A PN junction diode is a two-terminal semiconductor device. It is formed by joining a P-type semiconductor material with an N-type semiconductor material. The junction between these two materials creates a region that allows current to flow in only one direction.
In a PN Junction Diode, the P-type material has an excess number of holes (positive charge carriers) and the N-type material has an excess number of electrons (negative charge carriers). The junction between these N-type and P-type material is called the PN junction.
1. What are P-Type and N-Type Materials?
The process of adding impurities to a pure semiconductor material is called doping. Doping controls the electrical conductivity of the material.
1.1 N-Type Semiconductor
When silicon or germanium is doped with a pentavalent element like phosphorus or arsenic we get N-Type semiconductor materials.
Silicon and germanium has 4 valance electrons and phosphorus or arsenic has 5 valance electrons. Four electrons (out of five) from phosphorus or arsenic form covalent bonds with silicon atoms, and the fifth electron is free to move. This makes the N-type material rich in free electrons.
1.2 P-Type Semiconductor
When silicon or germanium is doped with a trivalent element like boron or aluminum, P-type semiconductor material is formed. These trivalent elements have only three valence electrons and germanium or silicon has four valence electrons. This creates vacancies and these vacancies are called holes. A hole behaves like a positive charge carrier.
2. Formation of the PN Junction
When the P-type and N-type materials are joined together, free electrons from the N-side cross over to the P-side and recombine with holes.
Similarly, holes from the P-side move to the N-side to recombine with free electrons. This process of recombination creates a region near the junction where there are no free charge carriers. This region is called the depletion region or the depletion layer.
The depletion region acts like an insulating barrier. As electrons leave the N-side, that side becomes positively charged near the junction. As holes leave the P-side, that side becomes negatively charged near the junction. This creates a built-in electric field that points from the N-side to the P-side.
This electric field stops further diffusion of charge carriers and the internal charge in the depletion layer creates a voltage barrier. In silicon, the built-in voltage barrier is approximately 0.7 volts. In germanium, it is about 0.3 volts.
As shown in the diagram above, the depletion region sits at the center of the PN junction between the P-type material (filled with empty holes) and the N-type material (filled with free electrons). The positive and negative ions are fixed on either side of the junction. The voltage barrier formed across this region measures between 0.3V and 0.7V depending on the semiconductor material used.
3. Structure and Symbol of the PN Junction Diode
The diode has two terminals:
- Anode (A): Connected to the P-type side
- Cathode (K): Connected to the N-type side
The standard circuit symbol of a diode is a triangle pointing toward a vertical bar as shown in the figure above. The base of the triangle is the anode, and the bar is the cathode. Current flows from anode to cathode in the conventional current direction.
Physically, most diodes are small cylindrical components as shown above. The cathode end is marked with a silver or grey band. This marking helps to identify the correct orientation of the diode when placing in a circuit.
4. Biasing of a PN Junction Diode
Biasing means applying an external voltage to the diode. There are two types of biasing:
4.1 Forward Bias
In forward bias, the positive terminal of a battery is connected to the P-side (anode) of a diode and the negative terminal of the battery is connected to the N-side (cathode) of the diode.
The external voltage pushes holes from the P-side toward the junction and pushes electrons from the N-side toward the junction. This reduces the width of the depletion layer. When the external voltage exceeds the built-in potential barrier (0.7V for silicon), the depletion region collapses and allows current to flow through the diode.
In forward bias, the diode conducts. The current increases rapidly after the threshold voltage is crossed. For silicon diodes, this threshold is around 0.7V. For germanium, it is around 0.3V.
The forward bias circuit diagram above shows a 5V DC supply connected with the positive terminal to the anode (+) and the negative terminal to the cathode (−). The diode current \(I_D\) flows through the circuit and through the load resistance. Since 5V is well above the 0.7V barrier voltage of silicon, the diode conducts and current flows without any restriction.
4.2 Reverse Bias
In reverse bias, the positive terminal of the battery is connected to the N-side (cathode) and the negative terminal is connected to the P-side (anode) of the PN Junction Diode.
This pulls the electrons toward the positive terminal and holes toward the negative terminal and hence the depletion region becomes wider. The barrier voltage increases and no current flows under normal conditions.
However, a very small amount of current flows even in reverse bias. This is called reverse saturation current or leakage current and is caused by thermally generated minority carriers.
The reverse bias circuit diagram above shows the same 5V DC supply but with reversed connections. The positive terminal is now connected to the cathode (−) and the negative terminal to the anode (+). As shown in the diagram, the diode current \(I_D = 0\). No current flows through the load resistance. The widened depletion region blocks majority carrier flow completely.
5. V-I Characteristics of a PN Junction Diode
The V-I (Voltage-Current) characteristic curve shows how current varies with applied voltage.
5.1 Forward Bias Region
At low forward voltages (below 0.7V for silicon), current is very small. After the voltage crosses the threshold (called the knee voltage or cut-in voltage), current increases very sharply as shown in the above plot. The diode behaves almost like a closed switch.
5.2 Reverse Bias Region
In reverse bias, the current remains very small and nearly constant regardless of voltage and this current is called the reverse leakage current or reverse saturation current as shown in the graph above. For reverse bias, the VI characteristics curve stays nearly flat along the voltage axis.
5.3 Breakdown Region
If the reverse voltage is increased beyond a certain limit, suddenly the current increases very rapidly. This is called breakdown. The voltage at which this happens is called the breakdown voltage.
There are two types of breakdown:
- Zener Breakdown: Occurs at lower breakdown voltages (below 6V). It is due to the strong electric field in the depletion region which pulls electrons directly out of covalent bonds.
- Avalanche Breakdown: Occurs at higher voltages. The free carriers gain enough energy to knock out electrons from other atoms during collisions, creating more carriers. This process multiplies rapidly.
Normal diodes should not operate in breakdown, as it can damage them. However, Zener diodes are specially designed to operate in the Zener breakdown region.
6. Diode Equation (Shockley Diode Equation)
The mathematical relationship between voltage and current in a diode is given by the Shockley diode equation:
\(I = I_0 (e^{\frac{V}{\eta V_T}} – 1)\)
Where:
- \(I\) = Diode current
- \(I_0\) = Reverse saturation current
- \(V\) = Applied voltage
- \(\eta \) = Ideality factor (1 for germanium, 2 for silicon in some cases)
- \(V_T\) = Thermal voltage ≈ 26mV at room temperature
At room temperature, if you apply a forward voltage of 0.7V across a silicon diode, the exponential term becomes very large and the current rises sharply.
7. Types of PN Junction Diodes
There are several types of PN junction diodes, each designed for a specific purpose.
1. Rectifier Diode
Used to convert AC to DC. Common in power supplies. Example: 1N4007.
2. Zener Diode
Operates in the reverse breakdown region. Used for voltage regulation. It maintains a constant voltage across its terminals.
3. Schottky Diode
Made from a metal-semiconductor junction instead of a P-N junction. Has a lower forward voltage drop (0.2V to 0.3V) and very fast switching speed. Used in high-frequency circuits.
4. Light Emitting Diode (LED)
Emits light when forward biased. The color of light depends on the semiconductor material used. GaAs emits infrared, GaP emits red or green, GaN emits blue.
5. Photodiode
Works in reverse bias. When light falls on the junction, it generates electron-hole pairs and produces current. Used in optical communication and light detection systems.
6. Varactor Diode (Varicap)
The junction capacitance changes with reverse voltage. Used in tuning circuits, VCOs (Voltage Controlled Oscillators), and FM radio tuners.
7. Tunnel Diode
Heavily doped on both sides. Exhibits a negative resistance region in its V-I characteristics due to quantum tunneling. Used in very high-frequency oscillators.
8. Applications of PN Junction Diode
1. Rectification
Rectification is the most common application of a PN junction diode. A diode passes only the positive half of an AC signal. A combination of four diodes in a bridge configuration converts full AC signal to pulsating DC signal. This is used in every AC-to-DC power adapter.
2. Clipping Circuits
A clipper circuit removes portions of a signal above or below a reference level. Diodes are used in AM radio receivers and waveform shaping circuits.
3. Clamping Circuits
A clamper shifts the DC level of a signal. PN junction diodes can clamp the signal to the required voltage level without changing the shape of the singal.
4. Voltage Multiplier Circuits
Using a combination of diodes and capacitors, you can multiply an AC voltage. Used in television sets (CRT type) and laser power supplies.
5. Voltage Regulation
Zener diodes maintain a fixed output voltage. This is in voltage regulators and reference circuits.
6. Demodulation
In AM radio, the diode is used to detect (demodulate) the audio signal from the carrier wave.
7. Protection Circuits
Diodes protect sensitive components from reverse voltage. A diode placed across a relay coil protects the transistor driver from back-EMF.
8. Logic Gates
Diodes can form basic AND and OR logic gates. This was used in early computers (DTL — Diode Transistor Logic).
9. Conclusion
A PN junction diode is formed by joining P-type and N-type semiconductors which creates a depletion region that controls flow of current. It conducts in forward bias after the voltage barrier is reached and blocks current in reverse bias condition.
The core operating principle of a PN junction diode is unidirectional conduction and this property makes it useful in rectifiers, voltage regulators, clipping circuits, clamping circuits, and protection circuits.
Silicon diodes are the most widely used type in modern electronic systems.
10. Frequently Asked Questions (FAQs)
A PN junction diode allows current to flow in only one direction. It conducts when forward biased and blocks current when reverse biased.
The depletion region creates a built-in potential barrier at the junction. The external forward voltage must overcome this barrier before the diode starts conducting. For silicon, this barrier is approximately 0.7V. For germanium, it is around 0.3V.
If the reverse voltage exceeds the breakdown voltage, current increases sharply. For normal rectifier diodes, this can cause permanent damage due to excessive heat. For Zener diodes, operating in breakdown is intentional and the diode is designed to handle it.
A regular PN junction diode is not designed for reverse breakdown and can get damaged in that region. A Zener diode is specially doped to have a well-defined breakdown voltage and is designed to operate safely in reverse breakdown for voltage regulation purposes.
At higher temperatures, charge carriers (electrons and holes) have more thermal energy. This makes it easier for them to cross the junction barrier and reduces the voltage needed to forward bias the diode.
No. A PN junction diode cannot amplify signals.
The depletion region is the area near the PN junction where mobile charge carriers are absent. Depletion capacitance is the capacitance associated with this region because the depletion layer behaves like a parallel plate capacitor with charge stored on either side.
Reverse saturation current flows in reverse bias due to thermally generated minority carriers — electrons in the P-side and holes in the N-side. Because these minority carriers are present in very small numbers at room temperature, the reverse current is very small.