In a circuit, the primary function of a diode is rectification, allowing forward current to flow freely while effectively impeding reverse current. How is this achieved?
Microscopically, this is associated with the P-N junction within the internal structure of the diode.
The current-moving regions within the diode can be divided into the P-region, N-region, and the internal electric field. Working together, they ultimately facilitate current conduction and restriction. In this article, TechSparks will introduce the working principle of a diode by explaining the formation of the P-N junction.
Working Principle of a Diode
In the initial formation of semiconductors, pure materials such as silicon and germanium are utilized, priced at four. At this stage, with an equal number of electrons and holes, the material is considered electrically neutral, exhibiting poor conductivity. To address this, trivalent elements (such as boron) and pentavalent elements (such as phosphorus or arsenic) are introduced to replace some silicon or germanium atoms. Boron introduces holes, while phosphorus or arsenic introduces free electrons, forming the P-region and N-region.
Note: Taking boron as an example, due to the fact that silicon’s lattice has four electrons, the addition of boron substitutes silicon atoms and creates a compound. At this point, the fourth electron is missing in the covalent bond, resulting in the formation of a hole. A hole is a vacancy left in the atomic structure when an atom loses an electron and is considered a “hole” with a positive charge.
At this stage, the concentration of holes in the P-region is higher than in the N-region, and conversely, the concentration of electrons in the N-region is higher than in the P-region. Due to the concentration difference, high-concentration regions diffuse to low-concentration regions, causing holes from the P-region to move towards the N-region, and electrons from the N-region to move towards the P-region. During this process, holes and electrons encounter each other and recombine.
At this point you may have a question: What to do with the impurity ions (introduced trivalent and pentavalent elements) in the P-region and N-region.?
In fact, when impurities are doped, impurity ions are bound by surrounding silicon or germanium atoms. The intersection between the P-region and N-region forms a thin space charge region, or depletion region. In this region, holes and free electrons have diffused to each other and combined, while positive and negative charges interact to create an internal electric field.
This internal electric field prevents further charges from crossing the intersection, forming a barrier-like structure by:
Blocking Diffusion Movement: Creating a potential barrier by impeding charged particles through the space charge region, preventing the free diffusion of charge carriers.
Minority Carrier Drift: In the N-region, free electrons move towards the P-region due to the electric field, while in the P-region, holes move towards the N-region due to the electric field.
Once diffusion and drift reach dynamic equilibrium, the P-N junction is formed, laying the foundation for diode operation. Subsequently, by adding appropriate electrode leads and packaging to the P-N junction, a diode can be constructed.
How a Diode Works in a Circuit
The operation of a diode in a circuit is closely tied to the connection of its P-region and N-region with positive and negative power sources, known as forward bias and reverse bias.
Forward bias involves connecting the P-region of the diode to the positive power source and the N-region to the negative power source. In this configuration, electrons move from the N-region to the P-region, while holes move in the opposite direction, from the P-region to the N-region. The direction of the current is opposite to the movement of electrons, flowing from the P-region to the N-region.
In the zero-bias state, a potential barrier exists within the P-N junction, preventing free movement of electrons and holes. When a forward bias is applied, the external electric field gradually counteracts the potential barrier, causing the depletion region to thin and eventually disappear. During this process, charge carriers can cross the P-N junction, allowing current to flow. There is a forward threshold voltage in this process, also known as the forward voltage drop.
When the diode is reverse-biased, meaning the P-region is connected to the negative power source, and the N-region is connected to the positive power source, the potentials on both sides of the P-N junction become opposite, increasing the height of the barrier. This hinders the movement of electrons and holes, resulting in the diode presenting a high-resistance state.
When the reverse voltage reaches a certain value, the barrier breaks down, causing a sharp increase in current within the P-N junction. This eventually leads to reverse breakdown, often causing local damage near the P-N junction and rendering the diode nonfunctional or damaged.
Sometimes, to protect other components, a diode is intentionally reverse-biased to create a so-called reverse polarity protection diode. When the power supply is connected in reverse, this diode starts to conduct, providing a low-impedance path for the reverse current, thus protecting other components.
You might wonder: When connected to the power supply in the normal, isn’t the diode in reverse bias? Doesn’t this cause damage?
Yes, that’s why in circuit design, it’s essential to ensure that diodes connected in reverse bias have a sufficiently high reverse breakdown voltage to prevent breakdown during normal operation.
All in all, the symbol of a diode almost succinctly elucidates its working principle: current can smoothly flow from the larger end but is unable to enter from the smaller end, much like the funnel we use in daily life.
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