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ECE 305 Spring 2015
ECE-‐305 Spring 2015 1
ECE 305 Homework SOLUTIONS: Week 15
Mark Lundstrom Purdue University
1) Use the Ebers-‐Moll model to determine IC for an NPN BJT biased at IB = 100 nA and
VCE = 0.10 V . Assume that the Ebers-‐Moll model parameters are IF0 = 1.25 ×10−16A ,
IR0 = 2.50 ×10−16A , and αF = 0.996 .
1a) Determine the collector current.
Solution: The Ebers-‐Moll equations are: IC VBE ,VBC( ) =αF IF0 e
qVBE kBT −1( )− IR0 eqVBC kBT −1( ) (*)
IE VBE ,VBC( ) = IF0 eqVBE kBT −1( )−α RIR0 eqVBC kBT −1( ) (**)
Use KVL to write:
VCE =VBE +VCB VCE =VBE −VBC We are given VCE , but we need VBE and VBC (the voltages across the two PN junctions). If we knew VBE , then we could determine VBC from KVL. Use the given base current and VCE to determine VBE . KCL: IB = IE − IC IB = 1−αF( ) IF0 eqVBE kBT −1( ) + 1−α R( ) IR0 eqVBC kBT −1( ) = 100 ×10−9
VBC =VBE −VCE
IB = 1−αF( ) IF0 eqVBE kBT −1( ) + 1−α R( ) IR0 eq VBE−VCE( ) kBT −1( ) = 100 ×10−9
IB = 1−αF( ) IF0eqVBE kBT − 1−αF( ) IF0 + 1−α R( ) IR0eq VBE−VCE( ) kBT − 1−α R( ) IR0 IB = 1−αF( ) IF0 + 1−α R( ) IR0e−qVCE kBT{ }eqVBE kBT − 1−αF( ) IF0 + 1−α R( ) IR0{ }
eqVBE kBT =IB + 1−αF( ) IF0 + 1−α R( ) IR0{ }1−αF( ) IF0 + 1−α R( ) IR0e−qVCE kBT
(***)
From the given base current and saturation currents, it is clear that
1−αF( ) IF0 + 1−α R( ) IR0{ } << IB
so the numerator in (***) can be simplified.
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HW Week 15 Solutions (continued) The result is that (***) becomes (to a good approximation)
eqVBE kBT = IB1−αF( ) IF0 + IR0e−qVCE kBT −αF IF0e
−qVCE kBT
where we have used the “reciprocity relation” αF IF0 =α RIR0 . Also, from the given VCE e−qVCE kBT = e−0.1/0.026 = 2.14 ×10−2 Putting in numbers:
eqVBE kBT = 10−7
1− 0.996( ) 1.25 ×10−16( ) + 2.50 ×10−16( ) 2.14 ×10−2( )− 0.996 1.25 ×10−16( ) 2.14 ×10−2( )
eqVBE kBT = 3.11×1010
VBE =kBTqln 3.11×1010( ) = 0.026 × 24.2 = 0.628 V
Now we can go back to (*) and compute the collector current: IC =αF IF0 e
qVBE kBT −1( )− IR0 eqVBC kBT −1( ) VBC =VBE −VCE = 0.628− 0.10 = 0.528
Both junctions are forward biased, this transistor is operating in the saturation region.
IC =αF IF0 e0.628 0.026 −1( )− IR0 e0.528 0.026 −1( )
IC = 0.996 1.25 ×10−16( ) 3.09 ×1010( )− 2.50 ×10−16( ) 6.60 ×108( ) IC = 3.85 ×10−6 −1.65 ×10−7 IC = 3.68 ×10−6A
1b) Compare the collector current to base current ratio computed in 1a) to βF
Solution:
In the active region, ICIB
= βF
βF =αF
1−αF
= 0.9961− 0.996
= 249 in problem 1a) ICIB
= 3.68 ×10−6
100 ×10−9 = 36.8
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HW Week 15 Solutions (continued) ICIB
= 37 1a)
ICIB
= 249 active region
The moral of this story is that IC = βF IB should only be used in the active region.
2) What is βR for a bipolar transistor described by an Ebers-‐Moll model with
IF0 = 1.25 ×10−16A , IR0 = 2.50 ×10
−16A and αF = 0.996 ?
Solution:
βR =
α R
1−α R
but we are not given α R
According to the reciprocity relation: αF IF0 =αRIR0 , so
α R =αFIF0IR0
= 0.996 1.25 ×10−16
2.50 ×10−16 = 0.498
βR = 0.498
1− 0.498= 0.992
βR = 0.992
Note: for this transistor,
βF =α F 1−α F( ) = 249 . 3) In general, the common emitter IV characteristics of a BJT do not go through the
origin. That is, IC VBE ,VCE = 0( ) ≠ 0 . Instead, the collector current goes to zero at some
positive value of VCE known as the collector-‐emitter offset voltage, VOS . Use the Ebers-‐Moll equations to answer the following questions for an NPN transistor. 3a) Derive an expression for VOS . Hint: Since the transistor is in saturation, we can
assume eqVBE kBT >>1and eqVBC kBT >>1 .
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HW Week 15 Solutions (continued)
Solution: Begin with: IC VBE ,VBC( ) =αF IF0 e
qVBE kBT −1( )− IR0 eqVBC kBT −1( ) = 0 Assume both junctions are forward biased significantly: 0 =αF IF0e
qVBE kBT − IR0eqVBC kBT
Use KVL: VCE =VOS =VBE +VCB =VBE −VBC VBC =VBE −VOS αF IF0e
qVBE kBT = IR0eq VBE−VOS( ) kBT
αF IF0 = IR0e
−qVOS kBT
Use reciprocity: αF IF0 =α RIR0 α RIR0 = IR0e
−qVOS kBT
eqVOS kBT = 1
α R
VOS =kBTqln 1
α R
⎛⎝⎜
⎞⎠⎟
3b) Give a physical explanation for the answer of part 3a).
Solution: Both junctions are forward biased. Assume that the base transport factor, αT , is one (which makes the explanation simpler, but you are invited to think about the case where αT <1). In the limit, α R →1 , our result says that VOS = 0 . Why? In this case, both junctions are forward biased the same. Both inject the same electron currents into the base, and they cancel out. Because the emitter-‐base junction is forward biased, holes are injected into the emitter and contribute to the emitter current. If α R = 1, and αT = 1 , then the emitter injection efficiency of the base collector junction is one, so no holes are injected from the base into the collector. Since there is no electron current and no hole current at the BC junction, IC = 0 . If α R <1, then it means that at VCE = 0 , holes are injected from the base to the collector, and IC < 0 . To make IC = 0 , we need a positive voltage on the collector to reverse bias it and shut off the hole injection. This positive voltage is the collector-‐emitter offset voltage.
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HW Week 15 Solutions (continued) 3c) What is VOS for a bipolar transistor described by an Ebers-‐Moll model with
IF0 = 1.25 ×10−16A , IR0 = 2.50 ×10
−16A and αF = 0.996 . Solution:
VOS =kBTqln 1
α R
⎛⎝⎜
⎞⎠⎟
α R =αFIF0IR0
= 0.996 1.25 ×10−16
2.50 ×10−16 = 0.498
VOS = 0.026 ln1
0.498⎛⎝⎜
⎞⎠⎟ = 0.018 V
VOS = 18 mV
4) Assume that the measurements listed below are performed on a NPN BJT. Determine which two Ebers-‐Moll parameters can be found, and determine them.
VBE (Volts) VBC (Volts) IB (µA) IC (mA) 0.75 -‐3.5 2.9 0.55
Solution: The device is clearly biased in the forward active region, so IC VBE ,VBC( ) =αF IF0 e
qVBE kBT −1( )− IR0 eqVBC kBT −1( ) simplifies to IC =αF IF0e
qVBE kBT
(in the active region, we can ignore IR0 , the reverse leakage current of the BC junction). So we can determine two parameters: αF and IF0 .
βF =
IC
IB
= 5502.9
= 190
α F =
βF
βF +1= 190
191= 0.9948
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HW Week 15 Solutions (continued)
IC =αF IF0eqVBE kBT
or
IF0 =ICαF
e−qVBE kBT = 0.55 ×10−3
0.9948× e−0.75/0.026 = 1.64 ×10−16 A
so the final answers are:
α F = 0.9948, IF 0 = 1.64×10−16 A
5) The transistor show below has a voltage, VC = 5 V , applied to the collector, the emitter is grounded, and the base terminal is open-‐circuited. Use the Ebers-‐Moll model to answer the following questions. Assume that IF0 = 1.25 ×10
−16A , IR0 = 2.50 ×10
−16A and αF = 0.996 and αR = 0.498 .
5a) What is the base voltage?
Solution: The Ebers-‐Moll equations are: IC VBE ,VBC( ) =αF IF0 e
qVBE kBT −1( )− IR0 eqVBC kBT −1( ) (*) IE VBE ,VBC( ) = IF0 eqVBE kBT −1( )−α RIR0 e
qVBC kBT −1( ) (**) Use KCL: IB = IE − IC IB = 1−αF( ) IF0 eqVBE kBT −1( ) + 1−αR( ) IR0 eqVBC kBT −1( ) = 0 1−αF( ) IF0 eqVBE kBT −1( ) = − 1−αR( ) IR0 eqVBC kBT −1( ) (+)
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HW Week 15 Solutions (continued)
VBE =VB −VE =VB
VBC =VB −VC =VB − 5 Equation (+) becomes:
eqVB kBT −1( ) = −1−αR( )1−αF( )
IR0IF0
⎛⎝⎜
⎞⎠⎟eqVB kBT e−q5/kBT −1( )
Solve for the base voltage:
eqVB kBT 1+1−αR( )1−αF( )
IR0IF0
⎛⎝⎜
⎞⎠⎟e−q5/kBT
⎛
⎝⎜⎞
⎠⎟= 1+
1−αR( )1−αF( )
IR0IF0
⎛⎝⎜
⎞⎠⎟
eqVB kBT =1+
1−αR( )1−αF( )
IR0IF0
⎛⎝⎜
⎞⎠⎟
1+1−αR( )1−αF( )
IR0IF0
⎛⎝⎜
⎞⎠⎟e−q5/kBT
⎛
⎝⎜⎞
⎠⎟
The quantity, e
−q5/kBT , is really, really small. e−q5/kBT = e−5/0.026 = 3.30 ×10−84 , so the denominator simplifies
eqVB kBT =1+
1−αR( )1−αF( )
IR0IF0
⎛⎝⎜
⎞⎠⎟
1
Putting in numbers:
eqVB kBT = 1+ 1− 0.498( )1− 0.996( )
2.501.25
⎛⎝⎜
⎞⎠⎟ = 252
VB = 0.026ln 252( ) = 0.144 V
VB = 0.144 V
This makes sense. Most of the voltage drops across the reverse-‐biased (high resistance junction) – not the forward-‐biased (low resistance junction).
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HW Week 15 Solutions (continued) 5b) What is the collector current?
Solution: IC VBE ,VBC( ) =αF IF0 e
qVBE kBT −1( )− IR0 eqVBC kBT −1( ) IC 0.144,0.144 − 5( ) =αF IF0 e
0.144/0.026 −1( )− IR0 eq 0.144−5( ) kBT −1( ) IC 0.144,−4.86( ) =αF IF0 e
0.144/0.026 −1( )− IR0 e−q 4.86( ) kBT −1( ) IC 0.144,−4.86( ) =αF IF0 e
0.144/0.026 −1( ) + IR0 Now put in the other numbers: IC = 0.996 1.25 ×10−16( ) 254 −1( ) + 2.50 ×10−16 = 3.18 ×10−14
IC = 31.8 fA
Note that this is a very small current, but larger than the saturation currents of the two junctions.