02/06/2015 8.1 Introduction to the BJT Chapter 8 Bipolar - PDF document

02/06/2015 8.1 Introduction to the BJT Chapter 8 Bipolar Junction Transistors NPN BJT: B E C N + P N • Since 1970, the high density and low-power advantage of Emitter Base Collector the MOS technology steadily eroded the BJT’s early dominance. • BJTs are still preferred in some high-frequency and analog V BE V CB applications because of their high speed and high power output. I C is an exponential function of forward V BE and independent Question : What is the meaning of “bipolar” ? of reverse V CB . Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-135 Slide 8-136 Common-Emitter Configuration 8.2 Collector Current depletion layers N + N P emitter base collector x 0 W B  B : base recombination lifetime D B : base minority carrier (electron) diffusion constant Boundary conditions : Question : Why is I B often preferred as a parameter over V BE ? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-137 Slide 8-138 8.2 Collector Current 8.3 Base Current Some holes are injected from the P-type base into the N + emitter. The holes are provided by the base current, I B . n  ------------- n  0  2 n i  qV BE kT  -------e – 1 (a) contact emitter base collector contact N B 1 electron flow – I E I C hole flow + It can be shown I B p E ' n B ' 0 x/ x / W B 1 (b) G B (s·cm 4 ) is the base Gummel number W E W B Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-139 Slide 8-140 1

02/06/2015 8.3 Base Current 8.4 Current Gain (a) contact contact emitter base collector Common-emitter current gain,  F : electron flow – I E I C Common-base current gain: hole flow + I B For a uniform emitter, It can be shown that How can  F be maximized? Is a large I B desirable? Why? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-141 Slide 8-142 8.4.3 Poly-Silicon Emitter EXAMPLE: Current Gain A high-performance BJT typically has a layer of As-doped N + A BJT has I C = 1 mA and I B = 10  A. What are I E ,  F and  F ? poly-silicon film in the emitter. Solution :  F is larger due to the large W E , mostly made of the N + poly- silicon. (A deep diffused emitter junction tends to cause emitter- collector shorts.) N + -poly-Si emitter We can confirm SiO 2 P-base and N-collector Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-143 Slide 8-144 8.5 Base-Width Modulation by Collector Voltage 8.5 Base-Width Modulation by Collector Voltage V BE Output resistance : N + N P V CE emitter base collector W B 3 } V CE 1 < V CE 2 < V CE 3 W B 2 W B 1 n' I C I B 3 Large V A (large r o ) I B 2 is desirable for a V A : Early Voltage large voltage gain I B 1 x How can we reduce the base-width modulation effect? V CE 0 V A Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-145 Slide 8-146 2

02/06/2015 8.5 Base-Width Modulation by Collector Voltage 8.6 Ebers-Moll Model V BE N + P N The base-width modulation V CE emitter base collector effect is reduced if we W B 3 } W B 2 V 1 < V 2 < V (A) Increase the base width, CE CE CE 3 W B 1 (B) Increase the base doping n' concentration, N B , or (C) Decrease the collector doping concentration, N C . x The Ebers-Moll model describes both the active Which of the above is the most acceptable action? and the saturation regions of BJT operation. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-147 Slide 8-148 8.6 Ebers-Moll Model 8.6 Ebers-Moll Model In general, both V BE and V BC are present : I C is driven by two two forces, V BE and V BC . When only V BE is present : In saturation, the BC junction becomes forward-biased, too. Now reverse the roles of emitter and collector. V BC causes a lot of holes to be injected When only V BC is present : into the collector. This uses up much  R : reverse current gain of I B . As a result, I C drops.  F : forward current gain V CE (V) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-149 Slide 8-150 Ebers-Moll NPN BJT Model BJT Modes of Operation BE junction BC junction Mode Reverse Reverse Cutoff (OFF) Forward Reverse Forward active (FA) Forward Forward Saturation (SAT) Reciprocity theorem Reverse Forward Reverse active (RA) transport saturation current 3

02/06/2015 Reduced models of the operation modes 8.7 Transit Time and Charge Storage When the BE junction is forward-biased, excess holes are stored in the emitter, the base, and even in the depletion layers. Q F is all the stored excess hole charge (b) Forward active (a) Cutoff  F is difficult to be predicted accurately but can be measured.  F determines the high-frequency limit of BJT operation. (c) Saturation (d) Reverse active Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-154 8.7.1 Base Charge Storage and Base Transit Time EXAMPLE: Base Transit Time Let’s analyze the excess hole charge and transit time in the base only. What is  FB if W B = 70 nm and D B = 10 cm 2 /s? Answer: 2.5 ps is a very short time. Since light speed is 3  10 8 m/s, light travels only 1.5 mm in 5 ps. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-155 Slide 8-156 8.7.2 Drift Transistor–Built-in Base Field 8.12 Chapter Summary The base transit time can be reduced by building into the base • The base-emitter junction is usually forward-biased while a drift field that aids the flow of electrons. Two methods: the base-collector is reverse-biased. V BE determines the • Fixed E gB , N B decreases from emitter end to collector end. collector current, I C . B - E C E c E f E v • Fixed N B , E gB decreases from emitter end to collector end. - E B C • G B is the base Gummel number, which represents all the 1 dE  c E subtleties of BJT design that affect I C . E c E f q dx E v Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-157 Slide 8-158 4

02/06/2015 8.12 Chapter Summary 8.12 Chapter Summary • The base (input) current, I B , is related to I C by the • Due to the forward bias V BE , a BJT stores a certain amount common-emitter current gain,  F . This can be related to of excess carrier charge Q F which is proportional to I C . the common-base current gain,  F .  F is the forward transit time. • The Gummel plot shows that  F falls off in the high I C If no excess carriers are stored outside the base, then region due to high-level injection in the base. It also falls , the base transit time. off in the low I C region due to excess base current. • Base-width modulation by V CB results in a significant slope of the I C vs. V CE curve in the active region (known as the Early effect). Modern Semiconductor Devices for Integrated Circuits (C. Hu) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8-159 Slide 8-160 5