4.1 Semiconductions - Transistors Characteristics Flashcards
what are the two types of characteristics regarding a transistor’s limitations?
static
dynamic
what is residual current/reverse leakage current?
the current that still flows to the p-n junction when a reverse bias voltage is applied
how is residual current affected by temperature?
it can lead to a shift in the operation point if appropriate switching measures are not implemented
delta of 10c = residual current doubles
what are used instead of absolute values?
levels (measured in decibels)
what happens when the temperature rises in a semi conductor?
additional charge carrier pairs become free in a semiconductor
increasing its intrinsic conductivity
how many times does the residual current increase when its temperature is raised from 25 to 150 degrees c?
2000 times
why is it important to modulate the temperature of a transistor?
temperature changes causes a shift in operating point, causing both half waves to be distored
what is the max permissible temp of a germanium transistor?
75 to 90 degrees c
what is the max permissible temp of a silicon transistor?
150 to 200 degrees c
what is the position of the operating point determined by?
the magnitude of the base-emitter voltage
how is the operating point shift prevented when there is a temperature change?
when the base-emitter voltage is decreased as the temperature increases
how is base current adjusted to temperature?
means of a base swamping resistor
if a resistor is added in the emitter circuit, what is created?
negative current feedback
how does a negative current feedback circuit work?
the voltage divider and additional resistor adjust the emitter current in such a way that it is almost independent of the rest
what else is used in a negative current feedback? why?
a capacitor connected in parallel,
to prevent the emitter from creating negative feedback and only stabilises the AC input voltage
in a voltage feedback circuit, where is the voltage divider?
at the collector
why is a voltage feedback circuit a good method of stabilising the operating point?
because it provides good stability with minimal circuitry
what other method can be used to stabilise the operating point?
temperature-sensitive base voltage divider
how does a temperature-sensitive voltage divider circuit work?
NTC resistors are connected in a series-parallel network with a resistor.
as temperature increases, the resistance of the NTC resistor decreases
what must be done to keep the transistor and NTC resistor at the same temperature to ensure correct function?
installing them both on the same heat sink
how many types and how are transistors classified into types?
2
bipolar junction transistors (BJT)
field effect transistors (FET)
how are BJT transistors split into different types?
NPN and PNP Junction
how are FET transistors split into different types?
FET to Junction FET and Metal Oxide Semiconductor FET
JFET to depletion mode, then to N and P channel
MOSFET to depletion mode and enhancement mode, then both into n and P channel
how are BJT transistors controlled?
by current.
small current at base = large current from emitter to colelctor
what 3 regions can BJT transistors operate in? for what reason?
- cut-off region (off state, zero current)
- active region (acts as an amplifier)
- saturation region (fully on state, acts as closed switch)
what are the 3 terminals of a FET transistor?
- gate
- drain
- source
what do FET transistors have control of?
the size and shape of a channel between the source and drain which is created by applied voltage
what type of transistors are FET transistors?
uni-polar, performing single channel operation
what are JFET transistors used for? (3)
switches,
amplifiers
resistors
how are JFET transistors controlled?
by voltage
voltage applied between gate and source allows current flow between source and drain
how many terminals does a MOSFET transistor have?
4
gate, drain, source and body/substrate
what is the advantages of MOSFET over BJT and JFET?
high input impedance and low output impedance
what is a small signal transistor?
a transistor than amplifies small signals using small voltage and currents
(few milli volts and few milli amps)
where might small signal transistors be used? (6)
led diode drive
relay driver
audio mute function
timer circuits
infrared diode amplifier
bias supply circuits
what is the hFE value of a small switching transistor?
10 to 200
how are collector terminals connected in a power transistor?
connected to the base of a metal device, which creates a heat sink for excess power
what value of power do power transistors range from?
10 to 300W
how are high-frequency transistors used?
for small signals which operate at high frequency,
high speed switching application
what is the maximum frequency value of a high-frequency transistor?
2000MHz
where are high frequency transistors used?
HF, VHF, UHF, CATV, MATV oscillator and amplifier circuits
how are phototransistors different to standard transistors?
They have a light-sensitive area instead of a base terminal
how do phototransistors work?
-when the sensitive area is dark, no current flows, off state.
-a small amount of current flows at the base when the sensitive area is covered in light,
-a large amount of current flows from the collector to emitter
how are photo-FETs different to photo-BJTs?
photo-FETs generate current at the gate by light, which controls current between drain and source
photo-FETs more sensitive to light
how are unijunction transistors only used?
as electronically controlled switches
how do unijunction transistors work?
-when no potential difference between emitter and base, a small current flows between both base terminals
when sufficient voltage is applied to the emitter, the currents added to the base current leading to a large current flow in the transistor
