DIODES

Introduction to Diode

A diode is defined as a two-terminal electronic component that only conducts current in one direction (as long as it is operated within a specified voltage level). An ideal diode will have zero resistance in one direction, and infinite resistance in the reverse direction. A diode is an electrical device that allows current to flow only in one direction. A diode is effectively like a valve for an electrical circuit.

The symbol of a diode is shown below. The arrowhead points in the direction of conventional current flow in the forward biased condition. That means the anode is connected to the p side and the cathode is connected to the n side. A diode is the simplest and most fundamental non –linear circuit element, like a resistor it has two terminals anode and cathode. Diodes are formed by joining P-TYPE and N-TYPE semiconductors.

WORKING PRINCIPLE OF DIODE 

A diode’s working principle depends on the interaction of n-type and p-type semiconductors. An n-type semiconductor has plenty of free electrons and a very few numbers of holes. 

In other words, it can be said that the concentration of free electrons is high and that of holes is very low in an n-type semiconductor. Free electrons in the n-type semiconductor are referred to as majority charge carriers, and holes in the n-type semiconductor are referred to as minority charge carriers.

A p-type semiconductor has a high concentration of  holes and a low concentration of free electrons. Holes in the p-type semiconductor are majority charge carriers, and free electrons in the p-type semiconductor are minority charge carriers.

Semiconductor diodes are the most common type of diode.These diodes begin conducting electricity only if a certain threshold voltage is present in the forward direction (i.e.the “low resistance” direction). The diode is said to be forward biased when conducting current in forward direction. When connected within a circuit in the reverse direction (i.e the “high resistance” direction), the diode is said to be “reversed biased.”

There are two types of diode operations:

Forward Biased   

In case of forward-biased diode operations, if a positive terminal of a source is connected to the p-type side and the negative terminal of the source is connected to the n-type side of the diode and one can increase the voltage of this source slowly from zero.

In the beginning, there is no current flowing through the diode. It is known that the depletion region acts as a potential barrier against the majority charge carriers called forward potential barriers. 

The majority charge carriers start crossing the forward potential barrier only when the value of externally applied voltage across the junction is more than the potential of the forward barrier. For silicon diodes, the forward barrier potential is 0.7 volts and for germanium diodes, it is 0.3 volts.

When the externally applied forward voltage across the diode becomes more than the forward barrier potential, the free majority charge carriers start crossing the barrier and contribute towards forward diode current. 

In that situation, the diode would behave as a short-circuited path, and the forward current gets limited by only externally connected resistors to the diode.

Reverse Biased

In reverse biased mode we connect the negative terminal of the voltage source to the p-type side and positive terminal of the voltage source to the n-type side of the diode. 

In this condition, due to electrostatic attraction of the negative potential of the source, the holes in the p-type region would be shifted more away from the junction leaving more uncovered negative ions at the junction.

In the same way, the free electrons in the n-type region would be shifted away from the junction towards the positive terminal of the voltage source leaving more uncovered positive ions at the junction. 

As a result of this phenomenon, the depletion region becomes wider. This condition of a diode is called the reverse biased condition. At that condition, no majority carriers cross the junction, and they instead move away from the junction. In this way, a diode blocks the flow of current when it is reverse biased and conducts current in one direction (as long as it is operated within a specified voltage level). 

Semiconductors

Working principle of diode

A diode is defined as a two terminal electronic component that only conducts current in one direction.

Types of Diodes

1. Small Signal Diode

A small signal diode is basically used in applications involved with high frequency and low current devices. One would recognize this diode from a glass enveloped casing which ensures that the diode stays protected from being contaminated. One commonly known model is 1N4148. Commonly known applications are radios and televisions.

2. Large Signal Diode

In case of large signal diodes, conversion of AC to DC voltages is limitless which increases the current forwarding capacity as well as the reverse blocking voltage. The range of forward resistance for these diodes is in Ohms and the reverse blocking resistance can range till mega Ohms. Moreover, the high current and voltage performance of these large signal diodes are useful in electrical devices in order to limit high peak voltages. A very common usage of these diodes is in the battery charging applications e.g. inverters.

3. Zener Diode

Zener diodes were first developed by American physicist Clarence Zener in 1934. Zener diodes are passive elements in nature. In the forward direction, the function of Zener diode is same as the normal diode but its uniqueness is that it allows current to flow in the reverse direction as well. This happens when the applied voltage of the Zener diode becomes equal to the breakdown voltage and the phenomena is known as Zener breakdown. Zener diodes are designed to prevent the other semiconductor devices from transient voltage pulses and therefore it acts as a voltage regulator.

4. Light Emitting Diodes (LEDs)

LEDs convert electrical energy to light energy and were first produced in 1968. LEDs undergo the process of electroluminescence where holes and electrons are recombined that results in energy in the form of light in the forward bias state. LED applications range from lamps to street lights to traffic lights and camera flash. 

5. Schottky Diodes

Schottky diode also known as Schottky barrier diode is named after the German physicist Walter H. Schottky. It is formed when the junction of a semiconductor material comes in contact with metal. This causes a forward voltage drop to decrease to minimum value. This ensures that diode performance enhances and the power loss is reduced. Metallic materials used here range from chromium to platinum and tungsten. Schottky diode is used in switching applications mainly high frequency rectifiers.

6. Shockley Diodes

Shockley diode is also known as PNPN diode. This diode was manufactured and commercialized in the late 1950s by Shockley Semiconductor Laboratory. This diode is a four layered semiconductor device and is similar to a thyristor except without the gate terminal. In fact, when the gate terminal of thyristor is disconnected, it can act as a PNPN diode. There is no trigger input and the diode can conduct only with forward voltage. This diode has two operating states i.e. conducting and non-conducting. In a non-conducting state, the diode operates with less voltage. The diode when turned “ON” will stay on unless it is turned “OFF.” Its applications include trigger switches in SCR and it also behaves as a relaxation oscillator.

7. Step Recovery Diodes

Step recovery diodes are special types of diodes that store the charge from the positive pulse and use it in the negative pulse of the sinusoidal signals. The rise time of the current pulse is equal to the snap time. Due to this phenomenon it has speed recovery pulses. The applications of these diodes are in higher order multipliers and in pulse shaper circuits. The cut-off frequency of these diodes is very high which are nearly at Gigahertz order. As a multiplier this diode has the cut-off frequency range of 200 to 300 GHz. In the operations which are performing at 10 GHz range these diodes play a vital role. The efficiency is high for lower order multipliers. 

8. Tunnel Diodes

Tunnel diodes work on the principle of tunneling effect. Tunnel diodes can be tuned in both mechanically and electrically. Tunnel diodes work with fast operation in the microwave frequency region. It is a two terminal device where concentration of dopants is too high. The transient response is being limited by junction capacitance plus stray wiring capacitance. It is mostly used in microwave oscillators and amplifiers.  

9. Varactor Diodes

Varactor diodes also known as Varicap diodes act like variable capacitors. It operates mainly in reverse bias state. These diodes are very famous due to its capability of changing the capacitance ranges within the circuit in the presence of constant voltage flow. They are able to vary capacitance up to high values. In a varactor diode, by changing the reverse bias voltage we can decrease or increase the depletion layer. These diodes have many applications as voltage controlled oscillators for cell phones, satellite pre-filters etc.

10. PIN Diode

In PIN diode doping is not necessary as intrinsic material can be inserted between the P and N regions. This increases the area of the depletion layer. When forward bias voltage is applied, the holes and electrons are forced into the intrinsic layer. At some point due to this high injection level, the electric field will conduct via intrinsic material as well thereby making the carriers to flow from two regions.

11. GUNN Diode

Gunn diode is also known as Transferred Electron Device (TED). It is based on the Gunn effect by physicist J.B. Gunn. The diode is fabricated with n-type semiconductor material only. In Gunn diode when voltage increases in the circuit the current also increases after which at a certain level of voltage, the current will exponentially decrease and exhibit negative differential resistance. Gunn diode has two electrodes with Gallium Arsenide and Indium Phosphide. It produces microwave RF signals and its applications include Microwave RF devices and amplifier.

12. Laser Diode

Laser diodes are similar to LED.  In a laser diode, the active region is formed by p-n junction and it is also called p-i-n diode in which the active region is in intrinsic region. It is used in fiber optics communications, bar code readers, laser pointers, CD/DVD/Blu-ray reading and recording, laser printing. Laser diodes are distributed, bragged, double hetro-layered structured, separately confined heterostructures and quantum based.

13. Transient Voltage Suppression Diode

Suppression diodes are used to suppress voltage transients that affect output response. Its operation is similar to Zener diode operation. At transient voltage in normal condition, the impedance gets high and the diode enters avalanche breakdown region where low impedance is provided. Its applications are in the telecommunication fields, medical, microprocessors and signal processing. 

14. Gold Doped Diodes

In these diodes, gold is used as a dopant for faster recombination of the minority carriers making them faster than other diodes. The leakage current in reverse bias condition is less and at higher voltage drop the diode operates at signal frequencies.

15. Super Barrier Diodes

Super barrier diodes are rectifier diodes that have a low forward voltage drop. These have high surge handling capability and low reverse leakage current as compared to a p-n junction diode. These diodes are designed for high power, fast switching and low-loss applications.

16. Current Limiting Diodes 

Current limiting diodes regulate voltage at a particular current and acts as a two terminal current limiter. These are used in achieving high output impedance.

17. Peltier Diode

In this type of diode, heat is produced due to electric charge produced by the recombination of minority charge carriers which flows from one terminal to another terminal. The flow is unidirectional and is equal to the direction of current flow. These types of diodes are used in sensors and heat engines for thermo-electric cooling.

18. Crystal Diode

Crystal diode is a point contact diode. Its operation depends on the pressure of contact between the semiconductor crystal and point. In this diode a metal wire is present which is pressed against the semiconductor crystal. In this the semiconductor crystal acts as cathode and metal wire acts as anode. These diodes are obsolete in nature. Its applications are mainly used in microwave receivers and detectors including crystal diode rectifier, crystal diode detector and crystal radio receiver.

19. Avalanche Diode

This passive element works under the principle of avalanche breakdown. It works in reverse bias condition. It results in large currents due to the ionization produced by p-n junction during reverse bias condition. These diodes are specially designed to undergo breakdown at specific reverse voltage to prevent the damage. Its applications include RF Noise Generation, Microwave Frequency Generation and Single Photon Avalanche Detector used in light level applications.

20. Silicon Controlled Rectifier

SCR is a three terminal device having anode, cathode and a gate. It is similar to the Shockley diode. Its purpose is to control small voltages that are applied in the circuit. It has a three mode operation i.e. forward blocking (off state), forward conduction (on state) and reverse blocking mode (off state). 

21. Vacuum Diodes

Vacuum diodes consist of two electrodes which will act as anode and cathode. Cathode is made up of tungsten which emits the electrons in the direction of anode. The electron flow will always be from cathode to anode only and therefore it acts like a switch. If the cathode is coated with oxide material then the electrons emission capability is high. The diode will conduct in only one case, that is when the anode is positive regarding the cathode terminal.