Slide 8-1 Chapter 8 Bipolar Junction Transistors Since 1970, the high density and low-power advantage of the MOS technology steadily eroded the BJT’s early dominance. BJTs are still preferred in some high-frequency and analog applications because of their high speed and high power output. Question: What is the meaning of “bipolar” ? Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Introduction to the BJT I C is an exponential function of forward V BE and independent of reverse V CB. Modern Semiconductor Devices for Integrated Circuits (C. Hu) NPN BJT: N+N+ PN EC B V BE V CB EmitterBase Collector

Slide 8-3 Common-Emitter Configuration Question: Why is I B often preferred as a parameter over V BE ? Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-4 Modern Semiconductor Devices for Integrated Circuits (C. Hu) 8.2 Collector Current  B : base recombination lifetime D B : base minority carrier (electron) diffusion constant Boundary conditions : N + P N emitter basecollector x 0 W depletion layers B

Slide 8-5 It can be shown G B (s·cm 4 ) is the base Gummel number 8.2 Collector Current n i 2 N B e qV BE kT  1–  n n x/ x/W B Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-6 At low-level injection, inverse slope is 60 mV/decade High-level injection effect : High Level Injection Effect At large V BE, When p > N B, inverse slope is 120mV/decade. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 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. p E ' n B ' W E W B (b) emitter basecollector contact I E I C electron flow – + hole flow I B (a) contact Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-8 Is a large I B desirable? Why? 8.3 Base Current emitter basecollector contact I E I C electron flow – + hole flow I B (a) contact Modern Semiconductor Devices for Integrated Circuits (C. Hu) For a uniform emitter,

Slide Current Gain How can  F be maximized? Common-emitter current gain,  F : Common-base current gain: It can be shown that Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-10 EXAMPLE: Current Gain A BJT has I C = 1 mA and I B = 10  A. What are I E,  F and  F ? Solution: We can confirm and Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Emitter Bandgap Narrowing Emitter bandgap narrowing makes it difficult to raise  F by doping the emitter very heavily. Modern Semiconductor Devices for Integrated Circuits (C. Hu) To raise  F, N E is typically very large. Unfortunately, large N E makes (heavy doping effect). Since n i is related to E g, this effect is also known as band-gap narrowing.  E gE is negligible for N E < cm -3, is 50 meV at cm -3, 95 meV at cm -3, and 140 meV at cm -3.

Slide 8-12 To further elevate  F , we can raise n iB by using an epitaxial Si 1-  Ge  base. With  = 0.2, E gB is reduced by 0.1eV and n iE 2 by 30x Narrow-Bandgap Base and Heterojuncion BJT Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-13 Assume D B = 3D E, W E = 3W B, N B = cm -3, and n iB 2 = n i 2. What is  F for (a) N E = cm -3, (b) N E = cm -3, and (c) N E = cm -3 and a SiGe base with  E gB = 60 meV ? (a) At N E = cm -3,  E gE  50 meV, (b) At N E = cm -3,  E gE  95 meV (c) EXAMPLE: Emitter Bandgap Narrowing and SiGe Base Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-14 A high-performance BJT typically has a layer of As-doped N + poly-silicon film in the emitter.  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-collector P-base SiO 2 emitter N + -poly-Si Poly-Silicon Emitter Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-15 Why does one want to operate BJTs at low I C and high I C ? Why is  F a function of V BC in the right figure? FF From top to bottom: V BC = 2V, 1V, 0V Gummel Plot and  F Fall-off at High and Low I c Hint: See Sec. 8.5 and Sec SCR BE current Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Base-Width Modulation by Collector Voltage Output resistance : Large V A (large r o ) is desirable for a large voltage gain IB3IB3 ICIC V CE 0 V A V A : Early Voltage IB2IB2 IB1IB1 Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-17 How can we reduce the base-width modulation effect? 8.5 Base-Width Modulation by Collector Voltage N + P N emitter base collector V CE W B3 W B2 W B1 x n' } V CE1 <V 2 <V 3 V BE Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-18 The base-width modulation effect is reduced if we (A) Increase the base width, (B) Increase the base doping concentration, N B, or (C) Decrease the collector doping concentration, N C. Which of the above is the most acceptable action? 8.5 Base-Width Modulation by Collector Voltage N + P N emitter base collector V CE W B3 W B2 W B1 x n' } V CE1 <V 2 <V 3 V BE Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Ebers-Moll Model The Ebers-Moll model describes both the active and the saturation regions of BJT operation. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-20 I C is driven by two two forces, V BE and V BC. When only V BE is present : Now reverse the roles of emitter and collector. When only V BC is present :  R : reverse current gain  F : forward current gain 8.6 Ebers-Moll Model Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-21 In general, both V BE and V BC are present : In saturation, the BC junction becomes forward-biased, too. V BC causes a lot of holes to be injected into the collector. This uses up much of I B. As a result, I C drops. V CE (V) 8.6 Ebers-Moll Model Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 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  F determines the high-frequency limit of BJT operation.  F is difficult to be predicted accurately but can be measured. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Base Charge Storage and Base Transit Time Let’s analyze the excess hole charge and transit time in the base only. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-24 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. EXAMPLE: Base Transit Time Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-25 The base transit time can be reduced by building into the base a drift field that aids the flow of electrons. Two methods: Fixed E gB, N B decreases from emitter end to collector end. Fixed N B, E gB decreases from emitter end to collector end. dx dE q c 1  E Drift Transistor–Built-in Base Field Modern Semiconductor Devices for Integrated Circuits (C. Hu) - EBC EcEc EvEv EfEf - E B C EcEc EvEv EfEf

Slide Emitter-to-Collector Transit Time and Kirk Effect Top to bottom : V CE = 0.5V, 0.8V, 1.5V, 3V. To reduce the total transit time, emitter and depletion layers must be thin, too. Kirk effect or base widening: At high I C the base widens into the collector. Wider base means larger  F. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-27 Base Widening at Large I c s dx dEdE  /  x  E base N collector N + base width depletion layer x  E base N N + collector “base width” depletion layer collector Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Small-Signal Model Transconductance: At 300 K, for example, g m =I C /26mV. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-29 This is the charge-storage capacitance, better known as the diffusion capacitance. Add the depletion-layer capacitance, C dBE : 8.8 Small-Signal Model Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-30 EXAMPLE: Small-Signal Model Parameters A BJT is biased at I C = 1 mA and V CE = 3 V.  F =90,  F =5 ps, and T = 300 K. Find (a) g m, (b) r , (c) C . Solution: (a) (b) (c) Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-31 Once the model parameters are determined, one can analyze circuits with arbitrary source and load impedances. The parameters are routinely determined through comprehensive measurement of the BJT AC and DC characteristics. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Cutoff Frequency The load is a short circuit. The signal source is a current source, i b, at frequency, f. At what frequency does the current gain fall to unity? Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-33 f T is commonly used to compare the speed of transistors. Why does f T increase with increasing I C ? Why does f T fall at high I C ? f T = 1/2  (  F + C dBE kT/qI C ) 8.9 Cutoff Frequency Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-34 Poly-Si emitter Thin base Self-aligned poly-Si base contact Narrow emitter opening Lightly-doped collector Heavily-doped epitaxial subcollector Shallow trench and deep trench for electrical isolation BJT Structure for Minimum Parasitics and High Speed Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-35 In order to sustain an excess hole charge in the transistor, holes must be supplied through I B to susbtain recombination at the above rate. What if I B is larger than ? Step 1: Solve it for any given I B (t) to find Q F (t) Charge Control Model For the DC condition, Step 2: Can then find I C (t) through I C (t) = Q F (t)/  F. I C (t) = Q F (t)/  F Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-36 Visualization of Q F (t) Q F (t) Q F /  F  F I B (t) Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-37 EXAMPLE : Find I C (t) for a Step I B (t) The solution of is What is E BC n t QFQF Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Model for Large-Signal Circuit Simulation Compact (SPICE) model contains dozens of parameters, mostly determined from measured BJT data. Circuits containing tens of thousands of transistors can be simulated. Compact model is a “contract” between device/manufacturing engineers and circuit designers. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide 8-39 A commonly used BJT compact model is the Gummel-Poon model, consisting of Ebers-Moll model Current-dependent beta Early effect Transit times Kirk effect Voltage-dependent capacitances Parasitic resistances Other effects 8.11 Model for Large-Signal Circuit Simulation Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Chapter Summary The base-emitter junction is usually forward-biased while the base-collector is reverse-biased. V BE determines the collector current, I C. G B is the base Gummel number, which represents all the subtleties of BJT design that affect I C. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Chapter Summary The base (input) current, I B, is related to I C by the common-emitter current gain,  F. This can be related to the common-base current gain,  F. The Gummel plot shows that  F falls off in the high I C region due to high-level injection in the base. It also falls 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)

Slide Chapter Summary Due to the forward bias V BE, a BJT stores a certain amount of excess carrier charge Q F which is proportional to I C.  F is the forward transit time. If no excess carriers are stored outside the base, then The charge-control model first calculates Q F (t) from I B (t) and then calculates I C (t)., the base transit time. Modern Semiconductor Devices for Integrated Circuits (C. Hu)

Slide Chapter Summary The small-signal models employ parameters such as transconductance, input capacitance, and input resistance. Modern Semiconductor Devices for Integrated Circuits (C. Hu)