Lecture Slides on Bipolar Junction Transistors - Microelectronic Devices and Circuits | EL ENG 105, Study notes of Electrical and Electronics Engineering

Download Lecture Slides on Bipolar Junction Transistors - Microelectronic Devices and Circuits | EL ENG 105 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Lecture 21: BJTs (Bipolar Junction Transistors) Prof J. S. Smith Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Context In Monday’s lecture, we discussed minority injection in forward biased PN junctions. Today we will discuss three terminal devices which use this effect for amplification, called: BJTs (Bipolar Junction Transistors) Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Reading Today’s lecture will covering chapter 7, Bipolar Junction Transistors (BJT’s) Next , we will start looking at amplifiers, chapter 8 in the text. Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Lecture Outline Review of minority current injection in PN Diode The BJT (7.1) BJT Physics (7.2) BJT Ebers-Moll Equations (7.3) BJT Small-Signal Model 2 Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith PN diode As you increase the forward bias on a PN junction, the barrier keeping the holes from diffusing into the N side, and keeping the electrons from diffusing to the P side, is reduced As the barrier height decreases, the diffusion of carriers across the barrier increases exponentially. p-type n-type DN AN - - - - - - - - - - - - - + + + + + + + + + + + + + 0E biqφ ,p diffJ ,p driftJ ,n diffJ ,n driftJ − − + + − − Thermal Generation Recombination Carrier with energybelow barrier height Minority Carrier Close to Junction Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Band edge diagram (forward bias) the density of electrons, at this energy, over here The number of Electrons here is determined by The number of holes over here, at this energy determines the number of holes over here Forward bias → reduced barrier height, so more minority carriers on both sides Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Forward bias → Increased population of minority carriers The minority carrier concentrations at the edges of the depletion region will be given by: kTVq Ann DBeNxxp /)()( −−== φ kTVq Dpp DBeNxxn /)()( −−=−= φ Note: NA and ND are the majority carrier concentrations on the other side of the junction, with the assumption that pn << ND and np << NA Also note: This neglects net recombination inside the depletion region Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Quasi-Neutrality Since the regions outside the depletion regions are going to be very close to electrically neutral (called quasi-neutrality) the number of majority carriers will increase slightly as well (slightly because there are usually many more majority carriers than minority carriers anyway) Carrier concentrations distance Nd Minority carriers 5 Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Currents in the BJT A BJT is ordinarily designed so that the minority carrier injection into the base is far larger than the minority carrier injection into the emitter. It is also ordinarily designed such that almost all the minority carriers injected into the base make it all the way across to the collector Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Band edge diagram The band edge diagram for an NPN transistor in operation N (heavily doped) P (Lightly doped) N Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Current controlled So the current is determined by the minority current across the emitter-base junction But since the majority of the minority current goes right through the base to the collector: And so the amount of current that must be supplied by the base is small compared to the current controlled: C EI I≈ − C BI I>> BEqV kT C SI I e≈ Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Actual BJT Cross Section Vertical npn sandwich (pnp is usually a lateral structure) n+ buried layout is a low resistance contact to collector Base width determined by vertical distance between emitter diffusion and base diffusion 6 Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith BJT Layout Emitter area most important layout parameter Multi-finger device also possible for reduced base resistance Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith BJT Schematic Symbol Collector current is control by base current linearly, a typical value would be , because only one in 100 electrons would stop in the base instead of making it across to the collector Collector is controlled by base-emitter voltage exponentially BI EI− BEV + − CEV + − BEqV kT C SI I e≈ C BI Iβ= The arrow on the symbol shows the controlling diode. 100=β Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Simple NPN BJT model A simple model for a NPN BJT: →)(tIB − + )(tVBE )(tiBβ B E C Real diode, not an ideal diode BI EI− BEV + − CEV + − C Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith BJT Collector Characteristic Ground emitter Fix VCE Drive base with fixed current IB Measure the collector current 7 Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith BJT operating modes Forward active – Emitter-Base forward biased – Base-Collector reverse biased Saturation – Both junctions are forward biased Reverse active – Emitter-Base reverse biased – Base-Collector forward biased – Transistor operation is poor in this direction, becauseβ is low: lighter doping of the layer designed to be the collector means that there is a lot of minority carrier injection out of the Base. Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Collector Characteristics (IB) Forward Active Region (Very High Output Resistance) Saturation Region (Low Output Resistance) Reverse Active (poor Transistor) Breakdown Linear Increase Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Base-Emitter Voltage Control Exponential Increase Forward Active Region (High Output Resistance) Reverse Active (Crappy Transistor) Saturation Region (Low Output Resistance) ~0.3V Breakdown Department of EECS University of California, Berkeley EECS 105 Spring 2004, Lecture 21 Prof. J. S. Smith Diffusion Currents Minority carriers in base form a uniform diffusion current. Since emitter doping is higher, this current swamps out the current portion due to the minority carriers injected from base