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Transistor (BJT)

Transistor (BJT). Introduction. BJT (Bipolar Junction Transistor) Vaccum tubes It comes because it is most advantageous in amplification Why it is called transistor? Transistor = Transfer + Resistor Why it is called BJT? Types of BJT. Introduction(cont.). npn. pnp. n. p. n. p. n.

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Transistor (BJT)

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  1. Transistor (BJT)

  2. Introduction • BJT (Bipolar Junction Transistor) • Vaccum tubes • It comes because it is most advantageous in amplification • Why it is called transistor? Transistor = Transfer + Resistor • Why it is called BJT? • Types of BJT

  3. Introduction(cont.) npn pnp n p n p n p C E C C C Cross Section B B B B Schematic Symbol Schematic Symbol E E • Collector doping is usually ~ 109 • Base doping is slightly higher ~ 1010 – 1011 • Emitter doping is much higher ~ 1017

  4. Junction Transistor • Sandwich structure. • Base is always in between E & C. • B is lightly doped. • E & C are heavily doped. E is more heavily doped than C and area of C is more than E. • Two PN junctions. E C E C BE CB BE CB B B P-N-P N-P-N

  5. Unbiased transistor • No external supply is applied. • Penetration of depletion region (less in E & C, more in B)

  6. Transistor Biasing • Mode BE junction BC junction • cutoff reverse biased reverse biased • linear(active) forward biased reverse biased • saturation forward biased forward biased • Inverse active reverse biased forward biased

  7. Transistor Biasing (cont.) • Transistor biasing in active region. • EB junction is forward biased and CB junction is reversed biased.

  8. Transistor operation in active region (NPN) B N N P E C v

  9. Large current

  10. Transistor operation (cont.) • Electrons will flow from E to B. • Now electrons have three options • Recombine with holes (IB) • Diffuse through base and out of the base connection. • Remaining e- will go in C (Ic).

  11. Transistor operation (cont.) B IE=IB+IC N N P E C Electrons emitted Electrons collected Emitter current Recombination current Collector current

  12. Transistor current • Emitter current, Base current, Collector current. • IE = IB + IC. (IE ≈ IC) • IE = IPE + INE (for NPN INE for PNP IPE). • IB = IPE - IPC. • Reverse saturation current (ICBO) : It is the reverse sat. current when EB junction is open. • IC = IPC + ICBO.

  13. Parameters relating to current components • Emitter efficiency (ɣ) = • Transport factor (ß) = • Large signal current gain (α) = • α = = • α = ß * ɣ

  14. With E.g. for common-base configuration transistor: Transistor as an amplifier Discussion of an amplification effect

  15. Transistor construction technologies • Grown type. • Alloy type. • Electrochemically etched type. • Diffusion type • Epitaxial type.

  16. Transistor configuration • Made one of three terminal common to i/p and o/p. • Depending on which terminal is made common. There are three possibilities 1. Common base configuration (CB). 2. Common emitter configuration (CE). 3. Common collector configuration (CC).

  17. Common Base configuration (CB) Rc Re Rc Re IC IC IE IE Vcc Vee Vcc Vee IB IB Common base configuration for NPN transistor Common base configuration for PNP transistor

  18. Common Base configuration (CB) • Input • Output • Current relations in CB configuration 1. Ic = IC(INJ) + ICBO 2. IC(INJ) (practically) 3. ICBO (with emitter open) ICB= collector to base current IO = emitter is open

  19. Common Base configuration (CB) 4. current amplification factor/current gain (αdc) αdc =IC(inj)/IE So Ic=(αdc * IE )+ ICBO Expression for IB: IB = (1- α)IE.

  20. Transistor char in CB configuration • Input char. 2. Output char. 3. Transfer char. • I/P char : graph of I/P current versus I/P voltage. • O/P char : graph of O/P current versus O/P voltage. • Transfer char: graph of O/P current versus I/P current

  21. CB I/P char. • I/P current is emitter current(IE) and I/P voltage is emitter to base voltage(VBE). 1. Its identical to VI char of diode inFB. 2. Up to cut-in V 3. I/P resistance 4. Effect of VCBon I/P VI char (Early effect) IE (mA) VCB = 8V VCB = 4V VBE

  22. Early effect / Base width modulation • Effect on β and α. E E C C B B Increase VCB Total base width = width of depletion region at CB junction + width of region which contains free charge carriers

  23. Output char of transistor in CB Operating region Cutoff region Active region Saturation region IC Active Region IE VCB Saturation Region Cutoff Region IB = 0

  24. Output char of transistor in CB • Cut off region : region below the curve IE =0 • Active region : IC ≈ IE (Const. current source) • Dynamic O/P resistance • Saturation region • Current controlled current source.

  25. Breakdown voltage and punch-through effect • Increasing VCB causes CB junction to breakdown. • Reach through / Punch through effect IC E C B VCB

  26. Potential variation through transistor Without biasing With external bias

  27. Transfer characteristic • Linear rela.tionship IC IE

  28. Common Emitter (CE) configuration RC RC RB RB VCE VCE VBE VBE VBB VBB Vcc Vcc Common emitter config. for PNP transistor Common emitter config. for NPN transistor

  29. CE Configuration • Input • Output • Current relation IE = IB + IC IC = α * IE + ICBO IC = IB (α/1- α) + ICBO / (1- α) But (α/1- α) = β So IC= IB * β + (β+1) ICBO IC = IB * β+ ICEO

  30. CE Configuration • Reverse leakage current in CE configuration (ICEO) • Thermal instability, so thermal stabilizing circuit is required. • Relation between α and β. α = β/(1+ β) and β = α /(1- α)

  31. CE characteristics (Input) • Same as conventional PN junction diode • Dynamic i/p resistance • Base current reduces as VCE increases. IB(μA) VCE = 4V VCE = 8V VBE

  32. CE characteristics (output) 1. Cut off region 2. Active region 3. Saturation region 4.Dynamic O/P resistance 5. Definition of β 6. Maximum VCE and breakdown

  33. Transfer characteristic • Why CE o/p char is more sloping than CB o/p char??? IC VCE = 5V VCE = 2V IB

  34. Typical transistor junction voltage values • Cutoff region • Short-circuited base • Open-circuited base • Cut-in voltage • Saturation voltage

  35. Standard test for regions • Saturation region 1. find IC, IB then check IB >= IC/β 2. measure VCB, positive for PNP and negative for NPN. • Active region measure VBE = 0.7 and measure VCB is negative (reverse biased) • Cutoff VBE is < 0.5 and VCB is negative

  36. Common collector configuration RE RE RB RB VECs VEC VBC VBC VBB VBB Vcc Vcc

  37. Practical way to draw CC config. VCC I/P voltage O/P voltage

  38. Current relation • IE = IB + IC IC = α * IE + ICBO IE = (β+1) * IB Current gain γ = IE/IB Maximum use of CC is for impedance matching (I/P is high and O/P is low).

  39. I/P char. And O/P char. IE IB(μA) VEC = 1V VEC = 2V Active Region IB VEC VBC Saturation Region Cutoff Region IB = 0

  40. Transfer char. IE VCE = 5V VCE = 2V IB

  41. Comparison of configurations

  42. Analytic expression for transistor char. • IC = -αN * IE – ICO () subscript N to α is transistor is being used in normal mode. For the inverted mode of operation IE = -αI* IC – IEO ()

  43. Base spreading resistance VCE VCB = VC + rbb * IB E C VE VC VCB VEB B

  44. Ebers – moll model αNIE αIIC IE IC VE VC IB

  45. Ebers – moll model (cont.) αNIE • I = IC + αNIE IC= - αNIE + I (I is diode current) IC I VC I = I0 () Now Take, I0 = -ICO , V = VC and n=1 So I = -ICO ()

  46. Why can’t we construct a transistor by connecting back to back diodes? VE VC

  47. Photo transistor 80 mW/(cm* cm) 60 mW/(cm* cm) 40 mW/(cm* cm) 20 mW/(cm* cm) Dark current

  48. Photo transistor (cont.) • Advantages: • Photo current multiplied by β • High sensitivity • Good switching speed • No memory effect.

  49. Phototransistor (cont.) • Disadvantages • Not so fast as conventional transistor because of photo-conducting material • Poor linearity • Temperature sensitive device • External voltage source is needed for operation

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