Early effect - wikipedia Early effect Figure 1. Top : NPN base width for low collector -- base reverse bias ; Bottom : narrower NPN base width for large collector -- base reverse bias. Hashed areas are depleted regions. Figure 2. The Early voltage ( V ) as seen in the output - characteristic plot of a BJT. The Early effect, named after its discoverer James M. Early, is the variation in the effective width of the base in a bipolar junction transistor ( BJT ) due to a variation in the applied base - to - collector voltage. A greater reverse bias across the collector -- base junction, for example, increases the collector -- base depletion width, thereby decreasing the width of the charge carrier portion of the base. Contents 1 Explanation 2 Large - signal model 3 Small - signal model 3.1 Current -- voltage characteristics 4 References and notes 5 See also Explanation In Figure 1, the neutral ( i.e. active ) base is green, and the depleted base regions are hashed light green. The neutral emitter and collector regions are dark blue and the depleted regions hashed light blue. Under increased collector -- base reverse bias, the lower panel of Figure 1 shows a widening of the depletion region in the base and the associated narrowing of the neutral base region. The collector depletion region also increases under reverse bias, more than does that of the base, because the collector is less heavily doped. The principle governing these two widths is charge neutrality. The narrowing of the collector does not have a significant effect as the collector is much longer than the base. The emitter -- base junction is unchanged because the emitter -- base voltage is the same. Base - narrowing has two consequences that affect the current : There is a lesser chance for recombination within the `` smaller '' base region. The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases. Both these factors increase the collector or `` output '' current of the transistor with an increase in the collector voltage, but only the second is called Early effect. This increased current is shown in Figure 2. Tangents to the characteristics at large voltages extrapolate backward to intercept the voltage axis at a voltage called the Early voltage, often denoted by the symbol V. Large - signal model In the forward active region the Early effect modifies the collector current ( I C ( displaystyle I_ ( mathrm ( C ) ) ) ) and the forward common - emitter current gain ( β F ( displaystyle \beta _ ( mathrm ( F ) ) ) ), as typically described by the following equations : I C = I S e V B E V T ( 1 + V C E V A ) β F = β F 0 ( 1 + V C E V A ) ( displaystyle ( \begin ( aligned ) I_ ( mathrm ( C ) ) & = I_ ( mathrm ( S ) ) e ^ ( \frac ( V_ ( mathrm ( BE ) ) ) ( V_ ( mathrm ( T ) ) ) ) left ( 1 + ( \frac ( V_ ( mathrm ( CE ) ) ) ( V_ ( mathrm ( A ) ) ) ) \right ) \beta _ ( mathrm ( F ) ) & = \beta _ ( mathrm ( F0 ) ) left ( 1 + ( \frac ( V_ ( mathrm ( CE ) ) ) ( V_ ( mathrm ( A ) ) ) ) \right ) end ( aligned ) ) ) Where V C E ( displaystyle V_ ( mathrm ( CE ) ) ) is the collector -- emitter voltage V T ( displaystyle V_ ( mathrm ( T ) ) ) is the thermal voltage k T / q ( displaystyle mathrm ( kT / q ) ) ; see thermal voltage : role in semiconductor physics V A ( displaystyle V_ ( mathrm ( A ) ) ) is the Early voltage ( typically 15 -- 150 V ; smaller for smaller devices ) β F 0 ( displaystyle \beta _ ( mathrm ( F0 ) ) ) is forward common - emitter current gain at zero bias. Some models base the collector current correction factor on the collector -- base voltage V ( as described in base - width modulation ) instead of the collector -- emitter voltage V. Using V may be more physically plausible, in agreement with the physical origin of the effect, which is a widening of the collector -- base depletion layer that depends on V. Computer models such as those used in SPICE use the collector -- base voltage V. Small - signal model The Early effect can be accounted for in small - signal circuit models ( such as the hybrid - pi model ) as a resistor defined as r O = V A + V C E I C ≈ V A I C ( displaystyle r_ ( O ) = ( \frac ( V_ ( A ) + V_ ( CE ) ) ( I_ ( C ) ) ) approx ( \frac ( V_ ( A ) ) ( I_ ( C ) ) ) ) in parallel with the collector -- emitter junction of the transistor. This resistor can thus account for the finite output resistance of a simple current mirror or an actively loaded common - emitter amplifier. In keeping with the model used in SPICE and as discussed above using V C B ( displaystyle V_ ( CB ) ) the resistance becomes : r O = V A + V C B I C ( displaystyle r_ ( O ) = ( \frac ( V_ ( A ) + V_ ( CB ) ) ( I_ ( C ) ) ) ) which almost agrees with the textbook result. In either formulation, r O ( displaystyle r_ ( O ) ) varies with DC reverse bias V C B ( displaystyle V_ ( CB ) ), as is observed in practice. In the MOSFET the output resistance is given in Shichman -- Hodges model ( accurate for very old technology ) as : r O = 1 + λ V D S λ I D = 1 I D ( 1 λ + V D S ) ( displaystyle r_ ( O ) = ( \frac ( 1 + lambda V_ ( DS ) ) ( lambda I_ ( D ) ) ) = ( \frac ( 1 ) ( I_ ( D ) ) ) left ( ( \frac ( 1 ) ( lambda ) ) + V_ ( DS ) \right ) ) where V D S ( displaystyle V_ ( DS ) ) = drain - to - source voltage, I D ( displaystyle I_ ( D ) ) = drain current and λ ( displaystyle lambda ) = channel - length modulation parameter, usually taken as inversely proportional to channel length L. Because of the resemblance to the bipolar result, the terminology `` Early effect '' often is applied to the MOSFET as well. Current -- voltage characteristics The expressions are derived for a PNP transistor. For an NPN transistor, n has to be replaced by p, and p has to be replaced by n in all expressions below. The following assumptions are involved when deriving ideal current - voltage characteristics of the BJT Low level injection Uniform doping in each region with abrupt junctions One - dimensional current Negligible recombination - generation in space charge regions Negligible electric fields outside of space charge regions. It is important to characterize the minority diffusion currents induced by injection of carriers. With regard to pn - junction diode, a key relation is the diffusion equation. d 2 Δ p B ( x ) d x 2 = Δ p B ( x ) L B 2 ( displaystyle ( \frac ( d ^ ( 2 ) Delta p_ ( \text ( B ) ) ( x ) ) ( dx ^ ( 2 ) ) ) = ( \frac ( Delta p_ ( \text ( B ) ) ( x ) ) ( L_ ( \text ( B ) ) ^ ( 2 ) ) ) ) A solution of this equation is below, and two boundary conditions are used to solve and find C 1 ( displaystyle C_ ( 1 ) ) and C 2 ( displaystyle C_ ( 2 ) ). Δ p B ( x ) = C 1 e x L B + C 2 e − x L B ( displaystyle Delta p_ ( \text ( B ) ) ( x ) = C_ ( 1 ) e ^ ( \frac ( x ) ( L_ ( \text ( B ) ) ) ) + C_ ( 2 ) e ^ ( - ( \frac ( x ) ( L_ ( \text ( B ) ) ) ) ) ) The following equations apply to the emitter and collector region, respectively, and the origins 0 ( displaystyle 0 ), 0 ′ ( displaystyle 0 ' ), and 0 '' ( displaystyle 0 ' ' ) apply to the base, collector, and emitter. Δ n B ( x '' ) = A 1 e x '' L B + A 2 e − x '' L B Δ n c ( x ′ ) = B 1 e x ′ L B + B 2 e − x ′ L B ( displaystyle ( \begin ( aligned ) Delta n_ ( \text ( B ) ) ( x ' ' ) & = A_ ( 1 ) e ^ ( \frac ( x ' ' ) ( L_ ( \text ( B ) ) ) ) + A_ ( 2 ) e ^ ( - ( \frac ( x ' ' ) ( L_ ( \text ( B ) ) ) ) ) Delta n_ ( \text ( c ) ) ( x ' ) & = B_ ( 1 ) e ^ ( \frac ( x ' ) ( L_ ( \text ( B ) ) ) ) + B_ ( 2 ) e ^ ( - ( \frac ( x ' ) ( L_ ( \text ( B ) ) ) ) ) end ( aligned ) ) ) A boundary condition of the emitter is below : Δ n E ( 0 '' ) = n E O ( e 1 k T q V EB − 1 ) ( displaystyle Delta n_ ( \text ( E ) ) ( 0 ' ' ) = n_ ( ( \text ( E ) ) O ) left ( e ^ ( ( \frac ( 1 ) ( kT ) ) qV_ ( \text ( EB ) ) ) - 1 \right ) ) The values of the constants A 1 ( displaystyle A_ ( 1 ) ) and B 1 ( displaystyle B_ ( 1 ) ) are zero due to the following conditions of the emitter and collector regions as x '' → 0 ( displaystyle x ' ' \rightarrow 0 ) and x ′ → 0 ( displaystyle x ' \rightarrow 0 ). Δ n E ( x '' ) → 0 Δ n c ( x ′ ) → 0 ( displaystyle ( \begin ( aligned ) Delta n_ ( \text ( E ) ) ( x ' ' ) & \rightarrow 0 Delta n_ ( \text ( c ) ) ( x ' ) & \rightarrow 0 end ( aligned ) ) ) Because A 1 = B 1 = 0 ( displaystyle A_ ( 1 ) = B_ ( 1 ) = 0 ), the values of Δ n E ( 0 '' ) ( displaystyle Delta n_ ( \text ( E ) ) ( 0 ' ' ) ) and Δ n c ( 0 ′ ) ( displaystyle Delta n_ ( \text ( c ) ) ( 0 ' ) ) are A 2 ( displaystyle A_ ( 2 ) ) and B 2 ( displaystyle B_ ( 2 ) ), respectively. Δ n E ( x '' ) = n E 0 ( e q V EB k T − 1 ) e − x '' L E Δ n C ( x ′ ) = n C 0 ( e q V CB k T − 1 ) e − x ′ L C ( displaystyle ( \begin ( aligned ) Delta n_ ( \text ( E ) ) ( x ' ' ) & = n_ ( ( \text ( E ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( EB ) ) ) ( kT ) ) - 1 \right ) e ^ ( - ( \frac ( x ' ' ) ( L_ ( \text ( E ) ) ) ) ) Delta n_ ( \text ( C ) ) ( x ' ) & = n_ ( ( \text ( C ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( CB ) ) ) ( kT ) ) - 1 \right ) e ^ ( - ( \frac ( x ' ) ( L_ ( \text ( C ) ) ) ) ) end ( aligned ) ) ) Expressions of I E n ( displaystyle I_ ( ( \text ( E ) ) n ) ) and I C n ( displaystyle I_ ( ( \text ( C ) ) n ) ) can be evaluated. I E n = − q A D E d Δ E ( x '' ) d x x '' = 0 '' I C n = − q A D C L C n C 0 ( e q V CB k T − 1 ) ( displaystyle ( \begin ( aligned ) I_ ( ( \text ( E ) ) n ) & = left. - qAD_ ( \text ( E ) ) ( \frac ( d Delta _ ( \text ( E ) ) ( x ' ' ) ) ( dx ) ) \right _ ( x ' ' = 0 ' ' ) I_ ( ( \text ( C ) ) n ) & = - qA ( \frac ( D_ ( \text ( C ) ) ) ( L_ ( \text ( C ) ) ) ) n_ ( ( \text ( C ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( CB ) ) ) ( kT ) ) - 1 \right ) end ( aligned ) ) ) Because insignificant recombination occurs, the second derivative of Δ p B ( x ) ( displaystyle Delta p_ ( \text ( B ) ) ( x ) ) is zero. There is therefore a linear relationship between excess hole density and x ( displaystyle x ). Δ p B ( x ) = D 1 x + D 2 ( displaystyle Delta p_ ( \text ( B ) ) ( x ) = D_ ( 1 ) x + D_ ( 2 ) ) The following are boundary conditions of Δ p B ( displaystyle Delta p_ ( \text ( B ) ) ). Δ p B ( 0 ) = D 2 Δ p B ( W ) = D 1 W + Δ p B ( 0 ) ( displaystyle ( \begin ( aligned ) Delta p_ ( \text ( B ) ) ( 0 ) & = D_ ( 2 ) Delta p_ ( \text ( B ) ) ( W ) & = D_ ( 1 ) W+ Delta p_ ( \text ( B ) ) ( 0 ) end ( aligned ) ) ) with W the base width. Substitute into the above linear relation. Δ p B ( x ) = − 1 W ( Δ p B ( 0 ) − Δ p B ( W ) ) x + Δ p B ( 0 ) ( displaystyle Delta p_ ( \text ( B ) ) ( x ) = - ( \frac ( 1 ) ( W ) ) left ( Delta p_ ( \text ( B ) ) ( 0 ) - Delta p_ ( \text ( B ) ) ( W ) \right ) x+ Delta p_ ( \text ( B ) ) ( 0 ) ) With this result, derive value of I E p ( displaystyle I_ ( ( \text ( E ) ) p ) ). I E p ( 0 ) = − q A D B d Δ p B d x x = 0 I E p ( 0 ) = q A D B W ( Δ p B ( 0 ) − Δ p B ( W ) ) ( displaystyle ( \begin ( aligned ) I_ ( ( \text ( E ) ) p ) ( 0 ) & = left. - qAD_ ( \text ( B ) ) ( \frac ( d Delta p_ ( \text ( B ) ) ) ( dx ) ) \right _ ( x = 0 ) I_ ( ( \text ( E ) ) p ) ( 0 ) & = ( \frac ( qAD_ ( \text ( B ) ) ) ( W ) ) left ( Delta p_ ( \text ( B ) ) ( 0 ) - Delta p_ ( \text ( B ) ) ( W ) \right ) end ( aligned ) ) ) Use the expressions of I E p ( displaystyle I_ ( ( \text ( E ) ) p ) ), I E n ( displaystyle I_ ( ( \text ( E ) ) n ) ), Δ p B ( 0 ) ( displaystyle Delta p_ ( \text ( B ) ) ( 0 ) ), and Δ p B ( W ) ( displaystyle Delta p_ ( \text ( B ) ) ( W ) ) to develop an expression of the emitter current. Δ p B ( W ) = p B 0 e q V CB k T Δ p B ( 0 ) = p B 0 e q V EB k T I E = q A ( ( D E n E 0 L E + D B p B 0 W ) ( e q V EB k T − 1 ) − D B W p B 0 ( e q V CB k T − 1 ) ) ( displaystyle ( \begin ( aligned ) Delta p_ ( \text ( B ) ) ( W ) & = p_ ( ( \text ( B ) ) 0 ) e ^ ( \frac ( qV_ ( \text ( CB ) ) ) ( kT ) ) Delta p_ ( \text ( B ) ) ( 0 ) & = p_ ( ( \text ( B ) ) 0 ) e ^ ( \frac ( qV_ ( \text ( EB ) ) ) ( kT ) ) I_ ( \text ( E ) ) & = qA left ( left ( ( \frac ( D_ ( \text ( E ) ) n_ ( ( \text ( E ) ) 0 ) ) ( L_ ( \text ( E ) ) ) ) + ( \frac ( D_ ( \text ( B ) ) p_ ( ( \text ( B ) ) 0 ) ) ( W ) ) \right ) left ( e ^ ( \frac ( qV_ ( \text ( EB ) ) ) ( kT ) ) - 1 \right ) - ( \frac ( D_ ( \text ( B ) ) ) ( W ) ) p_ ( ( \text ( B ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( CB ) ) ) ( kT ) ) - 1 \right ) \right ) end ( aligned ) ) ) Similarly, an expression of the collector current is derived. I C p ( W ) = I E p ( 0 ) I C = I C p ( W ) + I C n ( 0 ′ ) I C = q A ( D B W p B 0 ( e q V EB k T − 1 ) − ( D C n C 0 L C + D B p B 0 W ) ( e q V CB k T − 1 ) ) ( displaystyle ( \begin ( aligned ) I_ ( ( \text ( C ) ) p ) ( W ) & = I_ ( ( \text ( E ) ) p ) ( 0 ) I_ ( \text ( C ) ) & = I_ ( ( \text ( C ) ) p ) ( W ) + I_ ( ( \text ( C ) ) n ) ( 0 ' ) I_ ( \text ( C ) ) & = qA left ( ( \frac ( D_ ( \text ( B ) ) ) ( W ) ) p_ ( ( \text ( B ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( EB ) ) ) ( kT ) ) - 1 \right ) - left ( ( \frac ( D_ ( \text ( C ) ) n_ ( ( \text ( C ) ) 0 ) ) ( L_ ( \text ( C ) ) ) ) + ( \frac ( D_ ( \text ( B ) ) p_ ( ( \text ( B ) ) 0 ) ) ( W ) ) \right ) left ( e ^ ( \frac ( qV_ ( \text ( CB ) ) ) ( kT ) ) - 1 \right ) \right ) end ( aligned ) ) ) An expression of the base current is found with the previous results. I B = I E − I C I B = q A ( D E L E n E 0 ( e q V EB k T − 1 ) + D C L C n C 0 ( e q V CB k T − 1 ) ) ( displaystyle ( \begin ( aligned ) I_ ( \text ( B ) ) & = I_ ( \text ( E ) ) - I_ ( \text ( C ) ) I_ ( \text ( B ) ) & = qA left ( ( \frac ( D_ ( \text ( E ) ) ) ( L_ ( \text ( E ) ) ) ) n_ ( ( \text ( E ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( EB ) ) ) ( kT ) ) - 1 \right ) + ( \frac ( D_ ( \text ( C ) ) ) ( L_ ( \text ( C ) ) ) ) n_ ( ( \text ( C ) ) 0 ) left ( e ^ ( \frac ( qV_ ( \text ( CB ) ) ) ( kT ) ) - 1 \right ) \right ) end ( aligned ) ) ) References and notes Jump up ^ R.C. Jaeger and T.N. Blalock ( 2004 ). Microelectronic Circuit Design. McGraw - Hill Professional. p. 317. ISBN 0 - 07 - 250503 - 6. Jump up ^ Massimo Alioto and Gaetano Palumbo ( 2005 ). Model and Design of Bipolar and Mos Current - Mode Logic : CML, ECL and SCL Digital Circuits. Springer. ISBN 1 - 4020 - 2878 - 4. Jump up ^ Paolo Antognetti and Giuseppe Massobrio ( 1993 ). Semiconductor Device Modeling with Spice. McGraw - Hill Professional. ISBN 0 - 07 - 134955 - 3. Jump up ^ Orcad PSpice Reference Manual named PSpcRef. pdf, p. 209. ( archived from this URL ) This manual is included with the free version of Orcad PSpice. Jump up ^ R.C. Jaeger and T.N. Blalock ( 2004 ). Microelectronic Circuit Design ( Second ed. ). McGraw - Hill Professional. pp. Eq. 13.31, p. 891. ISBN 0 - 07 - 232099 - 0. Jump up ^ The Shichman - Hodges Enhancement MOSFET Model and SwitcherCAD III SPICE, Report NDT14 - 08 - 2007, NanoDotTek, 12 August 2007 Jump up ^ RS Muller, Kamins TI & Chan M ( 2003 ). Device electronics for integrated circuits ( Third ed. ). New York : Wiley. p. 280 ff. ISBN 0 - 471 - 59398 - 2. See also Small - signal model Retrieved from `` https://en.wikipedia.org/w/index.php?title=Early_effect&oldid=846527257 '' Categories : Transistor modeling Hidden categories : Use dmy dates from July 2013 All articles with unsourced statements Articles with unsourced statements from November 2007 Talk Contents About Wikipedia Deutsch Español فارسی Français Italiano Polski Română Русский Українська 中文 3 more Edit links This page was last edited on 19 June 2018, at 09 : 16 ( UTC ). Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: the early effect in a bjt is caused by
Answer: a variation in the applied base-to-collector voltage