The excess minority carriers on both sides of the junction have two effects. First, the carriers create an electric field. The excess holes in N-type silicon make it positively charged, and the excess electrons in P-type silicon make it negatively charged. In this way, an electric field with a high potential in the N region and a low potential in the P region is established along the PN junction.
When the carriers diffuse along the junction, an equal number of ionized impurity atoms are generated at the same time. These atoms are fixed in the crystal structure and cannot move. In the P region of the junction, there are acceptors with negative charge after ionization. In the N region of the junction, there are donors with positive charge after ionization. In this way, an electric field with high potential in the N region and low potential in the P region is established. This electric field will be superimposed on the electric field established by the carriers.
Carriers tend to drift in an electric field. Holes are attracted to the low potential P region of the junction. Similarly, electrons are attracted to the high potential N region of the junction. The drift of carriers is in opposition to their diffusion. Holes that diffused from the P region of the junction to the N region are drifted back. Electrons that diffused from the N region of the junction to the P region are also drifted back. When the diffusion current and drift current are equal in magnitude and opposite in direction, equilibrium is established. As the voltage along the junction reaches equilibrium, the excess minority carrier concentration on both sides of the junction also reaches an equilibrium value.
The voltage difference along the PN junction at equilibrium is its internal voltage, or contact voltage. In a typical silicon PN junction, the value of the internal voltage ranges from a few tenths of a volt to 1 volt. The internal voltage of a heavily doped junction is larger than that of a lightly doped junction. Since more carriers diffuse along the heavily doped junction when heavily doped, the diffusion current is larger. In order to achieve equilibrium, a larger drift current is required, thus forming a stronger electric field. Therefore, the internal voltage of a heavily doped junction is larger than that of a lightly doped junction.
Although the internal voltage is real, it cannot be measured with a voltmeter. This difficulty can be explained by a circuit containing a PN junction and a voltmeter (Figure 1.9). The two probes of the voltmeter are metal, not silicon. The contact points between the metal probes and the silicon also form junctions, and each junction has its own contact voltage. Since the silicon under the two probes has different doping levels, the contact voltages at the two contact points are different. The difference between the two contact voltages exactly cancels the internal voltage of the PN junction, so no current flows in the external circuit. This situation is inevitable, otherwise any current would mean a free energy source, or some kind of perpetual motion machine. The cancellation of the internal electric field ensures that no energy can leak out in a balanced PN junction, and it is impossible to violate the laws of thermodynamics. 
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Figure 1.9 Illustration of the impossibility of directly measuring the internal electric field. The contact voltages V1 and V3 exactly cancel the internal electric field V2.
The internal electric field has two causes: scattered ionized impurity atoms and scattered charged carriers. The carriers are free to move, but the impurity atoms are fixed in the crystal structure. If the impurity atoms were able to move, they would attract each other because they have opposite charges to the carriers. They are separated from each other because they are fixed in the crystal structure. The region occupied by these charged atoms forms a strong electric field. Any carrier that enters this region must pass through quickly or it will be swept away by the electric field. As a result, this region has very few carriers at any time. Because of the presence of charged impurity atoms, this region is sometimes called the space charge layer. But usually, it is called the depletion region because of the relatively low carrier concentration here.
If there are only a few carriers in the depletion region, then excess minority carriers must accumulate on both sides of it. Figure 1.10 shows the distribution of excess minority carriers graphically. The concentration gradient causes the carriers to diffuse beyond the neutral region of the junction. The electric field created by the charged carriers pushes them back into the junction. Soon equilibrium is established, forming a fixed distribution of minority carriers like the one in Figure 1.10.
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Figure 1.10 The concentration of excess minority carriers is balanced on both sides of the PN junction.
The characteristics of the PN junction can be summarized as follows: The diffusion of carriers along the junction produces excess minority carrier concentrations on both sides of the depletion region. The scattered ionized impurity atoms generate an electric field along the depletion region. This electric field prevents majority carriers from crossing the depletion region, and those carriers that cross are eventually pushed back to the other side by the electric field.
The thickness of the depletion region depends on the doping level on each side of the junction. If both sides are lightly doped, then a very thick depletion region must be present in the silicon in order to have enough impurity atoms to establish the internal electric field. If both sides are heavily doped, then only a very thin depletion region is needed to establish the necessary electric field. Therefore, heavily doped junctions have thin depletion regions and lightly doped junctions need thick depletion regions. If one side of the junction is much more doped than the other, then the depletion region in the lightly doped region will be much thicker. In this case, the lightly doped silicon needs a very thick depletion region to get enough ionized impurity atoms. In the heavily doped region, only a very thin depletion region is needed to get the ionized impurity atoms for balance. Figure 1.10 shows the case where the N region of the junction is lightly doped than the P region.
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