levels close to the valence band. Here, however, when the potential difference is applied across the semiconductor, the electrons tear off, leading to the presence of holes in the valence band. Connecting the P- and N-type semiconductors together leads to interesting properties that depend on the type of material, temperature, applied bias, or the potential difference applied. The fundamental feature of the PN junction is that it conducts electrical current only when the junction is forward biased. A simple analogy can be a dry riverbed. When we make effort to draw water into the mouth of the river, it will result in a stream of water („electrons“) flowing through the channel. However, if we draw water into a trough in the lowland, where the river already flows into the sea, the water („electrons“) would not flow through the channel "uphill". The whole area of PN junctions is more complex than assumed based on knowledge from a secondary school. The phenomena related to PN junctions are complex and interesting. I experimented with the PN junctions in the laboratory under different conditions.

First, it was necessary to adapt to the new rules in the lab. Instructions and information about experiments were very vague. I had great freedom as an experimentalist, but it also often meant to improvise. Although I always formulated a hypothesis based on theory, experiment results did not correspond to my a priori assumptions frequently. It was also complicated to build an apparatus. There were several skills which I had to learn such as soldering, drilling threads or handling liquid nitrogen to prepare the experiments. It reminded me of my studies at a technical secondary school, where these practical skills were taught as a part of the curriculum. At that time, it was torture for me. Unfortunately, I miss sometimes the intuition for seemingly easy practical tasks. I have to admit that laboratory assistants, PhD students, provided me with great support during the lab work. More precisely, they help us with the organization of experiments and with a variety of practical tasks. They also give us good feedback on our thoughts related to the experiments. In addition, sometimes we can laugh together. A great sense of humor is very important in challenging experiments.

The most difficult measurement was the analysis of the spectrum of the LED at a temperature of about 80 Kelvin (-193.15 degrees). The LED operates on the PN junction principle. I had to first pour the liquid nitrogen into a container with the shape of a cylinder with decreasing diameter. This was not easy because the liquid nitrogen container did not have a shape as a teapot, and I did not want liquid nitrogen to spill over my desk. An advantage was that liquid nitrogen evaporates relatively quickly. The disadvantage was that touching liquid nitrogen with the bare hand would most likely lead to serious frostbite. In general, I often struggle with teasing tea, especially when the kettle has an atypical shape. Fortunately, I managed to carefully pour liquid nitrogen into the cylinder eventually. The experiment was a great test of electronic connections which I soldered with tin. Meanwhile, liquid nitrogen in the cylinder bubbled as it evaporated, and it was challenging to make measurements. It was necessary to precisely focus the beam of visible light emitted from the LED to the spectrometer. In addition, the diode properties at such a low temperature changed, so I suddenly needed to set a high voltage at the power supply. I succeeded in destroying several LEDs because when the LED was removed from liquid nitrogen, the applied voltage was too high. I managed to successfully measure the spectrum in the end. The LED in liquid nitrogen emitted light of different colour than when LED was placed in the air. This is because the colour of the emitted light depends on the wavelength of emitted photons. Atoms in LED held at low temperatures needed high energy to excite their electrons into the conduction band. I felt excited when this colourful experiment succeeded! The electrons in the conduction band fall back into the valence band subsequently and photons were emitted.

I think that labs are an important part of the physics studies, although many of my coursemates have given up on them. They allow us to verify some physical principles in practice. It is then much easier to understand lecture courses, in this case, solid state physics course. Moreover, scientific experiments are the basic way of verifying different theories. I always have a good feeling after a lab session, because I frequently discover new relations between theory and practice. I would not ever be able to deduce some of the findings by studying theory only.