what a black of P-type material is joined to a block of N-type material. a very useful structure result. The region where join is called a PN junction and is a fundamental component (If many electronic devices transistor’>. The junction is not formed by slmrlV r acing the 0 materials adjacent to each otl er, but rather through a manufactu ‘1It; Ct,” cess that creates a transition from P to N within a single crystal. Nevertheless, It IS Instructive to view the formation of the junction in terms of the charge redistribution that would occur if two dissimilar materials were. in fact, suddenly brought Into very close contact with each other.
Let us suppose that a block of P-type material on the left is suddenly joined to a block of N-type material on the right as illustrated in Figure 2-8(a). In the figure. the accept or atoms and their associated “excess” holes are shown in the P region. Remember that the P region is initially neutral because each acceptor atom has the same number of electrons as protons. Similarly, the donor atoms are shown with their associated “excess” electrons in the N region, which is likewise electrically neutral. Remember also that diffusion current flows whenever there is a surplus of carriers in one region and a corresponding lack of carriers of the same kind in another region. Consequently, at the instant the P and N blocks are joined, electrons from the N region diffuse into the P region, and holes from the P region diffuse into the N region. (Recall Ihat this hole current is actually the repositioning of holes due to the motion of valence-band electrons.)
For each electron that leaves the N region to cross the junction into the P region. a donor atom that now has a net positive charge is left behind. Similarly. for each hole that leaves the P region (that is, for each accept or atom that captures an electron), an accept or atom acquires a net negative charge. The upshot of this process is that negatively charged accept or atoms begin to line the region of the junction just inside the P block. and positively charged donor atoms accumulate just inside the N region. This charge distribution is illustrated in Figure 2-8(b) and is often called space charge
It is well known-that accumulations of electric charge of opposite polarities in two separated regions cause an electric field to be established between those regions. In the case of the PN junction, the positive ions in the N material and the negative ions in the P material constitute such accumulations of charge, and an electric field is therefore established. The direction of the field (which by convention is the direction of the force on a positive charge placed in the field) is fromthe positive N region to the negative P region. Figure 2-9 illustrates the field E developed across a PN junction.
Note that the direction of the field is such that it opposes the flow of electrons from the N region into the P region and the flow of holes from the P region into the N region. In other words. the positive and negative charges whose locations
were caused by the original diffusion current across the junction are now inhibiting the further flow of current across the junction. An equivalent interpretation ‘s that the accumulation of negative charge in the’ P region prevents additional neg, tive charge from entering that region (like charges repel), and, similarly, the positively charged N regionrepels additional positive charge. Therefore, after the initial surge of charge across the junction, the diffusion current dwindles to a negligible amount. The direction of the electric field across the PN junction enables the flow of drift current from the P to the N region, that is, the flow of electrons from left to right and of holes from right -to left. in Fig re 2-9. There is therefore a small drift of minority carriers (electrons in the P material and holes in the N material) in the opposite direction from the diffusion current. This drift current is called reverse current, and when equilibrium conditions have been established, the small reverse drift current exactly cancels the diffusion current from N to P. The net current across the junction is therefore O.
In the region of the junction where the ,charged atoms are located, there are no mobile carriers (except those that get swept immediately to the opposite side).region holes have been annihilated by electrons, and the electrons have migrated to the P side. Because all Charge carriers have been depleted (removed) from this region, it is called the depletion region. See Figure 2-9. It is also called the barrier region because the electric field therein acts as a barrier to further diffusion current, as we have already described. The width of the depletion region depends on how heavily the P and N materials have been doped. If both sides have been doped to have the same impurity densities (not the usual case), then the depletion region will extend an equal distance into both the P and N sides. If the doping levels are not equal, the depletion region will extend farther into the side having the smaller impurity concentration. The width of a typical depletion region is on the order of 10 16 m. In the practical PN junction, there is not’ necessarily the abrupt transition from P- to N-type material shown in Figure 2-8. The junction may actually be formed, for example, by a gradual increase in the donor doping level of one block of P-type material, so that it gradually changes its nature from Pstypeto N-type with increasing distance through the block.
The electric field shown in. Figure 2-9 is the result of the potential difference that exists across the junction due to the oppositely charged sides of, the junction. This potential is called the barrier potential because it acts as a barrier to diffusion current. (It is also called a junction potential, or diffusion potential.) The value of the barrier potential, Vo, depends on the doping levels in the P and N regions, the type of material (Si or Ge), and the temperature. Equation 2-10 shows how these variables affect V.
Note that the barrier potential is directly proportional to temper utu re. As we shall see throughout the remainder of our study of semiconductor devices, temperature plays;a very important role in determining device characteristics and therefore has an important bearing on circuit design’ techniques. The quantity kT/q in equation
2- \0 has (he units of volts and is called the thermal.
