4.1.1 Semiconductors Flashcards
A diode is known to have how many terminals?
Two
A diode conducts current in a ________ direction
Single
(Diode) Single direction is achieved how?
Applying low resistance in one direction and high resistance in the other direction
What are the characteristics of a diode?
(6)
One Directional conductor
Small size and weight
Low operating voltage
Lower power dissipation
Higher reliability
Long service life
what are diode vulnerable to?
ESD (electrostatic discharge) during handling and operation
What are diodes made from?
Semiconducting material
How can conductivity be varied in a semiconductor?
Changes in temperature, light intensity and purity content
What is a fully integrated circuit?
Multiple discrete devices manufactured onto single semiconductive substrate
How is semiconductor device fabricated to become a positive/negative junction diode?
By doping together two sides of a single crystal semiconductor with opposite types of impurities. This forms a junction at the interface between the positive and negative sides.
A semiconductor diode is said to act as?
Check valve for current either block it or let it through
What is the difference between conventional flow and electron flow?
Conventional is positive to negative
Electron flow is negative to positive
Two examples of semiconductors
Germanium and silicon
How is the depletion layer formed?
By bonding positive type and negative type conductive materials together. Electrons diffuse from N material into P material to fill holes there.
What happens to the depletion layer in a forward bias PN junction circuit?
Depletion layer narrows
Negative type material is connected to the positive terminal on a battery in a forward bias junction circuit
true or false?
False
negative type material is connected to negative terminal on the battery and positive type material is connected to the positive terminal on a battery (forward bias)
A reverse bias is also known as what?
Insulator
In reverse bias, what happens to the depletion layer and is there a current flow?
Depletion layer widen and there is no current flow
What is true about the Ford current flowing through each diode in a series and what must it not do?
Current flying through each diode is the same and it must not exceed any individual current rating
What are the voltage drop values for silicon and germanium?
Silicon 0.7 V
Germanium 0.3 V
In a parallel circuit, what is true about the potential difference and current for the diodes?
Potential difference is the same for all diode but current is different for each diode
What happens if one diode fails in a parallel circuit?
The current is then shared by the remaining diodes causing them to fail
What is a SCR and what is it? Also known as?
Silicon controlled rectifier a.k.a. Thyrister
What is a SCR?
A multi layered semiconductor device with four layers of positive and negative type material. They are fast electronic switches made from silicon with 3PN junctions.
What are the three different operating states of a thyrister?
Reverse blocking mode
Forward blocking mode
Forward conducting mode
What operating mode causes the SCR to conduct?
Forward conducting mode- voltage is applied that would forward bias an equivalent diode and SCR conducts
When will a thyrister stop conducting?
Anode to cathode voltage is removed or reverse bias
Current falls below manufacturers specified holding current
“ voltage applied so anode is positive with respect to cathode” when will conduction occur?
When thyrister breakdown voltage occurs, SCR will conduct
“ Electrons combined with a hole and emit energy in form of photons “ is the principle behind what diode?
LED (light emitting diode)
What are photoconductive diode used in?
Light detecting circuits such as proximity detectors
What is a VDR?
Varistor (voltage dependent resistor)
How is a VDR constructed?
Many small, highly conductive crystal particles covered with extremely thin barrier layer and scented together
What is the relationship between voltage and resistance in a VDR?
They are proportional to each other
Higher voltage - low resistance,
low voltage - higher resistance
Where would rectifiers be used and what is a recognisable feature of them?
Rectifiers are used in applications that require high current such as power supplies used to convert AC to DC
One feature is that they are encased in metal, which act as a heat sink
In a serviceable condition, what should a diode show on the multimeter when tested in the forward bias direction?
0.6 V to 0.7 V
What happens when a dog is tested in the reverse bias direction?
Shows the internal voltage and higher resistance as overload
What indicates a faulty diode in an open circuit?
High resistance in both directions and internal voltage in both directions
