P-N Junction and Semiconductor Diode Characteristics

P-N Junction

The p-n junction is the fundamental building block of almost all semiconductor electronic devices like diodes, transistors, LEDs, and solar cells. It is formed when a p-type semiconductor material is joined with an n-type semiconductor material. The unique electrical properties of the p-n junction arise from the behaviour of charge carriers (electrons and holes) at the interface between the two types of semiconductors.

P-N Junction Formation (Depletion Region, Barrier Potential)

When a p-type semiconductor is brought into close contact with an n-type semiconductor, the region near the interface undergoes significant changes. Let's consider a p-type region (majority carriers are holes, minority are electrons, fixed negatively charged acceptor ions) and an n-type region (majority carriers are electrons, minority are holes, fixed positively charged donor ions).

Diagram illustrating the formation of a p-n junction, showing diffusion of carriers and formation of depletion region

Formation of a p-n junction. (a) Initially, majority carriers diffuse across the junction. (b) Formation of immobile ions and depletion region. (c) Built-in electric field and barrier potential.

Diffusion: Due to the concentration difference of charge carriers, holes from the p-side diffuse into the n-side, and electrons from the n-side diffuse into the p-side, across the junction.
Recombination: As holes diffuse into the n-side and electrons diffuse into the p-side, they encounter and recombine with the majority carriers present there.
Formation of Immobile Ions: When an electron from the n-side diffuses into the p-side and recombines with a hole, the positively charged donor ion left behind in the n-side near the junction becomes uncovered (it's no longer neutralised by the electron). Similarly, when a hole from the p-side diffuses into the n-side, the negatively charged acceptor ion left behind in the p-side near the junction becomes uncovered (it's no longer neutralised by the hole).
Depletion Region Formation: The region near the junction, on both the p-side and n-side, where the mobile charge carriers (electrons and holes) have diffused away, leaving behind the fixed, immobile charged ions (positive ions on the n-side and negative ions on the p-side), is called the depletion region or depletion layer. It is called "depletion" because it is depleted of mobile charge carriers.
Formation of Barrier Potential and Electric Field: The accumulation of fixed positive ions on the n-side and fixed negative ions on the p-side near the junction creates a built-in electric field ($\vec{E}_i$). This field is directed from the positive charge (n-side) to the negative charge (p-side) across the depletion region. This electric field opposes further diffusion of majority carriers across the junction. It acts as a barrier.
The potential difference across the depletion region due to this built-in electric field is called the barrier potential ($V_B$) or contact potential or built-in potential. The n-side becomes positive relative to the p-side.

Energy band diagram of a p-n junction in equilibrium, showing the bending of bands and the barrier potential.
Drift Current: The built-in electric field in the depletion region also causes minority charge carriers (electrons from the p-side and holes from the n-side) to drift across the junction. This drift current is in the opposite direction to the diffusion current.

In thermal equilibrium, the diffusion current of majority carriers across the junction is balanced by the drift current of minority carriers across the junction, resulting in zero net current. The width of the depletion region and the height of the barrier potential depend on the doping concentrations of the p and n regions and the temperature. The barrier potential is typically around 0.7 V for silicon and 0.3 V for germanium at room temperature.

Semiconductor Diode

A semiconductor diode is a two-terminal electronic component formed by a p-n junction. The two terminals are connected to the p-type region and the n-type region. Diodes have the important property of allowing electric current to flow predominantly in one direction while blocking it in the opposite direction. This behaviour is called rectification.

P-N Junction Diode Under Forward Bias (I-V Characteristics)

A p-n junction diode is said to be forward-biased when a positive potential is applied to the p-type side and a negative potential is applied to the n-type side. The applied voltage is $V_f$.

Diagram of a forward-biased p-n junction diode

Forward-biased p-n junction diode. Applied voltage reduces the barrier.

Effect of Forward Bias:

The applied forward voltage $V_f$ opposes the built-in barrier potential $V_B$. The net potential barrier across the junction reduces to $(V_B - V_f)$.
The electric field in the depletion region is reduced.
The width of the depletion region decreases as more majority carriers are pushed towards the junction.
The reduced barrier allows a large number of majority carriers (holes from p-side and electrons from n-side) to diffuse across the junction. This diffusion current constitutes the forward current.
The forward current increases exponentially with the applied forward voltage $V_f$ (above a certain voltage, the knee voltage or cut-in voltage, which is close to the barrier potential $V_B$).

P-N Junction Diode Under Reverse Bias (Breakdown Voltage)

A p-n junction diode is said to be reverse-biased when a negative potential is applied to the p-type side and a positive potential is applied to the n-type side. The applied voltage is $V_r$.

Diagram of a reverse-biased p-n junction diode

Reverse-biased p-n junction diode. Applied voltage increases the barrier.

Effect of Reverse Bias:

The applied reverse voltage $V_r$ adds to the built-in barrier potential $V_B$. The net potential barrier across the junction increases to $(V_B + V_r)$.
The electric field in the depletion region is increased.
The width of the depletion region increases as majority carriers are pulled away from the junction.
The increased barrier almost completely blocks the diffusion of majority carriers across the junction.
A very small current flows due to the drift of minority carriers (electrons from p-side drifting to n-side, and holes from n-side drifting to p-side) across the junction, aided by the reverse bias voltage. This is called the reverse saturation current ($I_s$). It is typically in the order of microamperes ($\mu A$) or nanoamperes ($nA$) and is largely independent of the reverse bias voltage.

I-V Characteristics of a P-N Junction Diode

The relationship between the voltage across the diode and the current flowing through it is represented by its I-V characteristic curve.

I-V characteristic curve of a p-n junction diode.

Forward Bias Characteristics:

When forward biased, the current is very small initially (below the barrier potential).
Once the applied forward voltage exceeds the barrier potential (knee voltage $\approx 0.7 V$ for Si, $0.3 V$ for Ge), the current increases sharply and exponentially with increasing voltage.
The dynamic resistance ($dv/di$) in forward bias is low.

Reverse Bias Characteristics:

When reverse biased, only a very small reverse saturation current ($I_s$) flows initially, which is almost constant with increasing reverse voltage. This current is due to minority carriers and is very small.
The dynamic resistance ($dv/di$) in reverse bias is very high.

Breakdown Voltage

If the reverse bias voltage across a p-n junction is increased beyond a certain value, the reverse current increases very rapidly. This voltage is called the breakdown voltage or Zener voltage (in the case of a Zener diode).

I-V characteristic showing reverse breakdown.

Mechanism of Breakdown: Breakdown occurs due to one of two mechanisms:

Zener Breakdown: Occurs in heavily doped junctions with narrow depletion regions. The strong electric field in the depletion region can break covalent bonds and generate electron-hole pairs through a quantum mechanical tunnelling effect. This is dominant at lower breakdown voltages.
Avalanche Breakdown: Occurs in lightly doped junctions with wider depletion regions. Minority carriers drifting across the depletion region gain high kinetic energy from the strong electric field. These energetic carriers collide with semiconductor atoms, breaking covalent bonds and generating new electron-hole pairs. These new carriers are also accelerated, causing further collisions and carrier generation, leading to a cumulative "avalanche" of carriers and a rapid increase in current. This is dominant at higher breakdown voltages.

While breakdown can damage a standard diode if the current is not limited, it is a controlled and reversible phenomenon in special diodes like Zener diodes, which are designed to operate in the breakdown region and are used as voltage regulators.

The unidirectional conduction property of the p-n junction diode makes it useful for applications such as rectification (converting AC to DC).

Example 1. The barrier potential of a silicon diode is approximately 0.7 V at room temperature. If a forward bias voltage of 0.5 V is applied, will the diode conduct significant current? Explain why.

Answer:

Given:

Barrier potential of silicon diode, $V_B \approx 0.7 \, V$

Applied forward bias voltage, $V_f = 0.5 \, V$

In forward bias, the applied voltage opposes the built-in barrier potential. For the diode to conduct significant current, the applied forward voltage must overcome the barrier potential, effectively reducing the net barrier. The voltage required to start significant conduction (knee voltage) is typically close to the barrier potential.

In this case, the applied forward bias voltage (0.5 V) is less than the barrier potential (0.7 V).

Therefore, the net potential barrier $(V_B - V_f = 0.7V - 0.5V = 0.2V)$ is still positive, although reduced. While some small current might flow due to carriers with thermal energies high enough to cross the remaining barrier, significant diffusion of majority carriers will not occur.

So, the diode will not conduct significant current when a forward bias voltage of 0.5 V is applied, as the applied voltage is below the knee voltage.