4.1 Semiconductors - Diodes Flashcards
What are semiconductors?
element or compound that contain relatively few free moving electrons / have a small band gap between valence band and conduction band
Which elements are the most important semiconductors? (4)
- Silicon
- Germanium
- Selenium
- Copper oxide
What is doping?
the shifting of balance of electrons by adding (doping) other atoms to silicon.
what is an n-type semi conductor?
atoms with one more valence electron than the pure semiconductor are added to it (n-type = negative because the global charge is negative)
what is an p-type semiconductor?
atoms with one less valence electrons than the pure semiconductor are added to it (p-type = positive charge)
what is the temperature limit of an impure material?
+75c germanium
+150c silicon
What is the diffusion voltage of germanium and silicon?
germanium = 0.3V
silicon = 0.7V
what can a semiconductor do?
block or let current through
if the positive pole of a current is in the n-zone and negative pole in the p-zone, what happens?
a depletion zone forms at the junction (no free electrons or empty spaces) and the semiconductors blocks the charge / Reverse Biased
if the negative pole is in the n-zone and positive pole in the p-zone, what happens?
the depletion layer breaks down and the semi conductor will let the current through / Forward biased
if no voltage is applied to a semi conductor, what happens?
a depletion zone is formed that has to be overcome before current can flow in the forward-biased direction
0.7V silicon
0.3V germanium
what is the threshold voltage?
the voltage required to breakdown the depletion layer at junctions
0.7V silicon
0.3 Germanium
when a semiconductor diode doesn’t fully block, what happens?
a residual reverse-biased current still flows (Leakage current)
why does a reverse-biased current occur?
caused by residual impurities in the crystal which are broken down by heat waves
what does the amount of residual reverse-biased current depend on?
the type of semiconductor diode type
what is maximum forward current?
the maximum forward current allowed to flow through the device
what will happen if the maxiumum forward current is exceed?
cause more heat to be generated across the junction and the diode will fail due to thermal overload
what is peak inverse voltage/maximum reverse voltage?
the maximum allowable reverse operating voltage that can be applied across the diode without reverse breakdown or damage
what is peak inverse voltage/maximum reverse voltage usually less than?
avalanche breakdown level
what is a typical value for peak inverse voltage/maximum reverse voltage?
a few volts to thousands of volts
when might a peak inverse voltage/maximum reverse voltage parameter be used?
rectifying diodes in AC Rectifier circuits
what are the three types of single crystal semiconductor diodes?
- microdiodes
- rectifier diodes
- heavy current diodes
what are the characteristics of a microdiode?
- mainly used in communication technology
- max forward-biased current below 1 amp
- have different names according to construction (junction diodes, point-contact diodes, planar diodes)
what are the characteristics of a rectifier diode?
- primariliy used for electrical power supply devices
- used to construct rectifier circuits for small outputs
- max forward-biased current up to 10 amps
what are the characteristics of a heavy current diode?
- max forward-biased current of 10-100 amps
- used in rectifier circuits for large power outputs
what is a freewheeling diode?
provides protection against back-EMF that occured when switching inductive loads
which bias is a freewheeling diode when the switching device is switched on?
reverse biased, (blocking state)
which bias is a freewheeling diode when the switching device is switched to off?
forward bias (allow current flow)
without the protection of a freewheeling diode, what would occur?
damage to the switching device due to high di/dt currents causing high voltage spike or transient currents to flow
when diodes are connected in series, what happens to the forward current?
why must care be taken when selecting your diode values?
the forward currents remains the same.
forward current must not exceed the lowest diode current rating
small voltage drop across each diode, smaller voltage to load
when diodes are connected in parallel, what happens to the forward current?
current divided between the diodes, possible for a single diode to exceed max forward current in a rectifer
what resistance will a diode show when wired in reverse bias direction?
high resistance (open circuit)
what resistance will a diode show when wired in the forward bias direction?
very low resistance
how can you tell a diode is faulty when tested?
high resistance in both directions
if a diode fails in a short circuit condition, what will be shown when tested?
low resistance in both directions
