ELECTRONICS 1home.elka.pw.edu.pl/.../electronics1-zrue/2-transistors.pdf · 2021. 3. 10. · •No gate voltage –the channel conducts •At negative gate polarity, the reverse diode`s

ELECTRONICS 1SEMICONDUCTOR ELEMENTS

QUESTIONS

• What are possible values for current 𝑰𝑫 ?

• A) 𝐼𝐷 = 50𝑚𝐴 B) 𝐼𝐷 = 0𝑚𝐴 C) 𝐼𝐷 = −1𝐴 D) 𝐼𝐷 = 0,05𝑢𝐴

• Consider diode characteristics on the right:

• What is the maximum reverse voltage,

that can be applied to the diode?

𝑽𝑹𝑹𝑴 = 𝑽𝑩𝑹 = −𝟕𝟎𝑽

• If the current flowing through the diode

is 𝑰𝑭 = 𝟔𝟎𝒎𝑨, then what is the forward

voltage 𝑽𝑭 of this diode?

𝑽𝑭 = 𝟎. 𝟖𝑽

𝑰𝑫

𝑽𝑹𝟏

QUESTIONS

• Consider a simplified diode model, where forward voltage is 𝑽𝑭 = 𝟎. 𝟕𝑽 when conducting

(characteristics on the right). Source voltage and resistance are:

• 𝑽𝑺 = 𝟕𝑽

• 𝑹𝟏 = 𝟐. 𝟏𝒌𝛀

• What is the voltage drop across the resistor 𝑽𝑹𝟏 ?

• 𝑽𝑹𝟏 = 𝟎𝑽 (as no current flows)

• What is the power dissipated in resistor 𝑽𝑹𝟏 ?

• 𝑷𝑹𝟏 = 𝑽𝑹𝟏 ⋅ 𝑰, 𝑰 =𝑽𝑹𝟏

𝑹𝟏⇒ 𝑷𝑹𝟏 =

𝑽𝑹𝟏𝟐

𝑹𝟏

• 𝑽𝑺 = 𝑽𝑹𝟏 + 𝑽𝑭 ⇒ 𝑽𝑹𝟏 = 𝑽𝑺 - 𝑽𝑭

• 𝑷𝑹𝟏 =𝑽𝑺 − 𝑽𝑭

𝟐

𝑹𝟏=

𝟕𝑽−𝟎.𝟕𝑽 𝟐

𝟐𝟏𝟎𝟎𝛀= 0.0189W = 18.9mW

• 𝑰 =𝑽𝑹𝟏

𝑹𝟏=

𝑽𝑺 − 𝑽𝑭

𝑹𝟏=

𝟕𝑽 −𝟎.𝟕𝑽

𝟐𝟏𝟎𝟎𝛀= 𝟑𝒎𝑨

𝑽𝑹𝟏 𝑽𝑭

𝑰

TRANSISTORS

TRANSISTOR

• Transistor is a semiconductor element used as an amplifier of voltage or current,

or as a switch

• Transistor controls a high power signal with

a low power control signal

• It consists of three connecotrs

• Invented in 1947

TRANSISTOR DIVISION

• Bipolar - 2 kinds of charge carriers

• Bipolar transistor (BJT – bipolar junction transistor)

• Unipolar – 1 kind of charge carrier

• Unipolar transistor (FET – field effect transistor)

• Complex

• Bipolar transistor with insulated gate (IGBT – insulated gate bipolar transistor)

TRANSISTOR OPERATION

• Current flowing through the device is controlled by an input signal

• BJT is controlled with current – base current determines collector current

• Contrary to FETs, which are controlled by voltage

BIPOLAR TRANSISTOR (BJT)

BIPOLAR TRANSISTOR (BJT)

• Utilizes the properties ot npn or pnp junction

• The arrangement is similar

to two diodes facing each

other forward or backward,

but the properties

are different !

• Markings

• C – collector

• B – base

• E – emitter

Bipolar transistor symbol

Arrow determines direction of

the current

NPN BIPOLAR TRANSISTOR

• Collector is the largest area

• Emitter is the most doped area

• Base is thin and little doped

CURRENT FLOW

• Collector current is tens or

hundreths times larger

than base current

TRANSISTOR AMPLIFICATION

• Bipolar transistor realizes current amplification

• The relations between collector and base currents is (approximately) linear

• The gain is β =𝐼𝐶

𝐼𝐵

• 𝐼𝐶 = 𝛽𝐼𝐵

• 𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵 = (𝛽 + 1)𝐼𝐵

• Typical β values

• 50

• 100

• 200

INPUT CHARACTERISTICS

• Input is the base of a transistor

• Base-emiter is actually a pn junction

• With positive polarity the input is a diode of a given characteristics

OUTPUT CHARACTERISTICS

• In active region the collector current IC is almost independent from VCE and

is β times larger than base current IB

• In saturation region VCE is low (~0.3V)

• In cut-off state (IB=0) collector current ICE0 is very(very) low

ACTIVE STATE

• Base-emitter is polarized forward

• Collector current is dependent on base current

• Collector voltage depends on the whole curcuit

SATURATION STATE

• Collector current IC reaches its maximum value

• The collector current is limited by external conditions, further rise of base

current IB does not result in collector current IC rise

• Collector-emitter voltage VCE reaches its minimum value VCESAT (usually around

0.3V)

CUT-OFF STATE

• Transistor does not conduct

• Base voltage is zero or negative, base current is zero (IB=0)

• Collector current ICE0 is very(very) little (micro-amps)

GRAPHICAL ANALYSIS - INPUT

• Base voltage source VBB and its corresponding series resistor RB make

the black characteristics of an non-ideal voltage source

• Base to emitter junction has a characteristics of a diode conducting forward

• The operating point of the base (IB & VBE) is the intersection of both lines

GRAPHICAL ANALYSIS - OUTPUT

• VCC and RC both make an non-ideal voltage source

• The output characteristics of the transistors collector is given at a particular

base current level IB

• The operating point of the output (IB & VCE) is the intersection of both lines

ON-OFF OPERATION

• The transistor may operate only as a switch

• In saturation area the collector current is maximum and the collector-emitter

voltage is almost zero

• In cut-off state the collector current is almost zero and collector-emitter voltage

results from the circuit

0

IC

VCE

IB = 0 Cutoff

VCE(sat) VCC

IC(sat)

Saturation

RB

0 V

RC IC = 0

+VCC

RC

C

E

+VCC

IB = 0

–

+RB

RC IC(sat)

+VCC

RC

C

E

+VCC

IB

+VBB

IC(sat)

MODEL LIMITATIONS

• The output supply voltage VCE

influences the input characteristics

• Parasitic capacitances

at high frequencies!

TRANSISTOR NPN VS. PNP

• pnp vs. npn transistors

• Different polarities

• Opposite current directions

PHOTOTRANSISTOR

• In phototransistor the base current is generated by the light reaching the base

• Phototransistor is more sensitive than photodiode

(it amplifies the senses current)

Collector

Emitter

Light

n p

n

Base

Dark current

50 10 15 20 25 30VCE (V)

10

8

6

4

2

IC (mA)

50 mW/cm2

40 mW/cm2

30 mW/cm2

20 mW/cm2

10 mW/cm2

OPTICAL COUPLER

• LED diode (transmitter) and phototransistor (receiver) are placed in common

package

• This allows signal handling without any galvanic connection (primary and

secondary sides may reside at different potentials), potential interferences are

cut-off

• Such a configuration may be used as an encoder.

BJT – SUMMARY I

TRANSISTOR PACKAGES

TRANSISTOR POWER PACKAGES

TRANSISTOR BASIC PARAMETERS

• VCEO: The maximum voltage that can be handled across the collector (C)and emitter(E) when the base (B) is open. (It may be shown as VCE)

• IC: The maximum collector (C) current

• PC: Maximum dissipated power (results mostly from collector voltage & current - P = U * I)

• hFE: The current gain (IC/IB)

• fT: The maximum DC switching frequency (the transition frequency)

Item 2SC

1815

2SD

880

VCEO

(V) 50 60

IC(mA) 15 3 A

PC(mW) 400 30W

hFE

70…

700

60…

300

fT(MHz) 80 3

FIELD EFFECT TRANSISTOR (FET)

UNIPOLAR TRANSISTOR (FET)

• Unipolar transistor (FET – Field Effect Transistor) utilizes electric field to affect

the conductivity of a semiconductor

• Two types of FET transistors

• Junction transistor (JFET – Junction FET)

• Insulated gate transistor (MOSFET - Metal-Oxide-Semiconductor FET)

JFET

• Channel in between of two gate parts

• Different naming compared to BJT

• Drain (D)

• Source (S)

• Gate (G)

JFET OPERATION

• No gate voltage – the channel conducts

• At negative gate polarity, the reverse diode`s depletion region expands and

blocks the channel – the resistance rises!

• At some voltage the channel is completely blocked

VOLTAGE DEPENDENCE

• Drain to source voltage VDS rise has a similar effect

• The higher the voltage the narrower the channel

• The resistance rises with voltage, the current remains at some maximum level

JFET CHARACTERISTICS

• Left – drain current ID as a function of drain to source voltage VDS for different

gate to source voltage VGS levels

• Right – drain current as a function of gate voltage

Input-to-output

characteristics

Output

characteristics

JFET AS A SWITCH

• JFET controls the output current with voltage input signal

• Transistor conducts without any voltage applied to the input

• Given a required polarization voltage on the input, the transistor stops conducting

• After taking the voltage away – disconnecting - the charge remains in the gate

• A discharging resistor is required for the gate

JFET - SUMMARY

• Control by applying

voltage

• Normally conducting

• Applying voltage

disables conduction

• Active gate

discharge required

E-MOSFET STRUCTURE

MOSFET

• Two types:

• D-MOSFET (depleted channel) – with non-polarized gate, the transistor has some

non-infinite resistance, applying voltage may increase or decrease this resistance

(depending on which way)

• E-MOSFET (enhanced channel) – with non-polarized gate, the output resistance is

„infinite”, applying voltage decreases the resistance

• Symbol differs – solid line, or dashed line

CHANNEL TYPE

• Depending on the body

substrate, charge carriers

are holes or electrones

• The difference in symbol

is in the direction of

the arrow

E-MOSFET

• E-MOSFET works by enhancing the channel

• No input voltage – no channel

• Voltage greater than threshold voltage level results in channel creation

VGG–

+

RD

–

+VDD

n

n

++++

––––

ID

Induced

channeln

n

SiO2

Source

p substrateGate

Drain

E-MOSFET

E-MOSFET OPERATION

• The gate is insulated

• At gate voltage VGS greater than threshold voltage (VT lub VGS(th)), a conducting

channel occurs

E-MOSFET CHARACTERISTICS

• Until the gate voltage reaches VGS(th) the transistor does not conduct

• Above this voltage, channel resistance decreases

Input to outputOutput

charactersitics

D-MOSFET

• Consider N-channel mosfet

• Different gate polarities result in:

• Negative – depletes channel

• Positive – enhances channel

D-MOSFET

––––––

n

nVGG

+

–

RD

–

+VDDp

++++++

n

nVGG–

+

RD

–

+VDDp

––––––

++++++

operating in D-mode operating in E-mode

D-MOSFET CHARACTERISTICS

• With zero gate voltage, the resistance is non-zero & non-infinite

• Applying voltage one way or another causes resistance change (up or down)

Input to outputOutput

charactersitics

MOSFET BODY DIODE

• There is a pn junction between

between source and drain

• It has some non-zero capacitance

GATE CAPACITANCE

• A dielectric layer resides between gate electrode and the substrate

• This causes a parasitic capacitance, which is charged and discharged by the

driver circuit

ESD PROTECTION

• MOSFET gate has very thin layer of the dielectric, thus it is sensitive to any

electrostatic discharde

• The circuits are designed to protect MOSFET gates (TVS or Zener diodes)

• Special storage packages (anti-static)

• Protection when working with such components

MOSFET – SUMMARY I

• MOSFET is faster than BJT

• In saturation state the channel has resistance of RDSon

• MOSFETs are easy to paralel

• MOSFETs have body diodes

• FET symbols:

FET – SUMMARY II

THANK YOU