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Semiconductor Electronics: Materials, Devices and Simple Circuits

Semiconductor Diode

Imagine a p-n junction with metallic contacts at both the ends for application of external voltage. This is a semiconductor diode.

semiconductor diode

This is the symbolical representation of a semiconductor diode:

p-n junction diode

In the image above, the arrow indicates the direction of current when the diode is under forward bias. It is also important to note here that the equilibrium barrier potential can be altered.

This is achieved by applying an external voltage across the diode. Depending on how this voltage is applied, the diode is a forward-bias or a reverse-bias diode.

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Semiconductor Diode under Forward Bias

p-n junction diode under forward bias

In the image above, you can see that an external voltage is applied across the semiconductor diode where the p-side of the diode is connected to the positive terminal and the n-side is connected to the negative terminal of the battery. This diode is forward biased.

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Formation of a Forward Bias Diode

Since the resistance of the depletion region (the region where there are no charges) is very high, the applied voltage drops primarily across this region. The drop in voltage across the p and n side of the junction is relatively negligible.

Also, the direction of the applied voltage (V) being opposite to that of the built-in potential (V0), the depletion layer’s width decreases and the barrier height reduce.

If the applied voltage is small, then the barrier potential is reduced marginally below the equilibrium value. This results in a small number of carriers crossing the junction.

Hence, the current is small. On the other hand, for a significantly high value of voltage, more carriers have the energy to cross the junction. This leads to a higher current.

Another thing to note is that when the voltage is applied, some electrons crossover to the p-side and some holes cross to the n-side. Under forward bias, this process is the minority charge injection process (refer to image below).

Hence, the minority charge concentration (electrons on the p-side are a minority and holes on the n-side are a minority) is significantly higher at the junction boundary.

forward bias diode

This concentration gradient, the injected electron diffuse from the junction-end to the far-end of the p-side. Similarly, injected holes diffuse to the far end of the n-side. This gives rise to current too.

The total diode forward current = Hole diffusion current + Electron diffusion current (usually in mA)

Semiconductor Diode under Reverse Bias

reverse bias diode

In the image above, you can see that an external voltage is applied across the diode. The n-side of the diode connects to the positive terminal and the p-side connects to the negative terminal of the battery. This diode is a reverse-bias diode.

Formation of  a Reverse Bias Diode

Since the resistance of the depletion region (the region where there are no charges) is very high, the applied voltage drops primarily across this region. The drop in voltage across the p and n side of the junction is relatively negligible.

Also, the direction of the applied voltage (V) being the same as that of the built-in potential (V0), the depletion layer’s width widens and the barrier height increases.

This leads to a suppression of the flow of electron to the p-side and holes to the n-side. Hence, the diffusion current decreases to a great extent.

Due to the direction of the electric field, the electrons in the p-side and holes in the n-side are swept to their majority zones, if they come close to the junction. This causes drift current.

The drift current is usually of a few μA. This current is very low even in the forward-biased diode as compared to the current due to the injected carriers.

Critical Value of Reverse Bias Voltage

Also, a small amount of voltage applied to the diode is sufficient to sweep the minority charge carrier to the far side of the junction.

This diode reverses current is not dependent on the voltage but on the concentration of the minority charge carriers on both sides of the junction.

However, the current is independent up to a critical value of reverse bias voltage – the Breakdown Voltage (Vbr). When the voltage applied crosses Vbr, even a small change in the bias voltage causes a huge change in current.

There is also an upper limit of current for every diode, beyond which it gets destroyed due to overheating. This is the rated value of current.

Experimental Study of the V-I characteristics of a Semiconductor Diode

vi characteristic of diode

The above diagram shows a diode connected in forward bias. The battery connects to the diode through a potentiometer enabling us to change the voltage for the sake of the experiment. A milliammeter measures the current.

characteristic of reverse bias diode

V-I Characteristics of Diode

This diagram shows a diode connected in reverse bias. The battery connects to the diode through a potentiometer enabling us to change the voltage for the sake of the experiment. A microammeter (since the expected current is low) measures the current.

Here is what we observe:

vi characteristics of diode

As can be seen in the graph above, in the forward biased diode, initially the current increases almost negligibly till a certain value is reached.

Post that, the current increases exponentially even for a small increase in diode bias voltage. This voltage is the threshold voltage. (~0.7 V for silicon diode and ~ 0.2 V for germanium diode)

In the reverse biased diode, the current is very small and almost remains constant with a change in bias voltage. It is the Reverse saturation current. In some cases, beyond the breakdown voltage, the current increases suddenly.

Hence, from this experiment, we can conclude that the p-n junction diode allows the flow of current only in one direction, i.e. forward-bias. Also, the forward bias resistance is lower than the reverse bias resistance.

Solved Examples for You

Question:  Define a forward-bias diode

Solution: When you apply an external voltage across a p-n junction diode such that:

  • the positive terminal of the battery connects to the p-side and
  • the negative terminal to the n-side

It is a forward-bias diode.

Question: Define a reverse-bias diode

Solution: When you apply an external voltage across a p-n junction diode such that:

  • the positive terminal of the battery connects to the n-side and
  • the negative terminal to the p-side

It is a reverse-bias diode.

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