Bipolar Junction Transistors (BJT)

Transistors

Different types and sizes

First Transistor

Modern Electronics

• Purpose • To amplify and switch electronic signals on or off (high or

low)

• Modern Electronics

Microprocessor

Cell Phones

Motor Controllers

Invention • Evolution of electronics

• In need of a device that was small, robust, reliable, energy efficient and cheap to manufacture

• 1947 • John Bardeen, Walter Brattain and William Shockley

invented transistor • Transistor Effect

• “when electrical contacts were applied to a crystal of germanium, the output power was larger than the input.”

General Applications

• P-N junction • Controls current flow via external voltage

• Two P-N junctions (bipolar junction transistor, BJT) • Controls current flow and amplifies the current flow

Bipolar Junction Transistor • Bipolar Junction Transistors (BJT)

consists of three “sandwiched” semiconductor layers

• The three layers are connected to collector (C), emitter (E), and base (B) pins

• Current supplied to the base controls the amount of current that flows through the collector and emitter – “current controlled device”

• Basic structure and schematic symbol

E C

B

E

B

C

n npE C

B

E C

B

E

B

C

p pnE C

B

NPN type PNP type

approximate equivalents

transistor symbols

Bipolar Junction Transistor

Bipolar Junction Transistor Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping.

npn pnp

n p n E

B

C p n p E

B

C

Cross Section Cross Section

B

C

E

Schematic Symbol

B

C

E

Schematic Symbol

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Modes of operation MODE Emitter Base

Junction (EBJ) Collector Base Junction (CBJ)

Applications

Forward Active

Forward Biased Reverse Biased Amplifier

Cut-off Reverse Biased Reverse Biased Switch

Saturation Forward Biased Forward Biased Switch

Reverse Active

Reverse Biased Forward Biased Mostly not operated in this mode

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FORWARD BIASING EMITTER BASE JUNCTION

REVERSE BIASING COLLECTOR BASE JUNCTION

Transistor Operation

• It is observed that most diffusing electrons will reach boundary of collector-base depletion region.

• Because collector is more positive than base, these electrons are swept into collector.

• collector current (iC) is approximately equal to diffusion current.

• iC = Idiffusion

• Magnitude of iC is independent of vCB. • As long as collector is positive, with respect to

base.

Collector Current

The Base Current • base current (iB) – composed of two components:

• ib1 – due to holes injected from base region into emitter. • ib2 – due to holes that have to be supplied by external

circuit to replace those recombined.

• common-emitter current gain (β.) – is influenced by two factors:

• width of base region (W) • relative doping of base emitter regions (NA/ND)

• High Value of β • thin base (small W in nano-meters) • lightly doped base / heavily doped emitter

Base Current

The Emitter Current

• All current which enters transistor must leave.

• iE = iC + iB

Emitter Current

• Previously, small reverse current was ignored. • This is carried by thermally-generated minority carriers.

• However, it does deserve to be addressed. • The collector-base junction current (ICBO) is normally in the nano-

ampere range. • Many times higher than its theoretically-predicted value.

The Collector-Base Reverse Current (ICB0)

Structure of Actual Transistors

• More realistic BJT cross-section. • Collector virtually surrounds entire emitter region.

• This makes it difficult for electrons injected into base to escape collection. • Device is not symmetrical.

• As such, emitter and collector cannot be interchanged. • Device is uni-directional.

CB, CE AND CC CONFIGURATIONS Common-base (CB) Mode

In this mode, the base terminal is common to both the input and the output circuits. This mode is also referred to as the ground–base configuration.

Notation and symbols used for the common-base configuration of a p–n–p

transistor

Common-base configuration of an n–p–n

transistor

Notation and symbols for common-emitter configuration (a) n–p–n transistor (b) p–n–p transistor

Common-emitter (CE) Mode When the emitter terminal is common to both the input and the output circuits, the mode of operation is called the common-emitter (CE) mode or the ground–emitter configuration of the transistor.

CB, CE AND CC CONFIGURATIONS

When the collector terminal of the transistor is common to both the input and the output terminals, the mode of operation is known as the common-collector (CC) mode or the ground–collector configuration.

Common-collector (CC) Mode

Common-collector configuration

CB, CE AND CC CONFIGURATIONS

Circuit Symbols and Conventions

Figure : Voltage polarities and current flow in transistors biased in the active mode.

Transistor Characteristics

23

Common Emitter Characteristics

Graphical Representation of Transistor Characteristics

Figure : (left) The iC-vBE characteristic for an npn transistor. (right) Effect of temperature on the iC-vBE characteristic. Voltage polarities and current flow in transistors biased in the active mode.

Dependence of iC on Collector Voltage – The Early Effect

• When operated in active region, practical BJT’s show some dependence of collector current on collector voltage.

• As such, iC-vCE characteristic is not “straight”.

VA - Early Voltage (50 – 100 V)

Early Effect

• Early effect or base width modulation: is the variation in the width of the base due to a variation in the applied base-to-collector voltage.

• For example a greater reverse bias across the collector- base junction increases the collector-base depletion width.

Consequences of Early Effect • Reverse saturation current increases, increasing the collector

current. • Less chance for recombination in the base. • Charge gradient is increased and hence the minority carriers

injected inside the emitter will increase. • For extremely large voltages, base width = 0 , causing voltage

breakdown in transistor resulting in punchthrough.

Output resistance (r0)

𝑟𝑟0 = 𝜕𝜕𝑖𝑖𝑐𝑐𝜕𝜕𝑣𝑣𝐶𝐶𝐶𝐶 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑣𝑣𝐵𝐵𝐵𝐵

−1

𝑟𝑟0 = 𝑉𝑉𝐴𝐴 + 𝑉𝑉𝐶𝐶𝐶𝐶

𝐼𝐼𝐶𝐶 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝐸𝐸𝐸𝐸𝑟𝑟𝑖𝑖𝐸𝐸 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝑖𝑖𝐸𝐸

= 𝑉𝑉𝐴𝐴

𝐼𝐼𝐶𝐶 𝑖𝑖𝐸𝐸𝑖𝑖𝑖𝑖𝐸𝐸𝑖𝑖𝐸𝐸𝑖𝑖𝑖𝑖𝑖𝑖 𝐸𝐸𝐸𝐸𝑟𝑟𝑖𝑖𝐸𝐸 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝑖𝑖𝐸𝐸

Transistor Operating Point

B BEB

B

CE CCC

C C

CE CC C C

V VIR

V VIR R

V V I R

−=

= − +

= −

DC Load Line

VCC

VCC/RC

In graphical analysis of nonlinear electronic circuits, a load line is a line drawn on the characteristic curve, a graph of the current vs the voltage in a nonlinear device like a diode or transistor. It is used to determine the correct DC operating point, often called the Q point.

Summary of equations for BJT

For PNP transistor, replace vBE with vEB