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Outline for todayOhmic Contact
Metal–Oxide–Semiconductor Contact (MOS) Structure , Effect of voltageEnergy levels forms in MOS in different bias
Solving second homework Chapter Three: Bipolar junction transistor Transistor(Concept, mission, types)
Bipolar junction transistorStructureContact methods for BJT’s CircuitsCurrents and voltage and symbols in circles BJTBand Energy Description of BJT at (equilibrium and non equilibrium
Conditions) to npn & pnpModes of Operation for BJT
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Time of Periodic Exams The first periodic exam in
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Metal–Oxide–Semiconductor Contact (MOS)
structure
In this contact, a thin layer of oxide is put on the surface of a semiconductor n-
type or p-type. Then, pole metallic (metal) is put above the surface of the oxide
layer . We should choose a good electrical insulation of oxide which has a large
energy gap and isolates the metal from the semiconductor which no passing
electrical current between them.
In thermal equilibrium condition
In the absence of application of the electric
field or the voltage, the Fermi and connection
and valence levels are horizontal and flat.
When applying an electric field, there is
a bending in energy levels
Metal–Oxide–Semiconductor Contact (MOS)
Effect of voltage bias
According to the applied voltage on this contact, it will consist three different
situations such as what is shown in figure .
1- depletion 2- inversion 3- accumulation
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Metal–Oxide–Semiconductor Contact (MOS)
Effect of voltage bias (metal and n-type contact)
1 - Depletion :
When applying negative bias voltage at the surface of metal, a small amount of
negative charges is made. Then, the oxide layer prevent electric current from
passage to semiconductor . However, the electrons in substrate of semiconductor
n-type will be affected by these negative charges and moved away from the area
located under oxide and created the depletion region in semiconductor similar to
those that created in pn Junction
=
Metal–Oxide–Semiconductor Contact (MOS)
Effect of voltage bias (metal and n-type contact)
2 - Inversion
When increasing a negative bias voltage on surface of metal, Instead of
expanding more of depletion region within the semiconductor, inversion status is
formed which holes gather next to the surface of the oxide. Those holes is the
minority carriers in the semiconductor n-type.
3 - Accumulation
When applying positive bias voltage at the surface of metal, negative majority
carriers attract and accumulate at the surface of the oxide in semiconductor n-
type.
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Metal–Oxide–Semiconductor Contact (MOS)
Energy levels forms in MOS in different bias (metal and p-type contact)
1- Depletion :
When applying positive bias voltage, Fermi level move down from its first
location in thermal equilibrium condition. Also,
straight bend at the energy level in oxide
and energy levels of the semiconductor p-type
move down near the interface of oxide.
In addition, electrons drop down in potential
well. We notice that the distribution of carriers
density of per unit area in semiconductor p-type
equal in the metal
=
Metal–Oxide–Semiconductor Contact (MOS)
Energy levels forms in MOS in different bias (metal and p-type contact) 2 - Inversion When increasing positive bias voltage more than threshold voltage VT ; the semiconductor inverse and electrons occupyinversion layer. Fermi level move more down from its first location in thermal equilibrium condition. Also, straight bend at the energylevel in oxide and energy levels of semiconductor p-type move more down near interface of oxide. In addition, electrons dropmore down in potential well. We notice that thedistribution of carriers density of per unit area in semiconductor p-type for maximum depletion region Wmax in addition to carrier of inversion layer Qn equal in the metal
=
Metal–Oxide–Semiconductor Contact (MOS)
Energy levels forms in MOS in different bias (metal and p-type contact)
3- Accumulation
When applying negative bias voltage, Fermi level move up from its first location
in thermal equilibrium condition. Also,
straight bend at the energy level in oxide
and energy levels of the semiconductor p-type
move up near the interface of oxide.
In addition, holes climb up in potential
well. We notice that the distribution of carriers
density of per unit area in semiconductor p-type
equal in the metal
=
Transistor Brief its history
The discovery of the transistor in year 1947 in Bell Lab’s in United States of
America. Since then, this discovery is one of the most important discoveries and
that the performance of a global revolution in technology.
the first transistor in 1947 Now
TransistorIn the third chapter we studied and we focused on two types of contacts :
PN Junction: ( semiconductor of n-type & p-type ) which enters in the
structure of bipolar junction transistor and Junction gate field-effect transistor
(JFET)
MOS contact: (Metal, Oxide, Semiconductor of n-type or p-type) which enters
in the structure of metal–oxide–semiconductor field-effect transistor (MOSFET)
Concept of transistor
It is a piece of three parts and as PN Junction this parts contain extrinsic
semiconductor N-type & p-type
Transistor mission
1. Works as amplifier in electrical signals
2. works as switch in integrated circuits
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Transistor’s types
The most important types of transistor two types are:
Bipolar junction transistor (BJT):
Will be studied in detail in this chapter (Chapter four)
Field-effect transistor (FET) :
Will be studied in detail in (Chapter five)
Diffusion transistor
Unijunction transistors
Single-electron transistors
Nanofluidic transistor,
Transistor’s types
There are special types classified within provirus types which Within bipolar junction transistorHeterojunction bipolar transistor Schottky transistor Avalanche transistor Darlington transistors. Insulated-gate bipolar Photo transistorMultiple-emitter transistor Multiple-base transistor
Within field effect transistorCarbon nanotube field-effect transistor (CNFET)Junction gate field-effect transistor (JFET) metal–semiconductor field-effect transistor (MESFET (metal–oxide–semiconductor field-effect transistor (MOSFET)metal–Insulator–semiconductor field-effect transistor (MISFET)Organic field-effect transistor Ballistic transistor Floating-gate transistor etc…
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Bipolar junction transistorBJT structure
The Bipolar junction transistor contains
of npn or pnp
Which is distributed in three parts
Emitter, Base & Collector
emitter and collector contain from the same semiconductor type either n-type or p-type; but often emitter has moreimpurities. Therefore n+p n or p+n p
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Contact methods for BJT’s Circuits
The electronic circuits often have a signal or voltage inside and another outside,
and here in a BJT part of the three parts involved in each of the entrance and exit
thus
common emitter configuration base common configuration common collector configuration
The figure for (npn) type, however for the other type (pnp) just reverse the arrow
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Currents and voltage and symbols in circles BJT
IB Base current IE Emitter current IC Collector current
Always gather those three by relationship IE = IB + IC
VBE voltage between base & emitter VBC voltage between base & collector
VCE voltage between emitter & collector
Distribution in the two types npn و pnp
W1 The length of the first depletion region between base and emitter
W2 The length of the second depletion region between base and collector
WB The length of the base region
=
Currents and voltage and symbols in circles BJT
IB Base current IE Emitter current IC Collector current
Always gather those three by relationship IE = IB + IC
VBE voltage between base & emitter VBC voltage between base & collector
VCE voltage between emitter & collector
Distribution in the two types npn و pnp
In electronic circuits for system method for contact transistor we need to know
Vin Input voltage Vout Output voltage
Rin Input resistance Rout Output resistance (often called load resistance RL )
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Band Energy Description of BJT at non equilibrium Conditions to npn & pnp
In the first figure
in equilibrium condition for npn,
the second figure for pnp
In two figures, we noticeFermi level stability alongacross emitter, base & collector.
It must be recalled that the contactpotential between emitter and basejunction higher than the contact potential between , base andcollector junction .This is because impurities in emitter higher than impurities in base and collector.
=
Ec
Ev
Ef
P+ P
n
E CB
P
n+ n
Band Energy Description of BJT at non equilibrium Conditions
to npn & pnpIn the first figure in equilibrium condition for npn,the second figure for pnp
In two figures, we noticeFermi level variable alongacross emitter, base & Collector duo to forwardand reverse bias.
We will see in detail what is happening in the diffusion of electrons and holes and also the recombination process in BJT in the next topics.
=
Ef
P+
P
n
Ec
EvEC
B
P
n+
n
forward bias reverse bias
Modes of Operation for BJTWe saw that in equilibrium condition there will be two case of the forward and reverse bias. thus there will be four modes of operation BJT (we will only mention now)
Active mode :
The forward bias in base & emitter junction. The reverse bias in base & collector
junction.( which often we will take about)
Saturation mode : The forward bias in two junctions. the transistor in this case be a maximum connection status and operates as if it is closed switch in a circle
Cut – off mode : The reverse bias in two junctions. the transistor in this case is not leaking any current and operates as if it is open switch in a circle
Inverted mode :The reverse bias in base & emitter junction. The forward bias in base & collector junction