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what determines the electrical characteristics of each particular type of atom?
- the electrons in the outer shell
- aka valence electrons
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Two important semiconductor materials
- silicon (Si)
- germanium (Ge)
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What is the energy level?
the amount of energy required to extract an electron from the atom
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how are the energy levels of the orbiting electrons measured?
in electron volts (eV)
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the amount of energy required to move one electron through a voltage difference of one volt
electron volt
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What are the 3 energy bands?
- Valence band
- energy gap(forbidden gap)
- conduction band
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electrons in which band become disconnected from atoms and are drifting around in the material?
conduction band
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what is the distinction between conductors, insulators and semiconductors largely concerned with?
the relative widths of the forbidden gap
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When does conduction occur in materials?
when an applied voltage causes electrons in the material to move in a particular direction
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free electrons in the conduction band are moved under the influence of the applied electric field
electron motion
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electrons which are still attached to atoms in the valence band
hole transfer
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what may be the reason a flow of current occurs?
hole movement or hold transfer
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electrons and holes are referred to as
charge carriers
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current flow from positive to negative is referred to as
the conventional current direction
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electron flow from negative to positive is known as
the direction of electron flow
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What is the bonding force that holds atoms together in a conductor is
metallic bonding
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when an electron fills one of the holes in the valence shell of an adjacent atom
covalent bonding
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when valence shell electrons are so strongly attached to the atoms that no charge carriers are available for current flow
insulating materials
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When atoms give up outer shell electrons which are accepted into the orbit of nearby atoms
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insulators have a _________ forbidden gap
Semiconductors have a _______forbidden gap
Conductors have a _____forbidden gap
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how does conduction occur in a semiconductor?
by both electron movement in the conduction band and hold transfer in the valence band
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how does conduction occur in a conductor
with no forbidden gap the valence and conduction band overlaps so very large numbers of electrons are available for conduction
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pure semiconductor material is known as
intrinsic material
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what must be added to intrinsic material before material can be used in the manufacture of a device to improve its conductivity?
impurity atoms
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What is the process of adding impurity atoms to intrinsic materials?
doping
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two types of doping
- donor doping
- acceptor doping
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What is the material referred to after doping?
extrinsic material
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type of doping material where impurity atoms have five electrons and three holes in the valence shells
n-type material
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n-type materials
antimony, phosphorus, and arsenic
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what are antimony, phosphorus and arsenic referred too as?
pentavalent atoms
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type of doping material with 3 electrons and 5 holes
p-type material
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p-type donor material
boron, aluminum, and gallium
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which doping material is considered acceptor-doped material
p-type material from semiconductors
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term where thermal energy causes electrons to break free and enter the conduction band creating pairs of holes and electrons
hole-electron pair generation
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term where thermal energy causes electrons to fall into holes is
recombination
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what is the majority charge carriers in n-type material
electrons
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what are the minority carriers in n-type material?
holes
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what is the majority charge carriers in p-type material
holes
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what is the minority charge carriers in p-type material?
electrons
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What happens when a conductor is heated?
- * atoms vibrate
- * movement of surrounding electrons
- * reduction in flow
- * conductor's resistance increases
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resistance that increases with increasing temperature
positive temperature coefficient (PTC)
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what temperature coefficient does a conductor have?
positive
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at what temp does a semiconductor behave as an insulator?
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What is the dominating factor in a semiconductor's current?
thermal generation of electrons
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increase in current flow and a decrease in resistance with rising temperature
negative temperature coefficient (NTC)
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what causes a semiconductor to behave more like a conductor?
when it's heavily doped
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what is a material called that has few free electrons available for conduction when not illuminated?
high dark resistance
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what happens when the semiconductor material is illuminated?
- resistance decreases
- comparable to a conductor
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when a electron going from negative to positive collides with atoms and rebounds towards the positive terminal due to the electric field
drift current
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electrical current that occurs when a concentration of one type of charge carrier is at one end of a semiconductor material causing repulsion towards a low concentration area
diffusion current
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which has greater mobility? electrons or holes?
electrons
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what determines the electron and hole velocities under the influence of an electric field?
mobility constants
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what does the initial diffusion of charge carriers in a pn-junction create?
barrier voltage
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What can the magnitude of the barrier voltage at the pn-junction be calculated with?
- * doping densities
- * electronic charge
- * junction temperature
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what does the barrier voltage oppose and assist across the pn-junction?
- opposes majority carriers
- assists minority carriers
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The layer on each side of the junction that is depleted of charge carriers
depletion region
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how is the depletion region created?
by movement of charge carriers across the junction
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what is on the n-side of the depletion region?
donor impurity atoms that lost free electrons and have become positively charged
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what is on the p-side of the depletion region?
acceptor impurity atoms that have become negatively charged by losing the hold associated with them (filled with an electron)
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What happens to the depletion region when either side is more heavily doped?
penetrates deeper into the opposite side
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a two-electrode, or two-terminal, device
diode
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one-way device, offering a low resistance when forward-biased, and behaving almost as an open switch when reverse-biased
diode
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what are low-current diodes used for?
switching circuits
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what are high-current diodes most often used for?
rectifiers for ac to dc conversion
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what is the positive terminal of a diode for forward bias called?
anode
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what is the negative terminal of a diode when forward biased?
cathode
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what can destroy a pn-junction diode?
- high level of forward current
- large reverse voltage
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another name for high-current diodes
power diodes
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what do power diodes generate a lot of?
heat
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what are power diodes generated for?
to be connected mechanically to a metal heat sink
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can pass forward currents of many amperes and can survive several hundred volts of reverse bias
power diodes
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is largely a minority charge carrier reverse saturation current
Ir(reverse-current)
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what happens if theres a small increase in Ir?
minority charge carriers leak into the junction surface
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why is it that a reverse-biased diode may be treated almost as an open switch?
because the reverse current is 1/10000 of the lowest normal forward current level
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what happens when diode reverse voltage (Vr) is sufficiently increased?
goes into reverse breakdown which can destroy a diode unless current is limited by suitable series-connect resistor
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in what device is reverse break-down useful
Zener diodes
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Ir
reverse saturation current
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Vbr
reverse breakdown voltage
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If(max)
maximum forward current
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the constant resistance (or dc resistance) of the diode at a particular constant forward current
static quantity
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the resistance offered to changing levels of forward voltage
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reciprocal of the slope of the forward characteristics beyond the knee
incremental resistance or ac resistance
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a diode with zero forward resistance and zero forward voltage drop
ideal diode aka perfect diode
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the reverse current of a ideal diode
infinitely high reverse resistance resulting in zero reverse current
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what is the relationship between the diode forward voltage and current in the circuit defined by?
device characterisitics
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what is the point on the dc load line where the diode voltage and current are compatible with the circuit conditions
point Q or quiescent point or dc bias point
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What dictates the slope of the dc load line in a diode series circuit
R1 resistor
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what determines point A on the load line?
supply voltage E
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what will happen if the maximum power dissipation of a diode is exceeded?
overheat and may short-circuit or open-circuit
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factor that defines the slope of the power-vs-temperature graph
derating factor (D) can be employed to draw graph
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depletion region is layer depleted of charge carriers situated between two blocks of low-resistance material refers to what two different devices?
pn-junction and capacitor
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Cpn
depletion layer capacitance aka transition capacitance
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the capacitance of a reverse-biased pn-junction
depletion layer capacitance
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what is the process of diffusion capacitance Cd
- E is reversed
- If ceases immediately
- majority charge carriers in depletion region
- depletion region widened
- creates a large reverse current at first
- current decreases to level of reverse saturation current
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what are the factors to consider with reverse-biased diodes
high reverse resistance (Rr) in parallel with depletion layer capacitance (Cpn)
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what are the factors to consider with forward-biased diode
dynamic resistance rd in series with voltage cell representing Vf
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the time required for the current to decrease to the reverse saturation current level
reverse recovery time (trr)
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Vr or Vrrm
- peak reverse voltage (peak inverse voltage and dc blocking voltage)
- max reverse voltage that may be applied across the diode
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Io or If(av)
- steady-state forward current
- max current that may be passed continuously through the diode
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Ifsm
- Non-repetitive peak surge current
- A one time current that is very much higher than the normal max
- may be passed for a specified time or flow briefly when a circuit is first switched on
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Ifrm
- repetitive peak surge current
- peak current that may be repeated over and over again during each cycle of a rectified waveform
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Vf
- static forward voltage drop
- max forward volt drop for a given forward current and device temp
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P
- Continuous power dissipation at 25 C
- max power that the device can safely dissipate continuously
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how can you get a diode to operate continuously in reverse device
- reverse voltage is sufficiently increased
- junction breaks down
- large reverse current flow
- reverse current limited by a suitable series-connected resistor
- Ir returns to its normal level when the voltage is reduced below the reverse breakdown level
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name and process of a reverse biased pn-junction (breakdown diodes) with a very narrow depletion region
- ionization by electric field aka Zener breakdown
- electric field strength (volts/width) can be very high
- high-intensity electric field causes electrons to break away from atoms
- converting depletion region from insulating into conduction
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name and process of a reverse biased pn-junction (breakdown diodes) with a very wide depletion region
- ionization by collision-avalanche breakdown
- electrons in the reverse saturation current can be given sufficient energy
- electrons to break free when they strike atoms in depletion region
- collide with other atoms
- produces more free electrons
- avalanche effect
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Z
- Zener diode
- cathode bar approximately in the shape of a letter Z
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Vz
zener breakdown voltage
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Izt
test current for measuring Vz
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Izk
reverse current near the knee of the characteristic, minimum reverse current to sustain breakdown
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Izm
Max zener current, limited by the max power dissipation
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the zener diode may be operated at any reverse current level between :
Izk and Izm
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what is the diode normally operated at for the greatest voltage stability
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