TRANSISTORS AND THYRISTORS CHAPTER 18 TRANSISTORS AND THYRISTORS

BIPOLAR JUNCTION TRANSISTORS (BJTs) BJT is constructed with three doped semiconductor regions separated by two pn junction There are three regions : Emitter Base Collector There are two type of BJT: npn pnp

Basic construction of BJT

Transistor Biasing The BE junction is forward-biased The BC junction is reverse-biased

Transistor Operation

Transistor Currents

Alpha and Bata The collector current is equal to times the emitter current The collector current is equal to the base current multiplied by has a value between 0.950 and 0.995 has a value between 20 and 200

Transistor Voltages

VOLTAGE-DIVIDER BIAS Use only a single dc source to provide forward-reverse bias to the transistor Resistor R1 and R2 form a voltage divider that provides the base bias voltage

Input Resistance at the Base

Base Voltage V

THE BIPOLAR JUNCTION TRANSISTOR AS AN AMPLIFIER

THE BIPOLAR JUNCTION TRANSISTOR AS AN AMPLIFIER When both junction are forward-biased, the transistor is in the saturation region of its operation When VCE exceeds 0.7 V, the base-collector junction becomes reverse-biased and the transistor goes into the active or linear region When IB=0 the transistor is in the cutoff region. Collector leakage current, B-E and B-C are reverse-biased I C E O

Load Line Operation

Quiescent or Q-Point

Quiescent or Q-Point

Signal (ac) Operation of an Amplifier The circuit which produces an output signal with the same waveform as input signal but with a greater amplitude is called amplification

Signal Operation on the Load Line

THE BJT AS A SWITCH Conditions in cutoff Conditions in saturation

BJT PARAMETERS AND RATINGS IF the temperature goes up, goes up, and vice versa.

BJT PARAMETERS AND RATINGS Maximum Transistor Ratings PD(max) = maximum power dissipation

THE JUNCTION FIELD-EFFECT TRANSISTOR (JFET) The JFET is a type of FET that operates with a reverse-biased junction to control current in the channel

Categories FET METAL-OXIDE semiconductor JUNCTION Depletion Depletion Enhancement P channel N channel P channel N channel P channel N channel

JFET Basic Operation The JFET is always operated with the gate-to-source pn junction reverse-biased

JFET Symbols

JFET CHARACTERISTICS IDSS (Drain to Source current with gate shorted) is maximum drain current occurring for VGS =0 V, and the value of VDS which ID becomes constant is the pinch-off voltage

Cutoff Voltage The value of VGS that makes ID approximately zero is the cutoff voltage VGS(off)

VGS controls ID. JFET must be operated between VGS =0 and VGS(off)

Comparison of Pinch-Off and Cutoff Vp is the value of VDS at which the drain current becomes constant and is always measured at VGS =0 V VGS(off) and Vp are always equal in magnitude but opposite in sign

THE METAL-OXIDE SEMICONDUCTOR FET(MOSFET) Depletion MOSFET (D-MOSFET) D-MOSFET can be operated either the depletion mode or the enhancement mode

Depletion Mode (D-MOSFET)

Enhancement Mode (D-MOSFET)

D-MOSFET Symbols

Enhancement MOSFET(E-MOSFET)

E-MOSFET Symbols

FET BIASING Self-Biasing a JFET

D-MOSFET Bias

Example

E-MOSFET Bias Drain-feedback bias Voltage-Divider bias (a) Drain-feedback bias (b) Voltage-Divider bias

The Other Types of MOSFET LDMOSFET VMOS Dual-gate n-channel TMOS

UNIJUNCTION TRANSISTORS (UJTs)

UJT operation In normal UJT operation, base 2 (B2) and the emitter are biased positive with respect to base 1 (B1), and total resistance between B1 and B2 is RBB The ratio RB1/RBB is designated η and is defined as the intrinsic standoff ratio It take an emitter voltage of VB + η *VBB to turn the UJT on, called the peak voltage When the emitter voltage decreases, it reaches a value called the valley voltage, and the UJT turns off

UJT operation

UJT Applications

THYRISTORS Thyristors are devices constructed of four layers of semiconductive material such as the silicon-controlled rectifier (SCR), the diac, and the triac When turned on (triggered), they become low-resistance current paths and remain, although the trigger is removed, until the current is reduced to a certain level or until they are turned off

Silicon-Controlled Rectifier (SCRs) The SCR has three terminals

SCR application Phase control circuit

Triacs The triac is also known as a bidirectional triode thyristor

Diacs The diac us a bidirectional device that does not have a gate It conducts current in either direction when a sufficient voltage, called the breakover potential

Transistor packages

FIGURE 4-26 Metal cases for general-purpose/small-signal transistors. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

FIGURE 4-27 Typical multiple-transistor packages. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

FIGURE 4-28 Typical power transistors. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

FIGURE 4-29 Examples of RF transistors. Thomas L. Floyd Electronic Devices, 6e and Electronic Devices: Electron Flow Version, 4e Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.