Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
[email protected] # 10 BJT breakdown • Possible mechanisms Avalanche breakdown of the collector-base junction Base pinch-through SDM 2, ©Michael Shur 1999-2009 1 [email protected] # 10 Breakdown for Common Base For a common-base configuration, the avalanche breakdown voltage, BVcb, can be found by equating the maximum electric field at the collector-base interface to the breakdown field, Fbr. Using the depletion approximation, we obtain: 2 ε s Fbr BVcb ≈ 2q SDM 2, ©Michael Shur 1999-2009 2 ⎛ 1 ⎞ F ε 1 s br + ⎜ ⎟≈ ⎝ N ab N dc ⎠ 2qN dc 2 [email protected] # 10 Breakdown Field • • • • 300 kV/cm for Si 400 kV/cm for GaAs > 2,500 kV/cm or more for SiC > 2,500 kV/cm or more for GaN SDM 2, ©Michael Shur 1999-2009 3 [email protected] # 10 More accurate approach then using a breakdown field Ic = Mcb (Icbo + αIe ) where Mcb = 1 1 − (Vcb / BVcb )mb SDM 2, ©Michael Shur 1999-2009 4 [email protected] # 10 Common Emitter Configuration Ic = Mcb (Icbo + αIe ) Substituting Ic = Ie, we find Mcb Ic = Icbo 1 − αMcb Hence, the breakdown occurs when αM cb = 1 BVce = BVcb (1 − α) 1/ mb SDM 2, ©Michael Shur 1999-2009 5 [email protected] # 10 Example Find the ratio BVce/BVcb for a transistor with α = 0.99 and mb = 3. Solution: From BVce = BVcb (1 − α) 1/ mb BVce = 0.215 BVcb. SDM 2, ©Michael Shur 1999-2009 6 [email protected] # 10 Multiplication rate for CE configuration Since We define Mce as I c = M ce I ceo Icbo Iceo = 1− α 1− α Mce = Mcb 1 − αMcb BVce = BVcb (1 − α )1/ mb SDM 2, ©Michael Shur 1999-2009 7 [email protected] # 10 Breakdown for Common Emitter Configuration Example Find the ratio BV ce /BV cb for a transistor with α - 0.99 and m b = 3. Solution BV ce - 0.215 BV cb. SDM 2, ©Michael Shur 1999-2009 8 [email protected] # 10 Multiplication coefficient 6 V bc + Mce 5 + V ce – 4 3 2 – Mcb 1 0 0 10 SDM 2, ©Michael Shur 1999-2009 20 30 Voltage (V) 40 50 9 [email protected] # 10 Leakage Currents 250 open base 200 150 open emitter 100 50 0 0 10 SDM 2, ©Michael Shur 1999-2009 20 30 Voltage (V) 40 50 10 [email protected] # 10 BJT Operation Look at the depletion regions From http://www.research.ibm.com/journal/rd/501/zutic7.gif SDM 2, ©Michael Shur 1999-2009 11 [email protected] # 10 Punch-through breakdown Merging depletion regions Emitter Base SDM 2, ©Michael Shur 1999-2009 Collector 12 [email protected] # 10 Punch-through equations V pth ≈ 2⎛ qN ab W 2ε s 2 2 ⎞ qN abW N ab ⎜1 + ⎟≈ Ndc ⎠ 2ε s N dc ⎝ BVcb ⎛ ε sFbr ⎞ ≈⎜ ⎟ Vp th ⎝ qnG ⎠ For Si SDM 2, ©Michael Shur 1999-2009 2 ⎛ BVcb ⎜ 2×1012 ≈ Vp th ⎜ nG cm −2 ⎝ ( ⎞ ⎟ ⎟ ⎠ 2 ) 13 [email protected] Example # 10 For silicon, the breakdown field, Fbr - 3x10 7 V/m, the dielectric permittivity, εs - 1.05x10 -10 F/m. At what base thickness will a silicon BJT with the base doping level Nab = 1017 cm-3 have equal breakdown voltages BV cb and Vpth ? Solution: The ratio of the avalanche breakdown voltage for the common-base configuration, BV cb (see eq. (5-3-1)), and the punch-through voltage, Vpth , is given by BVcb ⎛ ε s Fbr ⎞ ≈⎜ ⎟ Vpth ⎝ qnG ⎠ 2 where n G = NabW is called the Gummel number . For the silicon BJT we find 2 BVcb ⎛ 2 x 1012 ⎞ ≈⎜ ⎟ Vpth ⎜ n (cm − 2) ⎟ ⎝ G ⎠ level Nab = 1017 cm -3, For the base doping Vpth is equal to BV cb for the base width W = 0.2 µm. For thinner bases, punch through breakdown occurs at voltages smaller than BV cb. For thicker bases, avalanche breakdown occurs at voltages smaller than Vpth . SDM 2, ©Michael Shur 1999-2009 14 [email protected] # 10 Example (temperature coefficient) Assuming that the collector current is kept constant, estimate the temperature coefficient of the emitterbase voltage, dVbe/dT, for a typical silicon transistor. SDM 2, ©Michael Shur 1999-2009 15 [email protected] # 10 Constants and parameters (SI units) (*Constants*) q = 1.602 10^-19; kB = 1.38 10^-23; me = 9.109 10^-31; hbar=1.055 10^-34; (*Silicon material parameters*) mneff = 1.28; (*density of states mass at T = 300 K*) mpeff = 0.81; (*density of states mass at T = 300 K*) EG =1.12; mob = 0.08; (*Device parameters*) W = 1. 10^-7; Nab = 1. 10^23; A = 1. 10^-6; (*area in m^2*) Ic = 1.; T = 300.; SDM 2, ©Michael Shur 1999-2009 16 [email protected] # 10 Solution 2 i n V be 1 jc = qDn exp N ab Vth W EG n = N c N v exp− Vth 2 i SDM 2, ©Michael Shur 1999-2009 17 [email protected] # 10 Solution (continued) Nc = 2 (q mn Vth/(2 Pi hbar^2))^1.5; Nv = 2 (q mp Vth/(2 Pi hbar^2))^1.5; ni2 = Nc Nv Exp[-EG/Vth]; VBE= Vth Log[Ic Nab W/(q ni2 A Vth mob)]; SDM 2, ©Michael Shur 1999-2009 18 [email protected] # 10 dVbe/dT (mV/degree K) versus Ic (A) -1.60 -1.65 -1.70 -1.75 -1.80 -1.85 2 SDM 2, ©Michael Shur 1999-2009 4 6 8 10 19 [email protected] # 10 Safe Operating Area Saturation Maximum current limitation Safe Operating Area Maximum power limitation Break down voltage limitation Log(V ce) SDM 2, ©Michael Shur 1999-2009 20 [email protected] SiC Diode Breakdown # 10 From P. Rabkin, R. Cottle, P. A. Blakey, and M. S. Shur, 2D Simulation of DC, AC, and Breakdown Characteristics of Bipolar and Unipolar Silicon Carbide Devices, in Proceedings of International Semiconductor Device Research Symposium, Charlottesville, VA, Dec. 1-3, pp. 569-572 (1993) SDM 2, ©Michael Shur 1999-2009 21 [email protected] # 10 Cross-section of SiC thyristor Anode Anode Gate p+ SiC p+ SiC n SiC p- SiC n+ SiC n+ SiC substrate Cathode From M. S. Shur, SiC Transistors, in "SiC Materials and Devices", ed. Y. S. Park, Academic Press, Semiconductors and Semimetals, Vol. 52, pp. 161-193 (1998) SDM 2, ©Michael Shur 1999-2009 22 [email protected] # 10 Forward conduction and blocking I-V characteristics of a 30A, 1kV 4H-SiC epitaxial emitter BJT with an active area of 3.4x3.4mm2 35 IB = 1 A IC (A) 30 25°C 800 mA 25 600 mA 20 400 mA 15 10 BVCEO = 1000 V @ 50 μA 200 mA 5 0 mA 0 0 2 4 6 8 10 12 VCE (Volts) From S. Krishnaswami, A.K. Agarwal, S.-H. Ryu, J. Richmond, C. Capell, J.W. Palmour, S. Balachandran, T.P. Chow, B. Geil, S. Bayne, C. Scozzie, and K.A. Jones, “1000V, 30A SiC Bipolar Junction Transistors and Integrated Darlington Pairs,” Paper ThP2-28, European Conference on Silicon Carbide and Related Materials, Bologna, Italy, August 31-September 4, 2004 SDM 2, ©Michael Shur 1999-2009 23 [email protected] Collector-base voltage Summary breakdown Increase in collector current for voltages higher than BVcb Multiplication factor due to avalanche breakdown in the collector-base junction Collector current under avalanche multiplication (open base) Collector-emitter breakdown voltage Base punch-through voltage BV cb ≈ 2 εs F br ⎛ 2q 1 ⎜N ⎝ # 10 2 εs F br 1 ⎞ ≈ N ⎟⎠ 2qN dc dc + ab Ic = Mcb (Icbo + αIe ) Mcb = 1 1 − (Vcb / BVcb )mb 1− α Ic = Mce Iceo where Mce = Mcb 1 − αMcb 1/ m b BV ce = BV cb (1 − α ) 2 2 2 qN ab W ⎛ N ab ⎞ qN abW V pth ≈ ⎜1 + ⎟≈ 2ε s ⎝ Ndc ⎠ 2 ε s N dc SDM 2, ©Michael Shur 1999-2009 24