4.1 Semi Conductors Flashcards
How many electrons in the valence shell of a silicon atom? and what is its attraction value?
4 - attraction value +4
Explain doping
The process of adding impurities to the instrinsic materials.
How is an N type semiconductor created?
By adding phosphorous to the silicon chain, phosphorous has 5 electrons making each atom have combined 9 electrons, one of these electrons is set free and allowed to roam.
How is a p-type semi conductor created?
By adding the atom of boron to the chain, this atom has a total of 3 electrons in the valence shell for a combined total of 7 which leaves a hole for an electron to be filled from the N side.
What are some trivalent impurities that can be added to P-type semi conductors?
Boron, indium, gallium
What are some pentavalent atoms that can be added to N-type semi conductors?
Arsenic, Phosphorus, Bismuth, Antimony
Explain the basic operation of a Diode AKA rectifier diode.
Allows current flow in one direction but will oppose, or stop, current flow in the opposite direction.
What happens in forward bias mode?
Allow current flow, act as a conductor
What happens in reverse bias?
Blocks current flow entirely.
How is the cathode (-) side of a diode found?
By the band on the resistor or a tab of some sort.
How do you start the process of electrons flowing across the depletion region?
The potential difference (voltage) must build up enough for the electrons to jump across the depletion range, usually 0.7 volts
What are typical barrier potentials for silicon and germanium?
Silicon = 0.7V, Germanium = 0.3V
What happens to the diode with an increase in temperature?
Temperature increase will increase the conductance causing the barrier potential to DECREASE and conductivity of the diode increases!
What happens with temperature drops of a diode.
Kinetic energy of the charge carriers decreases therefor the potential barrier will increase.
What is the purpose of the resistor in a circuit.
Limits the current to a value that will not damage the PN structure.
In a forward bias diode what happens when the voltage across it reaches 0.7V?
This point is called the “knee”, voltage almost stops at 0.7 however current starts to very rapidly increase.
What happens internally of a diode with it is put into Reverse Bias?
Voltage is applied across the device in reverse bias condition, the electric field across the junction increases, this increase in distance between the P and n regions, decreases the probability that a current carriers/electrons can jump from one side to the other.
What happens to current & voltage when a voltage is applied in reverse bias?
When voltage is a applied a very very small amount of current(uA or nA) goes through the PN junction, as voltage comes to the point of breakdown, Current will begin to rapidly increase and voltage starts to slow down. At this point the diode will be destroyed and effectively act as a conductor.
What is the usual minimum voltage for diode breakdown in reverse bias?
50V
What conditions must exist for forward bias?
Applied voltage must be greater than the barrier potential of the diode, the diode must be orientated anode to +, cathode to -.
During manufacture of diodes what is not carefully controlled?
the reverse leakage current cant be accurate for every diode.
When diodes are placed in series for voltage regulation they could potentially break down due to manufacturing tolerances? what can be done to combat this?
Adding 1 resistor in parallel to every diode. Due to voltage sharing and with the correct size resistors, the reverse voltage that appears across the diodes can be brought to the same value and the circuit will work as intended.
Without parallel resistors to diodes what will happen to the diodes?
Reverse voltages across each individual diode could vary drastically damaging one which will in turn cause damage to the other remaining diodes.
Connecting diodes in parallel will increase?
the forward current rating
If diodes are installed in parallel with varying current capacities, the diode with the lowest forward voltage drop will try to carry a larger current, potentially damaging the diode. to combat this what must be done?
Resistors can be installed in series with the diodes but in parallel to each other.
What range do rectifiers operate in?
1 to 1000 amps or even higher.
Primary used of a rectifier diode?
Half wave rectifiers, full wave rectifiers, DC blockers. They can also be used to convert AC to DC
What are the 2 main functions of a half wave rectifier?
Step down of voltage and voltage rectification.
When both alternations of the input voltage cycle are used the circuit has been said to be?
Full wave rectififed.
When four diodes are connected in a circuit what type of rectifier is this?
Bridge rectifier. Full wave rectifier
What is a diode that acts as an open or close type switch?
Silicon controlled rectifiers (thyristors)
Describe the make up of a thyristor.
An SCR is a four layered semiconductor device (PNPN), it has 3 electrodes: an anode, cathode and a gate (Control electrode)
No current will flow in a SCR thyristor until what condition is met.
Current is applied to the gate, flooding the cathode side with electrons until a time when the depletion region collapse and allows the free flow of electrons.
once a SCR thyristor is energised how can you stop current flow?
Until the voltage between the cathode and the anode is reversed, or current is reduced below a certain value. Another common method is to short anode to the cathode.
What is the benefit of a SCR thyristor and where are they used?
Large amounts of power can be switched or controlled using a small triggering current or voltage, used in motor speed controls, light dimmers, pressure control systems
Describe the operation of a light emitting diode.
Forward biased electrons cross the pn junction from the n type material and recombine with covalent holes in the p type material, when the conduction electron becomes a valence electron in the p paterial the recombing electrons release energy in the form of light and some heat.
How is light emitted from an LED
Large exposed surface area of the extremely thin p layer of the semi conductive material permits photons to be emitted. AKA ELECTROLUMINESCENCE
How do you change the colour of a LED?
By changing the material by which is it made of.
GaAsP = Red or Yellow
GaP = Red or Green
With an LED the amount of power output translated into light is directly?
proportional to the forward current, and increase in current = increase in radiated light.
What is the forward voltage of an LED? and Reverse breakdown?
1.2 - 3.2V, Breakdown = 3V to 10V
The photoconductive diode is a device that operates in?
Reverse bias
What is the basic operating principles of a photodiode?
High reverse bias current when struck by light, when photons are absorbed by the window a current is allowed to pass.
Photodiodes react best with what type of light?
Infrared light.
How is the breakdown voltage of a zener diode set?
carefully controlling the doping level during manufacture,, it can be adjusted to reach 1.2 to 200 volts
When a zener diode reaches reverse breakdown what happens across the terminals?
Voltage across its terminals remain almost constant, even though current may change drastically. this is referred to as the zener voltage.
What are varistors used for?
Surge protection.
Why are varistors good for surge protection?
They have a very fast response time and low leakage current.
What happens to the varistor when a voltage spike is sensed?
Resistance instantly decreases, creating a shunt path for the over voltage, the shunt path creates a short circuit condition, causing the circuit protection device (fuse/CB) to blow.
What range of voltage will a DMM provide you for testing diodes?
2.5 - 2.6V
Explain the test results from a DMM with a diode working.
Black to cathode, red to anode, if diode is good you will receive a reading between .5 to .9V, next turn the diode around to reverse bias it. If the diode is working properly you will get a reading similiar to your DMM supplied voltage. some DMM will displat OL instead.
If a diode has failed OPEN Circuit, what readings will you receive on the DMM?
Open circuit voltage reading (2.6V) typical or an OL indication both fwd and rev bias conditions
If a diode is shorted what results will you receive on a DMM?
Meter reads 0V in both fwd. and rev bias tests.
In some conditions of a failed diode may exhibit small resistances what reading will you receive?
Small voltage much less than the correct readings, may result in a reading of 1.1 in both directions.
