At very low values of the p-d product, electrons must accelerate beyond the ionization limit to produce an avalanche process because the likelihood of impacts is too few. In this region, the breakdown voltage decreases with increasing value of the pressure-gap product. This occurs until a minimum condition is reached. At very high values of the pressure-gap product, the number of inelastic collisions is higher and the breakdown voltage increases. This U-shaped dependence is characteristic of gas phenomenon. At the high gas pressure, secondary processes, such as light emissions occur. Figure 1.3 shows the Paschen curve highlighting its U-shaped characteristic.