Transistores Bjt

Bipolar-Junction (BJT) transistors
References: Barbow (Chapter 7), Hayes & Horowitz (pp 84-141), Rizzoni (Chapters 8 & 9) A bipolar junction transistor is formed by joining three sections of semiconductors with alternatively different dopings. The middle section (base) is narrow and one of the other two regions (emitter) is heavily doped. Two variants of BJT are possible: NPN and PNP.
C n B p n+E B E Circuit Symbols

NPN Transistor
C B E C B p n

C

PNP Transistor
C B E Circuit Symbols B E C

p+ E

We will focus on NPN BJTs. Operation of a PNP transistor is analogous to that of a NPN transistor except that the role of “majority” charge carries reversed. In NPN transistors, electron flow is dominant while PNP transistors rely mostly on the flow of “holes.” Therefore, to zerothorder, NPN and PNP transistors behave similarly except the sign of current and voltages are reversed. i.e., PNP = − NPN ! In practice, NPN transistors are much more popular than PNP transistors because electrons move faster in a semiconductor. As a results, a NPN transistor has a faster response time compared to a PNP transistor. At the first glance, a BJT looks like 2 diodes placed back to back.Indeed this is the case if we apply voltage to only two of the three terminals, letting the third terminal float. This is also the way that we check if a transistor is working: use an ohm-meter to ensure both diodes are in working conditions. (One should also check the resistance between CE terminals and read a vary high resistance as one may have a burn through the base connecting collector andemitter.) The behavior of the BJT is different, however, when voltage sources are attached to both BE and CE terminals. The BE junction acts like a diode. When this junction is forward biased, electrons flow from emitter to the base (and a small current of holes from base to emitter). The base region is narrow and when a voltage is applied between collector and emitter, most of the electrons that wereflowing from emitter to base, cross the narrow base region and are collected at the collector region. So while the BC junction is reversed biased, a large current can flow through that region and BC junction does not act as a diode. The amount of the current that crosses from emitter to collector region depends strongly on the voltage applied to the BE junction, vBE . (It also depends weakly onvoltage applied
ECE65 Lecture Notes (F. Najmabadi), Winter 2006

75

between collector and emitter, vCE .) As such, small changes in vBE or iB controls a much larger collector current iC . Note that the transistor does not generate iC . It acts as a valve controlling the current that can flow through it. The source of current (and power) is the power supply that feeds the CE terminals. A BJT hasthree terminals. Six parameters; iC , iB , iE , vCE , vBE , and vCB ; define the state of the transistor. However, because BJT has three terminals, KVL and KCL should hold for these terminals, i.e., iE = i C + i B vBC = vBE − vCE
iC iB _ + vBE vCB + +

vCE

_ _ iE

Thus, only four of these 6 parameters are independent parameters. The relationship among these four parameters represents the “iv”characteristics of the BJT, usually shown as i B vs vBE and iC vs vCE graphs.

The above graphs show several characteristics of BJT. First, the BE junction acts likes a diode. Secondly, BJT has three main states: cut-off, active-linear, and saturation. A description of these regions are given below. Lastly, The transistor can be damaged if (1) a large positive voltage is applied across the CEjunction (breakdown region), or (2) product of iC vCE exceed power handling of the transistor, or (3) a large reverse voltage is applied between any two terminals. Several “models” available for a BJT. These are typically divided into two general categories: “large-signal” models that apply to the entire range of values of current and voltages, and “small-signal” models that apply to AC signals...