OMU-409MECHATRONICS

SEMICONDUCTOR ELECTRONICS

Asst.Prof. Özgür ÜNVER November 13th, 2019

CONDUCTORS & INSULATORS• Conductors: Metals have a large number of weakly bound electrons, when an

electric field is applied to a metal (voltage), the electrons migrate freely (producing current).

• Insulators: Some materials have atoms which valence electrons that are tightly bound, when an electric field (voltage) is applied, the electrons do not move easily (Pushing force of the voltage is not enough to overcome bounding force, therefore electrons cannot move and there is no current)

SEMICONDUCTORS• Semiconductors: A very useful class of materials, elements in

group IV of the periodic table, have properties somewherebetween conductors and insulators.

SEMICONDUCTORS• Semiconductors such as silicon and germanium have current-carrying

characteristics that depend on temperature or the amount of light falling on them.

• When a voltage is applied across a semiconductor, some of the valence electrons easily jump to the conductance band and then move in the electric field to produce a current, although smaller than that which would be produced in a conductor.

• In a semiconductor crystal, a valence electron can jump to the conduction band, and its absence in the valence band is called a hole.

• A valence electron from a nearby atom can move to the hole, leaving another hole in its former place.

• This chain of events can continue, resulting in a current that can be thought of as the movement of holes in one direction or electrons in the other.

http://www.youtube.com/watch?v=MCe1JXaLEwQ

DOPANTS• Dopants: The properties of pure semiconductor crystals can be

significantly changed by inserting small quantities of elements from group III or group V of the periodic table into the crystal lattice of the semiconductor.

• Dopants can be diffused or implanted into semiconductorsresulting in devices that are the basis of all modern electronics.

• Si (Silicon) has four valence electrons (4e)

• if Arsenic or Phosphorous from group V (5e) is added to the crystal lattice, one electron remains free to move around.

• Resulting semiconductor is called n-type (negative) silicon

Dopants

• if the Silicon is doped with Boron or Gallium from group III (3e), holesform due to missing electrons in the lattice.

• Electrons move to occupy the holes,

• Resulting semiconductor is called p-type (positive) silicon

• The purpose for doping a semiconductor such as silicon is to elevate and control the number of charge carriers in the semiconductor.

• In an n-type (negative, 1 excess electron) semiconductor, the charge carriers are electrons,

• In a p-type (positive, 1 missing electron) semiconductor, they are holes.

• The interaction between n-type and p-type semiconductor materials is the basis for most semiconductor electronic devices.

JUNCTION DIODE• If a p-type region of silicon is created adjacent to an n-type

region, a pn junction is the result

• The p-type side of the diode is referred to as the anode, and the n-type side is called the cathode.

• Electrons from the n-type silicon can diffuse to occupy the holes in the p-type silicon creating what is called a depletion region.

JUNCTION DIODE• A small electric field develops across this thin depletion region

due to the diffusion of electrons.

• This results in a voltage difference across the depletion region called the contact potential.

• For silicon, the contact potential is in the order of 0.6–0.7 V.

• If a voltage source is connected with the positive side to the anode and the negative side to the cathode it is said to be forward biased.

• The applied voltage overcomes the contact potential and shrinks the depletion region.

DIODE EQUATION• As the applied voltage approaches the value of the contact

potential (0.6–0.7 V for silicon), the current increases exponentially (diode equation)

• ID: current through the junction,

• I0: reverse saturation current,

• q: charge of one electron (1.60 x 10e-19 C),

• k: Boltzmann’s constant (1.381 x 10e23 J/K),

• VD: forward bias voltage across the junction,

• T: absolute temperature of the junction in Kelvin.

RECTIFIER• If the anode is connected to the n-type silicon and the cathode

to the p-type silicon, the depletion region is enlarged, inhibiting diffusion of electrons and thus current; and we say the junction is reverse biased.

• A reverse saturation current ( I0 ) does flow, but it is extremely small (on the order of 10e-9 to 10e-15 A).

• Therefore, a pn junction passes current in only one direction.

• It is known as a silicon diode and is sometimes referred to as a rectifier.

• For most diodes the breakdown value is at least 50 V

http://www.youtube.com/watch?v=cyhzpFqXwdA&feature=related

http://www.youtube.com/watch?v=aCfAdIRRw7M&feature=related

Video 3.1

RECTIFIER

DIODE• The diode is analogous to a fluid check valve, which allows fluid to flow only in

one direction• A diode is useful as a rectifier, they are used in the design of power supplies,

where AC power must be transformed into DC power for use.

Flyback Diode

IMPORTANT SPEC’S FOR DIODES

maximum forward current

maximum reverse bias voltage (where breakdown occurs)

instantaneous surge current

average current

frequency (diodes require nanoseconds to switch between their on and off states, therefore it may be a constraint for high speed circuits)

To select the right diode for your application, you need to consider (check the manual of the diode);

ZENER DIODE

• Zener diodes (voltage-regulator diodes)

• They can maintain a nearly constant voltage over a wide range of currents

• This characteristic makes them good candidates for building simple voltage regulators,

• In the presence of a variable supply voltage and variable load resistance they can maintain a stable DC voltage

Even when the current through the zener diode changes, the output voltage remains relatively constant.

Break down voltage

Connected reverse!

ZENER DIODE EXAMPLEIdeal zener diode!

• Voltage will be Vz as long as the zener diode is subject to reverse breakdown.• As long as Vz is constant and the load does not change, IL remains constant. • This means that the diode current changes to absorb changes from the

unregulated source.• R is known as a current-limiting resistor because it limits the power

dissipated by the zener diode. If Iz gets too large, the zener diode fails.

VOLTAGE REGULATORSAlthough the zener diode voltage regulator is cheap and simple to use, it has some drawbacks:

• The output voltage cannot be set to a precise value,

• Regulation against source ripple and changes in load is limited.

Special semiconductor devices are designed to serve as voltage regulators;

• some for fixed positive or negative values

• others easy to adjust to a desired, nonstandard value.

Three-terminal regulator designated as the 78XX, where the last two digits (XX) specify a voltage with standard values: 5 (05), 12, or 15 V.

VOLTAGE REGULATORS

• The 78XX can deliver up to 1 amp of current,

• It is internally protected from overload,

• Using this device, the designer need not perform the design calculations shown with the zener diode regulator.

NON-STANDARD VOLTAGE REGULATORS

• In some cases, you may need a regulated voltage source with a value not provided in a manufacturer’s standard sequence.

• Then you may use a three-terminal regulator designed to be adjustable by the addition of external resistors.

• The LM317L can provide an adjustable output with the addition of two external resistors

VOLTAGE REGULATORS

Three-terminal voltage regulators are;

• accurate,

• reject ripple on the input,

• Reject voltage spikes,

• have roughly a 0.1% regulation,

• quite stable,

Therefore; useful in mechatronic system design.

QUESTIONThe typical automobile has a 12 V DC electrical system where a lead-acid battery is charged by a belt-driven AC alternator whose frequency and voltage vary with engine speed.

What type of signal conditioning must be performed between the alternator and the battery, and how can this be done?

AC to AC voltage drop Transformer

Blocking back efm Diodes

AC to DC Full Wave Rectifier

Ripple Free Capacitors

Voltage Regulation Voltage Regulators

BOOST CONVERTER (DC TO DC STEP UP)• DC-to-DC power converter with an output voltage greater

than its input voltage

• Since power (P = VI) must be conserved, the output current is lower than the source current.

• Toyota Prius HEV uses a 500 V motor. Without a boost converter, the Prius would need nearly 417 cells to power the motor. However, it actually uses only 168 cells and boosts the battery voltage from 202 V to 500 V

• A white LED typically requires 3.3 V to emit light, and a boost converter can step up the voltage from a single 1.5 V alkaline cell to power the lamp.

• Charging a battery with a smaller voltage than the battery needs (i.e. solar panel creates 10 volts, but you need to charge up a 24V battery via step up DC/DC converter)

OPTOELECTRONIC DIODES• Light-emitting diodes (LED) are diodes that emit photons when forward biased,

• The positive lead (anode) is usually the longer of the two leads,

• The intensity of light is related to the amount of current flowing through the device,

• LED has a voltage drop of 1.5 to 2.5 V when forward biased,

• It is important to include a series current-limiting resistor in the circuit to prevent excess forward current, which can quickly destroy the diode.

• Usually a 330 Ω resistor is included in series with an LED when used in digital (5V)

9 mA = (3V/330 Ω)

PHOTODIODESPhotodiodes, are designed to detect photons and can be used in circuits to sense light,

Note that it is the reverse current that flows through the diode when sensing light.

It takes a considerable number of photons to provide detectable voltages with these devices.

http://www.youtube.com/watch?v=SFc673lEyQAhttp://www.youtube.com/watch?v=U6Wvmrc3akchttp://www.youtube.com/watch?v=FmBVNac4fls