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2-3. Conductivity modulation

In the previous subsections, we have discussed the pn and metal-semiconductor junctions. A pn junction is a bipolar junction because both electrons and holes act as charge carriers whereas a metal-semiconductor junction is a unipolar junction (also called a monopolar junction) because either electrons or holes act as charge carriers.

Figure 2-5 pn junction forward-biased at V<sub>F</sub> — Figure 2-5 pn junction forward-biased at V_F

Figure 2-6 Metal-semiconductor junction forward-biased at V<sub>F</sub> — Figure 2-6 Metal-semiconductor junction forward-biased at V_F

To create a pn junction, a lightly doped n-type (or p-type) semiconductor substrate is heavily doped with boron (B) or other p-type dopant using a diffusion, epitaxial growth, or ion implantation process. Therefore, the lightly doped n-type (or p-type) substrate acts as a series resistor. Note that the conductivity modulation of the pn junction causes its series resistance to decrease.
Typical pn junction diodes consist of heavily doped p-type (p⁺) and n-type (n⁺) regions on either side of a lightly doped n (n^–) region. Diodes with an extremely lightly doped n^– region are called PIN diodes. Typical pn junction diodes have a structure similar to that of PIN diodes although I layers of pn junction diodes are more heavily doped.

Electrons from a power supply flow into the n^– region via the n⁺ region. To maintain electrical neutrality, holes are injected into the n^– region from the p⁺ region. These electrons and holes recombine and disappear ultimately. Carrier lifetime is defined as the average time required for this recombination. Because both electrons and holes exist in the n^– region during this period, it exhibits low resistance as if it were a heavily doped region.

Figure 2-8 pn junction diode immediately after the application of a forward bias

Figure 2-9 Forward-biased pn junction diode in a steady state

The longer the carrier lifetime, the stronger the conductivity modulation effect, yet at the expense of an increase in reverse recovery time (i.e., the time required for a diode to stop conducting). Toshiba provides a type of diodes with a reduced reverse recovery time (i.e., a reduced carrier lifetime) called fast-recovery diodes (FRDs). The FRD has a shallower forward voltage-vs-forward current curve (i.e., higher forward resistance) than the typical pn junction diode as shown in Figure 2-10.