bipolar junction transistors (bjts) · simplified cross section of a planar npn bipolar junction...

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William Bradford Shockley Jr. William Bradford Shockley Jr. (February 13, 1910 – August (February 13, 1910 – August 12, 1989) was an American12, 1989) was an Americanphysicist and inventor. physicist and inventor.

Along with John Bardeen and Along with John Bardeen and Walter Houser Brattain, Walter Houser Brattain, Shockley co-invented theShockley co-invented theTransistor in 1948, for Transistor in 1948, for which all three which all three were awarded the 1956 Nobel were awarded the 1956 Nobel Prize in PhysicsPrize in Physics

BIPOLAR JUNCTION TRANSISTORS (BJTS)BIPOLAR JUNCTION TRANSISTORS (BJTS)

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Provide a reservoir of water for "C" (the "power Provide a reservoir of water for "C" (the "power supply voltage") but it can't move because there's a supply voltage") but it can't move because there's a big black plunger thing in the way which is big black plunger thing in the way which is blocking the outlet to "E". The reservoir of water is blocking the outlet to "E". The reservoir of water is called the "supply voltage". If we increase the called the "supply voltage". If we increase the amount of water sufficiently, it will burst our amount of water sufficiently, it will burst our transistor just the same as if we increase the voltage transistor just the same as if we increase the voltage to a real transistor. We don't want to do this, so we to a real transistor. We don't want to do this, so we keep that "supply voltage" at a safe level.keep that "supply voltage" at a safe level.

If we pour water current into "B" this current flows along the "Base" pipe and If we pour water current into "B" this current flows along the "Base" pipe and pushes that black plunger thing upwards, allowing quite a lot of water to flow pushes that black plunger thing upwards, allowing quite a lot of water to flow from "C" to "E". Some of the water from "B" also joins it and flows away. from "C" to "E". Some of the water from "B" also joins it and flows away. If we pour even more water into "B", the black plunger thing moves up further If we pour even more water into "B", the black plunger thing moves up further and a great torrent of water current flows from "C" to "E"and a great torrent of water current flows from "C" to "E"

How transistors work, fluid analogyHow transistors work, fluid analogy

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With an electrical current With an electrical current applied to the center layerapplied to the center layer(called the base), electrons (called the base), electrons will move from the N-type will move from the N-type side to the P-type side. The side to the P-type side. The initial small trickle acts as a initial small trickle acts as a switch that allows much switch that allows much larger current to flow.larger current to flow.


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Simplified cross section of a planar NPN bipolar junction transistorSimplified cross section of a planar NPN bipolar junction transistor

Transistors can be created by Transistors can be created by forming a sandwich out of forming a sandwich out of three regions of doped siliconthree regions of doped silicon

In 1948, Shockley invented a In 1948, Shockley invented a new device called a new device called a Bipolar Junction TransistorBipolar Junction Transistor

In the analogue world, a In the analogue world, a transistor can be used as a transistor can be used as a voltage amplifier, a current amplifier, or a switch; in the digital world, voltage amplifier, a current amplifier, or a switch; in the digital world, a transistor is primarily considered to be a switcha transistor is primarily considered to be a switch

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While the n-type has a surplus of electrons, the p-type has holes While the n-type has a surplus of electrons, the p-type has holes where electrons should be. Normally, the holes in the base actwhere electrons should be. Normally, the holes in the base actlike a barrier, preventing any significant current flow flowing from like a barrier, preventing any significant current flow flowing from the emitter to the collector and the transistor is in its "off" state.the emitter to the collector and the transistor is in its "off" state.

Suppose we attach a small positive voltage to the base, make Suppose we attach a small positive voltage to the base, make the emitter negatively charged, and make the collector positively the emitter negatively charged, and make the collector positively charged. Electrons are pulled from the emitter into the base—charged. Electrons are pulled from the emitter into the base—and then from the base into the collector. And the transistor and then from the base into the collector. And the transistor switches to its "on" stateswitches to its "on" state


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The small current that we turn on at the base makes a big The small current that we turn on at the base makes a big current flow between the emitter and the collector. By current flow between the emitter and the collector. By turning a small input current into a large output current, the turning a small input current into a large output current, the transistor acts like an amplifier. But it also acts like a switch transistor acts like an amplifier. But it also acts like a switch at the same timeat the same time


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Junction Field-Effect TransistorsJunction Field-Effect Transistors

In field-effect transistors (FETs), depletion mode and enhancement In field-effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the mode are two major transistor types, corresponding to whether the transistor is in an ON state or an OFF state at zero gate–source voltage.transistor is in an ON state or an OFF state at zero gate–source voltage.

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Formed by the junction of P-type and N-type siliconFormed by the junction of P-type and N-type silicon

The drain and source as form “data” terminals and the gateThe drain and source as form “data” terminals and the gateacting as the “control” terminal.acting as the “control” terminal.

When no signal is being applied to the gate terminal, the defaultWhen no signal is being applied to the gate terminal, the defaultdepletion region still leaves a channel of N-type silicon betweendepletion region still leaves a channel of N-type silicon betweenthe source and the drain, thereby allowing electrons to flow.the source and the drain, thereby allowing electrons to flow.


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Applying a small negative potential to the gate terminal, (reverseApplying a small negative potential to the gate terminal, (reversebias) like pinching a garden hosepipe to reduce the flow of water, bias) like pinching a garden hosepipe to reduce the flow of water, reduces the size of the conducting channel betweenreduces the size of the conducting channel betweenthe source and the drain terminals. the source and the drain terminals.

In turn, this increases the resistance of the channel and reduces In turn, this increases the resistance of the channel and reduces the flow of current between the source and the drain terminals.the flow of current between the source and the drain terminals.

If we keep on increasing the negative potential on the gate If we keep on increasing the negative potential on the gate terminal, at some stage the depletion zone will completely block terminal, at some stage the depletion zone will completely block the channel.the channel.


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JFETs have good linearity and low noise, and they are used almost JFETs have good linearity and low noise, and they are used almost entirely for processing analog signals. entirely for processing analog signals.

Typical applications include low-level audio amplification and Radio Typical applications include low-level audio amplification and Radio Frequency (RF) circuits such as RF mixers. Frequency (RF) circuits such as RF mixers.

JFETs also have a very high input impedance, which makes them JFETs also have a very high input impedance, which makes them suitable for applications like test equipment because they have minimal suitable for applications like test equipment because they have minimal disturbance on the signals being measured.disturbance on the signals being measured.


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Enhancement mode - to turn ON the channel, you need to bias the gateEnhancement mode - to turn ON the channel, you need to bias the gatehigher than the Vt, the threshold voltage.higher than the Vt, the threshold voltage.

Depletion mode - the device is normally ON, you need to provide a bias Depletion mode - the device is normally ON, you need to provide a bias to turn off, or "Pinch OFF" the channel.to turn off, or "Pinch OFF" the channel.

All modern digital ICs use enhancement mode NMOS and PMOS in All modern digital ICs use enhancement mode NMOS and PMOS in CMOS. CMOS.

If you try to use depletion mode, the gate voltage must go beyond the If you try to use depletion mode, the gate voltage must go beyond the power supply rails to shut the devices off. power supply rails to shut the devices off.

Depletion mode devices are useful in analog circuits. Depletion mode devices are useful in analog circuits.

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The JFET was predicted by Julius Lilienfeld in 1925 and by the The JFET was predicted by Julius Lilienfeld in 1925 and by the mid-1930s its theory of operation was sufficiently well known to mid-1930s its theory of operation was sufficiently well known to justify a patent. justify a patent.

However, it was not possible for many years to make doped crystals However, it was not possible for many years to make doped crystals with enough precision to show the effect. with enough precision to show the effect.

In 1947, researchers John Bardeen, Walter Houser Brattain, and William In 1947, researchers John Bardeen, Walter Houser Brattain, and William Shockley were trying to make a JFET when they discovered the Shockley were trying to make a JFET when they discovered the point-contact transistor. point-contact transistor.

The first practical JFETs were made many years later, in spite of their The first practical JFETs were made many years later, in spite of their conception long before the junction transistor.conception long before the junction transistor.

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Depletion-mode n-channel MOSFETDepletion-mode n-channel MOSFET

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In the case of these devices, the drain and source form In the case of these devices, the drain and source form the data terminals and the gate acts as the control terminal. the data terminals and the gate acts as the control terminal. Unlike bipolar devices, the control terminal is connected Unlike bipolar devices, the control terminal is connected to a conducting plate, which is insulated from the silicon by ato a conducting plate, which is insulated from the silicon by alayer of non conducting oxide or a layer of polycrystalline layer of non conducting oxide or a layer of polycrystalline silicon (poly silicon)silicon (poly silicon)

Depletion-mode n-channel Depletion-mode n-channel MOSFET is a unipolar transistor because MOSFET is a unipolar transistor because only one kind ("polarity") of electric charge is involved in making it only one kind ("polarity") of electric charge is involved in making it work.work.


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When no signal is being applied to the gate, the channel of When no signal is being applied to the gate, the channel of N-type silicon between the source and the drain allows current N-type silicon between the source and the drain allows current to flow. This means that these devices are ON by default and to flow. This means that these devices are ON by default and we have to apply a signal to the gate terminal in order to turn we have to apply a signal to the gate terminal in order to turn them OFF.them OFF.

Due to their low noise figure in the RF region, and better gain, these Due to their low noise figure in the RF region, and better gain, these devices are often preferred to bipolars in RF front-ends such as in devices are often preferred to bipolars in RF front-ends such as in TV sets.TV sets.


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The control terminal is connected to a conducting plate, which is The control terminal is connected to a conducting plate, which is insulated from the silicon by a layer of non conducting oxide. In insulated from the silicon by a layer of non conducting oxide. In the original devices the conducting plate was metal—hence, the original devices the conducting plate was metal—hence, the term “metal-oxide”—but this is now something of a misnomer the term “metal-oxide”—but this is now something of a misnomer because modern versions tend to use a layer of polycrystalline because modern versions tend to use a layer of polycrystalline silicon (poly silicon). silicon (poly silicon).

When a signal is applied to the gate terminal, the plate, insulated When a signal is applied to the gate terminal, the plate, insulated by the oxide, creates an electromagnetic field, which turnsby the oxide, creates an electromagnetic field, which turnsthe transistor ON or OFF—hence, the term “field-effect.”the transistor ON or OFF—hence, the term “field-effect.”


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ENHANCEMENT-MODE MOSFETSENHANCEMENT-MODE MOSFETS

nMOS is made with a p-type substrate (active high)nMOS is made with a p-type substrate (active high)

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The gate has a voltage applied to it that makes it positive with respect to the source. This The gate has a voltage applied to it that makes it positive with respect to the source. This causes holes in the P type layer close to the silicon dioxide layer beneath the gate to be repelled causes holes in the P type layer close to the silicon dioxide layer beneath the gate to be repelled down into the P type substrate, and at the same time this positive potential on the gate attracts down into the P type substrate, and at the same time this positive potential on the gate attracts free electrons from the surrounding substrate material. These free electrons form a thin layer of free electrons from the surrounding substrate material. These free electrons form a thin layer of charge carriers beneath the gate electrode (they can't reach the gate because of the insulating charge carriers beneath the gate electrode (they can't reach the gate because of the insulating silicon dioxide layer) bridging the gap between the heavily doped source and drain areas. This silicon dioxide layer) bridging the gap between the heavily doped source and drain areas. This layer is sometimes called an "inversion layer" because applying the gate voltage has caused the layer is sometimes called an "inversion layer" because applying the gate voltage has caused the P type material immediately under the gate to firstly become "intrinsic" (with hardly any P type material immediately under the gate to firstly become "intrinsic" (with hardly any charge carriers) and then an N type layer within the P type substrate.charge carriers) and then an N type layer within the P type substrate.

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Enhancement-mode MOSFETs are formed fromEnhancement-mode MOSFETs are formed fromtwo p-n-junctions, which act like two back-to-back diodestwo p-n-junctions, which act like two back-to-back diodes

By default, this device is OFF and we have to apply an electrical By default, this device is OFF and we have to apply an electrical potential to its gate terminal to turn it ONpotential to its gate terminal to turn it ON

Enhancement-mode MOSFETs are the most widely used Enhancement-mode MOSFETs are the most widely used member of the FET familymember of the FET family


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The term "enhancement mode" refers to the increase of The term "enhancement mode" refers to the increase of conductivity with increase in oxide field that adds carriers to theconductivity with increase in oxide field that adds carriers to thechannel, also referred to as the inversion layer. channel, also referred to as the inversion layer.

The channel can contain electrons (called an nMOSFET or The channel can contain electrons (called an nMOSFET or nMOS), or holes (called a pMOSFET or pMOS), opposite in type nMOS), or holes (called a pMOSFET or pMOS), opposite in type to the substrate, so nMOS is made with a p-type substrate, and to the substrate, so nMOS is made with a p-type substrate, and pMOS with an n-type substrate.pMOS with an n-type substrate.


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Using Transistors to Build Logic GatesUsing Transistors to Build Logic Gates

MOSFETs are the most widely used transistors in the world, finding MOSFETs are the most widely used transistors in the world, finding application in both analogue and digital circuits, especially in today’s application in both analogue and digital circuits, especially in today’s digital integrated circuits, the largest of which may literally digital integrated circuits, the largest of which may literally contain billions.contain billions.

CMOS is also sometimes referred to as complementary-symmetry CMOS is also sometimes referred to as complementary-symmetry metal–oxide–semiconductor. The words "complementary-symmetry" metal–oxide–semiconductor. The words "complementary-symmetry" refer to the fact that the typical digital design style with CMOS uses refer to the fact that the typical digital design style with CMOS uses complementary and symmetrical pairs of p-type and n-type metal complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic oxide semiconductor field effect transistors (MOSFETs) for logic functions.functions.

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Two important characteristics of CMOS devices are Two important characteristics of CMOS devices are high noise immunity and low static power consumption. high noise immunity and low static power consumption.

Significant power is only drawn when the transistors in the CMOS device Significant power is only drawn when the transistors in the CMOS device are switching between on and off states. are switching between on and off states.

Consequently, CMOS devices do not produce as much waste heat as Consequently, CMOS devices do not produce as much waste heat as other forms of logic, for example transistor-transistor logic (TTL) or other forms of logic, for example transistor-transistor logic (TTL) or NMOS logic. NMOS logic.

CMOS also allows a high density of logic functions on a chip. It was CMOS also allows a high density of logic functions on a chip. It was primarily for this reason that CMOS became the most used technology to primarily for this reason that CMOS became the most used technology to be implemented in VLSI chips.be implemented in VLSI chips.

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The transition level between high and low logic states is usually The transition level between high and low logic states is usually half the supply voltage, which means CMOS has superior noise half the supply voltage, which means CMOS has superior noise immunity to TTL. immunity to TTL.

Unlike TTL and its critical +5 volt power supply requirement, Unlike TTL and its critical +5 volt power supply requirement, CMOS can operate over a wide supply voltage CMOS can operate over a wide supply voltage range, typically +3 to +15 volts. range, typically +3 to +15 volts.

To help differentiate between TTL and CMOS supply voltages, To help differentiate between TTL and CMOS supply voltages, the supply voltage for CMOS is referred to as Vdd and the ground the supply voltage for CMOS is referred to as Vdd and the ground connection is sometimes referred to as Vss.connection is sometimes referred to as Vss.

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CMOS has a major drawback, however, and that is its CMOS has a major drawback, however, and that is its susceptibility to damage due to static electricity. CMOS devices susceptibility to damage due to static electricity. CMOS devices are constructed by placing several thin layers atop a silicon are constructed by placing several thin layers atop a silicon substrate, and these layers can be punctured by a static substrate, and these layers can be punctured by a static electricity discharge. electricity discharge.

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CMOS implementation of a NOT gateCMOS implementation of a NOT gate

The small circle, or bobble, on the control input of transistor Tr1, indicatesThe small circle, or bobble, on the control input of transistor Tr1, indicatesa PMOS transistor. The bobble is used to indicate that this transistor has ana PMOS transistor. The bobble is used to indicate that this transistor has anactive-low control, which means that a logic 0 applied to the control inputactive-low control, which means that a logic 0 applied to the control inputturns the transistor ON and a logic 1 turns it OFFturns the transistor ON and a logic 1 turns it OFF

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Simplified process of fabrication of a CMOS inverter on p-type substrateSimplified process of fabrication of a CMOS inverter on p-type substrate

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CMOS implementation of a BUF gateCMOS implementation of a BUF gate

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Nothing will happen. Neither transistor willNothing will happen. Neither transistor will be able to turn on.be able to turn on.

The N-channel enhancement-mode transistorThe N-channel enhancement-mode transistor(active high) requires its gate to be at a higher (active high) requires its gate to be at a higher voltage than the source (or drain), which can't voltage than the source (or drain), which can't happen if it's connected to Vdd.happen if it's connected to Vdd.

Similarly, the P-channel transistor requires a Similarly, the P-channel transistor requires a negative voltage on its gate, which can't happen negative voltage on its gate, which can't happen if it's connected to ground.if it's connected to ground.

Note, the active low transistors are always in the pull up sectionNote, the active low transistors are always in the pull up section

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NAND gate’s operation represented using switchesNAND gate’s operation represented using switches

Because the Because the NAND function NAND function has functional has functional completeness all completeness all logic systems can logic systems can be converted into be converted into NAND gates. NAND gates. This is also true This is also true of NOR gates.of NOR gates.

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CMOS implementation of a 2-input NAND gateCMOS implementation of a 2-input NAND gate

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CMOS implementation of a 2-input AND gateCMOS implementation of a 2-input AND gate

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CMOS implementation of a 2-input NOR gateCMOS implementation of a 2-input NOR gate

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Implementing a 2-input XOR from NOT, AND, and OR gatesImplementing a 2-input XOR from NOT, AND, and OR gates

22 transistors - two each for the two NOT gates, six each for the 22 transistors - two each for the two NOT gates, six each for the two AND gates, and six more for the OR gatetwo AND gates, and six more for the OR gate

bipolar junction transistors (bjts) · simplified cross section of a planar npn bipolar junction...

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