9.2.3 Diodes, Junctions, and Recombination

The simplest semiconductor device is called a diode, a name meaning that it has two electrical terminals. The first electronic diodes were vacuum tubes that transmitted current in one direction but not the other. Semiconductor diodes, likewise, normally transmit current in only one direction, although there are exceptions.

A semiconductor diode consists of regions of p- and n-type material which meet at a junction layer. In practice, a diode often is made by diffusing an excess of one type of dopant into a slab of semiconductor doped with the other type. For example, an electron acceptor such as aluminum or gallium can be diffused into a slab of n-type silicon, forming a top layer of p-type material in which the holes outnumber the electrons. A junction region in which holes and electrons are equal in number separates the p-type region from the n-type region. The junction typically is only 0.1 to 1 μm thick, and it is the place where the current flow changes and important things happen. Exactly what happens depends on the voltage applied across the junction.

If there is no bias across the junction, charge carriers are distributed through the crystal in roughly the same way as impurities and not much happens. Near the junction, electrons from the n-type material can fall into holes in the p-type material, releasing energy by a process called recombination as the electron drops from the conduction band to fill the hole in the valence band. This process reaches an equilibrium, and no net current flows.

No current also flows if the diode is reverse biased by applying a positive voltage to the n side of a junction and a negative voltage to the p side. As shown in Figure 9-3, the positive electrode attracts electrons from the n-type material, and the negative electrode attracts holes from the p-type material. This draws carriers away from the junction, and virtually no current flows through the junction. The leakage current is slightly above zero, because the semiconductor's resistance is not infinite, and a much higher current can flow if a high voltage is applied across the diode, causing breakdown.

Current flows through the diode that is forward biased by applying a positive voltage to the p side and a negative voltage to the n side. As shown in Figure 9-4, this attracts the p carriers to the nside of the device and vice versa, making them recombine at the junction. The electron hole pair is called an exciton, which exists briefly before releasing its extra energy as the electron falls into the hole in the valence band, releasing the band-gap energy at the

junction. Once the applied voltage exceeds the band-gap energy- typically 0.5 to 2 electronvolts-current flows through the diode. This produces a voltage drop at the junction equal to the band-gap energy.

LEDs are forward-biased diodes in which the recombination energy is released as light, described in more detail later. It is also possible for light shining on a semiconductor diode to release energy if it has enough energy to excite a valence-band electron to the conduction band, creating an electron hole pair. This effect can be used to sense light in photodetectors (described in Section 5.8.1) or to generate electricity in solar cells.