Bipolar Transistor(BJT - bipolar junction transistor)

.14 תרגול מס'

המחלקה להנדסת חשמל ואלקטרוניקה מכללה האקדמית אורט בראודה 31350מוליכים למחצה

Bipolar TransistorBipolar transistor was invented by Walter Brattain, John Bardeen and William Shockley (Bell Labs) in 1949.

It is a 3-terminal device, which is usually used as an amplifier or a switch.

3D sketch of pnp bipolar transistor Examples of typical bipolar transistors

What is Bipolar Transistor ?Bipolar transistor is a semiconductor device with two interacting PN-

junctions, connected together in series back-to-back.

two types of bipolar transistors:

arrow symbol ↔ Emitterarrow direction ↔ current direction Outstanding guide on npn transistor

the Base HAVE TO be short enough to allow crosstalk between the Emitter and the Collector

Bipolar Transistor modessaturation cutoff

active reverse-active

EBV

BCV

EBV

BCV

EBV

BCV

EBV

BCV

With both junctions forward-biased, a BJT is in the saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

In cutoff, biasing conditions are opposite to those of saturation, namely, both junctions are reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch.

In the active mode, the emitter–base junction is forward biased and the base–collector junction is reverse biased. Most of BJTs are designed to afford the greatest common-emitter current gain, βF, in the active mode. If this is the case, the collector–emitter current is approximately proportional to the base current, but many times larger, for small base current variations.

By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. The βF in inverted mode is several times (2–3 times for the ordinary Ge BJT) smaller than that of active mode. (Seldomly used mode.)

Band diagram of BJT (active mode biased pnp)

holes

electrons

Emitter Base Collector

holes

p n p

EF quasi

Basic idea of BJTEmitter - Base junction is asymmetric: n

Emitterp

Emitter II Thus, the Emitter’ hole current is controlled by Emitter - Base junction

The width of the neutral region of the Base is small (much less than the holes diffusion length):

pneutral

Base LW )(

Therefore most holes diffusing into the Base will reach the Collector

pneutral

Base LW )(

pEmitterI

Current flow (active mode - pnp)

Emitter – Base currents (EB junction is forward-biased)

(1) holes diffusing from the Emitter to the Base

(2) electrons diffusing from the Base to the Emitter

Base currents(3) recombination of holes injected into the Base(4) most holes reach Collector since

Base – Collector currents (BC junction is under reverse bias)(5) electron (minority carrier) current from Collector to Base(6) hole (minority carrier) current from Base to Collector

Basep WL

can be neglected for most practical purposes

magnetta – current of holesrose – current of electrons

kT

qV

BaseD

i

Base

Basepp

Emitter

BaseEmitter

eN

n

W

DqAI

2

nEmitter

pEmitter II

BJT principle of operation (active mode - pnp)

magnetta – current of holesrose – current of electrons

- pnpבטרנזיסטור ( ב, רוב מטעני צומת( emitterחורים דרך - PN emitter-baseהוזרקים ) ה ) לאזור הקדמי בממתח הנמצאbase - ( ב. רוב מטעני אלקטרונים עם רקומבינציה עושים מהחורים - baseחלק ) " ( ל(, סחיפה י ע בחזרה חוזרים אחר חלק

emitter - וה. היות - baseאבל , , מה שהוזרקו החורים רוב מעט יחסית ומסומם מאד דק " emitterהוא ( י, ע להגיע מצליחיםבצומת( המחסור לאזור עד . base – collectorזה )PNצומת. collector-baseדיפוזיה של( החזק השדה האחורי בממתח נמצא

- , ( ב מיעוט מטעני הם חורים כי להזכיר החורים את לוקח - baseהצומת ) ל אותם ומעביר פתוח הוא הצומת ובשבילםcollector - ה. זרם - collectorלכן ה לזרם שווה - emitterכמעט ב ) שנאבד הקטן החלק ( : baseמלבד רקומבינציה עקב

.

- ה זרם בין המקשר - emitterהמקדם ה - collectorלזרם ה ) ( זרם של המעבר מקדם נקרא המקדםemitter .בין משתנה כלל -0.9בדרך . 0.999ל " , - יש . יותר יעילה היא טרנזיסטור י ע הזרם העברת כך יותר גדול ש ככל

. - ו, במתחים תלוי ולא כמעט כי לציין

- , ה זרם בין המקשר המקדם כי נראה - baseבהמשך ה המקדם ) ( collectorלזרם דרך מתבטא המקדם , , - . : מתחים של רחב בטווח גם כי נובע בצמתים במתחים תלוי ולא כמעט ש העובדה מעצם הבאה בצורה

" , . של החלש בזרם השינוי י ע לכן קבוע - baseנשאר ה, , של החזק בזרם לשלוט , . collectorניתן

CollectorBaseCollectorEmitter IIII

EmitterCollector II

BaseEmitterV CollectorBaseV

BaseCollector II 1

BaseICollectorI

EBV

BCV

Thus the (small) base current controls the (strong) collector current

PDF Document

Basic amplifier circuitscommon-base configuration

Emitter

Collector

I

I

= current amplification in common-base circuit Since , typical values for are: 999.09.0 EmitterCollector II

By setting and we control and

BaseEmitterV EmitterI

BaseCollectorV CollectorI

inpu

t

output

Characteristic Common Base

Input impedance Low

Output impedance Very High

Voltage Gain High

Current Gain Low

Power Gain Low

PDF Document

Basic amplifier circuitscommon-emitter configuration

Base

Collector

I

I

= current amplification in common-emitter circuit

is very sensitive to :

11

111

Collector

Collector

Collector

EmitterCollectorEmitter

Collector

Base

Collector

I

I

I

III

I

I

I

By setting and ,

we control and

BaseEmitterV BaseI

CollectorEmitterV CollectorI

inpu

t

output

Characteristic Common Emitter

Input impedance Medium

Output impedance High

Voltage Gain Medium

Current Gain Medium

Power Gain Very high

100099.01

99.099.0

2095.01

95.095.0

BaseD

EmitterA

Base

Emitter

Base

Collector

N

N

I

I

I

I~~

The gain, , is determined by doping

By setting and ,

we control and

Basic amplifier circuitscommon-collector configuration

(grounded collector or Voltage Follower or Emitter Follower)

Base

Emitter

I

I

= current amplification in common-emitter circuit

Base

Collector

Base

Emitter

I

I

I

I

Characteristic Common Emitter

Input impedance High

Output impedance Low

Voltage Gain Low

Current Gain High

Power Gain Medium

inpu

t output

CollectorBaseV BaseI

CollectorEmitterV EmitterI

B. Streetman, S. Banerjee, “Solid state electronic devices” (6th edition), Prentice Hall, 2005.

B. Streetman, S. Banerjee, “Instructor’s solutions manual to the “Solid state electronic devices” ” (6th edition), Prentice Hall, 2005.

B. Van Zeghbroeck, “Principles of semiconductor devices”, Lectures – Colorado University, 2004.

D. Neamen, “Semiconductor Physics and Devices: Basic Principles” (3rd edition), McGraw Hill, 2003.

D. Neamen, “Semiconductor Physics and Devices: Basic Principles” (3rd edition) – Solutions manual, McGraw Hill, 2003. 

References