Mod 4 (Adv Semiconductor Devices) - Homework

thyristors,

semiconductor devices that are primarily used to control and switch high power
in both AC and DC circuits

Thyristors function as?

precision switches capable of handling large currents by turning on or off in
response to a control signal

Structure of a Silicon-Controlled Rectifier (SCR).

a four-layer semiconductor device that consists of
alternating layers of p-type and n-type materials (PNPN structure). It has three terminals: the
anode, cathode, and gate.

function of a Silicon-Controlled Rectifier (SCR)

s a switch, allowing current to flow from the anode to the cathode only when a small current is applied to the gate. Once triggered, remains on until the current flowing through it drops below a certain threshold, known as the holding current.

operating principles of an SCR

• it must be forward biased (positive voltage on the anode relative to the cathode)
• and a triggering signal must be applied to the gate terminal

significance of the holding current in an SCR

minimum current required to keep the SCR in the conducting state
after it has been triggered. determines the point at
which the device will stop conducting.

forward-biased

when the anode is connected to a positive voltage
relative to the cathode, preparing the device for conduction

forward blocking state

when the SCR is forward biased but not yet triggered by
the gate signal, so it does not conduct current

forward conduction state

when the SCR has been triggered and is actively
conducting current from the anode to the cathode

Forward Voltage Triggering:

Applying a high forward voltage across the SCR's terminals
can cause it to conduct. This method is generally avoided in controlled applications as it
can lead to uncontrolled turn-on

Temperature Triggering:

Heating the SCR can increase leakage current, potentially
triggering the device. This method is typically avoided as it may indicate thermal
runaway or failure.

dv/dt Triggering:

A rapid change in voltage across the SCR can induce a current in its
junction capacitance, leading to conduction. This method is usually undesired and
managed to prevent accidental turn-on

Light Triggering

Light radiation can trigger a Light-Activated SCR (LASCR) by generating
electron-hole pairs at the junction

Gate Triggering:

The most common and reliable method, where a positive voltage pulse is applied to the gate relative to the cathode when the SCR is forward biased. This method allows for precise control over the SCR’s turn-on process

Natural Commutation:

Relies on the natural zero-crossing of the AC current, which
causes the current through the SCR to drop below the holding current, turning it off

Forced Commutation

External circuitry forces the current through the SCR to drop
below the holding current, turning off the device

Gate Turn-Off (GTO)

A negative gate current is applied to specially designed Gate Turn-
Off thyristors (GTOs) to reduce the holding current, turning off the SCR

Reverse Bias

Applying a reverse voltage across the SCR’s anode and cathode can cause
the current to drop below the holding current, turning the SCR off.

Phase control

technique used in thyristor power control where the triggering of the
thyristor is delayed within each AC cycle

trigger angle

the point in the AC waveform at which the thyristor is triggered.

A smaller trigger angle results in more power being delivered, while a larger angle reduces the
power

DIAC (Diode for Alternating Current) structure

a bidirectional semiconductor device with two
terminals and no gate. It has a symmetrical structure with two PN junctions

DIAC (Diode for Alternating Current)

primarily used in triggering circuits for TRIACs, where they help to ensure that the TRIAC receives sufficient gate current to switch on. commonly found in light dimmers and motor speed controllers.

TRIAC (Triode for Alternating Current)

bidirectional semiconductor device that can
control AC power by conducting current in both directions when triggered by a gate signal.
Unlike SCRs, which are unidirectional and only conduct in one direction, TRIACs can switch on
during both halves of an AC cycle, making them ideal for AC power control.

TRIAC (Triode for Alternating Current) structure

consists of five layers of alternating p-type and n-type semiconductor
materials, forming two back-to-back SCRs with a common gate terminal.

TRIACs in phase control circuits

triggered at a specific point in the AC cycle, allowing it to control the power delivered to the
load. By adjusting the trigger angle, the user can control the average power supplied to the
load

Shockley diode

a four-layer semiconductor device (PNPN structure) that acts as a bistable switch, used in switching
circuits, pulse generation, and relaxation oscillators

Insulated-Gate Bipolar Transistor (IGBT).

a semiconductor device that combines the high
input impedance of a MOSFET with the high current-carrying capability of a bipolar junction
transistor (BJT

Insulated-Gate Bipolar Transistor (IGBT). structure

multiple layers of semiconductor materials, three terminals: gate, collector, and emitter

Insulated-Gate Bipolar Transistor (IGBT). function

When a positive voltage is applied
to the gate relative to the emitter, a conductive channel forms, allowing current to flow from
the collector to the emitter.

Unijunction Transistor (UJT)

simple semiconductor device primarily used for triggering
and timing applications. has a unique characteristic known as negative resistance,
where an increase in emitter current leads to a decrease in emitter voltage

Unijunction Transistor (UJT) Structure

consists of a single PN junction with three terminals: the emitter (E),
base 1 (B1), and base 2 (B2)

Unijunction Transistor (UJT) function

device triggers when the emitter voltage exceeds a certain threshold, causing the PN
junction to become forward biased and allowing current to flow.

Zener diodes

specially designed diodes that allow current to flow in the reverse direction when a specific breakdown voltage, known as the Zener voltage, is reached

Zener diodes function

connected in reverse bias across a load. When
the input voltage exceeds the Zener voltage, the diode conducts, maintaining a stable output
voltage equal to the Zener voltage

significance of the Zener breakdown voltage

This voltage is critical because it determines the voltage level that the Zener diode will regulate in a circuit.

Varactor diode.

a semiconductor device designed
to act as a variable capacitor when reverse biased. Its capacitance varies with the applied
reverse voltage, making it ideal for frequency tuning applications

Varactor diode. structure

consists of a PN junction, with the width of the depletion region changing
as the reverse voltage varies

Varactor diode. function

change in depletion region width alters the diode's
capacitance. used to adjust the resonance frequency

Schottky diodes structure

PN junction constructed using a metal-semiconductor junction, typically between a metal like aluminum
and an n-type semiconductor

Schottky diodes

ideal for high-frequency applications due to their low forward voltage
drop and fast switching speed.

Schottky diodes in power rectification and RF circuits.

used to convert AC to DC with minimal power loss
due to their low forward voltage drop.

Laser Diode

a semiconductor device that emits coherent light through a process called
stimulated emission

Laser diode structure

similar to that of an LED but includes
additional layers and mirrors to create an optical resonant cavity

Laser diode function

When a forward voltage is
applied, electrons and holes are injected into the active layer, where they recombine to
produce photons. These photons stimulate further electron-hole recombination, producing
more coherent light. The light is amplified by the resonant cavity formed by mirrors on either
side of the active layer, creating a focused, coherent laser beam.

Photoresistors

devices whose resistance
decreases as the intensity of incident light increases

Photoresistor structure

made of semiconductor
materials like cadmium sulfide

Phototransistors.

more sensitive and have a faster response time compared to
Photoresistors. preferred in
applications requiring precise and rapid detection of light,

Phototransistor structure

similar in structure to a standard transistor but includes a transparent case
that allows light to reach the base-collector junction

Phototransistor functions

When light photons hit this junction, they
generate electron-hole pairs, increasing the base current and, consequently, the collector
current. This process amplifies the electrical signal generated by light exposure