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B - Diodes And Semicon > 4.1 Semiconductions - Transistors Characteristics > Flashcards

4.1 Semiconductions - Transistors Characteristics Flashcards

1
Q

what are the two types of characteristics regarding a transistor’s limitations?

A

static
dynamic

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2
Q

what is residual current/reverse leakage current?

A

the current that still flows to the p-n junction when a reverse bias voltage is applied

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3
Q

how is residual current affected by temperature?

A

it can lead to a shift in the operation point if appropriate switching measures are not implemented

delta of 10c = residual current doubles

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4
Q

what are used instead of absolute values?

A

levels (measured in decibels)

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5
Q

what happens when the temperature rises in a semi conductor?

A

additional charge carrier pairs become free in a semiconductor

increasing its intrinsic conductivity

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6
Q

how many times does the residual current increase when its temperature is raised from 25 to 150 degrees c?

A

2000 times

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7
Q

why is it important to modulate the temperature of a transistor?

A

temperature changes causes a shift in operating point, causing both half waves to be distored

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8
Q

what is the max permissible temp of a germanium transistor?

A

75 to 90 degrees c

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9
Q

what is the max permissible temp of a silicon transistor?

A

150 to 200 degrees c

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10
Q

what is the position of the operating point determined by?

A

the magnitude of the base-emitter voltage

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11
Q

how is the operating point shift prevented when there is a temperature change?

A

when the base-emitter voltage is decreased as the temperature increases

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12
Q

how is base current adjusted to temperature?

A

means of a base swamping resistor

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13
Q

if a resistor is added in the emitter circuit, what is created?

A

negative current feedback

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14
Q

how does a negative current feedback circuit work?

A

the voltage divider and additional resistor adjust the emitter current in such a way that it is almost independent of the rest

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15
Q

what else is used in a negative current feedback? why?

A

a capacitor connected in parallel,

to prevent the emitter from creating negative feedback and only stabilises the AC input voltage

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16
Q

in a voltage feedback circuit, where is the voltage divider?

A

at the collector

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17
Q

why is a voltage feedback circuit a good method of stabilising the operating point?

A

because it provides good stability with minimal circuitry

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18
Q

what other method can be used to stabilise the operating point?

A

temperature-sensitive base voltage divider

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19
Q

how does a temperature-sensitive voltage divider circuit work?

A

NTC resistors are connected in a series-parallel network with a resistor.

as temperature increases, the resistance of the NTC resistor decreases

20
Q

what must be done to keep the transistor and NTC resistor at the same temperature to ensure correct function?

A

installing them both on the same heat sink

21
Q

how many types and how are transistors classified into types?

A

2

bipolar junction transistors (BJT)
field effect transistors (FET)

22
Q

how are BJT transistors split into different types?

A

NPN and PNP Junction

23
Q

how are FET transistors split into different types?

A

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

24
Q

how are BJT transistors controlled?

A

by current.

small current at base = large current from emitter to colelctor

25
Q

what 3 regions can BJT transistors operate in? for what reason?

A
  1. cut-off region (off state, zero current)
  2. active region (acts as an amplifier)
  3. saturation region (fully on state, acts as closed switch)
26
Q

what are the 3 terminals of a FET transistor?

A
  1. gate
  2. drain
  3. source
27
Q

what do FET transistors have control of?

A

the size and shape of a channel between the source and drain which is created by applied voltage

28
Q

what type of transistors are FET transistors?

A

uni-polar, performing single channel operation

29
Q

what are JFET transistors used for? (3)

A

switches,
amplifiers
resistors

30
Q

how are JFET transistors controlled?

A

by voltage

voltage applied between gate and source allows current flow between source and drain

31
Q

how many terminals does a MOSFET transistor have?

A

4

gate, drain, source and body/substrate

32
Q

what is the advantages of MOSFET over BJT and JFET?

A

high input impedance and low output impedance

33
Q

what is a small signal transistor?

A

a transistor than amplifies small signals using small voltage and currents
(few milli volts and few milli amps)

34
Q

where might small signal transistors be used? (6)

A

led diode drive
relay driver
audio mute function
timer circuits
infrared diode amplifier
bias supply circuits

35
Q

what is the hFE value of a small switching transistor?

A

10 to 200

36
Q

how are collector terminals connected in a power transistor?

A

connected to the base of a metal device, which creates a heat sink for excess power

37
Q

what value of power do power transistors range from?

A

10 to 300W

38
Q

how are high-frequency transistors used?

A

for small signals which operate at high frequency,
high speed switching application

39
Q

what is the maximum frequency value of a high-frequency transistor?

A

2000MHz

40
Q

where are high frequency transistors used?

A

HF, VHF, UHF, CATV, MATV oscillator and amplifier circuits

41
Q

how are phototransistors different to standard transistors?

A

They have a light-sensitive area instead of a base terminal

42
Q

how do phototransistors work?

A

-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

43
Q

how are photo-FETs different to photo-BJTs?

A

photo-FETs generate current at the gate by light, which controls current between drain and source

photo-FETs more sensitive to light

44
Q

how are unijunction transistors only used?

A

as electronically controlled switches

45
Q

how do unijunction transistors work?

A

-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

B - Diodes And Semicon (14 decks)

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