Accounting for the body effect in the compact modeling of an “extrinsic” MOSFET drain current in the linear and saturation regimes V. Turin*a, R. Shkarlata, b, V. Poyarkovb, O. Kshenskyb, G. Zebrevc, B. Iñiguezd, M. Shure Orel State University named after I.S. Turgenev, 95 Komsomolskaya Street, Orel, Russia 302026; b JSC “Bolkhov Plant of Semiconductor Devices”, Bolkhov, Orel region, Russia; cNational Research Nuclear University “MEPHI”, Moscow, Russia; dRovira i Virgili University, Tarragona, Spain; e Rensselaer Polytechnic Institute, Troy, NY, USA a ABSTRACT We use a linear approximation for the threshold voltage dependence on the body bias to derive the equation for the equivalent output resistance of the “extrinsic” MOSFET in the saturation regime. Previously we derived an equation for the equivalent output resistance of the “extrinsic” MOSFET in the saturation regime, that is based on the “intrinsic” transistor finite output resistance in the saturation regime. But we did not account for the body effect, i.e., the threshold voltage dependence on the body bias applied between the source and the fourth (body) MOSFET terminal. For the earlier generations of MOSFETs, the theory predicts that threshold voltage is a sublinear function of the body bias. However, modern transistors with steep retrograde body doping profiles exhibit an approximately linear relationship between a threshold voltage and a body bias, which allowed us to include the body effect into the compact model of an “extrinsic MOSFET. In addition, we discuss the application of our results to the theory of a common-gate amplifier and a commonsource amplifier with an NMOS transistor with source degeneration. Keywords: MOSFET, body effect, contacts parasitic resistances, output resistance, saturation regime, source degeneration, compact modeling INTRODUCTION Our previously proposed smoothing function [1] for an “intrinsic” (without accounting for the contact parasitic resistances) MOSFET ensures a monotonic increase of the output resistance from the minimum value in the linear regime to the maximum finite value in the saturation regime, that is due to short-channel effects. In [2] we derived an equation for the equivalent output resistance of the “extrinsic” (with accounting for the contact parasitic resistances) MOSFET in the saturation regime, based on the “intrinsic” transistor finite output resistance. In [3] we used the “improved” smoothing function for compact modeling of an “extrinsic” MOSFET. So far, we have not accounted for the body effect, i.e., the threshold voltage dependence on the body bias applied between the source and the fourth (body) MOSFET terminal. In this paper, we will take into account the body effect in a linear approximation [4] in the equation for the output resistance of an “extrinsic” MOSFET in the linear regime and the saturation regime. THE BODY EFFECT IN THE LINEAR APPROXIMATION The fact that the threshold voltage ππ‘ is a function of the source-to-body bias πππ΅ (see Figure 1) is called the body effect. In earlier generations of MOSFETs, the body doping density was more or less uniform. In that case, the theory predicts that threshold voltage is a sublinear function of the source-to-body bias [4]-[9]: ππ‘ = ππ‘0 + πΎ(√2π·π΅ + πππ΅ − √2π·π΅ ) (1) with the back-gate transconductance parameter π = πππ‘ ⁄ππππ΅ = πΎ⁄(2√2π·π΅ + πππ΅ ) [9]. Equation (1) can be linearized by Taylor expansion, so that ππ‘ can be approximated as a linear function of πππ΅ in case when one is small enough (πππ΅ βͺ π·π΅ ): ππ‘ ≈ ππ‘0 + πππ‘ ⁄ππππ΅ |πππ΅=0 β πππ΅ = ππ‘0 + πΎ⁄(2√2π·π΅ ) β πππ΅ . *voturin@mail.ru; phone 7 920 825-0040; oreluniver.ru/employee/2481 (2) Figure 1. Schematic cross-section of an n-channel MOSFET with four terminals (including the fourth (body) terminal). Modern transistors employ steep retrograde body doping profiles (lightdoping in a thin surface layer and very heavy doping underneath). The depletion-layer thickness is basically the thickness of the lightly doped region. As πππ΅ increases, the depletion layer does not change significantly. As a result, modern transistors exhibit a more or less linear relationship between the threshold voltage and the source-to-body bias [4]: ππ‘ = ππ‘0 + πΌπ΅ πππ΅ . (3) Here πΌπ΅ is called the body-effect coefficient, which is constant and can be extracted from the slope of the ππ‘ (πππ΅ ) curve [4]. The polarity of the body bias is normally that which would reverse bias the body-source junction. Hence, for NMOS transistors, usually, for the source-to-body bias, we have πππ΅ > 0 and for the body-to-source: ππ΅π = −πππ΅ < 0 . THE EQUIVALENT “EXTRINSIC” TRANSISTOR WITHOUT BODY TERMINAL In the “intrinsic” case, the overdrive voltage ππΊπ is ππΊπ = ππΊπ − ππ‘ = ππΊπ0 − πΌπ΅ πππ΅ with ππΊπ0 = ππΊπ − ππ‘0 . (4) An “extrinsic” MOSFET has parasitic resistors π π in series with its source terminal and a π π· in series with its drain terminal (see Figure 2). The total parasitic resistance is π π = π π + π π· . For “intrinsic” and “extrinsic” gate-to-source and drain-tosource biases we have [10][11]: ππΊπ = πππ − πΌπ π , (5) ππ·π = πππ − πΌπ π . (6) For “intrinsic” and “extrinsic” body-to-source bias we have: ππ΅π = πππ − πΌπ π . (7) We can rewrite this equation in terms of source-to-body bias, accounting for the fact that ππ΅π = −πππ΅ and πππ = −ππ π : πππ΅ = ππ π + πΌπ π . (8) We can substitute (8) into (3) to obtain the equation for the threshold voltage in the “extrinsic” case: ππ‘ = (ππ‘0 + πΌπ΅ ππ π ) + πΌπ΅ πΌπ π . (9) ππΊπ = πππ − (ππ‘0 + πΌπ΅ ππ π ) − πΌ(1 + πΌπ΅ )π π . ( 10 ) After substitution (5) and (9) into (4) we have From (10) we can see that it is possible to introduce the equivalent transistor without the body terminal (see Figure 2) with the equivalent threshold voltage: ππ‘∗ = ππ‘0 + πΌπ΅ ππ π ( 11 ) Figure 2. A four-terminal “intrinsic” MOSFET with parasitic contact resistors and the body (fourth) terminal on the left and a three-terminal equivalent “intrinsic” MOSFET with an equivalent threshold voltage and equivalent parasitic contact resistors on the right. Corresponding “intrinsic” MOSFETs are in the boxes drawn with short-dashed lines and “extrinsic” MOSFETs are in the boxes drawn with long-dashed lines. and with equivalent source resistance π π∗ and drain resistance π π·∗ : π π∗ = (1 + πΌπ΅ )π π , ( 12 ) π π·∗ ( 13 ) = π π· − πΌπ΅ π π . For the “extrinsic” overdrive voltage we have: πππ‘ = πππ − ππ‘ = πππ‘0 − πΌπ΅ πππ΅ = πππ‘0 − πΌπ΅ ππ π − πΌπ΅ πΌπ π with πππ‘0 = πππ − ππ‘0 . ( 14 ) For the equivalent “extrinsic” overdrive voltage we have: ∗ πππ‘ = πππ − ππ‘∗ = πππ‘0 − πΌπ΅ ππ π . ( 15 ) ∗ Note, that πππ‘ = πππ‘ + πΌπ΅ πΌπ π . With the use of equation (12) and equation (15), we can rewrite equation (10) as ∗ ππΊπ = πππ‘ − πΌπ π∗ . ( 16 ) LINEAR REGIME The output resistance of an “intrinsic” MOSFET in the linear regime (see Figure 3(a)) is [10] ∗ πch = 1⁄π½πππ‘ . ( 17 ) Here π½- the transconductance parameter. The output resistance of an “extrinsic” MOSFET in the linear regime (see Figure 4(a)) is [10]: π πβ = πch + π π . ( 18 ) SATURATION CURRENT AND VOLTAGE The saturation current of an “intrinsic” MOSFET (see Figure 3(b)) is [10] πΌππ΄π = π½ππΏ2 πΌ (√1 + ( πΌππΊπ 2 ) − 1) . ππΏ ( 19 ) Here ππΏ is the characteristic voltage for velocity saturation. And πΌ is: πΌ= where ππ΅ = 1 + πΌπ΅ is the bulk-charge factor [4]. 1 1+πΌπ΅ = 1 ππ΅ . ( 20 ) Figure 3. (a) Static circuit model (equivalent circuit) of an “intrinsic” MOSFET operated in its linear (ohmic) regime. (b) A large-signal circuit model for an “intrinsic” MOSFET that is biased to operate in its saturation regime [12]. The transconductance is: ππ = ππΌππ΄π πππΊπ = πΌπ½ππΊπ ⁄√1 + ( πΌππΊπ 2 ππΏ ) . ( 21 ) The “intrinsic” MOSFET saturation voltage (see Figure 3(b)) is [10] πππ΄π = ππΏ (1 + πΌππΊπ ππΏ − √1 + ( πΌππΊπ 2 ) ) . ππΏ ( 22 ) We will use parameter ππ = ππππ΄π πππΊπ = πΌ (1 − ππ π½ππΏ ). ( 23 ) In case of an “extrinsic” MOSFET with πΌπ΅ = 0 in (11) an equation for the “extrinsic” saturation current is derived [11]: 2 π½πππ‘0 πΌπ ππ‘ = 1+π½πππ‘0π π +√1+2π½πππ‘0 π π +( πππ‘0 2 ) ππΏ . ( 24 ) For the equivalent “extrinsic” MOSFET in the saturation point, following (16), we have: ∗ ππΊπ = πππ‘ − πΌπ ππ‘ π π∗ . ( 25 ) By substituting (25) into (19), we can obtain an implicit equation for the equivalent “extrinsic” MOSFET saturation current: ∗ πΌπ ππ‘ = πΌππ΄π (πππ‘ − πΌπ ππ‘ π π∗ ) . ( 26 ) From (26) we can derive an explicit equation for the equivalent “extrinsic” MOSFET saturation current (see Figure 4(b)): πΌπ ππ‘ = ∗2 πΌπ½πππ‘ 2 π∗ππ‘ ) ππΏ . ( 27 ) ∗ π ∗ +√1+2πΌπ½π ∗ π ∗ +πΌ 2 ( 1+πΌπ½πππ‘ ππ‘ π π ∗ Note, that after substitution in this equation πππ‘ (15) and π π∗ (12), for the “extrinsic” MOSFET saturation current we have: πΌπ ππ‘ = πΌπ½ (πππ‘0 −πΌπ΅ ππ π ) 2 πππ‘0 −πΌπ΅ ππ π 1+ π½ (πππ‘0 −πΌπ΅ ππ π )π π +√1+2 π½ (πππ‘0 −πΌπ΅ ππ π ) π π + πΌ 2 ( ) 2 . ( 28 ) ππΏ In case πΌπ΅ = 0 we have πΌ = 1 and this equation is in full agreement with equation (24). For an “extrinsic” saturation voltage (see Figure 4(b)), following (6), and by substituting (25) into (22), we have: ∗ ππ ππ‘ = πππ΄π (πππ‘ − πΌπ ππ‘ π π∗ ) + πΌπ ππ‘ π π . ( 29 ) Figure 4. (a) Static circuit model (equivalent circuit) of an “extrinsic” MOSFET operated in its linear (ohmic) regime. (b) A large-signal circuit model for an “extrinsic” MOSFET that is biased to operate in its saturation regime. OUTPUT RESISTANCE IN THE SATURATION REGIME The “intrinsic” MOSFET output resistance due to short-channel effects in the saturation regime (see Figure 3(b)) is [13] π0 = 1⁄ππΌππ΄π . ( 30 ) Here π = 1⁄ππΈ and ππΈ is the Early voltage. For an “intrinsic” MOSFET in the saturation regime for entry-level compact models (Level 1, for example) a linear approximation for the dependence of the drain current on drain bias is used: πΌπ΄ππ1 = πΌππ΄π + ππ·π ⁄π0 = πΌππ΄π β (1 + πππ·π ). ( 31 ) In more advanced compact models (BSIM3/4, for example) more complex equation is used: πΌπ΄ππ2 = πΌππ΄π + (ππ·π − πππ΄π )⁄π0 = πΌππ΄π β (1 + π(ππ·π − πππ΄π )) . ( 32 ) For the “extrinsic” MOSFET in the saturation regime in [2] we considered a linear approximation for the dependence of the drain current on the “extrinsic” drain bias in two forms: 1. The first form for entry-level compact models (31): πΌππ π¦1 = πΌπ ππ‘ + πππ ⁄π 01. ( 33 ) We obtained the equation for the output resistance for the equivalent transistor: π 01 = π0 + π π + ππ π0 π π∗ with π0 = 1⁄ππΌπ ππ‘ . ( 34 ) 2. The second form for more advanced compact models (32): πΌππ π¦2 = πΌπ ππ‘ + (πππ − ππ ππ‘ )⁄π 02 . ( 35 ) And we obtained the output resistance for the equivalent transistor in this case (see Figure 4(b)): π 02 = π0 + π π + (ππ π0 − ππ )π π∗ with π0 = 1⁄ππΌπ ππ‘ . ( 36 ) Note, that in (34) and (36), in equations for ππ (21) and ππ (23), we use ππΊπ from (25). APPLICATION TO THE SOURCE DEGENERATION THEORY In the theory of a common-source amplifier the circuit that consists of an NMOS transistor with a resistor π S in series with its source terminal is known as a transistor with source degeneration [14][15][16]. The output resistance, in this case, is π 0 = π0 + π π + ππ π0 π π ( 37) Note, that this equation was derived in [15] for the output resistance of the common-gate amplifier and was applied directly to the case of a source-degenerated common-source amplifier. In addition, in [15] it is shown, that the body effect can be taken into account by simply replacing transconductance ππ by the effective transconductance πππ : πππ = (1 + π) ππ ( 38) Figure 5. Output characteristics for an “intrinsic” (a) and “extrinsic” (b) MOSFET with zero body bias (red solid) and without body bias (blue dash). The gate-to-source bias is 3 V for the upper curve and further down 2V and 1V. Figure 6. Output conductance for an “intrinsic” (a) and “extrinsic” (b) MOSFET with zero body bias (red solid) and with 2V body bias (blue dash). The gate-to-source bias is 3 V for the upper curve and further down 2V and 1V. Figure 7. Transconductance for an “intrinsic” (a) and “extrinsic” (b) MOSFET with zero body bias (red solid) and with 2V body bias (blue dash). The drain-to-source bias is 0.75 V for the upper curve and further down 0.5 V and 0.25 V. A. Zero drain resistance Let’s put π π· = 0 in equations (34) and (36): 1. From (34), with π π∗ = (1 + πΌπ΅ )π π , we have: π 01 = π0 + π π + ππ π0 (1 + πΌπ΅ ) π π . ( 39 ) 2. From (36), with ππ from (23) and with πΌ = 1⁄(1 + πΌπ΅ ), we have π 02 = π0 + ππ (π0 (1 + πΌπ΅ ) + 1 π½ππΏ ) π π . ( 40) B. Neglecting the body effect In addition to π π· = 0, let’s put πΌπ΅ = 0 (πΌ = 1) in equations (34) and (36): 1. From (34), we have π 01 = π0 + π π + ππ π0 π π . ( 41) 2. From (36), we have π 02 = π0 + ππ (π0 + 1 π½ππΏ ) π π . ( 42) Note, that the equation (41) for π 01 , obtained for the case when π π· = 0 and πΌπ΅ = 0, is in full agreement with equation (37) from [15] for an output resistance π 0 of a common-gate amplifier with a signal source with a resistance π π and of a common-source amplifier with a source degeneration resistor π π . In addition, equation (39) for π 01 , obtained for the case when π π· = 0 and πΌπ΅ ≠ 0, can be treated as equation (37) with the effective transconductance πππ = (1 + πΌπ΅ ) ππ instead of ππ , that is taking into account the transistor’s body effect [15]. PARAMETERS AND RESULTS Channel length πΏ = 150 nm; gate width π = 750 nm; characteristic electric field for velocity saturation πΈπ ππ‘ = 8 × 104 V/cm; characteristic voltage: ππΏ = πΈπ ππ‘ πΏ = 1.2 V; the body-effect coefficient πΌπ΅ = 0.2; ππ‘0 = 0.4 V; the Early voltage ππΈ = 3.3 V; π π = 2 kο; π π· = 1.33 kο; (parasitic contact resistances are chosen sufficiently large to emphasize one's effect); π π∗ = 2.4 kο; π π·∗ = 0.93 kο; SiO2 thickness πππ₯ = 14 nm and relative permittivity πππ₯ =3.9; πΆππ₯ = πππ₯ π0 ⁄πππ₯ = 2.5 fF/οm2. We suppose the process transconductance parameter π′ = ππ πΆππ₯ = 25 οA/V2. Hence, the transconductance parameter π½ = π′ π ⁄πΏ = 125 οA/V2; characteristic resistance 1⁄π½ππΏ = 6.7 kο. In Figures 5 and 6 output characteristics are presented for an “intrinsic” (a) and “extrinsic” (b) MOSFET with and without body bias. In Figure 5 drain current dependence on drain-to-source bias in piecewise approximation is presented. In Figure 6 corresponding output conductance, obtained by numerical differentiation, is presented (in an “intrinsic” case π = ππΌππ ⁄πππ·π and in an “extrinsic” case π = ππΌππ ⁄ππππ ). In Figure 7 transconductance, obtained by numerical differentiation, is presented (in an “intrinsic” case ππ = ππΌππ ⁄πππΊπ and in an “extrinsic” case ππ = ππΌππ ⁄ππππ ). CONCLUSION In this paper, we generalized the equation for the output resistance of an “extrinsic” MOSFET in the saturation regime with accounting for the body effect in a linear approximation, which is a good approximation for modern transistors employed steep retrograde body doping profiles. We found that it is possible to introduce the equivalent transistor without the body (fourth) terminal with the equivalent threshold voltage and with equivalent source and drain resistances to simplify compact modeling of the four-terminal transistor with body (fourth) terminal. We calculated piecewise output and transfer characteristics for “intrinsic” and “extrinsic” MOSFET with and without body bias based on the derived output resistance of an “extrinsic” MOSFET in the saturation regime with accounting for the body effect in a linear approximation with help of equations for the equivalent three-terminal transistor. In addition, we discussed the application of our results to the theory of a common-gate amplifier and a common-source amplifier with an NMOS transistor with source degeneration. ACKNOWLEDGMENTS The authors acknowledge the JSC “Bolkhov Plant of Semiconductor Devices” for support of Roman Shkarlat in his Ph.D. project carried at the Orel State University named after I.S. Turgenev, which we acknowledge also. REFERENCES [1] Turin, V. O., et al., “Intrinsic compact MOSFET model with correct account of positive differential conductance after saturation,” Proc. SPIE 7521, International Conference on Micro- and Nano-Electronics 2009, 75211H (2010). 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