BJT Basics
ROCHESTER INSTITUTE OF TECHNOLOGY MICROELECTRONIC ENGINEERING
Bipolar Junction Transistor - Basics Dr. Lynn Fuller Webpage: http://people.rit.edu/lffeee/ Microelectronic Engineering Rochester Institute of Technology 82 Lomb Memorial Drive Rochester, NY 14623-5604 Tel (585) 475-2035 Email:
[email protected] Department webpage: http://www.rit.edu/kgcoe/microelectronic
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
1-29-17 BJT_Basics.ppt Page 1
BJT Basics
OUTLINE
Definitions Schematic Symbols Theory Integrated BJT Structure Modes of Operation IC-VCE Family of Curves Modifications References Homework Questions
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
DEFINITIONS Bipolar Junction Transistor - (BJT) Both holes and electrons participate in the conduction of current, hence the name bipolar. Minority carrier - In a p-type semiconductor electrons are the minority carrier type, in an n-type semiconductor holes are the minority carrier type. Emitter - Emits minority carriers into the base region of a BJT. For example, in an NPN BJT the n-type emitter, emits electrons into the p-type base. The emitter usually has the highest doping levels of the three regions of a BJT. Base - Thin region which is used to control the flow of minority carriers from the emitter to the collector
Collector -Collects the minority carriers that make it through the base from the emitter. The collector usually has the lightest doping concentrations of the three regions. DC Beta ( bdc ) - The ratio of the collector current to the base current. bdc = IC / IB AC Beta ( bac ) - The ratio of the change in the collector current to the change in the base current. bac = D IC / D IB Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
BJT - BIPOLAR JUNCTION TRANSISTOR
Label
2N3904
Flat 1 2
Emitter Base Collector
3
Rochester Institute of Technology Microelectronic Engineering
John Bardeen, William Shockley, Walter Brattain, 1948
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
SCHEMATIC SYMBOLS npn Base
Collector n p n
pnp
Collector p n p
Base
Emitter Collector Base
Emitter Collector Base
Emitter Rochester Institute of Technology Microelectronic Engineering
Emitter The arrow on the emitter is in the direction that current will flow in the Base Emitter pn junction
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
IDEALIZED STRUCTURE
p-type Emitter SC
SC
N
P
n-type
Collector
N n-type
Base Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
ELECTRON CONCENTRATIONS IN AN NPN BJT
Base
Emitter
Collector E
n~Nde
n~Ndc n~very small but not zero ~ni2/Nab BE Space Charge Layer
x
BC
With the B-E junction forward biased, and B-C junction reverse biased. There is a concentration gradient in the base that forces electrons to flow toward the collector. Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
COMMENTS 1. The concentration of electrons in n-type silicon is ~ doping concentration in that region. 2. In p-type silicon the number of electrons is almost zero 3. A forward biased pn junction means more carriers of both types can cross the potential barrier. So a forward biased base-emitter junction (in an npn BJT) means more electrons on the base side than in equilibrium (no bias). 4. A reverse biased pn junction means less carriers of both types can cross the potential barrier. So a reverse biased base-collector junction (in an npn BJT) means less electrons on the base side than in equilibrium (no bias). Even closer to zero electrons in p-type base at the edge of the B-C space charge layer. 5. The base is so narrow that few electrons are lost as they diffuse across the base width. Diffusion is driven by a concentration gradient. So electrons move towards the collector and current flows in the opposite direction. Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
INTEGRATED BJT STRUCTURE Collector (n)
~ 1018 cm-3
Base (p)
Emitter (n+) ~ 1018 cm-3
p-type ~ 1016 cm-3 electrons current Lightly doped (~1015 cm-3) n-type silicon wafer
Since the emitter is more heavily doped compared to the base than the collector, the emitter-base junction has a lower breakdown voltage than the base-collector junction. n+ means heavily doped n-type n- means lightly doped n-type Rochester Institute of Technology p+ means heavily doped p-type Microelectronic Engineering p- means lightly doped p-type © January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
BJT TERMINAL CURRENTS From device physics IE = ISE eVBE/VT where ISE = AqDni2/NAW and VT = KT/q
IC = a IE where a represents the fraction of carriers from the emitter that make it to the collector IC = b IB where b represents the ratio of collector current to base current We can show that b = a/(1-a) or a = b / (b+1) WithRochester the B-E junction forward biased, and B-C junction Institute of Technology reverse biased. Microelectronic Engineering © January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics LARGE SIGNAL MODEL IN FORWARD ACTIVE MODE
Modes
Base/Emitter Cutoff Reverse Active Forward Inverse Reverse Saturation Forward
Base/Collector Reverse Reverse Forward Forward
Collector IC
With the B-E junction forward biased, and B-C junction reverse biased.
a IE = a Is eVBE/VT IB Base
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© January 24, 2017 Dr. Lynn Fuller, Professor
IE Emitter Page 11
BJT Basics
EBERS-MOLL MODEL OF NPN BJT This type of model works in all four regions of operation
Collector IC
IC = aF iDe – iDc IE = -iDe + aR iDc
aF iDe
iDc
The diode currents are: iDc = ISC (e Vbc/VT -1) iDe = ISE (e Vbe/VT -1)
IB
Base aR iDc
iDe
Transistors are modeled by determining appropriate values of: aF, aR, ISC and ISE
IE Emitter
Institute of Technology Note:Rochester b is often given instead of a but a = b/(1+b) Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
EARLY VOLTAGE Increasing VCE increases the reverse bias on the BC junction increasing the width of the BC space charge layer resulting in a decrease in the base width and increase in concentration gradient and an increase in collector current. To account for this the equation relating the collector current to the VBE can be modified slightly as shown: VA is the Early voltage after Dr. Jim Early of Fairchild Semiconductor. IC IC = IS 1+ VCE VA
eVBE/VT
-VA
VCE This isRochester one of the many modifications to make the BJT models more accurate. Institute of Technology Engineering Other Microelectronic modifications include resistors to account for series resistance in the collector, base and emitter. © January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
CHARACTERISTICS OF TWO TERMINAL DEVICES I I + V -
V
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
BE JUNCTION, BC JUNCTION, CE I
I
-8 0.7
V
I
-8 0.7
V
-8 0.7
Collector I Base + V-
V+ Base I
+ V -
Emitter
I Collector
Emitter
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
V
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BJT Basics
CHARACTERISTICS OF THREE TERMINAL DEVICES Iin
Iout
+ + Vin Vout Common
Iin
Iout
Vin Vout Each trace is for a different value for Iout or Vout Rochester Institute of Technology Microelectronic Engineering
Each trace is for a different value for Iin or Vin
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
BJT IC-VCE FAMILY OF CURVES IC Steps of base current - IB
10 mA 9 mA
IB = 30 mA
8 mA
10 mA increments
7 mA
b dc
6 mA 5 mA
DIC = 2.5 mA bac
4 mA 3 mA
IB = 20 mA DIB = 10 mA IB = 10 mA
2 mA 1 mA
VCE Beta (b ac ) =
Beta (b dc ) =
DIC 2.5x10-3 = 250 = DIB 10x10-6 IC 5.0x10-3 = 250 = IB 20x10-6
The two Beta values are not always the same!
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
NPN COMMON EMITTER IC-VCE CHARACTERISTICS Forward Active Mode Base-Emitter junction is forward biased Base-Collector junction is reverse biased npn
IC - +5 V
B-C junction is reverse biased
Vbc negative base - p
IB base current steps
Va > 0 is a forward biased junction Va < 0 is a reverse biased junction Va is defined as the voltage from p to n
0.7 V
Sweep from 0 to 5 Volts
Collector - n
+ +
Vbe positive
-
B-E junction is forward biased
Emitter - n
0V
Inverse Active Mode Base-Emitter junction is reverse biased Base-Collector junction is forward biased
Rochester Institute of Technology Microelectronic Engineering
January 24, 2017 Dr. collector Lynn Fuller, Professor Switch the connections to©the emitter and leads
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BJT Basics
TRIPLE DIFFUSED BJT STRUCTURE Collector
Emitter
Base
n+
p-base
n+ p-wafer
n-well collector
Doping Conc n+ emitter
1-D Doping Profile
p-base
§ Simple BJT structure § Large collector series resistance § Large dimensions § Isolation issues
n-well collector depth
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
WB Base Width WB ~ 0.5mm Page 19
BJT Basics
SHALLOW TRENCH ISOLATION Collector SiO2
n+ plug
p-wafer
Emitter
Base
n+
p-base
SiO2
SiO2 n-type epitaxial silicon
n+ buried layer
§ Process Enhancements § Oxide-plug isolation § Patterned buried sub-collector § Epitaxial silicon
§ Performance Improvements § Low collector series resistance § Improved collector/emitter isolation § smaller geometries
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
REFERENCES 1. Sedra and Smith, 5.1-5.4 2. Device Electronics for Integrated Circuits, 2nd Edition, Kamins and Muller, John Wiley and Sons, 1986. 3. The Bipolar Junction Transistor, 2nd Edition, Gerald Neudeck, Addison-Wesley, 1989.
Rochester Institute of Technology Microelectronic Engineering
© January 24, 2017 Dr. Lynn Fuller, Professor
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BJT Basics
HOMEWORK - BJT’S
1. Why won’t two back to back diodes behave like a BJT? 2. Sketch a figure like that on page 7 showing the hole concentration for a pnp transistor with B-E junction forward biased and B-C junction reverse biased. Show direction of current flow. 3. The Ic versus Vce family of Vce curves for a 2N3906 BJT is shown. What is the current gain, Beta, b Ib 4. Look up the 2N3906 and see what the typical b is. Emitter 5. Look up some information about John Bardeen. Write a few sentences. Rochester Institute of Technology Microelectronic Engineering
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