Bipolar Junction Transistors BJTs B C E

Table of Contents The Bipolar Junction Transistor_______________________________slide 3 BJT Relationships – Equations________________________________slide 4 DC  and DC  _____________________________________________slides 5 BJT Example_______________________________________________slide 6 BJT Transconductance Curve_________________________________slide 7 Modes of Operation_________________________________________slide 8 Three Types of BJT Biasing__________________________________slide 9 Common Base______________________slide 10-11 Common Emitter_____________________slide 12 Common Collector___________________slide 13 Eber-Moll Model__________________________________________slides 14-15 Small Signal BJT Equivalent Circuit__________________________slides 16 The Early Effect___________________________________________slide 17 Early Effect Example_______________________________________slide 18 Breakdown Voltage________________________________________slide 19 Sources__________________________________________________slide 20

The BJT – Bipolar Junction Transistor The Two Types of BJT Transistors: npn pnp n p n p n p E C E C C C Cross Section Cross Section B B B B Schematic Symbol Schematic Symbol E E Collector doping is usually ~ 106 Base doping is slightly higher ~ 107 – 108 Emitter doping is much higher ~ 1015

BJT Relationships - Equations IE IC IE IC - VCE + + VEC - E C E C - - + + VBE VBC IB VEB VCB IB + + - - B B npn IE = IB + IC VCE = -VBC + VBE pnp IE = IB + IC VEC = VEB - VCB Note: The equations seen above are for the transistor, not the circuit.

DC  and DC   = Common-emitter current gain  = Common-base current gain  = IC  = IC IB IE The relationships between the two parameters are:  =   =   + 1 1 -  Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

Using Common-Base NPN Circuit Configuration BJT Example Using Common-Base NPN Circuit Configuration C Given: IB = 50  A , IC = 1 mA Find: IE ,  , and  Solution: IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA = IC / IB = 1 mA / 0.05 mA = 20  = IC / IE = 1 mA / 1.05 mA = 0.95238  could also be calculated using the value of  with the formula from the previous slide.  =  = 20 = 0.95238  + 1 21 IC VCB + _ IB B VBE IE + _ E

BJT Transconductance Curve Typical NPN Transistor 1 Collector Current: IC =  IES eVBE/VT Transconductance: (slope of the curve) gm =  IC /  VBE IES = The reverse saturation current of the B-E Junction. VT = kT/q = 26 mV (@ T=300K)  = the emission coefficient and is usually ~1 IC 8 mA 6 mA 4 mA 2 mA VBE 0.7 V

Modes of Operation Active: Saturation: Cutoff: Most important mode of operation Central to amplifier operation The region where current curves are practically flat Saturation: Barrier potential of the junctions cancel each other out causing a virtual short Cutoff: Current reduced to zero Ideal transistor behaves like an open switch * Note: There is also a mode of operation called inverse active, but it is rarely used.

Operation of The npn Transistor Active Mode Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown.)

Operation of The npn Transistor Active Mode Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; vBE  0 and vCB  0.

Three Types of BJT Biasing Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions. Common-Base Biasing (CB) : input = VEB & IE output = VCB & IC Common-Emitter Biasing (CE): input = VBE & IB output = VCE & IC Common-Collector Biasing (CC): input = VBC & IB output = VEC & IE

Emitter-Current Curves Common-Base Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works. Emitter-Current Curves IC Active Region IE Saturation Region Cutoff IE = 0 VCB

Circuit Diagram: NPN Transistor Common-Base + _ IC IE IB VCB VBE E C B VCE Circuit Diagram: NPN Transistor The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor. Region of Operation IC VCE VBE VCB C-B Bias E-B Bias Active IB =VBE+VCE ~0.7V  0V Rev. Fwd. Saturation Max ~0V -0.7V<VCE<0 Cutoff ~0  0V None/Rev.

Collector-Current Curves Common-Emitter Circuit Diagram Collector-Current Curves VCE IC IC + _ Active Region VCC IB IB Region of Operation Description Active Small base current controls a large collector current Saturation VCE(sat) ~ 0.2V, VCE increases with IC Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0. VCE Saturation Region Cutoff Region IB = 0

Emitter-Current Curves Common-Collector Emitter-Current Curves The Common-Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE. Also, since  ~ 1, and  = IC/IE that means IC~IE IE Active Region IB VCE Saturation Region Cutoff Region IB = 0

Eber-Moll BJT Model IE IC E C RIC RIE IF IR IB B The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model: IE IC E C RIC RIE IF IR IB B

Eber-Moll BJT Model IC = FIF – IR IB = IE - IC IE = IF - RIR R = Common-base current gain (in forward active mode) F = Common-base current gain (in inverse active mode) IES = Reverse-Saturation Current of B-E Junction ICS = Reverse-Saturation Current of B-C Junction IC = FIF – IR IB = IE - IC IE = IF - RIR IF = IES [exp(qVBE/kT) – 1] IR = IC [exp(qVBC/kT) – 1]  If IES & ICS are not given, they can be determined using various BJT parameters.

Small Signal BJT Equivalent Circuit The small-signal model can be used when the BJT is in the active region. The small-signal active-region model for a CB circuit is shown below: iB iC B C r iB r = ( + 1) * VT IE iE E @  = 1 and T = 25C r = ( + 1) * 0.026 IE Recall:  = IC / IB

Circuit whose operation is to be analyzed graphically.

Graphical construction for the determination of the dc base current in the previous circuit

Fig. 4.36 Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE

Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB

Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE.

The Early Effect (Early Voltage) Orange = Actual IC (IC’) Note: Common-Emitter Configuration IB -VA VCE Green = Ideal IC Orange = Actual IC (IC’) IC’ = IC VCE + 1 VA

Early Effect Example Given: The common-emitter circuit below with IB = 25A, VCC = 15V,  = 100 and VA = 80. Find: a) The ideal collector current b) The actual collector current Circuit Diagram VCE IC = 100 = IC/IB a) IC = 100 * IB = 100 * (25x10-6 A) IC = 2.5 mA + _ VCC IB b) IC’ = IC VCE + 1 = 2.5x10-3 15 + 1 = 2.96 mA VA 80 IC’ = 2.96 mA

Breakdown Voltage The maximum voltage that the BJT can withstand. BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts. BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts. Breakdown Voltage is Determined By: The Base Width Material Being Used Doping Levels Biasing Voltage

Sources Web Sites http://www.infoplease.com/ce6/sci/A0861609.html Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New Jersey: 2001. (pp 84-153) 1 Figure 3.7, Transconductance curve for a typical npn transistor, pg 90. Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press, New York: 1998. Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill, Boston: 1997. (pp 351-409) Web Sites http://www.infoplease.com/ce6/sci/A0861609.html