Forward Biased Diode

Figure 10-198 illustrates a forward biased PN junction. When an external voltage is applied to a PN junction, it as called bias. In a forward biased PN junction or diode, the negative voltage source is connected to the N-type material and the positive voltage source is connected to the P-type material. In this configuration, the current can easily flow. If a battery is used to bias the PN junction and it is connected in such a way that the applied voltage opposes the junction field, it will have the effect of reducing the junction barrier and consequently aid in the current flow through the junction.

The electrons move toward the junction and the right end of the diode becomes slightly positive. This occurs because electrons at the right end of the diode move toward the junction and leave positively charged atoms behind. The positively charged atoms then pull electrons into the diode from the negative terminal of the battery.

When electrons on the N-type side approach the junction, they recombine with holes. Basically, electrons are flowing into the right end of the diode, while the bulk of the electrons in the N-type material move toward the junctions. The left edge of this moving front of electrons disappears by dropping into holes at the junction. In this way, there is a continuous current of electrons from the battery moving toward the junction.

When the electrons hit the junction, they then become valence electrons. Once a valence electron, they can then move through the holes in the P-type material. When the valence electrons move through the P-type material from the right to the left, a similar movement is occurring with the holes by moving from the left side of the P-type material to the right. Once the valence electron reaches the end of the diode, it will then flow back into the positive terminal of the battery.

In summary:

  • Electron leaves negative terminal of the battery and enters the right end (N-type material) of the diode.
  • Electron then travels through the N-type material.
  • The electron nears the junction and recombines and becomes a valence electron.
  • The electron now travels through the P-type material as a valence electron.
  • The electron then leaves the diode and flows back to the positive terminal of the battery.

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