Diodes: Function, Analogy and Symbols

• A diode, also called a rectifier diode, allows current to flow in one direction, opposing flow in the opposite direction
• An ideal diode acts as a conductor in forward bias and an insulator in reverse bias
• Solid-state devices utilize solid materials to control electrical current flow through electron manipulation, replacing vacuum tubes

Diode Analogy: Ball in a Funnel

• Applying liquid pressure to a funnel with a ball in the throat seals it, stopping flow, this mimics applying reverse polarity to a diode, increasing the conduction band gap and stopping electron flow
• Pushing the ball away from the funnel throat by applying liquid pressure from the other side allows liquid to flow, similarly, applying forward polarity to a diode decreases the conduction band gap, initiating electron flow
• Electrons can only flow when pushed from the narrow end (cathode), blocking current when pushed from the funnel (anode)

Diode Symbols

• Diodes have various types and uses, each represented by unique circuit symbols

Diode Configurations

• Diodes have physical configurations with anode and cathode markings
• The cathode is indicated by a band, tab, or another feature
• Some diodes have the anode/cathode identified by one lead connecting to the case
• Always consult the data sheet for specifications

The PN Junction

• Diodes are made by joining P-type and N-type semiconductors
• P-type semiconductors (p-region) contain holes and have a net positive charge
• N-type semiconductors have excess electrons and a negative charge
• At the junction, P-type holes attract N-type electrons, creating the PN junction
• P-type has a few thermally generated free electrons and n-region contains a few thermally generated holes

Formation of the Depletion Region

• Upon PN junction formation, free electrons near the junction in the n-region diffuse into the p-region, combining with holes
• N-region loses electrons, creating a layer of positive charges (ions) near the junction, while electrons moving into the p-region make the P impurity an Ion which creates a layer of negative charges near the junction.
• These layers form the depletion region, depleted of charge carriers due to diffusion across the junction
• The depletion region forms rapidly and is thin compared to the n-region and p-region

Barrier Potential

• Positive and negative charges near each other exert force as described by Coulomb's law
• There are positive charges and negative charges on opposite sides of the PN junction
• The forces between charges form an electric field, creating a barrier to free electrons in the n-region
• External energy is required to move electrons across the electric field in the depletion region
• The potential difference of the electric field across the depletion region becomes the barrier potential, expressed in volts
• Voltage equal to the barrier potential with proper polarity must be applied across a PN junction for electron flow to begin

Barrier Potential Variables

• The barrier potential of a PN junction depends on the semiconductor material, doping level and temperature

  • Silicon barrier potential: Approximately 0.7 V
  • Germanium barrier potential: Approximately 0.3 V at 25°C

• Barrier potential is inversely proportional to temperature, conductance increases with temperature and as a result the barrier potential decreases, whereas a decrease in temperature has the opposite effect

Diode Forward Bias

• Forward bias allows current through the PN junction
• A DC voltage source connected by conductive material across a diode produces forward bias.
• The external bias voltage is VBIAS
• A resistor limits current to prevent damage to the PN structure

Forward Bias Operation

• In forward bias, the negative side of VBIAS connects to the n-region (cathode), and the positive side connects to the p-region (anode)
• The bias voltage, VBIAS, needs to be greater than the barrier potential
• No forward current flows when there is 0 V applied across the diode
• There is no current until the forward-bias voltage exceeds the barrier voltage
• Forward current then gradually increases, with a portion of the voltage dropping across the limiting resistor (Ohms law)
• The forward current increases rapidly and the depletion region narrows when forward-bias voltage increases to approximately 0.7 V for silicon

Ideal Diode Model

• The ideal diode model is a simple switch: closed (on) when forward biased and open (off) when reverse biased
• In this model, the barrier potential, forward dynamic resistance, and reverse current are neglected

Practical Diode Model

• The practical model adds the barrier potential to the ideal switch model, which is more realistic
• When forward biased, the diode is equivalent to a closed switch in series with a voltage source equal to barrier potential (0.7V), with the positive side toward the anode
• This equivalent voltage source represents the fixed voltage drop across the forward-biased PN junction of the diode, and is not an active source of voltage.
• When reverse biased, the diode equals an open switch
• The barrier potential does not affect reverse bias
• With the barrier potential included and dynamic resistance neglected, the diode has a voltage across it when forward biased
• The silicon diode curve is offset by 0.7 V, and the germanium diode by 0.3 V

Forward Bias Characteristics

• In reality a diode is not a perfect switch.
• The diode forward voltage (VF) increases to the right along the horizontal axis, and the forward current (IF) increases upward along the vertical axis.
• Forward current increases very little until voltage reaches approximately 0.7 V at the knee of the curve
• After the point, forward voltage remains at approximately 0.7 V, but If increases rapidly
• There is a slight increase in VF above 0.7 V, mainly to voltage drop across the static or dynamic resistance
• Normal operation for a forward-biased diode is above the knee of the curve
• The If scale is typically in mA

Diode Resistance

• Diode resistance is the opposition the diode has to current flow
• In an ideal diode, a forward-biased diode has zero resistance and infinite resistance when reverse biased.
• Every real diode has a small resistance when forward biased, and a considerable resistance when reverse biased.
• Resistance of forward-biased diode is not constant over the entire curve and depends on the V-I curve, which is called dynamic resistance. Rd = ΔVF/ΔIF
• Resistance is greatest below the knee of the curve because the current increases very little for a given change in voltage.
• Resistance begins to decrease in the region of the knee of the curve and becomes smallest above the knee with a large change in current and voltage

Diode Reverse Bias

• Reverse bias prevents current through the diode
• The positive side of VBIAS connects to the n-region, and the negative side connects to the p-region
• In reverse bias, a voltage is applied across the device, increasing the electric field at the junction
• A higher electric field decreases the probability of current carriers moving from one side of the junction to the other
• This widens the depletion region so the electrostatic field equals the Bias voltage
• The only current is thermally produced electron-hole pairs in the depletion region

Reverse Bias Characteristics

• Only a small reverse current (IR) goes through the PN junction when a reverse-bias voltage is applied across it
• There is no reverse current with 0 V across the diode
• When the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches the breakdown value (VBR) the reverse current begins to increase rapidly
• The current continues to increase very rapidly.
• The voltage increases little above VBR
• Breakdown isn't normal for most PN junction devices and would destroy them
• Breakdown voltage is equivalent to the voltage that would break down an insulator into conduction

Reverse Leakage Current

• There is very small constant reverse current (usually uA or nA) until the reverse voltage across the diode reaches approximately the breakdown voltage (VBR) at the knee of the curve
• After the point, the reverse voltage stays at VBR and Ir increases rapidly, resulting in overheating and possible damage
• Breakdown voltage for a silicon diode varies, but a minimum value of 50 V is not unusual

Full Diode Characteristic Model

• Combining the forward bias and reverse bias curves gives the complete V-I characteristic curve for a diode
• The If scale is in mA as compared to the Ir scale in uΑ.
• Since the barrier potential and the forward dynamic resistance are included, the diode is assumed to have a voltage across it when forward biased
• The voltage (VF) is the barrier potential plus the small voltage drop across the dynamic resistance
• The curve slopes because the voltage drop due to dynamic resistance increases as the current increases.
• The current / voltage graph shown here is a diode at room temperature (approximately 25 degrees celsius).

Connecting Diodes

• Determine the biasing state of the diode to evaluate the circuit
• Requirements for a diode to be forward biased:
  • Applied voltage must be greater than the barrier potential of the diode
  • Diode must be oriented with the anode to the positive potential and the cathode to the negative potential
• Diode CR₁ is forward biased in the example below

Voltage Evaluation

• To determine the voltage applied to the series resistor R1, the voltage applied to the circuit is 10 V
• A silicon diode requires 0.7 V to overcome the barrier potential, assume the diode semiconductor material is silicon or an unspecified material is used
• Therefore VR1 = VAPPLIED - VF (Si), or = 10 V - 0.7 V, = 9.3 V

Current Evaluation

• To determine the current flowing through resistor R1 and diode CR1, use Ohms law

Series Connected Diodes

• Series connections raise the blocking capabilities (peak inverse voltage of the diode) as many high-voltage applications cannot meet required voltage rating using one commercially available diode
• In practice this diode formation is not practical as the voltage does not distribute evenly between diodes
• As the reverse leakage current isn't a carefully controlled parameter during manufacturing, diodes can vary substantially from diode to diode even within the same batch
• Diodes with the lowest leakage current will have the highest voltage across them which will cause them to fail which will in turn apply excessive voltage to the remaining diodes and also cause them to fail
• Connecting a high-value resistor in parallel with each diode (parallel voltage- sharing resistors) is a simple solution
• Due to voltage sharing, reverse voltage that appears across the diodes can be brought to the same value using the correct size resistors and the circuit will work as intended
• With equal resistances (R1 = R2) lower than the diode resistance, the two diode voltages are about the same and the value of resistor selected makes the diodes irrelevant
• Without parallel resistors, variation in the resistance characteristic of each diode could cause reverse voltages across each individual diode to vary drastically
• In the diagram, the voltage drop across D2 may be insignificant
• As a result, avalanche breakdown may occur in diode
• D1 due to most voltage being dropped across it

Parallel Connected Diodes

• Connecting diodes in parallel increases the forward current rating
• If possible to match the diodes so that approximately equal current sharing is achieved this should be done
• Diodes are put in parallel with varying current capacities, similar to the series circuit, the diode with the lowest forward voltage drop will try to carry a larger current, which may cause damage to the diode and overheat
• In the event that the exact characteristics are not known, sharing resistors (with associated losses) can be used
• Again, calculate resistance values with a simple method can be used if all resistors are set equal

Worked Example

• Calculate a circuit with three forward biased diodes in parallel with a current rating of 20mA for each diode and calculated voltage drop across the diodes of 0.7 V to what value resistors should be used
• If the resistors placed in series are of equal resistance, then the current through each diode should be equal
• Maximum current for one diode is 20mA. Therefore, the resistor value should be set to make the current differences irrelevant
• Calculate the values as follows: R1 = R2 = R3 = V/I = 0.7 V/20 mA
• .: R1 = R2 = R3 = 35 Ω
• The resistors should be at minimum 100 ohms each in order to make the diode resistance irrelevant

Rectifier Diodes

• The terms diode and rectifier diode are often used interchangeably
• A diode is a small signal device with current capacity typically in milliamp range
• A rectifier is a power device, conducting from 1 to 1000 amps or even higher
• Rectifier diodes are simply diodes redesigned to serve the purpose of rectifying alternating current. The circuit symbol for diodes and rectifier diodes are the same
• The Schottky diode is a variant of the rectifier diode and is particularly popular in the field of digital electronics. In standard diodes and rectifier diodes, the connections remain the same, it is just the applications that differ
• The primary uses of a rectifier diode include:
  • Half Wave Rectifiers
  • Full Wave Rectifiers
  • DC Blockers
• Rectifiers are also used to convert AC voltage to DC voltage

Half Wave Rectifiers

• One of the most important uses of a diode is rectification
• The normal PN junction diode conducts heavily when forward biased and slightly when reverse biased (high-resistance direction)
• If we place this diode in series with a source of ac power, the diode will be forward and reverse biased every cycle
• Since current flows more easily in one direction than the other, rectification is accomplished
• The simplest rectifier circuit is a half-wave rectifier which consists of a diode, an AC power source, and a load resistor
• Half Wave Rectifier circuits are cheaper than full wave rectifiers, so they are used in some insensitive devices that can withstand the larger voltage variations
• The output average voltage of half wave rectifier is less than the input voltage, so they perform two main functions; step down of voltage and voltage rectification
• An important use of half wave rectifiers is Low power simple battery charger circuits.

Full Wave Rectifiers

• A full-wave rectifier is a device that has two or more diodes arranged so that load current flows in the same direction during each half cycle of the AC supply
• The connections to the diodes are arranged so that the diodes conduct on alternate half cycles Since both alternations of the input voltage cycle are used, the circuit is called a full wave rectifier.

Bridge Rectifiers

• A circuit with four diodes connected is called a bridge rectifier
• The input is applied to diagonally opposite corners of the network, and the output is taken from the remaining two corners
• A bridge rectifier is a common part of electronic power supplies
• Many electronic circuits require a rectified DC power supply for powering various electronic basic components from available AC mains supply
• Bridge rectifiers produce a voltage output that is nearly twice that of the conventional full wave circuit

Diode Devices: Silcon Controlled Rectifiers (Thyristors)

• Thyristors are used as open/close switches in circuits.
• An SCR is a type of thyristor of alternating p type and n type materials (pnpn).
• A thyristor contains three electrodes: an anode, a cathode, and a gate (control electrode).
• When the cathode is negatively charged no current flows until a pulse of current is applied to the gate.
• The SCR begins to conduct and continues to conduct until the voltage is reversed or is reduced below a certain value by using a method to to short the anode to cathode, reducing current to switch off.
• With a small triggering current/voltage can switch/control large amounts of power.
• SCRs are used in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators
• An over-temperature circuit uses a bimetallic sensor trigger an SCR to energize a warning light which then remain on until cancelled by a reset switch

Light Emitting Diodes (LEDs)

• The operation of LEDs occurs when the device is forward-biased, electrons cross the pn junction and recombine with co-valent holes in p-type material
• When the conduction electron becomes a valence electron in the P material, the recombining electrons release energy in the form of light and some heat
• A large exposed surface area of the extremely thin P layer of the semi-conductive material permits photons to be emitted as light
• electroluminescence produces light
• Wavelength dictates the colour LEDs are made of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP) However, Silicon and germanium are not used due to them essentially being heat-producing materials and very poor at producing light
• GaAs LEDs emit infrared (IR) radiation, which is invisible
• GaAsP produces either red or yellow visible light
• GaP emits red or green visible light

LED Biasing

• LEDs have a large forward voltage, between 1.2 V and 3.2 V
• The reverse breakdown voltage is much less
• The LED emits light in response to a sufficient forward current
• The amount of power transformed is proportional to the forward current
• The circuit should translate light output for a forward current with the appropriate safety tolerance (mA)

Organic Light Emitting Diode (OLED)

• An Organic Light-Emitting Diode (OLED) is a type of LED in which the light emitting layer is a film of organic (carbon-bearing) compound
• The emissive layer becomes ionised and emits light in response to a DC electric current
• OLEDs can be thin, flexible sheets
• The applications include:
  • Diffuse and variable cabin lighting from large, flexible OLED panels that conform to the curved shape of cabin trim panels. The large area OLED panels produce no concentrated heat loads.
  • Smart cabin signs.
  • Lightweight displays with excellent contrast and wide viewing angles.
• Typical input DC voltages for OLED lighting panels are 6V or 8.5V from a current-limited power supply (driver). Energy efficiency is similar to conventional LED
• Modern OLEDs now have a service life of 100,000 hours similar to conventional LEDs

Photoconductive Diodes

• The photoconductive diode (also referred to as a photodiode) is a device that operates in reverse bias Photodiodes contain a transparent window for their PN junction to be hit with light
• They are a semiconductor PN junction device, allowing high rever bias current when struck by light
• When photons are absorbed a current is allowed to pass
• Filters, built-in lenses, or either large or small surface areas.
• They have they same small, reverse leakage current when reverse bias
• The electron-hole pairs in the depletion rejoin creates a reverse leakage current for a rectifier diode, however instead photodiode usse light
• Photodiodes react to infrared light more than other wavelengths

Zener Diodes

• The Zener diode, designed for operation in the reverse-breakdown region, is a silicon PN junction device different from rectifier diodes
• The voltage across its terminals remains almost constant even though the current may change drastically
• This is the Zener Voltage of the device
• The reverse breakdown voltage is set by carefully controlling the doping level during manufacture
• Doping is adjusted to obtain Zener voltage values from 1.2 volts to approximately 200 volts
  • Circuit symbol is different Zener diodes are commonly used in electronic circuits to provide a stable reference voltage
  • Circuit has a small, transparent window that allows light to strike the PN junction

Varistors

• A Varistor is a surge protection device that is connected directly across the AC input or a component to be protected
• They have a very fast response time and low leakage current
• They behave like back to back zener diodes and only conduct when the breakdown voltage is exceeded
• The rest state has a high impedance (several meg ohms) and does not change the characteristics of the circuit.
• When a voltage surge or voltage spike is sensed, the Varistor’s resistance instantaneously decreases, creating a shunt path for the over-voltage, saving the sensitive components
• Because the shunt path creates a short circuit, the circuit protection device typically operates
• The resetting of a circuit breaker or the replacement of a fuse is cheaper than the replacement of sensitive components
• Symbol shows common diode circuit symbols

Functional Testing Of Diodes

• Many digital multimeters (DMMs) have a diode test position that provides testing way
• A typical DMM has a diode symbol to mark the location of the function switch
  • When set to diode test, DMMs internal voltage to forward-bias and reverse-bias a diode
• internal voltage may vary among different makes of DMMs, but 2.5 V to 2.6 V is a typical range of values
• The meter shows either a voltage reading or an indication of the condition for diode and is under test

When A Diode Is Working

• A reading between approximately 0.5 V and 0.9 V and a 0.7 nominal being forward bias will occur with the red (positive) lead connected to the anode and the black (negative) lead to the cathode
• If the diode is turned around with the leads reversed, a properly working diode will get a internal voltage source
• This indicates that the diode has a extremely high reverse resistance with internal voltage to appear across it with 2.6v representing a typical value.
• Some meters will show Out of Limit (OL) with respective instead of the values.
• Forward Bias voltage

When The Diode Is Defective

• Open circuit: 2.6 V or OL indication in both forward and reverse bias
• Shorted: 0 V in both forward and reverse bias
• Some failed diodes may have small resistance for both bias.
• Example results for resistive diode with 1.1V reading

Checking A Diodes

• DMMs that lack a diode test position are capable of checking a diode by setting the function switch to an OHMs range
• Internal battery readings can determine forward-bias checks of good diodes
• Reverse-bias check will get “OL” because its reverse resistance is too high
• Even with relative forward ranges, the diode can be functioned properly
• Relative low function is appropriate for reverse biases

Basic Summary of Diode Bias

Forward Bias

• In forward bias current is permitted to flow.
• Voltage connections
  • Positive to the p-region
  • Negative to the n-region
  • The bias voltage must be greater than the barrier potential
  • The depletion region narrows
• The barrier potential should be 0.7V for silicon with electrons to pass through the depletion and are carried by by P type materials

Reverse Bias

• In reverse bias current isn’t premitted to flow and current is prevented
• Has opposite Voltage connections
• Positive to the n-region
• Negative to the p-region
• The bias voltage is less than the breakdown voltage.
• No current flows after small transition for depletion