ANALOGUE ELECTRONICS I

This is the process of connecting a DC voltage source across a diode so that the diode can conduct current in one direction and block it in the other direction.

Under no Bias conditions (i.e. bias voltage = 0 V), a diode doesn’t conduct current due to the barrier potential associated with the depletion region. That is, in the absence of an applied bias across a semiconductor diode, the net flow of charge in one direction is zero as shown below.

Diode can either be forward biased or reverse biased:

1.1.1 Forward Biasing (V_BIAS  > 0 V)

Forward bias is the condition that allows current through the pn junction. Figure 2–3 shows a dc voltage source connected by conductive material (contacts and wire) across a diode in the direction to produce forward bias. This external bias voltage is designated as V_BIAS. The resistor limits the forward current to a value that will not damage the diode. Notice that the negative side of V_BIAS is connected to the n region of the diode and the positive side is connected to the p region. This is one requirement for forward bias. A second requirement is that the bias voltage, V_BIAS, must be greater than the barrier potential.

Because like charges repel, the negative side of the bias-voltage source “pushes” the free electrons, which are the majority carriers in the n region, toward the pn junction. This flow of free electrons is called electron current. The negative side of the source also provides a continuous flow of electrons through the external connection (conductor) and into the n region.

The bias-voltage source imparts sufficient energy to the free electrons for them to overcome the barrier potential of the depletion region and move on through into the p region.
Once in the p region, these conduction electrons have lost enough energy to immediately
combine with holes in the valence band.

Now, the electrons are in the valence band in the p region, simply because they have lost too much energy overcoming the barrier potential to remain in the conduction band. Since unlike charges attract, the positive side of the bias-voltage source attracts the valence electrons toward the left end of the p region. The holes in the p region provide the medium or “pathway” for these valence electrons to move through the p region. The valence electrons move from one hole to the next toward the left. The holes, which are the majority carriers in the p region, effectively (not actually) move to the right toward the junction. This effective flow of holes is the hole current.

As the electrons flow out of the p region through the external connection (conductor) and to the positive side of the bias-voltage source, they leave holes behind in the p region; at the same time, these electrons become conduction electrons in the metal conductor. Recall that the conduction band in a conductor overlaps the valence band so that it takes much less energy for an electron to be a free electron in a conductor than in a semiconductor and that metallic conductors do not have holes in their structure. There is a continuous availability of holes effectively moving toward the pn junction to combine with the continuous stream of electrons as they come across the junction into the p region.

When forward bias is applied, the free electrons are provided with enough energy from the bias-voltage source to overcome the barrier potential and effectively “climb the energy hill” and cross the depletion region. The energy that the electrons require in order to pass through the depletion region is equal to the barrier potential. In other words, the electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region. This energy loss results in a voltage drop across the pn junction equal to the barrier potential (0.7 V). An additional small voltage drop occurs across the p and n regions due to the internal resistance of the material. For doped semiconductor materials, this resistance, called the dynamic resistance, is very small and can usually be neglected.

1.1.1.1  The Effect of Forward Bias on the Depletion Region

The depletion region narrows and a voltage drop is produced across the pn junction when the diode is forward-biased. As more electrons flow into the depletion region, the number of positive ions is reduced. As more holes effectively flow into the depletion region on the other side of the pn junction, the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow, as indicated in the figure below.