Semiconductor Diodes: Advanced Concepts

Advanced Diodes

• This section focuses on the inner workings of diodes using electron theory and diode functionality parameters

PN Junction Recap

• A PN junction forms when n-type and p-type doped silicon combine in a silicon semiconductor
• The p-region contains holes from impurity atoms and few thermally generated free electrons
• The n-region contains free electrons from impurity atoms and few thermally generated holes

PN Junction Formation and Depletion Region

• The n-region loses free electrons that diffuse across the junction when a PN junction forms
• This creates a layer of positive charges near the junction
• p-region loses holes that combine with electrons, creating a layer of negative charges near the junction
• The depletion region is formed by layers of positive and negative charges and is depleted of charge carriers at the p-n junction
• The depletion region forms quickly and is thin compared to the n and p regions

Depletion Region Formation and Barrier Potential

• Coulomb's law describes the force acting on positive and negative charges near each other
• An electric field forms in the depletion region from forces between opposite charges
• This electric field presents a barrier to free electrons in the n-region, requiring energy to move electrons through electric field

Barrier Potential Details

• Barrier potential, measured in volts, is the voltage required to move electrons through the electric field in the depletion region
• Barrier potential depends on semiconductive material, amount of doping and temperature
• Typical barrier potential is 0.7V for silicon and 0.3V for germanium at 25°C

Unbiased PN Junctions

• A PN junction with no external voltage applied is a zero bias or unbiased PN junction
• The electric field that forms is the unbiased condition of a diode
• Applying external voltage forward or reverse biases the diode or PN junction

Forward Biased PN Junctions

• Diodes conduct electricity when forward biased
• The negative side of the bias-voltage source repels free electrons (majority carriers in the n-region) toward the PN junction, creating electron current
• Bias-voltage gives free electrons sufficient energy to overcome depletion region barrier potential
• Electrons lose energy and combine with holes in the valence band once in the p-region

Electron Movement in Forward Bias

• The positive side of the bias-voltage attracts valence electrons toward the left end of the p-region
• Valence electrons move through the p-region using holes as a "pathway"
• Holes move to the right toward the junction: this is called hole current

Electron Flow in Forward Bias

• Electrons flow out of the p-region to the positive side of the bias-voltage source, leaving holes behind
• The conduction band overlaps the valence band in the metal conductor
• It takes less energy for an electron to be free in a conductor than in a semiconductor
• There is continuous availability of holes moving to the PN junction to combine with electrons

Effects of Forward Bias on the Depletion Region

• The number of positive ions falls with more electrons flowing into the depletion region
• As holes flow into the depletion region on the other side of the PN junction, the number of negative ions falls
• Reduction in positive and negative ions during forward bias causes the depletion region to narrow

Reverse Biased PN Junctions

• Reverse bias prevents current flow through the diode or PN junction
• Positive side of the bias-voltage source attracts free electrons (majority carriers in the n-region) away from the PN junction
• Additional positive ions are created as electrons flow to the voltage source, widening the depletion region and depleting majority carriers

Valence Electrons in Reverse Bias

• Electrons from the negative side of the voltage source enter the p-region as valence electrons
• Valence electrons move from hole to hole to the depletion region, creating additional negative ions
• This widens the depletion region and depletes majority carriers, holes are pulled toward the positive side

Reverse Bias and Transition Current

• Initial flow of charge carriers is transitional
• The availability of majority carriers decreases as the depletion region widens
• At the strength is increased until the potential across the depletion region equals the bias voltage
• Transition current ceases except for a negligible reverse current

Reverse Leakage Current

• A PN junction should not conduct in reverse bias, but increased barrier potential causes leakage
• Free electrons in the p-region drag to the battery's positive terminal
• Holes in the n-region drag to the battery's negative terminal
• This produces a current of minority charge carriers with an extremely small magnitude
• For constant temperatures, reverse current is almost constant as reverse voltage increases
• This is called reverse saturation current.

Reverse Breakdown

• Reverse current drastically increases when external reverse bias voltage increases to breakdown voltage
• All semiconductor types have a reverse breakdown voltage

Insulators and Breakdown Voltage

• The breakdown voltage of an insulator is the voltage at which the material conducts uncontrollably
• Conduction starts through a specific path in the material, causing molecular changes that ruin the insulator

Diodes and High Reverse Voltage

• High reverse voltage imparts energy to free minority electrons, resulting in collisions with atoms
• Atomic collisions knock valence electrons into the conduction band
• Newly created conduction electrons are high in energy and repeat the process and numbers multiply in a chain reaction
• High energy electrons go through the n-region as conduction electrons, rather than combining with holes

Avalanche Breakdown

• The multiplication of conduction electrons is called avalanche
• It results in high reverse current that damages diodes due to excessive heat dissipation
• Zener diodes are designed to work within the reverse bias breakdown voltage without damage

Temperature Effects on Diodes

• For forward-biased diodes, increased temperature causes an increase in forward current and forward voltage decreases
• For reverse biased diodes, increased temperature increases reverse current

Diode Parameters and Data Sheets

• Diode manufacturers uniquely identify diodes using a numbering system with the prefix "1N"
• Data sheets give detailed device information for proper application, including maximum ratings, electrical characteristics, mechanical data, and graphs

Thermal Effects on Semiconductors

• Temperature impacts any semiconductor device
• Temperature variations change characteristics of semiconductor diodes due to changes in the PN junction
• Increasing heat increases minority carriers, lowers forward resistance and reduces forward voltage drop
• In reverse bias, diodes develop increased leakage current and reverse breakdown potential with increased heat
• Diode's forward resistance rises with decreased temperature: this results in increased forward voltage drop
• The leakage current and reverse breakdown potential are reduced under reverse-biased conditions
• Diode normal operating temperature is referenced at room temperature, or 25°C

Datasheet Parameters

• Maximum forward voltage (VF max) is the maximum forward voltage drop of a diode at a specified forward current, normally specified at 25°C
• Maximum forward current (IFmax) is the maximum current allowed through a forward-biased diode before damage, specified at 25°C and must be de-rated at higher temperatures

Maximum Reverse Voltage

• Maximum reverse voltage determines the maximum reverse bias potential
• It must be safely applied to a diode before the diode enters the breakdown region and is less than the diode's reverse breakdown or avalanche potential
• This is also known as the Peak Reverse Voltage (PRV) or Peak Inverse Voltage (PIV)
• Maximum power dissipation is the maximum power a diode can dissipate before damage occurs
• It is calculated by VF and If measured at a point in the diode's operation
• If the product of these exceeds the maximum power rating then the device is damaged

Average and Peak Current

• With a diode conducting for half an AC cycle, the average current rating (Iav) is lower than the continuous (DC) current rating (IDC) of the device -This parameter is important in diode rectification
• Repetitive peak current is the maximum instantaneous value of repetitive forward current that the diode can withstand in a specified period of time

Frequency and Leakage

• Frequency threshold (ft) is the maximum operating frequency at which the diode can function correctly
• It depends on reverse recovery time capabilities during manufacture
• Leakage current (IR) is current that flows when a diode is reverse biased, specified at a reverse voltage (VR)

Reverse Recovery

• Reverse recovery time (trr) is the time for diode current to change direction, switching from forward to reverse bias
• Determined by the charge stored by the diode's internal capacitance
• Dynamic forward resistance (RF) is diode resistance when the diode is forward biased

Analyzing Maximum Ratings

• Absolute maximum ratings define the diode's operational limits without damage
• Diodes should operate well under these limits for greater reliability, which are specified at 25°C and adjusted downward for higher temperatures
• Understanding parameters is important for selecting a replacement diode

Understanding AC and DC Parameters

• Manufacturers maintain uniformity by measuring parameters at 25°C (unless stated)
• AC and DC parameters are distinguished using subscripts where AC parameters are denoted by a lower case subscript (Vf) and DC parameters by an upper case subscript (VF)
• VRRM is the maximum reverse peak voltage that can be applied repetitively and is the same as the PIV rating
• IF is the maximum average value of a 60 Hz rectified forward current, ratings are provided at 75°C
• IFSM is the maximum peak value of non-repetitive (one cycle) forward surge current and has relevant derating graphs
• TSTG is the storage junction temperature range
• TJ is the operating junction temperature
• PD is power dissipation

Power Dissipation and Junction Temperature

• As a diode dissipates power, its junction temperature rises and depends on -Amount of power dissipated (PD) -Ambient temperature (TA) -Thermal resistance (RΘJA) between the diode junction and ambient temperature
• Power rating is the power dissipation that raises the junction temperature from ambient temperature (usually 25°C) to its maximum value, TJMAX (175°C)

Electrical Characteristics

• VF is the instantaneous voltage across the forward-biased diode when forward current is 1A at 25°C
• Note the graphs showing forward voltage variations with forward current
• Irr is the maximum reverse current averaged over one cycle (when reverse biased with an AC voltage)
• IR is the maximum current when the diode is reverse biased with a DC voltage
• CT is the total junction capacitance with a 4V reverse-biased signal at 1 MHz

Half Wave Rectifiers

• Diodes are used in circuits called rectifiers
• Rectifiers convert AC voltage into DC voltage because diodes conduct current in one direction and block it in the other direction
• Rectifiers are in all DC power supplies that operate from an AC voltage source
• A power supply is an essential part of each electronic system

Half Wave Rectifier Circuit

• A diode is connected to any AC source and to a load resistor (RL) to form a half-wave rectifier
• Ground symbols represent the same point electrically
• When the sinusoidal input voltage (Vin) goes positive, the diode is forward biased and conducts current through the load resistor
• The current produces an output voltage across the load RL, with the same shape as the positive half-cycle of the input voltage (60 Hz signal)
• When the input voltage goes negative, the diode is reverse biased and there is no current, so the voltage across the load resistor is 0V

Half Wave Rectifier Output Voltage

• The net result is that only the positive half-cycles of the AC input voltage appear across the load and it is a pulsating DC voltage with frequency of 60Hz
• The average value of the half-wave rectified output voltage is the value measured on a DC voltmeter
• Mathematically, it is determined by finding the area under curve of a full cycle and then dividing by 2π
• Results in the equation, Vavg = Vp / π, where Vp is the peak value of the voltage
• Vavg is approximately 31.8% of Vp for a half-wave rectified voltage

Half Wave Rectifier and Barrier Potential

• Real diodes have a barrier potential that must be included in rectifiers
• When the practical diode model with a 0.7V barrier potential is used, the input voltage must overcome the barrier potential before the diode becomes forward-biased during the positive half-cycle
• This results in a half-wave output with a peak value that is 0.7V less than the peak value of the input
• The expression for the peak output voltage is: VP(out) = VP(in) - 0.7V

Ideal Diode Models

• It is acceptable to use the ideal diode model that neglects the barrier when the peak value of the applied voltage is much greater than the barrier potential (at least 10V)
• For consistency, the practical model of a diode, taking the 0.7V barrier potential into account unless stated otherwise

Half Wave Rectifier PIV

• The peak inverse voltage (PIV) equals the peak value of the input voltage
• The diode must withstand this amount of repetitive reverse voltage, PIV, occurs at the peak of each negative alternation of the input voltage when the diode is reverse biased

Full Wave Rectifiers

• Although half-wave rectifiers have uses, full-wave rectifiers are the most common type in DC power supplies
• Full-wave rectifiers allow unidirectional current through the load in the entire 360° of the input cycle
• Half-wave rectifiers allows unidirectional current through the load only during one half of the cycle
• Full-wave rectification results in an output voltage with a frequency twice the input frequency that pulsates every half-cycle of the input
• The number of positive alternations in the full-wave rectified voltage is twice that of the half-wave voltage for the same time interval

Full Wave, Average Voltage and Diodes

• The average value, measured on a DC voltmeter, for a full-wave rectified sinusoidal voltage is twice that of the half-wave with given formula
• Each diode in the full-wave rectifier is alternately forward biased and then reverse biased
• The maximum reverse voltage that each diode must withstand is the peak secondary voltage, with the assumption that D2 is reverse biased
• The PIV is twice the peak value of the output voltage plus a diode drop

Center-Tapped Full Wave Rectifier

• A centre-tapped rectifier uses two diodes connected to the secondary of a centre-tapped transformer
• The input voltage is coupled thru the transformer to the centre-tapped secondary, where half of the total secondary voltage seems between the centre tap and each end of the secondary winding
• For a positive half-cycle of the input voltage, the polarities forward bias diode D1 and reverse biases diode D2 where current path is through D1 and the load resistor RL

Center-Tapped Full Wave in Reverese

• For a negative half-cycle of the input voltage, the voltage polarities on the secondary are shown in the figure
• This condition reverse biases D1 and forward biases D2
• The current passes through D2 and RL as indicated
• Because the output current during both portions of the input cycle is in the same direction through the load, the output voltage developed across the load resistor is a full-wave rectified dc voltage
• The maximum reverse voltage that each diode must withstand is the peak secondary voltage where D2is assumed to be reverse biased

Bridge Rectifier and Operation

• When the input cycle is positive, diodes D1 and D2 are forward biased and conducts current where voltage is developed across RL, which looks like the positive half of the input
• During this period, diodes D3 and D4 are reverse biased
• When the input cycle is negative, diodes D3 and D4 are forward biased and conduct current where D1 and D2 are reverse biased and output appears across RL
• It is a full wave
• During positive half-cycle of total secondary voltage, diodes D1 and D2 are forward biased, secondary voltage appears across load resistor
• During negative half-cycle diodes D3 and D4 are forward biased, secondary voltage appaears across the load resistor

Bridge Rectifier PIV

• 2-diodes are always in series w- load resistor during both the positive and negative half-cycles and if these diode drops are included, the output voltage is given by a formula. The PIV is calculated assuming D1 and D2 are forward bias
• Peak inverse voltage equal to peak secondary voltage because the output voltage ideally matches secondary voltage

Rectifier Summaries

• The peak output voltage is peak section voltage minus the barrier potential of a diode using a silicon value for a half wave rectifier
• Average output voltage is peak output voltage divided by π (PIV is peak voltage
• The peak output voltage is the peak section voltage (Vp(sec)) halved, minus the barrier potential of a silicon diode for center tap full wave rectifiers
• To find the average output voltage multply the peak output voltage and divide by PI.
• For a centre tapped recitifer the PIV is given by Peak output Voltage Plus the Barrier Potential for a Diode
• For a bridge full wave the peak Output is the the Peak Secondary Voltage minus the barrier (The number of diodes (silicon values to find the AVERAGE output -Then we DIVIDE PI into 2 Peak Output For its PIV just add the Peak Output to the Barrier Voltage for a Full Rectified Circuit

Power Supply Filters

• A power supply filter eliminates fluctuations in output voltage of a half-wave or full-wave rectifier and produces a DC voltage
• Filtering is necessary because circuits require a DC source to provide biasing for proper operation
• Voltage regulation in power supplies is usually done with integrated circuit voltage regulators to prevents changes in DC voltage due to input voltage or load vars
• AC power is converted to a constant DC voltage where the 50 Hz pulsating DC output of the rectifier gets filtered to reduce voltage variations, small amount of fluctuation creates ripple effects

Filters

• A half-wave rectifier with a capacitor-input filter is shown, where the filter is simply a capacitor from the rectifier output to ground where (Rl is resistance)
• During the pos quarter cycle Diode is Bias so the capacitor voltage can charge within 0.7 voltage of the peak input

More on Filters

• The source decreases where the Capacitor retains charge and the diode has REVERSE based because its cathode is way more positive than the anode.
• This is because the capacitor can discharchge the laod resistanced so the RC time constant as its long to the period

Ripple Voltage

• Because the capacitor charges very fast the start of a cycle + slowly releases its charge into and with help of that its called the ripple, but the ripple makes it very undesirable and makes things look unappealing in all senses. -We do wnat to reduce the size + that Ripple for a much efficient and better filter

Ripple Voltage

• Because Full Wave recitifer that happens have frequency that is TWICE then its value in Hallf wave makes it easy and better filter because the SHORTER time that appears
• When something is filtered the rectifier voltage has something of smaller Rippel than a half cause both resistor/capacitor equal where capacitor releases but this happen since there small between each wave.

Ripple Current

• The measurement factor by determining if effeiceny in a filtered (what I do is indicate on effecteness into what the system and this can have many definition that is.
• *I -PP = peak to Peak voltag that is Ripple
• VDC = Is value or is for *DC = Which means the DC AVERAGE you get from your filter where L Lower the Rippe this leads into the Best _Factor for the Filter where by increasing the capacitor or the load you increase the resistors

Electrical Current & Surge

• Before the switch is engaged the capacitor is not used, it does not possess charge , so just before engaging to the switch of the circuit (Bridge of capacitors) its going to engage with the charge before the resistor gets connected + Diode Because its going to have a good ammount, so that being it does need a resistor that can stop by, there is always a case where we closed at something we call voltage

Varactor Diodes

• A varactor is a diode that always operates in reverse bias and is doped to maximise the inherent capacitance of the depletion region
• The depletion region which widens by reverse bias acts as a capacitor dielectric because of its nonconductive characteristic
• the p & n regions are conductive and acts as the capacitor plates.

Varactor Operations

• As reverse-bias voltage Increases the deletion gets stronger

The Shockly Diode

• These Diodes are used primarily from High-Frequency and the FAST switching that they're doing, where also know for Hot-Barrier that there's diodes that are Schottky-Diodes where its symbol shown by. The Diodes is performed connecting doped semiconductor region + usually metal such as Gold/ Platinum- So rather than using a PN-unction its now just METAL-TO-The semiconductor and is showed below then a typical foward is shown at 0.3 V

Tunnel Diodes

• Tunnel Diodes have a characteristic of a * negative resistance which in term gives use a oscillator of microwave application __ 2 different symbol the Diodes have germanium of Gallium arsehide were there doped a P and N