Principles & Applications Electronics Principles & Applications Sixth Edition Charles A. Schuler Chapter 5 Transistors (student version) ©2003 Glencoe/McGraw-Hill

INTRODUCTION Amplification Transistors Characteristic Curves Transistor Testing Other Transistor Types Transistors as Switches

Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to.

Concept Preview Amplifiers provide gain (the output is larger than the input). Transistors have gain. Transistors have a collector, a base, and an emitter. The C-B junction is reverse biased. The B-E junction is forward biased. Most of the emitter carriers reach the collector. The base current is relatively small but controls the larger currents.

Out In Amplifier Out Gain = In

NPN Transistor Structure The collector is lightly doped. C N The base is thin and is lightly doped. B P The emitter is heavily doped. E N

NPN Transistor Bias No current flows. C The C-B junction is reverse biased. P B N E

NPN Transistor Bias C B The B-E junction is forward biased. E Current flows.

NPN Transistor Bias IC Current flows everywhere. C B Note that IB is smaller than IE or IC. IC IB IE N C Most of the emitter carriers diffuse through the thin base region since they are attracted by the collector. P B When both junctions are biased.... N E

Gain is something small controlling something large Note: when the switch opens, all currents go to zero. IC N C Although IB is smaller it controls IE and IC. P B IB N E Gain is something small controlling something large (IB is small). IE

Transistor Structure and Bias Quiz The heaviest doping is found in the ___________ region. emitter The thinnest of all three regions is called the ____________. base The collector-base junction is ___________ biased. reverse The base-emitter junction is ____________ biased. forward The majority of the emitter carriers flow to the ___________. collector

Concept Review Amplifiers provide gain (the output is larger than the input). Transistors have gain. Transistors have a collector, a base, and an emitter. The C-B junction is reverse biased. The B-E junction is forward biased. Most of the emitter carriers reach the collector. The base current is relatively small but controls the larger currents. Repeat Segment

Concept Preview The base to collector gain is called b (beta). To find b, divide the collector current by the base current. The emitter current is the largest since it is the sum of the base and collector currents. PNP transistors have opposite polarity from NPN transistors. In an NPN transistor, the major flow is made up of electrons. In a PNP transistor, the major flow is made up of holes.

IC = 99 mA The current gain from base to collector C is called b. IB = 1 mA P B 99 mA IC N E b = = 99 IB 1 mA IE = 100 mA

IC = 99 mA Kirchhoff’s current law: C IB = 1 mA B IE = IB + IC P B IE = IB + IC = 1 mA + 99 mA N E = 100 mA IE = 100 mA

IC = 99 mA In a PNP transistor, holes flow from emitter to collector. C IB = 1 mA B Notice the PNP bias voltages. E IE = 100 mA

Transistor Currents Quiz  is the ratio of collector current to ______ current. base The sum of the base and collector currents is the __________ current. emitter In NPN transistors, the flow from emitter to collector is composed of _______. electrons In PNP transistors, the flow from emitter to collector is composed of _______. holes Both NPN and PNP transistors show __________ gain. current

Concept Review The base to collector gain is called b (beta). To find b, divide the collector current by the base current. The emitter current is the largest since it is the sum of the base and collector currents. PNP transistors have opposite polarity from NPN transistors. In an NPN transistor, the major flow is made up of electrons. In a PNP transistor, the major flow is made up of holes. Repeat Segment

Concept Preview The NPN schematic symbol shows the emitter arrow as Not Pointing iN. The collector curves are a graph of collector voltage versus collector current. Both dc beta (bdc) and ac beta (bac) can be determined from the collector curves. The collector circuit of a transistor can be modeled as a resistor, as a closed switch or as an open switch. The amount of base current determines which of the three models applies.

NPN Schematic Symbol Collector Base Emitter Memory aid: NPN means Not Pointing iN.

PNP Schematic Symbol Collector E B C Base Emitter

IC This circuit is used to collect IC versus VCE data for several values of IB. C IB VCE B E

When graphed, the data provide an NPN collector family of curves. 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 0 mA 2 4 6 8 10 12 14 16 18 VCE in Volts

IC b = IB 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 2 4 6 8 10 12 14 16 18 VCE in Volts 14 mA IC 6 mA This type of gain is called bdc or hFE. b = = 140 = 150 40 mA IB 100 mA

bac = 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 0 mA 2 2 4 6 8 10 12 14 16 18 VCE in Volts 2.5 mA DIC Another type of gain is called bac or hfe. bac = = 125 20 mA DIB

100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 0 mA 2 4 6 8 10 12 14 16 18 VCE in Volts The C-E model is a resistor. C E IB With these values of IB:

100 mA VCE @ 0 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 0 mA 2 4 6 8 10 12 14 16 18 VCE in Volts IB When IB >> 100 mA The model is a closed switch.

100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 0 mA IC = 0 2 4 6 8 10 12 14 16 18 VCE in Volts IB When IB = 0 The model is an open switch.

Transistor Operating Conditions Quiz When IB is large and VCE  0, the transistor acts as a ___________ switch. closed When IB = 0 and IC = 0, the transistor acts as an ___________ switch. open When IB > 0 and VCE > 0, the transistor acts as a ___________. resistor Two current gain measures are dc and __________. ac The symbol hfe is the same as _________. ac

Concept Review The NPN schematic symbol shows the emitter arrow as Not Pointing iN. The collector curves are a graph of collector voltage versus collector current. Both dc beta (bdc) and ac beta (bac) can be determined from the collector curves. The collector circuit of a transistor can be modeled as a resistor, as a closed switch or as an open switch. The amount of base current determines which of the three models applies. Repeat Segment

Concept Preview It is possible to test transistors out-of-circuit using an ohmmeter. The E-B and C-B junctions act as diodes during ohmmeter testing. The C-E test shows a high resistance because two junctions are involved; one of which is reverse biased by the ohmmeter. Gain can be verified by using a resistor in conjunction with the ohmmeter test.

The E-B junction is forward biased by the ohmmeter.  NPN V E mA B C The E-B junction is forward biased by the ohmmeter.

The C-E resistance is very high.  NPN V E mA B C The C-E resistance is very high.

The meter reading is < 100 kW due to gain.  NPN V E mA B C 100 kW The meter reading is < 100 kW due to gain.

Concept Review It is possible to test transistors out-of-circuit using an ohmmeter. The E-B and C-B junctions act as diodes during ohmmeter testing. The C-E test shows a high resistance because two junctions are involved; one of which is reverse biased by the ohmmeter. Gain can be verified by using a resistor in conjunction with the ohmmeter test. Repeat Segment

Concept Preview Bipolar junction transistors (BJTs) are controlled by base current. Junction field effect transistors (JFETs) are controlled by gate voltage. JFETs operate in the depletion mode (as normally on devices). Metal oxide semiconductor field effect transistors (MOSFETs) usually operate in the enhancement mode (as normally off devices). Insulated gate bipolar transistors (IGBTs) are modified MOSFETs and have very low on-resistance. Unijunction transistors (UJTs) are not used as amplifiers.

Current Amplifier Current Out Current In The BJT is a current

Voltage Amplifier Current Out Voltage In The JFET is a voltage controlled amplifier.

The channel has carriers so it conducts from source to drain. Gate Source Drain P N-channel P-type substrate Structure of an N-channel JFET Drain Source Gate The channel has carriers so it conducts from source to drain.

A negative gate voltage can push the carriers from Source Drain P N-channel P-type substrate A negative gate voltage can push the carriers from the channel and turn the JFET off. Drain Source Gate

0 V -1 V -2 V ID in mA VGS -3 V -4 V -5 V VDS in Volts This is known as a depletion-mode device. N-channel JFET drain family of characteristic curves

It’s possible to make enhancement type field effect transistors as well. Metal oxide insulator Drain n G S D VDD VGG Gate p n N-channel MOSFET Source Gate bias enhances the channel and turns the device on.

Enhancement mode MOSFET drain family of characteristic curves ID in mA VGS 2 V 1 V 0 V VDS in Volts Drain Enhancement mode MOSFET drain family of characteristic curves Gate Source

The IGBT (insulated gate bipolar transistor) Operation and structure similar to a MOSFET Voltage controlled (like the MOSFET) Has one more junction than a MOSFET Hole injection reduces the collector resistance RCE = 8.33 mW Faster turn off than BJTs but not as fast as MOSFETS

Three major device technologies Extra junction hole injection Courtesy of Powerex, Inc.

Typical IGBT Driver Circuit Typically +15 V for turn on Typically - 5 to -15 V for turn off IGBT Control signal

Powerex IGBT Module Structure Powerex high voltage IGBT package

The unijunction transistor fires when its emitter voltage reaches VP. Then, the emitter voltage drops due to its negative resistance characteristic. Emitter voltage Base 2 Emitter current Emitter The UJT is not useful as an amplifier. It is used in timing and control applications. Base 1

Other Transistor Types Quiz BJTs are _________-controlled amplifiers. current FETs are __________-controlled amplifiers. voltage JFETs operate in the __________ mode. depletion MOSFETs operate in the __________ mode. enhancement IGBTs are __________-controlled amplifiers. voltage UJTs are not useful as __________. amplifiers

Concept Review Bipolar junction transistors (BJTs) are controlled by base current. Junction field effect transistors (JFETs) are controlled by gate voltage. JFETs operate in the depletion mode (as normally on devices). Metal oxide semiconductor field effect transistors (MOSFETs) usually operate in the enhancement mode (as normally off devices). Insulated gate bipolar transistors (IGBTs) are modified MOSFETs and have very low on-resistance. Unijunction transistors (UJTs) are not used as amplifiers. Repeat Segment

Concept Preview BJTs can be used as switches. No base current = switch is off. High base current = switch is on. The dissipation is always zero in an ideal switch: off = no current flow and on = no voltage drop. MOSFETs can also be used as switches: no gate voltage = switch is off and high gate voltage = switch is on.

How do transistor switches work? Can be viewed as solid state relays: they are either ON or they are OFF. BJT switches are characterized by: high base current (switch is on) (or no base current … off) low resistance from collector to emitter (on) (or very high resistance … off) low collector dissipation (on or off) PC = VCE x IC PC = 0 x IC = 0 W (on) (or PC = VCE x 0 = 0 W … off)

NPN SWITCH LOAD LOAD LOAD RCE @  W RCE @ 0 W RCE @  W DRIVER The driver output is zero volts, IB = 0 and the load is off The driver output is positive, IB > 0 and the load is on The driver output is zero volts, IB = 0 and ILOAD = 0

PNP SWITCH LOAD LOAD LOAD The driver output goes low: the voltage drop across the resistor makes VBE negative The driver output is off (high Z): the resistor pulls the base voltage up so that VBE = 0 The driver output is off (high Z): the resistor pulls the base voltage up so that VBE = 0

PNP SWITCH WITH NPN DRIVER LOAD LOAD LOAD

NPN SWITCH WITH PNP DRIVER (NEGATIVE POWER SUPPLY) VBE  0 V VBE  -0.7 V VBE  +0.7 V LOAD LOAD LOAD

Enhancement mode power MOSFETs used as switches STEPPER MOTOR A B C D

Concept Review BJTs can be used as switches. No base current = switch is off. High base current = switch is on. The dissipation is always zero in an ideal switch: off = no current flow and on = no voltage drop. MOSFETs can also be used as switches: no gate voltage = switch is off and high gate voltage = switch is on. Repeat Segment

REVIEW Amplification Transistors Characteristic Curves Transistor Testing Other Transistor Types Transistors as Switches