Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
169 Cards in this Set
- Front
- Back
- 3rd side (hint)
|
How do we solve the problems with the Rutherford model |
We use the Bohr model in quantum mechanics. This states that the electrostatic force = The centrifugal force |
|
|
|
What does the Bohr Theory of the Atom tell us |
- Electrons exist only in distinct energy levels. - Electrons can only gain/lose discrete amounts of energy - Electrons must have a unique quantum number, only energy levels can hold only 2 electrons of opposite intrinsic spin. - Electrons spin round on themselves, they have two soon states: clockwise and anticlockwise |
|
|
|
How do energy levels fill according to full quantum theory |
- Electrons tend to fill the lowest energy level first. - Its energetically favourable for electrons to form pairs of opposite spin - Electrons can only jump to empty levels. |
|
|
|
How are chemical bonds formed |
Electrons of opposite spin will form pairs and hold atoms together. |
|
|
|
What happens to energy levels once a bond is formed between electrons |
There is a very small difference in the energy levels of the 2 electrons, so the energy levels split slightly. |
|
|
|
How do bands of energy levels form |
As more and more atoms bond together energy levels of the same origin will undergo further splitting.
This forms bands of energy levels with N levels.
Some of these levels will overlap. |
|
|
|
How does semiconducting behaviour occur |
Electrons held in chemical bonds cannot carry electrical current.
Electrons leave holes when they become excited.
The holes move between bonds always filling the empty state
|
|
|
|
What is a hole |
As electrons leave their bond they leave an empty bonding state, this empty state is a hole and is generally thought as being a positively charged particle. |
|
|
|
What is the conduction band |
The upper energy band containing electrons which are free to conduct. |
|
|
|
What is the Valance band |
The lower energy band containing the fixed electrons held in chemical bonds and the holes which are free to conduct. |
|
|
|
How are electrons and holes created in Intrinsic Semiconductors |
Chemical bonds are broken.
These bonds are broken through an energy input. This energy input is generally in the form of heat.
Electrons aquire enough thermal energy that they can jump into the conduction band, leaving a hole in the Valance band.
The electrons most likely to do this are those in energy states at the top of the Valance band. |
|
|
|
What is recombination |
The process of electrons in the conduction band falling back into the Valance band. |
|
|
|
What does conductivity depend on |
- The probability Pe(E) of an electron occupying the conduction band and the probability Ph(E) of a hole occupying the Valance band.
|
|
|
|
What does Electron/hole generation depend on |
The thermal energy. Therefore, the probability of an electron/hole occupying the conduction/valence band to be temperature dependent. |
|
|
|
What is the first observation we make about the probability of creating free electrons |
Pe(E) increases exponentially with kt (quantum of thermal energy) |
|
|
|
What is the second observation we make about the probability of creating free electrons |
Pe(E) decreases exponentially with E
Pe(E) depends on the energy difference E between the energy of an electron in the Valance band and the energy level it is being excited into. The bigger the energy jump less the likely an electron will occupy the higher energy level |
|
|
|
What is the third observation we make about the probability of creating free electrons |
At T=0 Electrons can only occupy Valance band states |
|
|
|
What is the Fermi-dirac distribution function |
Where E(F) is Fermi level of energy |
|
|
|
What is the density of states |
It is a concept used to describe the numbers of energy levels in a band
It is the number of energy levels per unit volume per unit energy range. |
|
|
|
Why does the Valence band contain very many levels closely spaced in energy |
As it must accommodate all of the electrons that bond together the atoms forming a solid. |
|
|
|
Why do we need to know how many energy levels exist in an energy range |
We need to know the number of energy levels that exist in an energy range Delta (E) per unit volume of semi conductor as many energy levels overlap in the valance and conduction bands and electrons become identifiable only by their quantum number. |
|
|
|
What is the formula for the density of electrons in the conduction band available to carry current. |
Where Nc(E) is the density of states per unit volume per unit energy range in the conduction band |
|
|
|
What is the formula for the density of holes in the valence band available to carry current. |
Where Nv(E) is the density of states per unit volume per unit energy range in the valence band. |
|
|
|
What are currents made up of, and what is it in a semiconductor. |
Fluxes of charged particles through a conductor. For a semiconductor this is a flux of electrons and a flux of holes |
|
|
|
What are the equations for flux of holes, flux of electrons and total current density |
|
|
|
|
How is mobility defined, and what are the equations for it |
Mobility is defined as the gradient of the velocity-electric field curve
|
|
|
|
What us the equation fr total current density using mobility |
|
|
|
|
What is the ohms law formula using current density and it's standard form and what current density formula does this derive |
|
|
|
|
What is the formula for conductivity |
This is not a -q |
|
|
|
What is the formula for total free electron density |
|
This is based of electron density |
|
|
What is the formula for the Total free hole density |
|
|
|
|
How do we derive electrons and holes in an intrinsic semiconductor |
Purely from breaking chemical bonds
Therefore, n = p = ni Where ni is the intrinsic density of electrons or holes. |
|
|
|
What is the formula for ni, the intrinsic density of electrons or holes |
|
|
|
|
What are the limitations of intrinsic semiconductors for use in devices |
- Semi-conductors must operate I've a temperature range of 290 +- 40K however ni varies rapidly with temperature, therefore conductivity is strongly temperature dependent. This would lead to a temperature dependent device performance, which is undesirable. Or Conductivity is low which is also undesirable. |
|
|
|
How do we make a semiconductor insensitive to temperature and increase it's conductivity |
We add impurities to increase the density of either electrons or holes. |
|
|
|
What is Doping |
The adding of impurities to a semiconductor |
|
|
|
What happens when an impurity with -1 electron is used |
There is bond missing an electron this is equal to have an extra hole for conduction.
The impurity (extra hole) will accept an electron from the valence band to leave a hole behind.
The effect of this is a valence band hole is created without simultaneously creating an electron in the conduction band. |
|
|
|
What is the trait of a p-type semiconductor |
Hole density (p) = acceptor density (Na) this uses the wavy almost equals sign. |
|
|
|
What are acceptors |
These are impurities that accept electrons from the valence band to leave a hole behind. |
|
|
|
How do acceptor accept an electron |
The acceptor energy level lies close to the valence band edge.
So only a tiny amount of energy is needed to excite an electron from a valence bond on to an acceptor atom. |
|
|
|
What happens to the Fermi level in an acceptor |
The Fermi level decreases as the probability of the valence band being unoccupied by electrons increases |
|
|
|
What happens when an impurity with +1 electrons is used |
The impurity donates an electron to the conduction band.
This has the effect of a conduction band electron being created without simultaneously creating a hole in the valence band. |
|
|
|
What is the trait of a n-type semiconductor |
Electron density (n) = donor density (Nd) this uses the wavy almost equals sign. |
|
|
|
What is a donor |
An impurity with an extra electron which donates an electron to the conduction band. |
|
|
|
How do donors donate at electron |
The donor energy level lies close to the conduction band edge. So only a tiny amount of energy is needed to excite an electron from a donor atom on to the conduction band. |
|
|
|
What happens to the Fermi level in a donor |
The Fermi level increases as the probability of electrons occupying the conduction band must increase. |
|
|
|
What happens in an N-type semiconductor |
Donors readily ionise, donating their extra electron to the conduction band, to give a high density of electrons.
Some electrons still originate by thermal excitation from the valence band leaving a few holes in the valence band.
The material is electrically neutral n = p + Nd where, Nd = density of ionized donors. |
|
|
|
What are the properties of N-type Semi-conductors |
- Density of holes (p) << density of electrons.
- Electrons are called the majority carrier (high density)
- Holes are the minority carrier (due to a low density) |
|
|
|
What happens in a P-type semiconductor |
Acceptor readily ionise, accepting electrons from the valence band, to leave a high density of holes in the valence band.
Some thermal excitation of electrons from the valence band still occurs, thus giving a few electrons in the conduction band.
The material is electrically neutral |
|
|
|
What are the properties of P-Type semiconductors |
- Density of electrons (n) << density of holes (p) - Holes are called the majority carrier (high density) - Electrons are the minority carrier (due to a low density) |
|
|
|
What happens when a P-type and N-type semiconductor are joined. |
Without a boundary, holes will diffuse out of the p-type region creating a -ve charged layer just inside, arising from -ve charged acceptors (which cannot move).
Electrons simultaneously diffuse out of the N-type region to leave a +ve charged layer, due to a fixed +ve charge on donors. |
|
|
|
What is the graph that depicts the charge density of the pn junction depletion region |
|
|
|
|
What does the depletion region determine |
Circuit performance of pn junctions both in diodes and transistors |
|
|
|
What is the equation for the depletion region |
|
|
|
|
Out is further outward diffusion prevented in the pn junction |
Space charge layers are formed just inside the p-type and n-type regions. This prevents holes from the p-type and electrons from the n-type from diffusion due to electrostatic repulsion. |
|
|
|
Why must energy bands bend |
After the depletion layer has formed:
- Holes gain potential energy (PE) to flow from p-type to n-type regions.
- Electrons gain potential energy to flow from n-type to p-type regions. |
|
|
|
What is the Fermi Level in a junction |
At zero bias it stays constant |
|
|
|
How do we represent carrier fluxes |
|
|
|
|
What formulas do we derive at zero bias for electron flux and hole flux |
As no electric current flows at zero bias we have. Zero net electron flux Jne = Jpe = Jeo Zero net hole flux Jph = Jnh = Jho |
|
|
|
What is the effect on the pn junction at forward bias |
The potential barter between the p-type and n-type regions is lowered by qV.
Here the fluxes of minority carriers stays the same but the majority carrier fluxes will increase as the majority carriers are more likely to get over the lower barrier. |
|
|
|
What is the formula for the net electrons into the p-type region from the n-type region under foward bias. |
|
|
|
|
What is the probability formula for an electron getting over the barrier |
It is the same as the probability of an electron having an energy q ( Vbi - V) above it usual energy |
|
|
|
What is the formula for electrons from n-type to p-type under standard conditions and zero bias |
|
|
|
|
What is the equation for net flux Je, from the n-type to p-type region |
|
|
|
|
What is the equation for net flux Jh from the p-type to n-type region For standard conditions and zero bias fluxes of holes |
|
|
|
|
What is the formula for the net current density flowing across a junction |
|
|
|
|
What is the effect on the pn junction at negative/reverse bias |
The potential barrier between p-type and n-type regions is raised by qVrev
The width of the depletion region increases as root ( Vrev ) increases.
The minority flux carriers are unaffected however the majority carriers are unable to diffuse over the higher barrier, therefore the fluxes J'ne and J'ph become negligibly small. |
|
|
|
What is the diode reverse current at a low-to-medium Vrev |
J = q ( Jho + Jeo ) = Jo |
|
|
|
What is the diode reverse current at a High Vrev |
We need to consider the effect of the voltage drop across the depletion region:
- Due to the existence of charged layers arising from the ionized impurities, there will be an electric field in the depletion region.
- The strength of the electric field varies the applied reverse bias as
Fmax = potential drop / width of depletion layer = ( Vbi + Vrev ) / W |
|
|
|
What is the effect of the rapid band bending under a High reverse biad |
It will create a High electric field
Electrons accelerated by the high field will gain kinetic energy.
When electron KE > Ec - Ev it creates an electron-hole pair if it collides with the lattice, this results in there being 2 electrons and 1 hole.
Further collisions with the lattice cause multiplication of the number of charge carriers, thus greatly increasing current.
Holes are accelerated too, adding to the current multiplication. |
|
|
|
What is Avalanche Multiplication and where is it likely to occur |
The current growth of electrons and holes colliding with the lattice.
It occurs rapidly at the breakdown voltage. |
|
|
|
What is the DC resistance for a diode |
|
|
|
|
What do we define small signal resistance as |
|
|
|
|
What is the formula for current in foward bias |
|
|
|
|
What is the formula for small signal resistance in foward bias |
|
|
|
|
What is the current of a diode in reverse bias |
I = Io = Constant |
|
|
|
What are the 2 main differences between a parallel plate capacitor and a pn junction |
- Charged are actually distributed through the junction.
- The depletion layer width, W, varies with the reverse biasm |
|
|
|
What is the equation for capacitance of a diode |
|
|
|
|
How is a diode used for Rectification |
|
|
|
|
How is a diode used for Full Wave rectification |
|
|
|
|
How is a diode used for a Voltage Reference |
|
|
|
|
How are diodes used as light emitting diodes |
A high density of electrons and holes is required in the conduction bands.
This is achieved by foward biasing a diode.
Electrons are injected from the n-type and holes are injected from the p-type. The electrons and holes combine at the depletion region and emit photons of energy Ep = Ec - Ev |
|
|
|
How are diodes used as photodiodes |
Light absorbtion acts as a basis for photodetection.
Photo-generated electrons and holes rapidly recombine UNLESS they are physically separated.
This separtion is achieved by absorbing light in a depletion region.
The photo-detected electrons and holes are pulled out of the depletion region by an electric field.
A reverse bias can also extend the depletion region to increase the efficiency of photodetection.
The extracted carriers create a photocurrent or photovoltage. |
|
|
|
What does a transistor act as |
1. An Amplifier A small change in the input voltage or current produces a large change in the output current.
2. A switch A large change in the input voltage or current produces a large change in the output current. |
|
|
|
What sections does a Bipolar Junction Transistor comprise of |
1. The Emitter This is the source of minority carriers which are injected into the base by foward biasing a pn junction.
2. The Base Acts as the controlling layer through which minority carriers are transported towards the collector.
3. The Collector Where the carriers successfully transported through the base are collected, by reverse biasing a pn junction, before forming the output current. |
|
|
|
What are the schematic and circuit representations of the 2 types of BIPOLAR transistors pnp and npn |
|
|
|
|
What is the Common Emitter Configuration and where is it's application |
Where one contact the emitter terminal is common to both the input circuit and output circuit. It's main area of application is in amplifiers. |
|
|
|
What is the Common Collector Configuration and where is it's application |
Where the collector terminal is common to the input and output circuits.
It's main area of application is with unity gain buffer amplifiers. |
|
|
|
What is the Common Base Configuration |
Where the base terminal is common to the input and output circuits. |
|
|
|
How can the BJT be used as a common emitter amplifier |
|
|
|
|
What does the graph for collector current versus voltage look like for a common emitter amplifier |
The sloping straight line is the curve for Vce = Vcc - Ic Rc and this is called the Load Line
With the DC base current IB , output voltage is given by the intersection of collector characteristic for IB with the Load Line, point Q, is called the Quiescent Operating Point
AC output voltage varies between VCE1 and VCE2 as shown |
|
|
|
What is the emitter injection efficiency |
The ratio of electron and hole currents across the emitter-base junction |
|
|
|
What is the base transport factor |
The ratio of the current associated with carriers reaching the collector to the current injected into the base from the emitter. |
|
|
|
For an npn transistor what is the emitter injection efficiency |
The Emitter efficiency helps determine the gain |
|
|
|
For an npn transistor (when finding the emitter injection efficiency) what is the formula for Ine |
|
|
|
|
For an npn transistor (when finding the emitter injection efficiency) what is the formula for Ipe |
|
|
|
|
What is a formula for the emitter injection efficiency using doping densities in the emitter and base |
Here NB is the density of acceptors in the base and NE is the density of donors in the emitter |
|
|
|
How do we find the current injected into the base from the emitter |
Due to recombination only a fraction of the total current from the emitter reaches the c-b junction. This fraction is the Base Transport Factor |
|
|
|
What is the equation for the base transport factor |
This Base Transport Factor will affect gain. |
|
|
|
What is the definition (semi-equation) for the Common base current gain |
|
|
|
|
What is the formula for the Common base current gain (in terms of measurable terminal currents) |
|
|
|
|
What is the formula for the Common base current gain (in terms of the emitter injection efficiency and the base transport factor) |
|
|
|
|
What is the formula for the Common Emitter current gain |
|
|
|
|
What is the formula for the Common Emitter current gain (in terms of the emitter injection efficiency and the base transport factor) |
|
|
|
|
What is thr 9 step process to make an npn silicon BJT |
1. Oxidise an n-type Si wafer
2 Define the base area by photolithography, use a mask as a template with the pattern on it being transferred into the photoresist.
3. Develop the resist and etch a window into the oxide
4. Grow an oxide layer containing acceptors and diffuse impurities into the Si. The diffusion of the acceptor through the window converts local areas of n-type Si to more heavily doped p-type material.
5. Repeats steps 1-3 to define the emitter area.
6. Grow an oxide laye4 containing donors and diffuse the impurities into the Si. The density of diffused donors must be sufficient to convert p-type Si in the base to more heavily doped n-type material.
7. Repeats steps 1-3 to define contact windows
8. Deposit a metal layer for contacts
9. Define the contacts by photolithography and etch away unwanted areas of metal. |
|
|
|
How do we define the input and output using terminal for a BJT in the Common Emitter Configuration |
The e-b terminals represent the Input , with the pn junction contributing a resistance Rpi and a capacitance Cpi . There is an access resistance rb to the junction region from the base contact
The e-c terminals represent the output, with a resistance ro dominated by the reverse biased c-b junction and Co dominated by the reversed biased c-b junction. We have a small signal current source ic |
|
|
|
What is the ewuaivalent circuit foe the Common Emitter Configuration |
|
|
|
|
What are the 3 issues with the Common Emitter Configuration |
1. ic will have a dependence on VBE
2. ro will have a dependence on VBE
3. The capacitance between the b and c terminals must be included. |
|
|
|
What is the formula for nB(0) the density of electrons injected into the base at zero |
|
|
|
|
What is the density of electrons at the collector end |
Zero, the high electric field in the c-b depletion region pulls any electrons arriving there into the collector. |
|
|
|
What is the formula for the Flux of electrons |
Where dn is the change in density, dx is the change in distance and D is th3 distribution. |
|
|
|
Why is there a difference in electron flux in the base |
1. A change in electron density gradient will give rise to the diffusion flux (current)
2. Recombination with holes causes a loss of electrons. |
|
|
|
What is the formula for the rate of recombination |
Where T is the average time an I ejected minority carrier survives before recombination. This is the Lifetime. n(x) is the sum of the injected and background electron densities n(x) = nb (x) + n Bo |
|
|
|
What is the formula for the rate of change in the density of electrons in the element of width delta x due to recombination |
|
|
|
|
What is the formula for the rate of change in the density of electrons in an element of width delta x due to the difference in the diffusion flux |
|
|
|
|
What is the formula for the Total rage of change in the electron density in the element |
|
|
|
|
What is the solution of the differential equation to give a formula for nB (x) |
|
|
|
|
What is the Minority Carrier Diffusion Length |
|
|
|
|
What is the formula for Ic |
|
|
|
|
What is transconductance |
For a small signal the strength of the output current in th3 circuit must be in small changes in VBE so we must express Ic in a small change. |
|
|
|
What if the formula for transconductance |
|
|
|
|
What is the Hybrid-pi equivalent circuit of a BJT |
|
|
|
|
How do MOSFETs operate |
By inducing a conducting channel between two contacts known as the SOURCE and the DRAIN then applying a Voltage to a GATE electrode. |
|
|
|
What is the DEPLETION MODE format of forming a conducting channel |
Use Majority Carriers
a -ve gate Voltage attracts holes to form a channel. This makes VG less, -ve DEPLETES the channel of charge carriers |
|
|
|
What is the ENHANCEMENT MODE format of forming a conducting channel |
Use Minority carriers A +ve gate Voltage attracts electrons to induce a channel. This increasing VG ENHANCES the density of charge carriers. |
|
|
|
What is the key requirement to form a channel for ENHANCEMENT MODE MOSFETs |
A voltage VT is first required to form a channel, therefore when Vg > Vt only the voltage Vg - Vt acts on the electrons. |
|
|
|
What is the formula to find the charge in element between x and x + dx |
Where Co, is the capacitance per unit area of the metal-oxide channel sandwich. W delta x is the area of the element where W is the width of the gate. |
|
|
|
What is the alternate formula to find the charge in element between x and x + dx |
Whre D is the average depth of thr channel |
|
|
|
What is the equation for electron density |
|
|
|
|
What is the formula for current density (J) |
|
|
|
|
What is the formula for Id the output current |
|
|
|
|
What is the formula for output current Id at saturation Idsat |
|
|
|
|
Why do MOSFETs act as true transconductance devices |
As the change in the input voltage VGS directly causes a change in the output current Id |
|
|
|
What is the transconductance below saturation |
|
|
|
|
What is the transconductance in saturation |
|
|
|
|
What is the formula for the Total capacitance for a MOS under gate |
|
|
|
|
What is the formula for the small signal transconductance between the source and drain gds |
In the linear region : Image
In the saturation region: it is zero on theory but small in practise |
|
|
|
What is the equivalent circuit of a common source connected MOSFET |
|
|
|
|
What is the formula for the instantaneous drain current |
|
|
|
|
What is the formula for the instantaneous drain current under small signal conditions |
|
|
|
|
What is thr id formula in the saturation region |
|
|
|
|
What is the formula for the voltage gain of a common source amplifier |
|
|
|
|
What is the formula for the voltage of a diode |
|
|
|
|
What is the formula for the conductivity in an N-type Semiconductor |
Here N is the doping density, e is the charge of an electron and mu is the electron mobility |
|
|
|
What is the equation for the cut off frequency for the low frequency response of an amplifier |
Ri + Rs or Rb |
|
|
|
What is the equation for the cut off frequency for the upper frequency response of an amplifier |
|
|
|
|
How do we find the value of Rol so that we ensure a maximum power transfer to the load |
The maximum power transfer is achieved when RL = Ro which is condition for impedance matching |
|
|
|
What is the superposition theorem and how does it relate to circuit theory |
The superposition theorem states that for a linear system the response in any branch of a linear circuit equals the sum of the responses caused by each independent source. This is helpful in circuit theory as it allows us to evaluate the current and voltage in any branch and at the output easily. |
|
|
|
What is the principle of linearity and how is it related to circuit theory |
The principle of linear relates closely to the superposition theorem. In a linear circuit both additivity and homogeneity apply.
This relates to circuit theory as it means that if two or more signals are summer at the input of a linear system, then the signals will emerge scaled in amplitude and/or delayed at the output. |
|
|
|
Resistance formula with conductivty |
|
|
|
|
IB Formula for a pn junction |
|
|
|
|
Diagram and equations to find the band bending |
|
|
|
|
Voltage gain at RL = Ro |
Av = 0.5 Av |
|
|
|
Equation to find RB / Internal Resistance ( Rs ) |
|
|
|
|
What is the transfer characteristic that shows the transistor as a switch |
|
|
|
|
Diagram of a the general equivalent circuit for a BJT in CE mode |
|
|
|
|
Diagram to show different combinations of transistor for CE, CB and CC modes |
|
|
|
|
Why do we change BJT circuits to make them better |
Without the change the circuits will have a small reverse leakage current. This current flows from the collector and is highly temperature dependent. This leads to a positive feedback loop which can cause thermal runaway in the transistor |
|
|
|
How are BJT circuits changed to make them better |
An emitter degeneration resistor is added, this is a resistor in series with the emitter terminal.
The Base is supplied with a potential divider this will provide a much lower impedance. |
|
|
|
Distinguish between small signal and large signal analysis as it applies to transistor circuits to be used in linear applications |
Most active circuits are non-linear devices. This means that the voltage / current relationship of terminal parameters are non-linear for large (~1V or more) excursions. To linearise devices we first choose an appropriate point on the DC characteristic away from the supply rails. We then restrict the input signal levels (to mV) so that the relevant arc of the DC curve approximates to a straight line. The process of liberalization is not exactly linear but with the correct design can be made to function for most practical applications. |
|
|
|
How do BJT circuits operate |
These circuits will perform small excursions about a DC operating point and establish a stable operating point Q. The input is decoupled by a DC blocking capacitor that is chosen to be sufficiently large that it's impedance is negligible across the whole frequency range of interest. The Emitter degeneration resistor Re is bypassed by Ce which again is sufficiently large that impedance is much less then Re for all relevant frequencies. |
|
|
|
What are the cross sectional diagrams of a transistor for the hybrid-pi circuit |
|
|
|
|
What is the proof for transconductance using the current equation |
|
|
|
|
What is the proof for the small signal voltage gain from instantaneous drain current |
|
|
|
|
What is the equation for current i(b) for a small signal transistor circuit |
|
|
|
|
What is the equation for current i(c) for a small signal transistor circuit |
|
|
|
|
What is the equation for current v(c) for a small signal transistor circuit |
|
|
|
|
How is the amplifier output found for a small signal transistor circuit when the capacitor C(E) has a value 0 |
It is given by -Rc / Re |
|
|
|
What is the drain current vs Vds graph for the output characteristics of an Enhancement Mode MOSFET |
|
|
