Module 1: Semiconductors Flashcards
Why can’t a semi-conductor device turn from ‘off’ to ‘on’ instantly?
When the voltage on a diode is reversed, for example, it takes time for the holes to return to their original sides and create the depletion region. During this time, there will still be some current flow.
What is a ‘FET’ transistor, and how is it different to a bipolar transistor?
‘FET” = Field Effect Transistor Works off the principle that a charge can attract nearby by objects (i.e. electromagnetic ‘field’). High input impedance (therefore very little current) - said to be ‘voltage controlled’ (unlike bipolar transistors which are current controlled)
What are some advantages and disadvantages of FETs vs BJTs?
advantages = easier to manufacture, high input impedance and little current input, more stable temperature wise disadvantages = poor frequency response
How does a FET work?
An applied voltage between the gate and source makes the depletion region smaller and thus allows electronics to flow through the channel.
What does ‘MOSFET’ stand for?
Metal Oxide Semiconductor Field Effect Transistor
What is an ‘active device’ (as opposed to a passive one?) Give examples of each.
An active device can control the flow of electrons (e.g. transistors). Passive devices cannot (e.g. resistors, diodes, capacitors, etc…)
What are the two types of MOSFETs and what is a major difference between the two?
Depletion and enhancement types. The enhancement type does not begin with a channel, whereas a the depletion type does.
Draw the construction of an n-channel, enhancement-type MOSFET. What are the major constructional features?
- ‘gate’ electrode not electrically connected to the p-type body - it is seperated by a thin insulating layer of metal oxide (hence MOSFET) - this is basically a capacitor
- does not begin with a channel between the source and drain
What are the majority and minority charge carriers for a p-type semiconductor?
Majority = holes; minority = electrons
Explain, with a diagram, how the channel is formed in an NMOS FET
- As VGS increases (applied voltage between gate and p-type body), the metal oxide insulator between the gate terminal and the p-type body creates a capacitor
- Negative electrons gather on the top of the plate, whilst holes are pulled away from the edge of the p-type body
- This creates a depletion region, thus created an n-channel joining both n-type regions
Draw the standard n-channel JFET symbol
Which was does current flow in an enhancement-type NMOS FET?
Drain - source
Which way does current flow in an NPN BJT and why? Explain with use of a diagram.
Current (not electron) flow is from collector to the emitter.
Electrons from VEB travel from the battery, through the first n-doped block, through the p-type block (the p-type section is very thin so most electrons go past), and into (and past) the final ‘n’ collector section. Current flow is the reverse of this.
PNP BJT current flow is from (collector/emitter) to (collector/emitter)
Emitter to collector
Draw the relationship between Irradiation (mW) and Output Current (uA) in a photo diode
