Term
| How do insulators and conductors react under an electric field? |
|
Definition
|
|
Term
What is a semi-conductor?
What is silicon's structure and how does it change with temperature?
What is a hole? |
|
Definition
(Most commonly Silicon) They are insulators at low temperatures, but when the temperature is increased they become conducting.
It has a tetrahedral structure, each atom sharing a covalent bond with four other atoms.
[image]
When heated, silicon's electrons gain enough energy to break free from their covalent bonds to each other and create current. This creates a hole. |
|
|
Term
| What happens to the electrons/holes in the silicon when an electric field is put across it? |
|
Definition
|
|
Term
| How can the conductivity of silicon be increased? |
|
Definition
By adding impurities into the silicon's structure, called 'dopants'.
N-type: When a dopant with five outer electrons is added, allowing for a freed electron.
P-type: A dopant with three electrons in it's outer shell is added, creating an extra positive hole.
[image] |
|
|
Term
|
Definition
This is a structure where the doping suddenly changes from p to n-type. This results in the transfer of holes and electrons between the two sides. Creating a region with no free electrons or holes, called the depletion region.
This produces an electric field which opposes diffusion, called the contact potential/ built-in potential.
[image]
When there is no voltage, there is no net flow of current across the junction. |
|
|
Term
| Reverse biasing and its uses on a p-n junction? |
|
Definition
The equilibrium no longer holds, electrons are removed from the n-type. Increasing the size of the depletion region and removing holes from the p-type. Inducing a current called the reverse saturation current (typically 10^-9 A).
[image]
If a reversed bias junction is exposed to light then the leakage current can increase creating a photodiode/phototransistor.
[image] |
|
|
Term
|
Definition
On the p-side, holes are pushed away from the terminal towards the junction decreasing the depletion region. The net current is then predominantly due to the diffusion current (can be about 10^-3 A).
[image]
This creates a diode: [image] |
|
|
Term
| Graph showing voltage against current for diode |
|
Definition
[image]
The forward bias relationship between I and V is often approximated to [image]. |
|
|
Term
| What is the load line of a circuit and how can it be used to find information about a circuit with a diode? |
|
Definition
|
|
Term
| N-channel enhancement mode MOSFET |
|
Definition
MOSFET's have a layer of insulating oxide which blocks current between the gate and the FET.
[image]
The voltage of the Gate can be used to push and pull electrons in/out of the depleted regions, thus controlling the current that can flow. |
|
|
Term
| How does the enhancement MOSFET work? |
|
Definition
If there is a voltage between the drain and source when there is no gate voltage, no drain current will flow as at least one of the p-n junctions will be reverse biased and there are no charge carriers in the space between them.
If a positive voltage is applied, the electric field induced attracts electrons into the depleted region (most of which from the n-type regions).
[image]
If [image] is high enough above the threshold value, [image], enough electrons will accumulate between the n-type junctions. Creating an inversion layer and thus an n-type conducting channel within the p-type.
[image]
This means that if a voltage ([image]) is applied between the drain and source, then a current, [image], will flow through the inversion layer. |
|
|
Term
| How does [image] change as [image] is increased? |
|
Definition
For small values of [image], [image] increases linearly and rapidly (ohmic region).
As [image] is increased, the difference in the voltage between the gate and the drain reduces, decreasing the number of electrons in the inversion layer near the drain resulting in a levelling off of the curve (saturation region).
[image] |
|
|
Term
| What is the difference between n-channel enhancement-mode and depletion-mode MOSFETs? |
|
Definition
Depletion-mode MOSFETs have a built-in n-type channel. Therefore when a negative gate voltage is applied electrons are pushed away resulting in a decrease in conductivity.
However, the built-in n-type channel means that this negative voltage must be applied to stop all current. Meaning that the channel is controlled by both positive and negative voltages (better at dealing with bipolar signals such as AC circuits).
[image] |
|
|
Term
| What is the difference between an n-channel and a p-channel MOSFET? |
|
Definition
Exactly the same BUT EVERYTHING IS NEGATIVE.
[image]
If the letter are then swapped around the values then become positive.
P-type junctions at the drain and source. |
|
|
Term
|
Definition
Known as the common source amplifier as the source (S) is grounded. The circuit has DC (or battery) power, [image] which is assumed to be constant with no impedance.
The FET can be in either enhancement or depletion mode (affecting the polarity of [image]). The resistor [image] is included as the source [image] has very low impedance, so it will set the input impedance of the circuit (ideally ~M[image]).
[image]
[image] could be visualised as a wire heading out of the D on this circuit. |
|
|
Term
| How to deal with a common source amplifier in a question? |
|
Definition
It is common practice to set [image] = [image]/2 for maximum voltage swing at the output.
The value of [image] then creates a load line and operating point (P). |
|
|
Term
| Gain of a common source amplifier |
|
Definition
The input for this circuit is set by the DC source [image] which sets the operating point.
[image]
The gain in this example is negative as the change in [image] is negative. This means that the amplifier inverts the signal (changes the signal's phase by 180). |
|
|
Term
| Self-biased depletion mode MOSFET amplifier |
|
Definition
[image]
All the voltages in the circuit are positive. But [image] is required to be negative since this is a depletion mode MOSFET. Therefore, if [image] is grounded and [image] is positive, [image] becomes negative.
[image] |
|
|
Term
| Self-biased enhancement mode MOSFET circuits |
|
Definition
|
|
