ECE 342 Solid-State Devices & Circuits 6. Bipolar Transistors

1. ECE 342 Solid-State Devices & Circuits 6. Bipolar Transistors Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu

2. Bipolar Junction Transistor • Bipolar Junction Transistor (BJT) • First Introduced in 1948 (Bell labs) • Consists of 2 pn junctions • Has three terminals: emitter, base, collector

3. BJT – Modes of Operation

4. BJT in Forward Active Mode (NPN)

5. Longitudinal Current Flow Electrons are minority carriers in the base (p-type) Minority electrons will diffuse in the p-type base AE: cross section area of BEJ W: Effective width of base NA: doping concentration base Dn: electron diffusivity q: electron charge Collector current: iCis independent of vCB

6. Base Current • Base current: Two components • Hole injection into emitter  iB1 • Electron recombination in base iB2 Dp: hole diffusivity in emitter Lp: hole diffusion length in emitter ND: doping concentration of emitter Qn: minority carrier charge in base tb: minority carrier lifetime From area under triangle

7. BJT Operation: Longitudinal and Base Currents • Base current has two functions • Feed recombination that occur in the base • Support reverse injection • Base current is small because • Base is thin • Has large lifetime • Emitter is much more heavily doped than base • Longitudinal current • 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

8. BJT Operation: Current Gain • Total Base current: b is the common-emitter current gain In order to achieve a high gain b we need Dn: large Lp: large ND: large NA: small W: small Define a current gain b such that Using previous relation for iC Typically 50 < b < 200 In special transistors, b can be as high as 1000

9. Current Gain Temperature Dependence

10. BJT Operation: Emitter Current • Emitter current: Define a such that Using previous relation for iC a is the common-base current gain a 0.99

11. Structure of BJT’s Collector surrounds emitter region  electrons will be collected

12. Ebers-Moll Model NPN Transistor Describes BJT operation in all of its possible modes

13. Common-Emitter Large-Signal Model • Common  terminal is common to input and output • Common  terminal is used as reference or ground

14. BJT – Common-Emitter Characteristics

15. BJT – Voltage-Current Characteristics

16. Common Emitter Configuration

17. Common Emitter I-V Characteristics

18. Early Voltage • Early Voltage VA • Dependence of collector current on collector voltage • Increasing VCE increases the width of the depletion region

19. Output Resistance ro is output resistance seen from collector terminal Alternatively, neglecting the Early effect on the collector current, we define The output resistance then becomes

20. Problem A transistor has b = 100, vBE= 0.7V with IC= 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. CBJ reversed biased  FAR Voltage drop across RC= 15-5 =10V IC = 2mA  RC= 10V/2mA = 5kW Since vBE=0.7V at IC= 1 mA Since base is at 0V, emitter voltage is at –0.717 volts =VE For b= 100, a= 100/101=0.99  IE = IC/a= 2/0.99 = 2.02 mA Now, This order of accuracy is not necessary

21. Operation in the Saturation Mode IV Characteristics Minority Carrier Profile • Forward active region can be maintained for negative vCB down to about -0.4V • Beyond that point, the transistor enters the saturation mode and iC decreases with decreasing vCB

22. Operation in the Saturation Mode If vBC increases, iC will decrease, as described by The base current iB will decrease, as described by The current gain will decrease to a value lower than bFdescribed as: We will also have:

23. Operation in the Saturation Mode • Blue: Gradient that gives rise to diffusion current • Gray: Minority carriers driving transistor deeper into saturation

24. NPN in Saturation Mode

25. Biasing Bipolar Transistors

26. BJT Bias 1. Base Current Bias

27. BJT Bias 2. Emitter Bias Thevenin Equivalent Provides good stability with respect to changes in b with temperature

28. BJT Emitter Bias Thevenin Equivalent

29. Bipolar Biasing Approach • Methods • First method is to find R1 & R2from Ethand Rth and IBQ • Second method is to select R2 to be 10 times to 20 times RE to provide good stability & then select R1 to give proper IBQ Remark: To keep collector voltage at the middle of the forward active region, use:

30. Stability Considerations Objective: Minimize effect of variations in b. Circuit must be stable with respect to changes in b. • Need to examine quiescent point in variations for interchanged BJT’s

31. Stability Considerations (A) If Rth>> (b+1) RE • Changes in b lead to significant changes in VCQ (B) If (b+1)RE>> Rth a varies only 1% to 2% for large b variations  (B) is good choice.

32. Bias Example • The circuit shown below has RC= 8.2 kW, RE= 1 kW, R2=20 kW, VCC= 12 V, b = 100, VBE= 0.7V • Select R1 to place VCQ at midpoint of the (forward) active region. • Find maximum symmetrical peak-to-peak output voltage that can be obtained before saturation or cutoff occurs.

33. Bias Example - Solution Minimum: Maximum: Midpoint:

34. Bias Example (con’t)

35. BJT Transistor Polarities NPN PNP