EE 434 Lecture 22. Properties of Bipolar Devices

EE 434 Lecture 22 Properties of Bipolar Devices

Quiz 16 A dc current source is shown. If the device has width W50u, lenth L1.2u, ucox100ua -2, T.75 and.04-1, determine a) The nominal output current b) The small sinal output impedance c) The minimum output voltae to maintain operation as a current source

1 And the number is. 8 6 9 7 5 4 2 3

Quiz 16 Solution A dc current source is shown. If the device has width W50u, lenth L1.2u, ucox100ua -2, T.75 and.04-1, determine a) The nominal output current b) The small sinal output impedance c) The minimum output voltae to maintain operation as a current source I I C DQ OX W 2L 50 2 1.2 ( ) 2 EB 2 ( 2 -.75) ma 3.25mA 4 DQ 10

Quiz 16 Solution A dc current source is shown. If the device has width W50u, lenth L1.2u, ucox100ua -2, T.75 and.04-1, determine a) The nominal output current b) The small sinal output impedance c) The minimum output voltae to maintain operation as a current source To maintain saturation reion operation, DS > GS - T OUT >2-.751.25

Review from Last Time Alternate equivalent small sinal model for BJT includes current dependent current source as alternative to transconductance source For a bipolar transistor >> >> m o m for the BJT is much larer than that of the MOSFET (ood) for BJT is much larer than that of the MOSFET (not so ood)

Other Properties of BJT Alternate Equivalent Small Sinal Model Relative manitude of small sinal parameters Simplified small sinal model Comparison of BJT and MOSFET

Comparison of MOSFET and BJT BJT MOSFET Assume BJT operatin in FA reion, MOSFET operatin in Saturation Assume same quiescent output voltae and same resistor R 1 One of the most widely used amplifier architectures

Comparison of MOSFET and BJT BJT MOSFET Assume BJT operatin in FA reion, MOSFET operatin in Saturation Assume same bias current One of the most widely used amplifier architectures in interated applications Special Case of Previous Architecture

Comparison of MOSFET and BJT BJT MOSFET

Comparison of MOSFET and BJT BJT G 1 R 1 1 v OUT o m + G 1 v IN A v v OUT IN m 0 + G 1 v A OUT OUT IN G m 1 v IN mr 1 m I CQ t A I CQ t R 1

Comparison of MOSFET and BJT MOSFET G 1 R 1 1 v OUT o m + G 1 v IN A v v OUT IN m 0 + G 1 v A OUT G OUT IN m 1 v IN mr 1 m 2I DQ EB A 2I DQ EB R 1

Comparison of MOSFET and BJT BJT MOSFET 2I ICQR DQ 1 1 AB AM t EB AB EB AM 2 t A B is typically much larer than A M for this basic amplifier architecture R

Comparison of MOSFET and BJT BJT MOSFET Assume same bias currents

Comparison of MOSFET and BJT BJT MOSFET A v v OUT IN Identical functional form Assumin same bias currents m 0 A A B M A A B M mb ob mm om mb mm om ob A v v OUT Typically A B >>A M EB [ λaf ] 2 t IN m 0

Comparison of MOSFET and BJT How do the areas required for implementin a MOSFET compare to those required for a BJT To make the comparison fair, will assume the same lithoraphy equipment and that the minimum feature size both processes is 2 Refer the desin rules for a bipolar process in the GAS text if additional details are needed The followin slides are adapted from the BJT technoloy presentation

1 mid-line of isolation 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 63λ 5 10 15 Emitter diffusion base diffusion 20 25 30 58λ 12λ 2λ 2λ 6λ 3λ 2λ 14λ 62λ 35 40 45 burried collector Note: 24λ required Between p-base and isolation diffusion 50 55 n-epi Minimum spacins for BJT (top view) Isolation diffusion

1 mid-line of isolation 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 5 10 15 20 25 Emitter diffusion 67λ base diffusion 62λ 30 35 40 45 burried collector Note: Not to vertical Scale Boundin Area 4154λ 2 50 55 n-epi Isolation diffusion

1 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 5 10 15 Minimum-Sized MOSFET 20 14λ 25 30 12λ 35 40 45 Boundin Area 168λ 2 Active Area 6λ 2 50 55

1 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 5 10 15 20 25 67λ 62λ 30 35 40 45 MOSFET BJT 50 55 Note: Not to vertical Scale

Other Properties of BJT Alternate Equivalent Small Sinal Model Relative manitude of small sinal parameters Simplified small sinal model Comparison of BJT and MOSFET Equivalent Circuit for dc biasin

Bipolar Models Simple dc Model Small Sinal Model Better Analytical dc Models Sophisticated Model for Computer Simulations Frequency-Dependent Small Sinal Model Better Models for Predictin Device Operation

Bipolar Models Simple dc Model for Biasin I B J S A E e BE t I β C I B I J C I S A β E e C I B BE t Althouh it may appear this is a model simplification, it is still hihly nonlinear because the diode from BE is nonlinear

Recall Transfer Characteristics J S.25fA/u 2 A E 400u 2 0.70 0.60 0.50 BE 0.40 0.30 0.20 0.10 0.00 0.1 1 10 I C (ma) BE close to 0.6 for a two decade chane in I C around 1mA

Bipolar Models Simple dc Model for Biasin I B JSA E e BE t I β C I B I β C I B BE is approximately 0.6 when in FA reion

Bipolar Models Simple dc Model for Biasin JSA I B I β E e C I B BE t I β C I B BE is approximately 0.6 when in FA reion BE 0.6 I β C I B I β C I B Dramatic Simplification in Model Widely used for biasin

Bipolar Models Simple dc Model for Biasin BE 0.6 I β C I B I β C I B When is this model justifiable? When it doesn t make much difference in the analysis of a circuit whether BE 0.6 or 0.7 When the CE dependence on the collector current is neliable