Charge Carriers in Semi-Conductors
One of the most important characteristics of semi- conductors is the fact that both positive and negative change carriers exist. When an electron is excited from the valence band to the conduction band, a positive "hole" is left in the valance band while the excited electron becomes a negative charge carrier in the conduction band. Another electron in a nearby position in the valence band is then free to fill the positive hole, leaving a hole in the spot where that electron originated. In this fashion, the hole appears to move about the valence band of the crystal and is functionally a positive charge carrier.
N-type and P-type Semi-Conductors: Doping
For a pure semi-conductor the concentration of holes and electrons can be determined using a simple equation from thermal physics, but this ratio can also be manipulated by adding a certain amount of impurity atoms to the semi- conductor crystals in a process called doping. By introducing impurities with a different number of valence electrons, the number of available charge carriers in the semi-conductor can be increased. An important consequence of doping is the creation of intermediate energy levels in the forbidden region because of the excess charge carriers.
By introducing an impurity with five valence electrons (one more than the normal four) such as arsenic or phosphorus, the semi-conductors will have additional electrons for excitation into the conduction band. Electrons will then become the majority charge carriers in the semi-conductor. These type of doped semi-conductors are called n-type semi-conductors. The extra electron is nearly free in the crystal and has an energy level that lies in the forbidden energy gap, just below the conduction band. This energy level is called the donor level and typically lies 0.05 eV below the conduction band in silicon.
By introducing an impurity with three valence electrons (one less than the normal four) such as gallium or indium, there will be a shortage of electrons available for covalent bonding in the crystal and an excess of "holes" in the valence level of the crystal. These are called p-type semi- conductors and have holes as the majority charge carriers. These holes produce acceptor energy levels in the forbidden region which lie above the valence band (approximately 0.05 eV higher for silicon). A representative drawing of these intermediate energy levels is shown in Fig. 16.
Np Junctions
When one n-type semi-conductor and one p-type semi- conductor are placed together, the resulting device has some very special properties. Due to the fact that each semi- conductor contains charge carriers of differing polarity, the negative electrons in the n-type semi-conductor will be drawn towards the positive holes in the p-type semi- conductor and vice versa. The charge carriers will subsequently diffuse into the neighboring area until a thermal equilibrium is reached in which the thermal energy of an individual charge carrier is no longer great enough to excite it over the newly formed depletion region around the np-junction. As shown in Fig. 17, this thermal equilibrium results in a depletion region surrounded by the remaining positive charge carriers in the p-type semi- conductor and the remaining negative charge carriers in the n-type semi-conductor. The resulting energy diagram is also shown in Fig. 17.
A fact that is often confusing to some is that, after the thermal equilibrium is reached, the n-type semi- conductor now possesses a net positive charge and the p-type side possesses a net negative charge. The explanation for this is fairly straightforward. Before the two semi- conductors were joined, each was independently neutral. After the two were joined, electrons in the n-type semi- conductor were attracted to the positive holes in the p-type semi-conductor and diffused over the junction. When the n- type semi-conductor lost some of its negative charge carriers, it was left with a resultant positive charge. The same argument holds for the p-type semi-conductor becoming negatively charged because it loses holes and gains electrons through diffusion.
The depletion region is thereby depleted of mobile charge carriers as a thermal equilibrium is reached, and a barrier is created between the n-type and p-type regions. As argued above, there is also an electric field produced in this region due to the produced net positive and negative charges. This field typically has a magnitude between 1,000 and 10,000 V/cm and is shown in Fig. 18.