CSE251 Lecture 8: Introduction to Bipolar Junction Transistor (BJT)

The Invention 2 Important Features ( compared to Vacuum tubes ): - three terminal solid-state device- requires less power - smaller and lightweight- lower operating voltage - has rugged construction- more efficient - no heater requirement The First Transistor: On Dec 23, 1947, three scientists led by Dr. William Shockley at the Bell Telephone Laboratories demonstrated the amplifying action of the first transistor. ( Courtesy Bell Telephone Laboratories.) Co-inventors: Dr. William Shockley (seated); Dr. John Bardeen (left); Dr. Walter H. Brattain. Honored with Nobel Prize in Physics in 1956

The Structure 3  Bipolar: both electrons and holes are involved in current flow.  Junction: has two p-n junctions.  Transistor: Transfer + Resistor.  It can be either n-p-n type or p-n-p type.  Has three regions with three terminals labeled as i. Emitter (E) ii. Base (B) and iii. Collector (C) The Bipolar Junction Transistor (BJT)

4 The Structure: npn & pnp  Base is made much narrow.  Emitter is heavily doped ( p +, n + ).  Base is lightly doped ( p -, n - ).  Collector is lightly doped ( p, n ).

5 The Structure: npn & pnp Transistors can be constructed as two diodes that are connected together.

6 Circuit Symbol  The arrow indicates the direction of current flow.  The current flows from collector to emitter in an n-p-n transistor.  The arrow is drawn on the emitter.  The arrow always points towards the n-type. So the emitter is n-type and the transistor is n-p-n type. Layout and Circuit Symbol: n-p-n Transistor

7 Circuit Symbol  The arrow indicates the direction of current flow.  The current flows from emitter to collector in an p-n-p transistor.  The arrow points towards the n- type.  So the base is n-type and transistor is p-n-p type. Layout and Circuit Symbol: p-n-p Transistor

Modes of Operation 8  Based on the bias voltages applied at the two p-n junctions, transistors can operate in three modes: 1. Cut-off (both EB and CB junctions are reversed biased) 2. Saturation (both EB and CB junctions are forward biased) 3. Active mode (EBJ is forward biased and CBJ is reversed biased)  Cut-off and Saturation modes are used in switching operation.  Active mode is used in amplification purposes.

9 Modes of Operation

10 Terminal Currents Reference Positive Current Directions I C Collector current I B Base current I E Emitter current

11 Modes of Operation  Both the junctions are reversed biased.  No current can flow through either of the junctions.  So the circuit is open. Cut-off V BC V BE Ideal model of BJT in cut-off.

12 Modes of Operation  Both the junctions are forward biased.  So the equivalent circuit can be represented by short-circuit between the base, emitter and collector. Saturation: Ideal Model V BC V BE Ideal model of BJT in saturation.

13 Modes of Operation Saturation: Practical Model V CE(sat) is in the range of 0.1 to 0.2 V, as V BC and V BE are both approximately equal to the diode forward drop.

14 Active Mode Operation EBJ: Forward Biased CBJ: Reverse Biased ◦ Forward bias of EBJ injects electrons from emitter into base (Emitter current). ◦ Most electrons shoot through the base into the collector (Collector current). ◦ Some emitted electrons recombine with holes in p-type base (Base Current)

NP N E B C electron Hole

NP N E C B V BE V CB E-Field Electron diffusion Hole diffusion

NP N E-Field E C B V BE V CB Electron hole recombination

Collector current The equation above shows that the BJT is indeed a voltage- dependent current source; thus it can be used as an amplifier. Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion to the collector because of the higher potential applied to the collector

20 Active Mode Operation Biasing for Active Mode Carriers injected from forward bias junction (from the emitter labeled E) travel through the intermediate layer (BASE, labeled B) and swept into the COLLECTOR, labeled C by the reverse biased voltage. EBJ: Forward Biased CBJ: Reverse Biased

21 Active Mode: Terminal Currents Current Relationships and Amplification As  is close to unity,  is very large, typically around 100.  represents the current amplification factor from base to collector. The base current is amplified by a factor of  in the collector circuit in the Active mode.  is called the Forward Current Gain, often written as  F.

22 Amplification Action Voltage Amplification: Active Mode As the base-emitter junction is forward biased, the source at the input between EBJ sees a low resistance. However, as the CBJ is reverse biased, the output resistance is very high, typically in the range of hundreds of k Ω to MΩ. Therefore, it is unlikely that the value of collector current I C will be affected by a load resistance usually in the range of a few kΩ. As such, a large load resistance will result in a large output voltage. Therefore, the transistor is capable of both voltage and current amplification. Voltage amplification is achieved by transferring the current from low resistance to high resistance circuit and, thereby, the name TRANSISTOR. Basic voltage amplification action of the common-base configuration.

Circuit Configuration Two sets of characteristics are required to describe the behavior of a three terminal devices such as BJT transistor.

Common-Base Configuration - base is common to both input and output of the configuration. - base is usually the terminal closest to or at ground potential. common-base amplifiers requires two set of characteristics: -Input or driving point characteristics. -Output or collector characteristics The output characteristics has 3 basic regions: -Active region –defined by the biasing arrangements -Cutoff region – region where the collector current is 0A -Saturation region- region of the characteristics to the left of V CB = 0V

-0.4V V CB IC=IEIC=IE ICIC Active region Saturation region -I C independent of V CB (ideal situation) -I C exists when V CB =0 - I C =  I E

Common-Base characteristics

Common-Emitter Configuration - emitter is common or reference to both input and output terminals. - emitter is usually the terminal closest to or at ground potential. Two set of characteristics are necessary to describe the behavior for CE ;input (base terminal) and output (collector terminal) parameters.

Proper Biasing common-emitter configuration in active region

Characteristics of Common-Emitter npn transistor (a)- Collector characteristics = output characteristics. (b)- Base characteristics = input characteristics. 30

I B is microamperes compared to miliamperes of I C. I B will flow when V BE > 0.7V for silicon and 0.3V for germanium Before this value I B is very small and no I B. Base-emitter junction is forward bias Increasing V CE will reduce I B for different values.

Effect of V CE on I C V BE +V CB =V CE ; for V BE kept constant say at 0.7V. V CB =V CE -0.7V; if V CE  then V CB  therefore CBJ depletion region width , Base region width  More electrons in the P type Base find themselves near the CBJ, which are subsequently swept across the junction into the collector. I E , and hence I C  with I B held constant. More pronounced slope observed. I C depends on V CE (though very slightly!!)

I C =0 when V CE =0, Explain how? V BE +V CB =V CE ; If V CE =0, then V BE +V CB =0, V BE =-V CB, for the BEJ forward biased V BE =0.7V. V CB =-0.7V; V BC =0.7V, V B -V C =0.7V which means BEJ forward biased. The transistor is in the saturation mode therefore I C =0.