The diode is one of the simplest and most fundamental semiconductor devices. Its unique ability to allow current flow predominantly in one direction makes it indispensable in countless electronic applications.

A semiconductor diode is essentially a P-N junction formed by bringing together two different types of semiconductor materials: a p-type material and an n-type material.

• P-type Semiconductor: Created by doping (adding impurities) a pure semiconductor (like silicon) with trivalent atoms (e.g., Boron, Gallium). These dopants create "holes" (absences of electrons) which act as the majority charge carriers. Electrons are minority carriers.
• N-type Semiconductor: Created by doping a pure semiconductor with pentavalent atoms (e.g., Phosphorus, Arsenic). These dopants contribute "free" electrons, which act as the majority charge carriers. Holes are minority carriers.

Formation of the P-N Junction:
When a p-type and an n-type material are joined:
1. Diffusion: Due to concentration gradients, majority carriers begin to diffuse across the junction. Electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side.
2. Recombination: As electrons move into the p-side, they quickly recombine with the holes present there. Similarly, holes moving into the n-side recombine with free electrons.
3. Depletion Region Formation: This recombination process near the junction creates a region that is depleted of mobile charge carriers (electrons and holes).
4. Immobile Ions and Electric Field: As electrons leave the n-side, they leave behind positively charged donor ions. As holes leave the p-side, they leave behind negatively charged acceptor ions. These fixed, immobile ions create an electric field across the depletion region, directed from the positive ions on the n-side to the negative ions on the p-side.
5. Barrier Potential (Built-in Voltage, V0 or Vbi): This electric field establishes a potential difference across the depletion region, acting as a natural barrier that opposes further diffusion of majority carriers. This potential difference is called the barrier potential or built-in voltage. For silicon (Si) diodes, V0 ≈0.7 V at room temperature. For germanium (Ge) diodes, V0 ≈0.3 V at room temperature. This barrier prevents unlimited diffusion and establishes equilibrium in the unbiased diode.

The current-voltage (I-V) characteristic curve graphically illustrates the relationship between the current flowing through a diode (ID) and the voltage applied across its terminals (VD). It reveals three distinct regions of operation:

1. Forward Bias Region:
2. Condition: The positive terminal of the external voltage source is connected to the p-side (anode) of the diode, and the negative terminal to the n-side (cathode). This connection pushes majority carriers towards the junction.
3. Operation: When the applied forward voltage (VD) is less than the barrier potential (V0), only a very small current flows. As VD increases and exceeds the barrier potential, current begins to flow exponentially, increasing rapidly. This voltage at which significant current begins to flow is often called the "knee voltage" or "turn-on voltage" (VON).
4. Reverse Bias Region:
5. Condition: The positive terminal of the external voltage source is connected to the n-side (cathode) and the negative terminal to the p-side (anode). This connection pulls majority carriers away from the junction.
6. Operation: The applied reverse voltage adds to the barrier potential, causing the depletion region to widen and presenting high resistance to the flow of majority carriers. Only a very small, almost constant, current flows (reverse saturation current).
7. Reverse Breakdown Region:
8. Condition: If the reverse bias voltage is continuously increased, it reaches a critical point known as the reverse breakdown voltage (VBR).
9. Operation: At VBR, an avalanche breakdown or Zener breakdown occurs, allowing a large current to flow, which can damage the diode if uncontrolled. Zener diodes are designed to operate safely in this region.

To simplify circuit analysis, diodes are often represented by simplified models. The choice of model depends on the required accuracy and the complexity of the problem.

1. Ideal Diode Model:
2. This model assumes the diode is a perfect conductor in forward bias (zero voltage drop) and a perfect insulator in reverse bias (infinite resistance).
3. Practical Diode Model:
4. Accounts for the forward voltage drop and assumes the diode behaves like a voltage source in series with small forward resistance once the applied voltage exceeds the turn-on voltage (VD). For silicon diodes, VD is about 0.7V.
5. Exponential Diode Model (Shockley Diode Equation):
6. Provides a more precise representation of diode behavior across its operating range. The equation ID = IS (e^(VD/ηVT) - 1) captures the exponential relationship between current and voltage.