How Do Semiconductor Diodes Work?


How Do Semiconductor Diodes Work?

Semiconductor diodes are often used to conduct damaging voltages away from sensitive circuits. This is accomplished by raising an external voltage across the device until it reaches its threshold voltage.

When the threshold voltage is reached, the p-n junction becomes depleted of majority charge carriers. This creates a thin intrinsic region with very high resistance.

Forward Bias

When a diode is connected in a circuit with the p-type (anode) attached to the positive end of the voltage source and the n-type (cathode) connected to the negative end, an external voltage can be applied that can supply free electrons and holes with the energy they need to cross the pn junction and decrease the width of the depletion layer. This is called forward bias.

When the applied voltage is greater than the potential barrier, electrons in the n-type material and holes in the p-type material are repelled by their respective regions of the semiconductor. The repulsion causes them to lose enough energy that they begin to move toward the junction. Eventually, they reach the junction and, if the applied voltage is great enough, they break through the depletion region and allow current to flow.

The process of electrons moving across the p-n junction and into the n-type region is known as diffusion. It can be accelerated by applying a high forward voltage or increasing the temperature of the device. However, if the forward voltage is applied to the semiconductor for an extended period of time or the current flows through it for too long, the device will dissipate more power in the form of heat than it was designed for and the current will stop flowing. This is why resistors are used in series with the device to limit the amount of current that can pass through it.

Reverse Bias

In reverse bias, the dc voltage source is connected across the diode in the opposite direction to forward bias, with the positive side of the source connected to the p region and the negative side to the n region. As with the forward voltage, this causes the depletion region to shrink. However, unlike the situation in forward bias where like charges repel, the negative side of the source essentially “pushes” the free electrons in the n region toward the junction and they combine with the holes being pushed in the same direction by the positive voltage to form current.

The high reverse-bias voltage gives these free electrons enough energy to knock valence electrons from their orbit and move into the conduction band. They are high in energy, so they continue to pass through the p-type material where they collide with atoms and knock more valence electrons dc to ac power inverter out of orbit and into the conduction band, resulting in a continuing multiplication of free electrons. This process continues until the density of conduction electrons becomes too great for the depletion zone and current stops flowing, though a small amount of leakage current may still flow through the diode.

The reduced width of the depletion region means that there is less resistance than in a non-biased state. Therefore, a lower voltage is needed to achieve the same current through the diode in reverse bias. However, if the voltage is increased past a point known as the breakdown voltage (Vbr) of the diode, the depletion zone breaks down and current starts flowing. This occurs by either the Zener or avalanche breakdown processes.

Forward Current

Forward Current is the amount of current that flows through a diode in the forward direction when a positive voltage is applied across the device. It is directly dependent on the Forward Voltage and can be plotted along with Forward Voltage in a graph called the Forward IV characteristic.

In a typical p-n diode, when a positive voltage is applied, the electrons in the n-type material are attracted Microprocessor away from the junction towards the positive electrode while holes in the p-type material are attracted toward the negative electrode and away from the junction. This creates a region depleted of charge-carriers at the junction and makes the junction less conductive. The width of this region determines the conductivity of the p-n junction.

A positive voltage applied to the n-type material pushes electrons into the valence band of the material and also raises the energy levels of the conduction bands in the p-type materials. This pushes the electron density up and the hole density down allowing free electrons to cross the p-n junction.

When the electrons reach the junction they give up their excess energy to the vacancies and the energy level of the vacancies drop significantly. This causes the recombination of the vacancies with electrons to lower the potential barrier and widen the depletion layer. As the width of the depletion region increases, the current through the p-n junction decreases.

Reverse Current

When there is no external voltage applied to the diode, free electrons in the p-type material diffuse into the near-neutral region where they combine with holes from the n-type material and become conduction electrons. This process is called minority diffusion. The negative voltage of the reverse bias causes this minor current to increase because it provides the energy to the free electrons to overcome the potential barrier and flow across the PN junction.

This is shown graphically in the Fig below where we plot the voltage vs current for forward and reverse bias. Ideally*, the diode would be completely “on” and conduct current in both directions as it has very low (ideally zero) resistance to current flow in one direction.

In reality, this is not the case, since a small amount of current is still able to flow from Cathode to Anode in the reverse direction. This current is very low and referred to as Leakage Current.

The wider the depletion region, the less current flows through a diode. This is because a large potential difference between the p and n regions increases the repulsion of the free electrons and decreases the enerty of the holes. When the potential difference reaches about half of the breakdown voltage, the depletion region becomes thinner and this allows for a higher current to flow through the diode.

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