Understanding Forward Biased Diode Current

A semiconductor diode is essentially a p-n junction. When a voltage is applied across this p-n junction, it is said to be biased. There are two main types of biasing: forward bias and reverse bias.

The total current in a semiconductor is typically made up of two components: drift current and diffusion current.

• Drift Current: This current is caused by the movement of charge carriers (electrons and holes) under the influence of an electric field. Carriers drift in the direction of the electric field (for holes) or against it (for electrons).
• Diffusion Current: This current is caused by the movement of charge carriers from a region of higher concentration to a region of lower concentration. This movement is due to the random thermal motion of the carriers.

In a p-n junction, without any external bias, there is an equilibrium state where the diffusion of majority carriers across the junction is balanced by the drift of minority carriers caused by the built-in electric field (due to the depletion region).

Forward Biasing a p-n Junction Diode

When a p-n junction diode is forward biased, the positive terminal of the external voltage source is connected to the p-type material, and the negative terminal is connected to the n-type material. This external voltage opposes the built-in potential barrier across the depletion region.

Applying a forward bias has the following key effects:

• The width of the depletion region decreases.
• The height of the potential barrier across the junction decreases.

As the potential barrier is reduced, it becomes easier for majority carriers to cross the junction.

• Majority holes from the p-side are pushed across the now-smaller barrier into the n-side.
• Majority electrons from the n-side are pushed across the barrier into the p-side.

This movement of majority carriers across the junction is driven by the concentration gradient that is enhanced by the forward bias pushing carriers towards the junction. This flow constitutes the diffusion current.

While the potential barrier is reduced, there is still an electric field within the depletion region. This field can cause minority carriers that wander into the depletion region to be swept across, contributing to a small drift current. However, the number of majority carriers being pushed across the junction due to diffusion under forward bias is vastly larger than the number of minority carriers contributing to drift current.

Therefore, in a forward biased p-n junction diode, the current flowing through the device is overwhelmingly due to the diffusion of majority carriers across the reduced potential barrier.

The forward biased diode current is predominantly diffusion current.

Forward Biased Diode Current Components

In summary, under forward bias:

• Diffusion Current: Large, due to majority carriers crossing the reduced barrier.
• Drift Current: Small, due to minority carriers being swept across the depletion region by the remaining electric field.

The total forward current is the sum of these, but the diffusion component is much, much larger.

Revision Table: Diode Biasing Currents

Additional Information on p-n Junction Diode Operation

The current-voltage (I-V) characteristic of a p-n junction diode illustrates the behavior under different biases.

• Under forward bias, current increases exponentially with voltage after overcoming the barrier voltage (typically around 0.7V for silicon, 0.3V for germanium). This large current is the predominant diffusion current.
• Under reverse bias, only a very small leakage current flows, which is the drift current caused by minority carriers generated within the depletion region and surrounding areas, swept across by the electric field.
• Beyond a certain high reverse voltage (breakdown voltage), the current increases rapidly again due to avalanche or Zener breakdown mechanisms, but this is a different phenomenon from the normal drift/diffusion currents at lower reverse voltages.

Understanding the dominance of diffusion current in forward bias and drift current in reverse bias is fundamental to analyzing diode circuits.