Types of Diodes Transistors

The function of the diode is regulating the voltage at a particular current.

1. Small Signal Diode It is a small device with disproportional characteristics and whose applications are mainly involved at high frequency and very low currents devices such as radios and televisions etc. To protect the diode from contamination it is enveloped with a glass so it is also named as Glass Passivated Diode which is extensively used as 1N4148.2.

2. Large Signal Diode These diodes have large PN junction layer. Thus the transformation of AC to DC voltages is unbounded. This also increases the current forward capacity and reverse blocking voltage. These large signals will disrupt the functional point also. Due to this it is not suitable for high frequency applications.

3. Zener Diode It is a passive element works under the principle of zener breakdown. First produced by Clarence zener in 1934.It is similar to normal diode in forward direction, it also allows current in reverse direction when the applied voltage reaches the breakdown voltage. It is designed to prevent the other semiconductor devices from momentary voltage pulses. It acts as voltage regulator.

4. Light Emitting Diode (LED)These diodes convert the electrical energy in to light energy. First production started in 1968. It undergoes electroluminescence process in which holes and electrons are recombined to produce energy in the form of light in forward bias condition.

5. Constant Current Diodes It is also known as current-regulating diode or constant current diode or current-limiting diode or diode-connected

6. Schottky Diode In this type of diode the junction is formed by contacting the semiconductor material with metal. Due to this the forward voltage drop is decreased to min. The semiconductor material is N-type silicon which acts as an anode and the metal acts as a cathode whose materials are chromium, platinum, tungsten etc.

7. Shockley Diode It was the invention of first semiconductor devices it has four layers. It is also called as PNPN diode. It is equal to a thyristor without a gate terminal which means the gate terminal is disconnected. As there is no trigger inputs the only way the diode can conduct is by providing forward voltage.

8. Step Recovery Diodes It is also called as snap-off diode or charge-storage diode. These are the special type of diodes which stores the charge from positive pulse and uses 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.

9. Tunnel Diode It is used as high speed switch, of order nano-seconds. Due to tunneling effect it has very fast operation in microwave frequency region. It is a two terminal device in which concentration of dopants is too high.

10. Varactor Diode These are also known as Varicap diodes. It acts like the variable capacitor. Operations are performed mainly at reverse bias state only. These diodes are very famous due to its capability of changing the capacitance ranges within the circuit in the presence of constant voltage flow.

VARACTOR DIODE APPLICATIONS

a. Voltage-controlled capacitors.

b. Voltage-controlled oscillators.

c. Parametric amplifiers.

d. Frequency multipliers.

e. FM transmitters and Phase locked loops in radio, television sets and cellular telephone.

11. Laser Diode Similar to LED in which active region is formed by p-n junction. Electrically laser diode is p-i-n diode in which the active region is in intrinsic region. Used in fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray reading and recording, Laser printing.

LASER DIODE TYPES:

a. Double Heterostructure Laser: Free electrons and holes available simultaneously in the region.

b. Quantum Well Lasers: lasers having more than one quantum well are called multi quantum well lasers.

c. Quantum Cascade Lasers: These are heterojunction lasers which enables laser action at relatively long wavelengths.

d. Separate Confinement Heterostructure Lasers: To compensate the thin layer problem in quantum lasers we go for separate confinement heterostructure lasers.

e. Distributed Bragg Reflector Lasers: It can be edge emitting lasers or VCSELS.

12. Transient Voltage Suppression DiodeIn semiconductor devices due to the sudden change in the state voltage transients will occur. They will damage the device output response. To overcome this problem voltage suppression diodes are used. The operation of voltage suppression diode is similar to Zener diode operation.

13. Gold Doped Diodes In these diodes gold is used as a dopant. These diodes are faster than other diodes. In these diodes the leakage current in reverse bias condition also less. Even at the higher voltage drop it allows the diode to operate in signal frequencies. In these diodes gold helps for the faster recombination of minority carriers.

14. Super Barrier Diodes It is a rectifier diode having low forward voltage drop as schottky diode with surge handling capability and low reverse leakage current as p-n junction diode. It was designed for high power, fast switching and low-loss applications. Super barrier rectifiers are the next generation rectifiers with low forward voltage than schottky diode

15. Peltier Diode In this type of diode, at the two material junction of a semiconductor it generates a heat which flows from one terminal to another terminal. This flow is done in only single direction that is as equal to the direction of current flow.

16. Crystal Diode This is also known as Cat’s whisker which is a type of point contact diode. Its operation depends on the pressure of contact between semiconductor crystal and point.

CRYSTAL DIODE APPLICATIONS

a. Crystal diode rectifier

b. Crystal diode detector

c. Crystal radio receiver

17. Avalanche Diode This is passive element works under principle of avalanche breakdown. It works in reverse bias condition. It results large currents due to the ionisation produced by p-n junction during reverse bias condition.

AVALANCHE DIODE USES

a. RF Noise Generation: It acts as source of RF for antenna analyzer bridges and also as white noise generators. Used in radio equipments and also in hardware random number generators.

b. Microwave Frequency Generation: In this the diode acts as negative resistance device.

c. Single Photon Avalanche Detector: These are high gain photon detectors used in light level applications.

18. Silicon Controlled Rectifier It consists of three terminals they are anode, cathode and a gate. It is nearly equal to the Shockley diode. As its name indicates it is mainly used for the control purpose when small voltages are applied in the circuit.

19. Vacuum Diodes Vacuum diodes consist of two electrodes which will acts as an anode and the cathode. Cathode is made up of tungsten which emits the electrons in the direction of anode. Always electron flow will be from cathode to anode only. So, it acts like a switch.

20. PIN Diode The improved version of the normal P-N junction diode gives the PIN diode. In PIN diode doping is not necessary. The intrinsic material means the material which has no charge carriers is inserted between the P and N regions which increase the area of depletion layer.

PIN DIODE APPLICATIONS:

a. Rf Switches: Pin diode is used for both signal and component selection. For example pin diodes acts as range-switch inductors in low phase noise oscillators.

b. Attenuators: it is used as bridge and shunt resistance in bridge-T attenuator.

c. Photo Detectors: it detects x-ray and gamma ray photons.

21. Point Contact Devices A gold or tungsten wire is used to act as the point contact to produce a PN junction region by passing a high electric current through it. A small region of PN junction is produced around the edge of the wire which is connected to the metal plate.

22. Gunn Diode: Gunn diode is fabricated with n-type semiconductor material only. The depletion region of two N-type materials is very thin. When voltage increases in the circuit the current also increases. After certain level of voltage the current will exponentially decrease thus this exhibits the negative differential resistance. Discuss, illustrate and derive the related equations of various rectifier circuits.A widely used application of this feature and diodes in general is in the conversion of an alternating voltage ( AC ) into a continuous voltage ( DC ). In other words, Rectification.But small signal diodes can also be used as rectifiers in low-power, low current (less than 1-amp) rectifiers or applications, but where larger forward bias currents or higher reverse bias blocking voltages are involved the PN junction of a small signal diode would eventually overheat and melt so larger more robust Power Diodes are used instead.

The power semiconductor diode, known simply as the Power Diode, has a much larger PN junction area compared to its smaller signal diode cousin, resulting in a high forward current capability of up to several hundred amps (KA) and a reverse blocking voltage of up to several thousand volts (KV).Since the power diode has a large PN junction, it is not suitable for high frequency applications above 1MHz, but special and expensive high frequency, high current diodes are available. For high frequency rectifier applications Schottky Diodes are generally used because of their short reverse recovery time and low voltage drop in their forward bias condition.

Power diodes provide uncontrolled rectification of power and are used in applications such as battery charging and DC power supplies as well as AC rectifiers and inverters. Due to their high current and voltage characteristics they can also be used as free-wheeling diodes and snubber networks.Power diodes are designed to have a forward “ON” resistance of fractions of an Ohm while their reverse blocking resistance is in the mega-Ohms range. Some of the larger value power diodes are designed to be “stud mounted” onto heatsinks reducing their thermal resistance to between 0.1 to 1oC/Watt.If an alternating voltage is applied across a power diode, during the positive half cycle the diode will conduct passing current and during the negative half cycle the diode will not conduct blocking the flow of current. Then conduction through the power diode only occurs during the positive half cycle and is therefore unidirectional i.e. DC as shown. Power Diode Rectifier Power diodes can be used individually as above or connected together to produce a variety of rectifier circuits such as “Half-Wave”, “Full-Wave” or as “Bridge Rectifiers”. Each type of rectifier circuit can be classed as either uncontrolled, half-controlled or fully controlled where an uncontrolled rectifier uses only power diodes, a fully controlled rectifier uses thyristors (SCRs) and a half controlled rectifier is a mixture of both diodes and thyristors.

Half Wave Rectification

A rectifier is a circuit which converts the Alternating Current (AC) input power into a Direct Current (DC) output power. The input power supply may be either a single-phase or a multi-phase supply with the simplest of all the rectifier circuits being that of the Half Wave Rectifier.

A. Half Wave Rectifier Circuit The current on the DC side of the circuit flows in one direction only making the circuit Unidirectional. As the load resistor receives from the diode a positive half of the waveform, zero volts, a positive half of the waveform, zero volts, etc, the value of this irregular voltage would be equal in value to an equivalent DC voltage of 0.318 x Vmax of the input sinusoidal waveform or 0.45 x Vrms of the input sinusoidal waveform.Then the equivalent DC voltage, VDC across the load resistor is calculated as follows. Where Vmax is the maximum or peak voltage value of the AC sinusoidal supply, and VS is the RMS (Root Mean Squared) value of the supply.

B. Half-wave Rectifier with Smoothing CapacitorWhen rectification is used to provide a direct voltage ( DC ) power supply from an alternating ( AC ) source, the amount of ripple voltage can be further reduced by using larger value capacitors but there are limits both on cost and size to the types of smoothing capacitors used. In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A multiple winding transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, (C). This configuration results in each diode conducting in turn when its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier so it is 100% efficient as shown below.

C. Full Wave Rectifier Circuit The Diode Bridge Rectifier The full wave rectifier circuit consists of two power diodes connected to a single load resistance (RL) with each diode taking it in turn to supply current to the load. When point A of the transformer is positive with respect to point C, diode D1 conducts in the forward direction as indicated by the arrows.As the spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and isabout 0.637Vmax of the peak voltage, assuming no losses. Where: VMAX is the maximum peak value in one half of the secondary winding and VRMS is the rms value.

The Full Wave Bridge Rectifier

Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output. The four diodes labelled D1 to D4 are arranged in “series pairs” with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.

The Positive Half-cycle During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch “OFF” as they are now reverse biased. The current flowing through the load is the same direction as before. The Negative Half-cycle As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax.Full-wave Rectifier with Smoothing Capacitor The smoothing capacitor converts the full-wave rippled output of the rectifier into a more smooth DC output voltage. If we now run the Partsim Simulator Circuit with different values of smoothing capacitor installed, we can see the effect it has on the rectified output waveform as shown. Discuss the block diagram of a power supply. You may illustrate it and the corresponding waveforms.

As illustrated in view B of figure 4-1, the first section is the TRANSFORMER. The transformersteps up or steps down the input line voltage and isolates the power supply from the power line. The RECTIFIER section converts the alternating current input signal to a pulsating direct current. However, as you proceed in this chapter you will learn that pulsating dc is not desirable. For this reason a FILTER section is used to convert pulsating dc to a purer, more desirable form of dc voltage. The final section, the REGULATOR, does just what the name implies. It maintains the output of the power supply at a constant level in spite of large changes in load current or input line voltages. Now that you know what each section does, let's trace an ac signal through the power supply. At this point you need to see how this signal is altered within each section of the power supply. Later on in the chapter you will see how these changes take place. In view B of figure 4-1, an input signal of 115 volts acis applied to the primary of the transformer. The transformer is a step-up transformer with a turns ratio of 1:3. You can calculate the output for this transformer by multiplying the input voltage by the ratio of turns in the primary to the ratio of turns in the secondary; therefore, 115 volts ac3 = 345 volts ac (peak-to-peak) at the output. Because each diode in the rectifier section conducts for 180 degrees of the 360-degreeinput, the output of the rectifier will be one-half, or approximately 173 volts of pulsating dc. The filter section, a network of resistors, capacitors, or inductors, controls the rise and fall time of the varying signal; consequently, the signal remains at a more constant dc level. You will see the filter process more clearly in the discussion of the actual filter circuits.

The output of the filter is a signal of 110 volts DC, with ac ripple riding on the dc. The reason for the lower voltage (average voltage) will be explained later in this chapter. The regulator maintains its output at a constant 110-volt dc level, which is used by the electronic equipment (more commonly called the load). State other applications of a diode Applications of Diodes While only two pin semiconductor devices, there are a number of applications of diodes that are vital in modern electronics. Diodes are known for only allowing current to move in one direction through the component. This lets a diode acts as a one-way valve, keeping signals where they need to be or routing them around components. While diodes only let current move in one direction, each type of diode acts differently, making a number of useful applications for diodes. Some of the typical applications of diodes include:• Rectifying a voltage, such as turning AC into DC voltages• Isolating signals from a supply• Voltage Reference• Controlling the size of a signal• Mixing signals• Detection signals• Lighting• Lasers diodes

Power Conversion One significant application of diodes is to convert AC power to DC power. A single diode or four diodes can be used to transform 110V household power to DC by forming a half-way (single diodes) or a full-wave (four diodes) rectifier. A diode does this by allowing only half of the AC waveform to travel through it. When this voltage pulse is used to charge a capacitor, the output voltage appears to be a steady DC voltage with a small voltage ripple. Demodulation of Signals The most common use for diodes is to remove the negative component of an AC signal so it can be worked with easier with electronics. Since the negative portion of an AC waveform is usually identical to the positive half, very little information is effectively lost in this process. Signal demodulation is commonly used in radios as part of the filtering system to help extract the radio signal from the carrier wave.

Over-Voltage Protections Diodes also function well as protection devices for sensitive electronic components. When used as voltage protection devices, the diodes are non-conducting under normal operating conditions but immediately short any high voltage spike to ground where it cannot harm an integrated circuit. Specialized diodes called transient voltage suppressors are designed specifically for over-voltage protection and can handle very large power spikes for short time periods, typical characteristics of a voltage spike or electric shock, which would normally damage components and shorten the life of an electronic product. Current Steering The basic application of diodes is to steer current and make sure it only flows in the proper direction. One area where the current steering capability of diodes is used to good effect is in switching from power from a power supply to running from a battery. When a device is plugged in and charging, for example, a cell phone or uninterruptible power supply, the device should be drawing power only from the external power supply and not the battery and while the device is plugged in the battery should be drawing power and recharging. As soon as the power source is removed, the battery should power the device so no interruption in noticed by the user.