מכללה האקדמית אורט בראודה המחלקה להנדסת חשמל ואלקטרוניקה מוליכים למחצה 31350

Bipolar transistor was invented by Walter Brattain, John Bardeen and William Shockley (Bell Labs) in It is a 3-terminal device, which is usually used as an amplifier or a switch. 3D sketch of pnp bipolar transistor Examples of typical bipolar transistors

Bipolar transistor is a semiconductor device with two interacting PN- junctions, connected together in series back-to-back. arrow symbol ↔ Emitter arrow direction ↔ current direction Outstanding guide on npn transistor the Base HAVE TO be short enough to allow crosstalk between the Emitter and the Collector

With both junctions forward-biased, a BJT is in the saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. In cutoff, biasing conditions are opposite to those of saturation, namely, both junctions are reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch. In the active mode, the emitter–base junction is forward biased and the base–collector junction is reverse biased. Most of BJTs are designed to afford the greatest common-emitter current gain, β F, in the active mode. If this is the case, the collector–emitter current is approximately proportional to the base current, but many times larger, for small base current variations. By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. The β F in inverted mode is several times (2–3 times for the ordinary Ge BJT) smaller than that of active mode. (Seldomly used mode.)

holes electrons Emitter Base Collector holes E F quasi

Emitter - Base junction is asymmetric: Thus, the Emitter’ hole current is controlled by Emitter - Base junction The width of the neutral region of the Base is small (much less than the holes diffusion length): Therefore most holes diffusing into the Base will reach the Collector

Emitter – Base currents (EB junction is forward-biased) (1)holes diffusing from the Emitter to the Base (2)electrons diffusing from the Base to the Emitter Base currents (3) recombination of holes injected into the Base (4) most holes reach Collector since Base – Collector currents (BC junction is under reverse bias) (5) electron (minority carrier) current from Collector to Base (6) hole (minority carrier) current from Base to Collector can be neglected for most practical purposes magnetta – current of holes rose – current of electrons

magnetta – current of holes rose – current of electrons בטרנזיסטור pnp, חורים ( מטעני רוב ב - emitter) הוזרקים דרך צומת PN emitter-base ( הנמצא בממתח הקדמי ) לאזור ה - base. חלק מהחורים עושים רקומבינציה עם אלקטרונים ( מטעני רוב ב - base), חלק אחר חוזרים בחזרה ( ע " י סחיפה ) ל - emitter. אבל היות וה - base הוא דק מאד ומסומם יחסית מעט, רוב החורים, שהוזרקו מה - emitter, מצליחים להגיע ( ע " י דיפוזיה ) עד לאזור המחסור בצומת collector-base. צומת PN זה (base – collector) נמצא בממתח האחורי. השדה החזק של הצומת לוקח את החורים ( להזכיר, כי חורים הם מטעני מיעוט ב - base ובשבילם הצומת הוא פתוח ) ומעביר אותם ל - collector. לכן זרם ה - collector כמעט שווה לזרם ה - emitter ( מלבד החלק הקטן שנאבד ב - base עקב רקומבינציה ) :. המקדם המקשר בין זרם ה - emitter לזרם ה - collector ( המקדם ) נקרא מקדם המעבר של זרם ה - emitter. בדרך כלל משתנה בין 0.9 ל ככל ש - גדול יותר, כך העברת הזרם ע " י טרנזיסטור היא יעילה יותר. יש לציין, כי כמעט ולא תלוי במתחים ו -. בהמשך נראה, כי המקדם המקשר בין זרם ה - base לזרם ה - collector ( המקדם ) מתבטא דרך המקדם בצורה הבאה :. מעצם העובדה ש - כמעט ולא תלוי במתחים בצמתים, נובע, כי גם בטווח רחב של מתחים נשאר קבוע. לכן, ע " י השינוי בזרם החלש של base,, ניתן לשלוט בזרם החזק של ה - collector,.

= current amplification in common-base circuit Since, typical values for are: By setting and we control and input output CharacteristicCommon Base Input impedanceLow Output impedanceVery High Voltage GainHigh Current GainLow Power GainLow

= current amplification in common-emitter circuit is very sensitive to : By setting and, we control and input output CharacteristicCommon Emitter Input impedanceMedium Output impedanceHigh Voltage GainMedium Current GainMedium Power GainVery high The gain,, is determined by doping

By setting and, we control and = current amplification in common-emitter circuit CharacteristicCommon Emitter Input impedanceHigh Output impedanceLow Voltage GainLow Current GainHigh Power GainMedium input output

B. Streetman, S. Banerjee, “Solid state electronic devices” (6 th edition), Prentice Hall, B. Streetman, S. Banerjee, “Instructor’s solutions manual to the “Solid state electronic devices” ” (6 th edition), Prentice Hall, B. Van Zeghbroeck, “Principles of semiconductor devices”, Lectures – Colorado University, D. Neamen, “Semiconductor Physics and Devices: Basic Principles” (3 rd edition), McGraw Hill, D. Neamen, “Semiconductor Physics and Devices: Basic Principles” (3 rd edition) – Solutions manual, McGraw Hill, 2003.