CHAPTER4 BIPOLAR JUNCTION TRANSISTOR 16 MARKS Visit for

CHAPTER-4 BIPOLAR JUNCTION TRANSISTOR 16 MARKS Visit for more Learning Resources

Chapter 4 – Bipolar Junction Transistors (BJTs) Introduction http: //engr. calvin. edu/PRibeiro_WEBPAGE/courses/engr 311/311_frames. html

Physical Structure and Modes of Operation A simplified structure of the npn transistor.

Physical Structure and Modes of Operation A simplified structure of the pnp transistor.

Operation of The npn Transistor Active Mode Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown. )

Operation of The npn Transistor Active Mode Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; v. BE 0 and v. CB 0.

Operation of The npn Transistor Active Mode Current Flow The Collector Current The Base Current The Emitter Current

What is FET? FET is uni-polar device i. e. operation depends on only one type of charge carriers (h or e). It is a Voltage controlled Device (gate voltage controls drain current) 8

ADVANTAGES OF FET 1. Very high input impedance ( 109 -1012 ) 2. Source and drain are interchangeable 3. Low Voltage Low Current Operation is possible (Low-power consumption) 4. Less Noisy 5. No minority carrier storage (Turn off is faster) 6. Very small in size, occupies very small space in ICs 10

Current Controlled vs Voltage Controlled Devices 11

Types of Field Effect Transistors (The Classification) » FET JFET n-Channel JFET p-Channel JFET MOSFET (IGFET) Enhancement MOSFET n-Channel EMOSFET p-Channel EMOSFET Depletion MOSFET n-Channel DMOSFET p-Channel DMOSFET 12

JFET Construction There are two types of JFET’s: n-channel and p-channel. The n-channel is more widely used. There are three terminals: Drain (D) and Source (S) are connected to n-channel 13 Gate (G) is connected to the p-type material

N-Channel JFET Operation The nonconductive depletion region becomes thicker with increased reverse bias. (Note: The two gate regions of each FET are connected to each other. ) 14

SYMBOLS Drain Gate Source n-channel JFET Source p-channel JFET 15

CHARATERISTICS At the pinch-off point: • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • ID is at saturation or maximum. It is referred to as IDSS. 16

ID IDSS As VGS becomes more negative: • the JFET will pinch-off at a lower voltage (Vp). • ID decreases (ID < IDSS) even though VDS is increased. • Eventually ID will reach 0 A. VGS at this point is called Vp or VGS(off). • Also note that at high levels of VDS the JFET reaches a breakdown situation. ID will increases uncontrollably if VDS > VDSmax. 17

Transfer Characteristics The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear): As a result, FET’s are often referred to a square law devices 18

Transfer (Transconductance) Curve From this graph it is easy to determine the value of ID for a given value of VGS It is also possible to determine IDSS and VP by looking at the knee where VGS is 0 19

Case Construction and Terminal Identification 20

p-Channel JFET operates in a similar manner as the n-channel JFET except the voltage polarities and current directions are reversed 21

P-Channel JFET Characteristics As VGS increases more positively • the depletion zone increases • ID decreases (ID < IDSS) • eventually ID = 0 A Also note that at high levels of VDS the JFET reaches a breakdown situation. ID increases uncontrollably if VDS > VDSmax. 22

BJT n JFET comparision Characteristi BJT c FET Carriers Electrons, Holes Only one type (Electrons or Holes) Control Terminal Current control device 3 Terminal (Emitter, Baise, Collector) Voltage control device 4 Terminal (Source, Gate, Drain, body (substrate)) Used Amplifiers, Regulators of Digital Electronis, IC currents Amplifiers Other Name ----- Unipolar transistor junctions Two 1 junctions , 23

BJT n JFET comparision Characteristic BJT FET Symbol impedance d. Vi/di Secondary break down operating frequency switching losses Yes large (in the range of 1010 -1015 Ω) No Low High Low 24

JFET Parameters The input resistance can be determined by the following formula: 25

MOSFET (Metal Oxide Semiconductor FET) 26

MOSFET There are two types of MOSFET’s: • Depletion mode MOSFET (D-MOSFET) • Operates in Depletion mode the same way as a JFET when VGS 0 • Operates in Enhancement mode like E-MOSFET when VGS > 0 • Enhancement Mode MOSFET (E-MOSFET) • Operates in Enhancement mode • IDSS = 0 until VGS > VT (threshold voltage) 27

Depletion Mode MOSFET Construction The Drain (D) and Source (S) leads connect to the to n-doped regions These N-doped regions are connected via an n-channel This n-channel is connected to the Gate (G) via a thin insulating layer of Si. O 2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS 28

D-MOSFET Symbols 29

Basic Operation A D-MOSFET may be biased to operate in two modes: the Depletion mode or the Enhancement mode 30

p-Channel Depletion Mode MOSFET The p-channel Depletion mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed 31

Enhancement Mode MOSFET’s 32

Enhancement Mode MOSFET Construction The Drain (D) and Source (S) connect to the to n-doped regions These n-doped regions are not connected via an n-channel without an external voltage The Gate (G) connects to the p-doped substrate via a thin insulating layer of Si. O 2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS 33

E-MOSFET Symbols 34

Basic Operation The Enhancement mode MOSFET only operates in the enhancement mode. VGS is always positive IDSS = 0 when VGS < VT As VGS increases above VT, ID increases If VGS is kept constant and VDS is increased, then ID saturates (IDSS) The saturation level, VDSsat is reached. 35

p-Channel Enhancement Mode MOSFETs The p-channel Enhancement mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed. 36

Summary Table JFET D-MOSFET For more detail contact us E-MOSFET 37