Autotronics - Diodes

Introduction


- electronic component which let current flow through a conductor in one direction and act as an insulator in the opposite direction.

- can be thought as an electronic version of check valve.

- act as a rectifier to convert AC current to DC current and remove modulation from radio signals in radio receivers.

- The term of diode related to the semiconductor characteristics of PN junction .



The Construction of Semiconductor Diode

- made of impurities P-N semiconductor material (mostly Silicon, Germanium, Galliumarsenide and Sillicon carbide).

- The conductivity of a conductor is depend on the polarity of potential difference i.e. Anode (p-type semiconductor) and Cathode (n – type semiconductor) or the direction of the current flowing across it.

- The PN junction is an area where the action of diode takes place.

- Here the positive charge carriers (holes) from the P-type semiconductor and the negative charge carriers (electron) from the n-type semiconductor combine together.

- The crystal conducts current in a direction from p-type (anode) to the n-type side(cathode) but not in opposite direction.



Mechanical Model of Diode

- The function of diode can be represented as a simple check valve.

- When the pressure( potential difference) of the check valve (diode) is given in the opposite direction the current flow will be blocked.

- Enough pressure must be given in this direction until the spring of the check valve spoil to let the current flow.

- The voltage which is needed to let current flow is called threshold voltage or forward voltage drop.

- To reach this situation voltage at such value must be put in the direction of forward voltage so that diode will become conductive at certain position.


Semiconductor Diode

- made of semiconductor material such as silicon which has been added with impurities in it.

- The impurities is very important to create region on one side which contains negative charge carriers (electrons) i.e. n-type semiconductor.

- On another side it is a region which contains positive charge carriers (holes), called p-type semiconductor.

- Each of these regions are attached by diodes terminals

- between them there is a border which is called PN junction where here all actions of diode takes place.

- Currents flow in a direction from p-type side (anode) to the n-type side (cathode) but not in the opposite direction.

- Another type of semiconductor diode is the Schottky Diode which is formed from the contact between a metal and a semiconductor rather than by p-n junction.





Current – voltage Characteristic

- The behaviour of a semiconductor diode can be observed by the current-voltage characteristic.

- The shape of the curve in the graph is influenced by the transport of charge carriers through the depletion layer in the p-n junction.

- At first when the p-n junction is created the electrons from the N-type region diffuse into the p-type region where there are more holes (places for electron in which no electron is present).

- When an electron from the n-type region combines together with a hole, both electron and hole disappear.

- Now on the N-side, there is a static positively charged donor and on the P-side there is a negatively charged acceptor.

- Therefore right now the area around the P-N junction becomes depleted of charge carriers and thus behaves as an insulator.

- However the width of the depletion region can grow with limit.

- Each time the electron-hole recombines, a positively-charged ion is left behind in N-type region and negatively charged ion is left in the P-type region.

- This process happens continuously and more ions are created.

- After some time an increasing electric field develops in the depletion region and make the process slower and finally stop the recombination process.

- Now there is a built-in potential across the depletion area.

- Now let say if we put an external voltage across the diode with the same polarity as the built-in potential

- the depletion area will act as an insulator to prevent any electric current flow.

- This phenomenon is called reverse bias.

- However if the polarity of the external voltage is put in the opposite direction of the built in potential, the recombination process happen again to create a current flow through the p-n junction.

- For silicon diodes the built-in potential is approximately 0.6V.

- Thus if an external current passed through the diode about 0.6V will be developed across the diode such that the P-doped region is positive with respect to the N-typed region.

- Therefore the diode is said to be turned on as it has forward bias.



Types of Semiconductor Diode

- There are several types of diode

- differentiated by geometric scaling, doping level, choosing the right electrodes and the application of diodes.

- Normally these diodes are made of doped silicon

- rarely germanium.


a) Avalanche Diodes

- It conducts in the reverse direction when the reverse bias voltage exceeds the breakdown voltage.

- Normally they are mistakenly called Zener diodes due to electrical similarity.

- Avalanche diode break down by different mechanism, the avalanche effect.

- It is designed to break down at a well defined reverse voltage without being destroyed.

- The only practical difference is that the two types have temperature coefficients of opposite polarities.


b) Crystal diode

- It is a type of point-contact diode.

- It consists of a thin or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of a coal.

- The wire forms the anode and the crystal forms the cathode.

- This type of diode is used normally as a crystal radio receivers.

- But nowadays crystal diode are generally obsolete, but may be available from a few manufacturers.


c) Constant current diode

- It is actually a JFET with the gate shorted to the source and function like a two-terminal current limiter analogue to the Zener diode, which is limiting voltage.

- Current is allowed to pass through then to rise to a certain value, and then level off at a specific value.

- It is also called CLDs, constant-current diode, diode-connected transistors or current-regulating diodes.


d) Esaki or tunnel diode

- It has a region of operation showing negative resistance caused by quantum tunnelling, thus allowing amplification of signals and very simple bistable circuits.

- This diode is also the type most resistant to nuclear radiation.


e) Gunn diode

- It is similar to tunnel diode in that it is made of materials such GaAs or InP that exhibit a region of negative differential resistance.

- With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built.


f) Light emitting diode (LED)

- In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side.

- Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced.

- The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 V to violet.

- The first LEDs were red and yellow, and higher-frequency diodes have been developed over time.

- All LEDs produce incoherent, narrow-spectrum light; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating.

- LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.


Laser diodes

- When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed.

- Laser diodes are commonly used in optical storage devices and for high speed optical communication.


Peltier diodes

- These diodes are used as sensors, heat engines for thermoelectric cooling.

- Charge carriers absorb and emit their band gap energies as heat.


Photodiodes

- All semiconductors are subject to optical charge carrier generation.

- This is typically an undesired effect, so most semiconductors are packaged in light blocking material.

- Photodiodes are intended to sense light(photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light).

- A photodiode can be used in solar cells, in photometry, or in optical communications.

- Multiple photodiodes may be packaged in a single device, either as a linear array or as a two-dimensional array.

- These arrays should not be confused with charge-coupled devices.


Point-contact diodes

- These work the same as the junction semiconductor diodes described above, but their construction is simpler.

- A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor.

- Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact.

- The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.


PIN diodes

- A PIN diode has a central un-doped, or intrinsic, layer, forming a p-type/intrinsic/n-type structure.

- They are used as radio frequency switches and attenuators.

- They are also used as large volume ionizing radiation detectors and as photodetectors.

- PIN diodes are also used in power electronics, as their central layer can withstand high voltages.

- Furthermore, the PIN structure can be found in many power semiconductor devices, such as IGBTs, power MOSFETs, and thyristors.



Schottky diodes

- Schottky diodes are constructed from a metal to semiconductor contact.

- They have a lower forward voltage drop than p-n junction diodes.

- Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation.

- They can also be used as low loss rectifiers although their reverse leakage current is generally higher than that of other diodes.

- Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down many other diodes — so they have a faster “reverse recovery” than p-n junction diodes.

- They also tend to have much lower junction capacitance than p-n diodes which provides for high switching speeds and their use in high-speed circuitry and RF devices such as switched-mode power supply, mixers and detectors.


Super Barrier Diodes

- Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p-n junction diode.


Gold-doped diodes

- As a dopant, gold (or platinum) acts as recombination centers, which help a fast recombination of minority carriers.

- This allows the diode to operate at signal frequencies, at the expense of a higher forward voltage drop.

- Gold doped diodes are faster than other p-n diodes (but not as fast as Schottky diodes).


Snap-off or Step recovery diodes

- The term step recovery relates to the form of the reverse recovery characteristic of these devices.

- After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform).

- SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.


Transient voltage suppression diode (TVS)

- These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients.

- Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.


Varicap or varactor diodes

- These are used as voltage-controlled capacitors.

- These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock.

- A PLL is faster than an FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal).

- They also enabled tuneable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.


Zener diodes

- Diodes that can be made to conduct backwards.

- This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference.

- In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero.

- Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see above).

- Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark).