CURRENTS THROUGH A p-n JUNCTION
We can understand the behavior of a p-re junction diode qualitatively on the basis of the mechanisms for conductivity in the two regions. Suppose, as in Fig. 44-27a. you connect the positive terminal of the battery to the p region and the negative terminal to the n region. Then the p region is at higher potential than the n, corresponding to positive V in Eq. (44-23), and the resulting electric field is in the direction p to n. This is called the forward direction, and the positive potential difference is called forward bias. Holes, plentiful in the p region, flow easily across the junction into the n region, and free electrons, plentiful in the n region, easily flow into the p region; these movements of charge constitute a forward current. Connecting the battery with the opposite polarity gives reverse bias, and the field tends to push electrons from to n and holes from n to p. But there are very few free electrons in the p region and very few holes in the n region. As a result, the current in the reverse direction is much smaller than that with the same potential difference in the forward direction.
Suppose you have a box with a barrier separating the left and right sides: You fill the left side with PF3 gas and the right side with N, gas. What happens if the barrier leaks? PF3 diffuses to the right, and Nz diffuses to the left. A similar diffusion occurs across a p-n junction. First consider the equilibrium situation with no applied voltage (Fig. 44-28). The many holes in the p region act like a hole gas that diffuses across the junction into the n region. Once there, the holes recombine with some of the many free electrons. Similarly, electrons diffuse from the n region to the p-region and fall into some of the many holes there. The hole and electron diffusion currents lead to a net positive charge in the n region and a net negative charge in the p region, causing an electric field in the direction from n to p at the junction. The potential energy associated with this field raises the electron energy levels in the p region relative to the same levels in the n region. There are four currents across the junction, as shown. The diffusion processes lead to recombination currents of holes and electrons, labeled ip, and in in Fig. 44-28. At the same time, electron-hole pairs are generated in the junction region by thermal excitation. The electric field described above sweeps these electrons and holes out of the junction; electrons are swept opposite the field to the n side, and holes are swept in the same direction as the field to the p side. The corresponding currents, called generation currents, are labeled ipg and ing• At equilibrium the magnitudes of the generation and recombination currents are equal:
Now we apply a forward bias, a positive potential difference V across the junction. A forward bias decreases the electric field in the junction region. It also decreases the difference between the energy levels on the p and n sides (Fig. 44-29) by an amount
IlE = -eV. It becomes easier for the electrons in the n region to climb the potentialenergy hill and diffuse into the p region and for the holes in the p region to diffuse into the n region. This effect increases both recombination currents by the Maxwell- Boltzmann factor e..•.E1kT= e’VikT. (We don’t have to use the Fermi-Dirac distribution because most of the available states for the diffusing electrons and holes are empty, so the exclusion principle has little effect.) The generation currents don’t change appreciably,
so the net hole current is in agreement with Eq. (44-23). This entire discussion can be repeated for reverse bias (negative V and l) with the same result. Therefore Eq. (44-23) is valid for both positive
and negative values. Several effects make the behavior of practical p-n junction diodes more complex than this simple analysis predicts. One effect, avalanche breakdown, occurs under large reverse bias. The electric field in the junction is so great that the carriers can gain enough energy between collisions to create electron-hole pairs during inelastic collisions. The electrons and holes then gain energy and collide to form more pairs, and so on. (A similar effect occurs in dielectric breakdown in insulators, discussed in Section 44-5.) A second type of breakdown begins when the reverse bias becomes large enough that the top of the valence band in the p region is just higher in energy than the bottom of the conduction band in the n region (Fig. 44-30). If the junction region is thin enough, the probability becomes large that electrons can tunnel from the valence band of the p region to the conduction band of the n region. This process is called Zener breakdown. It occurs in Zener diodes, which are widely used for voltage regulation and protection against voltage surges.