ECE 342 – Jose Schutt-Aine 1 ECE 342 Solid-State Devices & Circuits 6. Bipolar Transistors Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois 1

ECE 342 – Jose Schutt-Aine 2 Bipolar Junction Transistor (BJT) –First Introduced in 1948 (Bell labs) –Consists of 2 pn junctions –Has three terminals: emitter, base, collector Bipolar Junction Transistor 2

ECE 342 – Jose Schutt-Aine 3 BJT – Modes of Operation ModeEBJCBJ CutoffReverse Forw. ActiveForwardReverse Rev. ActiveReverseForward SaturationForward 3

ECE 342 – Jose Schutt-Aine 4 BJT in Forward Active Mode (NPN) 4

ECE 342 – Jose Schutt-Aine 5 Electrons are minority carriers in the base (p-type) i C is independent of v CB Collector current: Minority electrons will diffuse in the p-type base Longitudinal Current Flow A E : cross section area of BEJ W: Effective width of base N A : doping concentration base D n : electron diffusivity q : electron charge 5

ECE 342 – Jose Schutt-Aine 6 Base Current D p : hole diffusivity in emitter L p : hole diffusion length in emitter N D : doping concentration of emitter Base current: Two components –Hole injection into emitter  i B1 –Electron recombination in base  i B2 Q n : minority carrier charge in base  b : minority carrier lifetime From area under triangle 6

ECE 342 – Jose Schutt-Aine 7 Base current has two functions BJT Operation: Longitudinal and Base Currents Base current is small because Longitudinal current  Support reverse injection  Feed recombination that occur in the base  Has large lifetime  Base is thin  Emitter is much more heavily doped than base  Depends (exponentially) on emitter junction voltage  Is independent of collector junction voltage  Field due to collector-base voltage attracts carriers but has no effect on rate of attraction

ECE 342 – Jose Schutt-Aine 8 BJT Operation: Current Gain Total Base current: Define a current gain  such that Using previous relation for i C  is the common-emitter current gain In order to achieve a high gain  we need D n : large L p : large N D : large N A : small W: small Typically 50 <  < 200 In special transistors,  can be as high as

ECE 342 – Jose Schutt-Aine 9 Current Gain Temperature Dependence

ECE 342 – Jose Schutt-Aine 10 BJT Operation: Emitter Current Emitter current: Define  such that Using previous relation for i C  is the common-base current gain  

ECE 342 – Jose Schutt-Aine 11 Structure of BJT’s Collector surrounds emitter region  electrons will be collected 11

ECE 342 – Jose Schutt-Aine 12 Ebers-Moll Model NPN Transistor Describes BJT operation in all of its possible modes 12

ECE 342 – Jose Schutt-Aine 13 Common-Emitter Large-Signal Model  Common  terminal is common to input and output  Common  terminal is used as reference or ground

ECE 342 – Jose Schutt-Aine 14 BJT – Common-Emitter Characteristics

ECE 342 – Jose Schutt-Aine 15 BJT – Voltage-Current Characteristics

ECE 342 – Jose Schutt-Aine 16 Common Emitter Configuration 16

ECE 342 – Jose Schutt-Aine 17 Common Emitter I-V Characteristics 17

ECE 342 – Jose Schutt-Aine 18 Early Voltage Early Voltage V A –Dependence of collector current on collector voltage –Increasing V CE increases the width of the depletion region

ECE 342 – Jose Schutt-Aine 19 Output Resistance r o is output resistance seen from collector terminal Alternatively, neglecting the Early effect on the collector current, we define The output resistance then becomes 19

ECE 342 – Jose Schutt-Aine 20 A transistor has  = 100, v BE = 0.7V with I C = 1 mA. Design a circuit such that a current of 2 mA flows through the collector and a voltage of 5V appears at the collector. Problem CBJ reversed biased  FAR Voltage drop across R C = 15-5 =10V I C = 2mA  R C = 10V/2mA = 5k  Since v BE =0.7V at I C = 1 mA Since base is at 0V, emitter voltage is at –0.717 volts = V E For  = 100,  = 100/101=0.99  I E = I C /  = 2/0.99 = 2.02 mA Now, This order of accuracy is not necessary 20

ECE 342 – Jose Schutt-Aine 21  Forward active region can be maintained for negative v CB down to about -0.4V Operation in the Saturation Mode IV Characteristics Minority Carrier Profile  Beyond that point, the transistor enters the saturation mode and i C decreases with decreasing v CB

ECE 342 – Jose Schutt-Aine 22 Operation in the Saturation Mode If v BC increases, i C will decrease, as described by The base current i B will decrease, as described by The current gain will decrease to a value lower than  F described as: We will also have:

ECE 342 – Jose Schutt-Aine 23 Operation in the Saturation Mode  Blue: Gradient that gives rise to diffusion current  Gray: Minority carriers driving transistor deeper into saturation

ECE 342 – Jose Schutt-Aine 24 NPN in Saturation Mode

ECE 342 – Jose Schutt-Aine 25 Biasing Bipolar Transistors

ECE 342 – Jose Schutt-Aine 26 BJT Bias 1. Base Current Bias 26

ECE 342 – Jose Schutt-Aine Emitter Bias BJT Bias Provides good stability with respect to changes in  with temperature 27 Thevenin Equivalent

ECE 342 – Jose Schutt-Aine 28 BJT Emitter Bias 28 Thevenin Equivalent

ECE 342 – Jose Schutt-Aine 29 Methods –First method is to find R 1 & R 2 from E th and R th and I BQ –Second method is to select R 2 to be 10 times to 20 times R E to provide good stability & then select R 1 to give proper I BQ Bipolar Biasing Approach Remark: To keep collector voltage at the middle of the forward active region, use: 29

ECE 342 – Jose Schutt-Aine 30 Stability Considerations Objective: Minimize effect of variations in . Circuit must be stable with respect to changes in  –Need to examine quiescent point in variations for interchanged BJT’s 30

ECE 342 – Jose Schutt-Aine 31 Stability Considerations Changes in  lead to significant changes in V CQ (A) If R th >> (  +1) R E (B) If (  +1) R E >> R th  varies only 1% to 2% for large  variations  (B) is good choice. 31

ECE 342 – Jose Schutt-Aine 32 The circuit shown below has R C = 8.2 k , R E = 1 k , R 2 =20 k , V CC = 12 V,  = 100, V BE = 0.7V - Select R 1 to place V CQ at midpoint of the (forward) active region. - Find maximum symmetrical peak-to-peak output voltage that can be obtained before saturation or cutoff occurs. Bias Example 32

ECE 342 – Jose Schutt-Aine 33 Bias Example - Solution Minimum: Maximum: Midpoint: 33

ECE 342 – Jose Schutt-Aine 34 Bias Example (con’t) 34

ECE 342 – Jose Schutt-Aine 35 PNP NPN BJT Transistor Polarities