Diodes and Solid State Devices

• A diode, sometimes called a rectifier diode, permits current to flow in one direction while opposing it in the opposite direction.
• An ideal diode acts as a conductor in forward bias and as an insulator in reverse bias.
• Solid state is a term used to describe devices using solid materials to manage electrical current flow via electron manipulation, instead of vacuum tubes.

Diode Analogy - Ball in a funnel

• If liquid pressure is applied to one end of a funnel containing a ball, the ball seals the throat, stopping flow and when electrical potential is applied in reverse polarity to a diode the conduction band gap increases, stopping electron flow.
• If liquid pressure is applied to other side of the funnel the ball is pushed away from the funnel throat allowing the liquid to flow. If electrical potential is applied in forward polarity to a diode the conduction band gap decreases and electron flow commences.
• When electrons are pushed toward the arrow/anode, current is blocked; when pushed from the narrow end/cathode, current flows.

Diode Symbols

• Diodes have various forms for specific uses.
• Varistors use similar semiconductor principles but are not technically diodes.
• Varistors are for non-ohmic varying resistors.
• Variable resistors, like potentiometers and rheostats, have ohmic characteristics and operate differently.

Diode Configurations

• Diodes' physical configurations vary with package type.
• The cathode is usually marked by a band, tab, or another feature.
• Sometimes, the anode/cathode is identified by a lead connected to the case, but always refer to the datasheet.

The PN Junction

• A diode is created by joining equivalently doped P-type and N-type semiconductors.
• The P-type region has numerous holes (majority carriers) with a net positive charge.
• The N-type semiconductor has excess electrons (negative charge).
• At the contact point between the p-region and n-region, P-type holes attract N-type electrons.
• This contact point is the PN junction.
• The p-region contains a few thermally generated free electrons (minority carriers), and n-region contains a few thermally generated holes.

Formation of the Depletion Region

• Free electrons move randomly in the n-region.
• At PN junction formation, electrons near the junction diffuse into the p-region, combining with holes in the covalent bond near the junction.
• As the n-region loses free electrons to the p-region, it creates a layer of positive charges near the junction.
• As electrons move into the p-region, completing the covalent bond, the P impurity becomes an ion, creating a layer of negative charges near the junction.
• These positive and negative charge layers form the depletion region, an area near the PN junction depleted of charge carriers (electrons and holes) due to diffusion across the junction which forms rapidly and is thin.

Barrier Potential

• A force acts on positive and negative charges near each other.
• In the depletion region, numerous positive and negative charges on opposite sides of the PN junction create an electric field, which acts as a barrier to free electrons in the n-region, requiring energy to move an electron through it.
• The barrier potential is the potential difference of the electric field across the depletion region, reflecting the voltage needed to move electrons through the electric field.
• A voltage, equal to the barrier potential with proper polarity, must be applied across a PN junction before electron flow begins.

Barrier Potential Variables

• The PN junction's barrier potential depends on the semiconductor material, doping amount, and temperature.
• The standard barrier potential is around 0.7 V for silicon and 0.3 V for germanium at 25°C.
• Barrier potential is inversely proportional to temperature.
• Increased temperature boosts hole and electron velocity, raising conductance, decreasing the barrier potential, and increasing diode conductivity.
• Lower temperatures reduce charge carrier kinetic energy, increasing the potential barrier.

Diode Forward Bias

• Apply DC voltage to bias a diode.
• Forward bias allows current to flow through the PN junction.
• External bias voltage is designated as VBIAS, and the resistor limits current to prevent PN structure damage.
• In forward bias, the negative side of VBIAS connects to the diode's n-region (cathode), and the positive side connects to the p-region (anode).
• A bias voltage, VBIAS, must be greater than the barrier potential.

Forward Bias Operation

• With 0 V across the diode, there is no forward current.
• As forward-bias voltage is gradually increased, there will be no current until the voltage exceeds the barrier voltage, after which the forward current through the diode will increase gradually, and a portion of forward-bias voltage is dropped across the limiting resistor (Ohms law).
• As forward-bias voltage increases to the point where voltage across the diode hits around 0.7 V (barrier potential), forward current across the diode increases rapidly, and depletion region narrows.
• As forward-bias voltage continues to increase, current continues to rise rapidly, but voltage across the diode only increases gradually above 0.7 V.
• The slight increase in diode voltage above the barrier potential is due to the voltage drop across the semi-conductive material's internal dynamic resistance.

Ideal Diode Model

• The ideal diode acts as a simple switch.
• When forward biased, it acts like a closed (on) switch, and when reverse biased, it acts like an open (off) switch.
• This model neglects barrier potential, forward dynamic resistance, and reverse current.
• The ideal V-I characteristic curve graphically describes ideal diode operation.
• The diode has zero voltage when forward biased because barrier potential and forward dynamic resistance are neglected.

Practical Diode Model

• The practical model adds the barrier potential to the ideal switch model for better accuracy.
• When forward biased, the diode is like a closed switch in series with a small equivalent voltage source equal to the barrier potential (0.7V) with the positive side toward the anode.
• This equivalent voltage source is not an active voltage source but the voltage drop produced across the forward-biased PN junction of the diode.
• When reverse biased, a diode is equivalent to an open switch with no current flow.
• The barrier potential does not affect reverse bias.
• Since barrier potential is included and dynamic resistance is neglected, the diode is assumed to have a voltage across it when forward biased
• A germanium diode would be offset by 0.3 V instead of the curve being for a silicon diode

Forward Bias Characteristics

• The realistic V-I characteristic curve for a forward-biased diode shows that a diode isn't a perfect switch.
• Diode forward voltage (VF) rises right along the horizontal axis, forwarding current (IF) climbs up the vertical axis.

Diode Resistance

• Diode resistance is its effective opposition to current flow.
• An ideal forward biased diode offers zero resistance, while a reverse biased one offers infinite resistance.
• In reality, all diodes, offer some small resistance when forward-biased, and a significant resistance when reverse-biased.
• The resistance of a forward-biased diode is not constant over the curve; dynamic resistance changes along the V-I curve.

Diode Reverse Bias

• Reverse bias prevents current through the diode.
• DC voltage source connected across a diode produces reverse bias.
• The external bias voltage is designated as VBIAS.
• The positive side of VBIAS connects to the diode's n-region, and the negative side to the p-region.
• In reverse bias, voltage is applied across the device, increasing the electric field at the junction and decreasing probability of current carriers moving.
• This widens the depletion region, and when electrostatic field equals Bias voltage, the only current is from thermally produced electron-hole pairs.

Reverse Bias Characteristics

• With reverse-bias voltage across a diode, only a tiny reverse current (IR) flows through the PN junction.
• 0 V across the diode means no reverse current.
• As reverse-bias voltage rises, a small constant reverse current flows.
• When voltage increases to where reverse voltage hits breakdown value (VBR), reverse current increases rapidly.
• If the bias voltage continues to increase, current continues to increase drastically, but voltage across the diode increases very little above VBR.
• Breakdown is usually destructive, and breakdown voltage is equivalent to the voltage that would break down an insulator into conduction.

Reverse Leakage Current

• A very small constant reverse current (uA or nA) exists until the reverse voltage approximates the breakdown value (VBR) at the curve's knee.
• After, voltage remains at approximately VBR, but IR spikes rapidly, causing overheating and damage in a typical silicon diode, the breakdown voltage can vary, and a minimum value of 50 V is common.

Full Diode Characteristic Model

• Combining forward and reverse bias curves gives the full V-I characteristic curve.
• The If scale is in mA, and the Ir scale is in µA.
• The diode voltage (VF) includes barrier potential plus the small voltage drop across the dynamic resistance.

Simple Diode Circuit

• To evaluate a circuit, first decide on the biasing state of the diode.
• For a diode to be forward biased:
  • Voltage applied must exceed the diode's barrier potential.
  • The diode must have its anode oriented to the positive potential and the cathode to the negative potential.
• Using Ohm's law and the voltage values for the series resistor and diode, the current flowing is calculated.

Series Connected Diodes

• For high-voltage applications, multiple diodes are serially connected to meet required voltage ratings.
• Diode formation is impractical because voltage doesn't distribute evenly because reverse leakage current for diodes is not constant and can vary substantially.
• Diodes with lower leakage current (and thus, higher reverse bias resistance) will experience higher voltage, leading to failure and excessive voltage across remaining diodes.
• A simple fix is connecting a high-value resistor in parallel with each diode (parallel voltage-sharing resistors).
• Voltage sharing, with correctly sized resistors, evens the reverse voltage across the diodes.
• If resistances are equal, and lower than the diode resistance, the two diode voltages will be approximately the same, as the value of resistor selected makes the difference between the two diodes irrelevant.

Parallel Connected Diodes

• Connecting diodes in parallel increases the forward current rating.
• It is best to match diodes for equal current sharing.
• Placing varying current capacity diodes in parallel, the diode with the lowest forward voltage drop will draw more current, potentially causing damage or overheating.
• Sharing resistors can be used if characteristics are unknown.

Worked Example

• To achieve equal current distribution in three parallel forward-biased diodes, insert series resistors that have equal resistance.
• Set the resistor value to make the current differences irrelevant by calucalting using ohms law, considering the current rating of each diode, and the intended voltage drop across the diodes.

Rectifier Diodes

• The terms diode and rectifier diode are often used synonymously.
• A diode is a small signal device, while a rectifier is a power device.
• Rectifier diodes serve to rectify alternating current.
• Commonly used in:
  • Half Wave Rectifiers
  • Full Wave Rectifiers
  • DC Blockers
• Rectifiers also convert AC to DC voltage.

Half Wave Rectifiers

• One critical diode use is rectification.
• The normal PN junction diode works well because it conducts heavily when forward biased but only slightly when reverse biased.
• Using a diode in series with AC power rectifies because current flows more easily in one direction.
• 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, acting to step down of voltage and perform voltage rectification.
• Important uses of half wave rectifiers is low power simple battery charger circuits.

Full Wave Rectifiers

• A device with multiple diodes arranged so load current flows in the same direction during each AC supply half cycle.
• A transformer(L) supplies the source voltage for two rectifier diodes arranged to conduct on alternate half cycles to measure voltage through the output (represented by a resistor) is always in the same direction resulting in rectified AC pulses.
• The circuit is a full wave rectifier because it uses both input voltage cycle alternations.

Bridge Rectifiers

• A circuit with four diodes connected, taking input from diagonally opposite corners, and output from remaining corners is a bridge rectifier, commonly found in electronic power supplies.
• A transformer inputs AC voltage into the bridge rectifier.
• Diodes D1 and D2 are forward biased during the positive half-cycle, current flows through them from earth back to applied EMF's positive potential.
• During the negative half cycle, D1 and D2 are reverse biased, current flows from earth through the load, D3 and D4, and back to the EMF's positive, therefore Current always flows in one direction through the load, so it no longer alternates.
• A bridge rectifier produces nearly twice the voltage output of a conventional full-wave rectifier with a given transformer.

Silicon Controlled Rectifiers (Thyristors)

• Thyristors serve as open/closed switches.
• An SCR is a 4-layer semiconductor device with alternating p and n-type materials (pnpn) and has three electrodes: anode, cathode, and gate.
• When the cathode is negatively charged relative to the anode, the SCR is triggered by applying current to the gate, resulting in conductance until the cathode-anode voltage is reversed or current is reduced below a certain value.
• SCRs can be switched off by shorting anode to cathode, reducing current below the minimum specified value.
• SCRs are in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators.
• In over-temperature circuits, a bimetallic sensor could trigger an SCR energising a warning light until a reset switch cancels it.

Light Emitting Diodes (LEDs)

• The device must be forward-biased for electrons to cross the pn junction from the n-type material and recombine with co-valent holes in the p-type material.

LED Semiconductive Materials

• LEDs use gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP), but not silicon or germanium (poor at producing light).

LED Biasing

• LED forward voltage is higher than silicon diodes, typically between 1.2 V and 3.2 V.
• Reverse breakdown voltage is much lower (3V to 10V).
• The LED emits proportional to forward current, as does light output, but reduces lifespan.

Organic Light Emitting Diode (OLED)

• This type of LED uses organic (carbon-bearing) films as a light-emitting layer and can be made as a thin, flexible sheet.
• Applications in aircraft interiors may encompass:
  • Diffuse/variable cabin lighting via flexible panels.
  • Smart cabin signs.
  • Lightweight displays with excellent contrast/viewing angles.
• OLED lighting panels operate typically at 6V or 8.5V from a current-limited driver and The 55-60 lumens per Watt (lm/W) energy efficiency is similar to traditional LEDs, which consume less power and cool lighting components reduce air conditioning load.

Substrate (clear plastic, glass, foil)

• This material supports the OLED.

Anode (transparent)

• This material removes electrons when a current flows through the device.

Organic layers

• These layers are made of organic molecules or polymers.

Conducting layer

• Made to transport "holes" from the anode.
• One conducting polymer used in OLEDs is polyaniline.

Emissive layer

• This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode.

Cathode

• Injects electrons when a current flows through the device.

Photoconductive Diodes

• This device, also called a photodiode, operates in reverse bias, where I is the reverse current and has a small transparent window that allows light to strike the PN junction.
• When reverse-biased, a rectifier diode has a very small reverse leakage current (same is true for a photodiode).

Photodiodes

• A semiconductor PN junction device that allows a high reverse bias current when struck by light and when photons are absorbed, a current is allowed to pass.
• A photodetector may contain optical filters and integrated lenses, or both large and small surface areas.
• The reverse-biased current is produced by thermally generated electron-hole pairs in the depletion region, which are swept across the PN junction by the electric field; In a rectifier diode, reverse leakage current rises due to an increase in electron-hole pairs via temperature, Photodiodes use light instead.

Zener Diodes

• A silicon PN junction device designed for operation in the reverse-breakdown region, and when it reaches reverse breakdown, voltage across its terminals stays nearly constant despite drastic current changes.
• A Zener Voltage is carefully controlled by the doping level during manufacture to obtain values from 1.2 volts to approximately 200 volts, and are commonly used to provide a stable reference voltage in circuits.

Varistors

• A surge protection device connected directly across the AC input or component by using fast response time and low leakage current.
• Behaves like reverse-to-reverse (back-to-back) zener diodes, conducting only when breakdown voltage is exceeded.
• The rest state has a high impedance that does not change the characteristics of the circuit.

Normal AC Volt

• When a voltage surge/spike occurs, the Varistor's resistance rapidly decreases, creating immediate shunt path for over-voltage.
• Creating a short circuit means the circuit protection device usually operates, where resetting a circuit breaker or replacing a fuse is cheaper than replacing sensitive components.

Testing Diodes

• Internal components may be damaged when diodes are tested due to the nature of voltage usage.

DMM Diode Test Position

• A DMM commonly used has a small diode symbol labeling the function switch that indicates the function of the switch is set to a diode.
• The meter provides internal voltage and when set to diode test is internally forward bias and the opposite.
• As ranges vary a meter of 2.5 V to 2.6 V is commonly used when diodes are being tested.
• Diode quality is measured using voltage.

When the Diode Is Working

• A positive and red meter lead is properly connected to the anode, and on the other hand, the black lead is connected to the cathode to provide forward bias a reading is considered functional between 0.4V and 0.9V with a nominal of 0.7V.
• Diode operation requires the device be turned the opposite direction resulting in the reverse direction properly you should receive voltage that is a based on your meters internal source (2.6V).
• You should see if this displays OL(Out of Limit) readings for the device or a voltage reading depending on your meter.

When the Diode Is Defective

• When a diode has failed has failed open from the circuit, you (2.6 V is typical) on an Out of Limit OL indication is received for each of testing states.
• If shorted it will present 0V for each bias test, the meter might present small bias rather than a completely short voltage.
• A diode might still be able to resist correctly but may be different than standard, 1.1 volts is the presented bias.

Checking a Diode with the OHMs Function

• A diode can still be functionally tested If testing of the position isn't available with the OHMs switch.
• Diodes depend on a meters internal battery function which may require a check by setting the range on the switch.
• If your diode is still functional a resistance value might still depend in the range because meter voltage varies.
• The higher the diode value a meter to measure is still available for reverse bias.
• A functioning diode relative readings will present data, depending on internal voltage range in a few to several OHMs if you want to fully test the diode reverse bias functionality
• After you expect that indication has been tested by showing extremely data (OHMs range is properly working)