Special Purpose p-n Junction Diode

Let’s look at some p-n junction diodes, developed for specific applications. We will cover Zener diode and Opto-electronic junction devices including photodiodes, light emitting diode, and solar cells.

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Zener Diode

C. Zener designed a p-n junction diode in reverse bias to operate in the breakdown region. It is used as a voltage regulator. The symbol for a Zener diode is:

It is made by doping both p and n sides of the junction heavily. This leads to the formation of a very thin depletion region and an extremely high electric field across the junction even for a small reverse bias voltage (~5 V). Here is a quick look at the V-I characteristics of a Zener diode:

In the above graph, you can see that as the reverse bias voltage (V) reaches the breakdown voltage of the Zener diode (V_z), there is a large change in current. Also, note that for a negligible change in the reverse bias voltage, a large change in current is produced. Hence, in a Zener diode, the voltage remains nearly constant even when there is a large change in current. This property is used to regulate supply voltages.

Why does the current increase suddenly?

This is really interesting. We know that the reverse current is created due to the movement of the minority charge carriers. The electrons from the p-side move to the n-side and the holes from the n-side move to the p-side. As we increase the reverse bias voltage, the electric field at the junction becomes stronger.

At V=V_z, the electric field is strong enough to pull valence electrons from the host atoms on the p-side which are accelerated to n-side. This explains the sudden increase in current. This process of emission of electrons from the host atoms due to a high electric field is known as Internal Field Emission or Field Ionization.

Zener Diode as a Voltage Regulator

By now we know that even if we use a rectifier to convert AC input voltage, the output fluctuates too. A Zener diode is used to get constant DC voltage from a DC unregulated output of a rectifier.

This is a circuit diagram of a Zener diode used as a voltage regulator. Here the unregulated DC output of a rectifier is connected to it through a series of resistance (R_s). Also, it is in reverse bias. Here is how it works:

• An increase in input voltage increases the current through R_s and the diode
• The voltage drop across R_s increases
• The voltage across the Zener diode does not change

This happens because, in the breakdown region, Zener voltage remains constant despite the change in current.

• A decrease in the input voltage decreases the current through R_s and the diode
• The voltage drop across R_s decreases
• The voltage across the Zener diode does not change

Hence, an increase/decrease of the voltage drop across the Rs does not change the voltage across the Zener diode. Hence, it acts as a voltage regulator.

Optoelectronic Junction Devices

Till now, you have read about how semiconductor diodes behave under electrical inputs. Now, let’s look at how they react to photo-excitation. These are optoelectronic devices or devices in which carriers are generated by photons. Let’s look at three such devices:

1. Photodiodes to detect optical signals
2. Light Emitting Diodes (LEDs)
3. Solar cells

Photodiodes

This is a special p-n junction diode operated in reverse bias and designed with a transparent window to allow light to fall on it. When photons with energy (h_v) greater than the energy gap of the semiconductor fall on it, electron-hole pairs are generated. The diode is designed in a manner that these electron-hole pairs are generated in or near the depletion region.

Also, the electric field at the junction ensures that the electrons and holes are separated before they recombine. Further, the direction of the electric current is to ensure that the electrons reach the n-side and holes reach the p-side. This gives rise to an EMF.

Current flows on connecting an external load. Also, the magnitude of this photocurrent depends on the intensity of the incident light. The direct correlation between the two is easily observed on application of a reverse bias. Hence, a photodiode can detect optical signals. Here is a circuit diagram of a photodiode:

Light emitting diode (LED)

A LED is a heavily doped p-n junction which emits spontaneous radiation under forward bias. It is covered in a capsule with a transparent cover allowing the emitted light to come out. Being forward biased, electrons move from n to p-side and holes move from p to n-side.

Also, the concentration of the minority carriers is higher near the junction as compared to the equilibrium concentration. Hence, at the junction, on either side, excess minority carriers recombine with the majority carriers. This releases energy in the form of photons. The photons emitted have energies equal to or less than the band gap.

Another thing to note here is that the intensity of the emitted light is small when the forward current is small. The intensity of the emitted light increases as this current increases and reached a maximum value. Post this, an increase in forward current leads to a decrease in light intensity.

Commercially available LEDs can emit red, yellow, orange, green and blue light. LEDs are used extensively in remote controls, optical communication, etc. Here are some advantages of LEDs over incandescent low power lamps:

• Low operational voltage and less power.
• Fast action and no warm-up time required.
• The bandwidth of emitted light is 100 Å to 500 Å or in other words, it is nearly (but not exactly) monochromatic.
• Long life and ruggedness.
• Fast on-off switching capability.

Solar Cell

A solar cell is a p-n junction which generates EMF when solar radiation falls on it. The working principle is similar to the photodiode except that no external bias is applied and the junction area is much larger to enable solar radiation incidence. This is how a simple p-n junction solar cell looks:

When sunlight falls on a solar cell, the EMF is generated due to these three processes:

• Generation of electron-hole pairs due to light (with h_ν > E_g) close to the junction
• Separation of electrons and holes due to the electric field of the depletion region. Electrons are swept to n-side and holes to p-side.
• Collection – the electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact. Thus p-side becomes positive and n-side becomes negative giving rise to photo-voltage.

The above diagram shows an external load connected to a solar cell leading to a flow of photocurrent. The V-I characteristics can be graphically shown as follows:

The correlation is shown in the fourth quadrant because the solar cell does not draw current but supplies it to the load. Also, remember that sunlight is not always required for a solar cell. A light having a frequency greater than the band gap suffices.

Solved Examples for You

Question: How does a Voltage regulator using Zener diode work?

Solution: The unregulated DC output of a rectifier is connected to a Zener diode through a series of resistance (R_s). Also, it is in reverse bias. Here is how it works:

• An increase in input voltage increases the current through R_s and the diode
• The voltage drop across R_s increases
• The voltage across the Zener diode does not change

This happens because, in the breakdown region, Zener voltage remains constant despite the change in current.

• A decrease in the input voltage decreases the current through R_s and the diode
• The voltage drop across R_s decreases
• The voltage across the Zener diode does not change

Hence, an increase/decrease of the voltage drop across the Rs does not change the voltage across the Zener diode. Hence, it acts as a voltage regulator.