Chapter 8 Bipolar Junction Transistors Since 1970 the
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 8 -1
8. 1 Introduction to the BJT NPN BJT: B E N+ Emitter VBE P Base C N Collector VCB IC is an exponential function of forward VBE and independent of reverse VCB. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -2
Common-Emitter Configuration Question: Why is IB often preferred as a parameter over VBE? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -3
8. 2 Collector Current depletion layers N+ N P emitter base 0 x collector WB B : base recombination lifetime DB : base minority carrier (electron) diffusion constant Boundary conditions : Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -4
8. 2 Collector Current n¢ ------2 n ¢ ()0 1 ni q. V BE ¤ k. T () -------e – 1 NB It can be shown 0 x/ x/W B 1 GB (s·cm 4) is the base Gummel number Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -5
8. 2. 1 High Level Injection Effect At large VBE, 10 -2 Ik. F 10 -4 I C (A) • At low-level injection, inverse slope is 60 m. V/decade • High-level injection effect : 10 -6 60 m. V/decade 10 -8 10 -10 10 -12 0 0. 2 0. 4 0. 6 0. 8 1. 0 VBE When p > NB , inverse slope is 120 m. V/decade. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -6
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, IB. contact emitter (a) I collector contact base electron flow E + – hole flow I B IC p. E' n. B' (b) WE WB Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -7
8. 3 Base Current contact emitter (a) collector contact base electron flow I E + – I C hole flow I B For a uniform emitter, Is a large IB desirable? Why? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -8
8. 4 Current Gain Common-emitter current gain, F : Common-base current gain: It can be shown that How can F be maximized? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -9
EXAMPLE: Current Gain A BJT has IC = 1 m. A and IB = 10 m. A. What are IE, F and F? Solution: We can confirm and Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -10
8. 4. 1 Emitter Bandgap Narrowing To raise F, NE is typically very large. Unfortunately, large NE makes (heavy doping effect). Since ni is related to Eg , this effect is also known as band-gap narrowing. DEg. E is negligible for NE < 1018 cm-3, is 50 me. V at 1019 cm-3, 95 me. V at 1020 cm-3, and 140 me. V at 1021 cm-3. Emitter bandgap narrowing makes it difficult to raise F by doping the emitter very heavily. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -11
8. 4. 2 Narrow-Bandgap Base and Heterojuncion BJT To further elevate F , we can raise ni. B by using an epitaxial Si 1 -h. Geh base. With h = 0. 2, Eg. B is reduced by 0. 1 e. V and ni. E 2 by 30 x. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -12
EXAMPLE: Emitter Bandgap Narrowing and Si. Ge Base Assume DB = 3 DE , WE = 3 WB , NB = 1018 cm-3, and ni. B 2 = ni 2. What is F for (a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1020 cm-3 and a Si. Ge base with DEg. B = 60 me. V ? (a) At NE = 1019 cm-3, DEg. E 50 me. V, (b) At NE = 1020 cm-3, DEg. E 95 me. V (c) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -13
8. 4. 3 Poly-Silicon Emitter 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 WE , mostly made of the N+ poly- silicon. (A deep diffused emitter junction tends to cause emittercollector shorts. ) N+-poly-Si emitter Si. O 2 P-base N-collector Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -14
SCR BE current F 8. 4. 4 Gummel Plot and b. F Fall-off at High and Low Ic From top to bottom: VBC = 2 V, 1 V, 0 V Why does one want to operate BJTs at low IC and high IC? Why is F a function of VBC in the right figure? Hint: See Sec. 8. 5 and Sec. 8. 9. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -15
8. 5 Base-Width Modulation by Collector Voltage Output resistance : IB 3 IC IB 2 VA : Early Voltage VA IB 1 0 Large VA (large ro ) is desirable for a large voltage gain VCE Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -16
8. 5 Base-Width Modulation by Collector Voltage V BE N+ N P emitter base WB 3 WB 2 WB 1 V collector CE } VCE 1 < VCE 2 <VCE 3 n' x How can we reduce the base-width modulation effect? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -17
8. 5 Base-Width Modulation by Collector Voltage The base-width modulation effect is reduced if we (A) Increase the base width, (B) Increase the base doping concentration, NB , or (C) Decrease the collector doping concentration, NC. VBE N+ N P emitter base WB 3 WB 2 WB 1 n' VCE collector } VCE 1< VCE 2<VCE 3 x Which of the above is the most acceptable action? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -18
8. 6 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 -19
8. 6 Ebers-Moll Model IC is driven by two forces, VBE and VBC. When only VBE is present : Now reverse the roles of emitter and collector. When only VBC is present : R : reverse current gain F : forward current gain Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -20
8. 6 Ebers-Moll Model In general, both VBE and VBC are present : In saturation, the BC junction becomes forward-biased, too. VBC causes a lot of holes to be injected into the collector. This uses up much of IB. As a result, IC drops. VCE (V) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -21
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. QF is all the stored excess hole charge F is difficult to be predicted accurately but can be measured. t. F determines the high-frequency limit of BJT operation. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -22
8. 7. 1 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 -23
EXAMPLE: Base Transit Time What is FB if WB = 70 nm and DB = 10 cm 2/s? Answer: 2. 5 ps is a very short time. Since light speed is 3 108 m/s, light travels only 1. 5 mm in 5 ps. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -24
8. 7. 2 Drift Transistor–Built-in Base Field The base transit time can be reduced by building into the base a drift field that aids the flow of electrons. Two methods: • Fixed Eg. B , NB decreases from emitter end to collector end. E B - C Ec Ef Ev • Fixed NB , Eg. B decreases from emitter end to collector end. E Ec - B C Ef 1 d. Ec E = q dx Ev Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -25
8. 7. 3 Emitter-to-Collector Transit Time and Kirk Effect • To reduce the total transit time, emitter and depletion layers must be thin, too. • Kirk effect or base widening: At high IC the base widens into the collector. Wider base means larger F. Top to bottom : VCE = 0. 5 V, 0. 8 V, 1. 5 V, 3 V. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -26
Base Widening at Large Ic -E N collector base N+ collector base width x depletion layer -E d. E dx N = r /es base collector N+ collector x “base depletion width” layer Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -27
8. 8 Small-Signal Model Transconductance: At 300 K, for example, gm=IC /26 m. V. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -28
8. 8 Small-Signal Model This is the charge-storage capacitance, better known as the diffusion capacitance. Add the depletion-layer capacitance, Cd. BE : Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -29
EXAMPLE: Small-Signal Model Parameters A BJT is biased at IC = 1 m. A and VCE = 3 V. F=90, F=5 ps, and T = 300 K. Find (a) gm , (b) rp , (c) Cp. Solution: (a) (b) (c) Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -30
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 8 -31
8. 9 Cutoff Frequency The load is a short circuit. The signal source is a current source, ib , at frequency, f. At what frequency does the current gain fall to unity? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -32
8. 9 Cutoff Frequency f. T = 1/2 p( F + Cd. BEk. T/q. IC) f. T is commonly used to compare the speed of transistors. • Why does f. T increase with increasing IC? • Why does f. T fall at high IC? Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -33
BJT Structure for Minimum Parasitics and High Speed • 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 Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -34
8. 10 Charge Control Model • For the DC condition, IC(t) = QF(t)/ F • In order to sustain an excess hole charge in the transistor, holes must be supplied through IB to susbtain recombination at the above rate. • What if IB is larger than ? Step 1: Solve it for any given IB(t) to find QF(t). Step 2: Can then find IC(t) through IC(t) = QF(t)/ F. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -35
Visualization of QF(t) IB ( t) Q F (t) Q F / F F Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -36
EXAMPLE : Find IC(t) for a Step IB(t) The solution of is n E B C t What is QF Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -37
8. 11 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 -38
8. 11 Model for Large-Signal Circuit Simulation 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 Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -39
8. 12 Chapter Summary • The base-emitter junction is usually forward-biased while the base-collector is reverse-biased. VBE determines the collector current, IC. • GB is the base Gummel number, which represents all the subtleties of BJT design that affect IC. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -40
8. 12 Chapter Summary • The base (input) current, IB , is related to IC 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 IC region due to high-level injection in the base. It also falls off in the low IC region due to excess base current. • Base-width modulation by VCB results in a significant slope of the IC vs. VCE curve in the active region (known as the Early effect). Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -41
8. 12 Chapter Summary • Due to the forward bias VBE , a BJT stores a certain amount of excess carrier charge QF which is proportional to IC. F is the forward transit time. If no excess carriers are stored outside the base, then , the base transit time. • The charge-control model first calculates QF(t) from IB(t) and then calculates IC(t). Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -42
8. 12 Chapter Summary The small-signal models employ parameters such as transconductance, input capacitance, and input resistance. Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 8 -43
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