The SPB16.2 release now has new MOSFET device Model BSIM4 Support in PSpice
PSpice designers had been requesting support for the BSIM4 Mosfet Model. The BSIM4 model addresses the MOSFET physical effects into sub-100nm regime. It is a physics-based, accurate, scalable, robust and predictive MOSFET spice model for circuit simulation and CMOS technology development.
BSIM4 finds wide application in RF circuits since it's a physical device model.
The BSIM4 model supported by PSpice is BSIM4 version 4.1.0. To use the BSIM4 equations you need to use the keyword LEVEL=8 inside the model file.
Here's an example:
.MODEL N1 NMOS
+VERSION = 4.1.0
As specified by University of California, Berkeley, BSIM4 has the following major improvements and enhancements over BSIM3 model:
1. A new accurate channel thermal noise model and a noise partition model for the induced gate noise.
Complete noise model includes flicker noise (also known as 1/f noise), channel thermal noise and induced gate noise and their correlation, thermal noise due to physical resistances such as the source/ drain, gate electrode, and substrate resistances, and shot noise due to the gate dielectric tunneling current.
2. An accurate gate direct tunneling model
In BSIM4, the gate tunneling current components include the tunneling current between gate and substrate (Igb), and the current between gate and channel (Igc), which is partitioned between the source and drain terminals by Igc = Igcs + Igcd. The third component happens between gate and source/drain diffusion regions (Igs and Igd).
The figure below shows the schematic gate tunneling current flows.
3. A better model for pocket-implanted devices in Vth, bulk charge effect model, and Rout
BSIM4 uses Abulk to model the bulk charge effect. Several model parameters are introduced to account for the channel length and width dependences and bias effects.
Abulk is formulated by
4. An asymmetrical and bias-dependent source/drain resistance, either internal or external to the intrinsic MOSFET, at the user’s discretion
BSIM4 models source/drain resistances in two components: bias-independent diffusion resistance (sheet resistance) and bias-dependent LDD resistance. Accurate modeling of the bias-dependent LDD resistances is important for deep submicron CMOS technologies. In BSIM3 models, the LDD source/drain resistance Rds(V) is modeled internally through the I-V equation and symmetry is assumed for the source and drain sides. In addition, BSIM4 allows the source LDD resistance Rs(V) and the drain LDD resistance Rd(V) to be external and asymmetric (i.e. Rs(V) and Rd(V) can be connected between the external and internal source and drain nodes, respectively; furthermore, Rs(V) does not have to be equal to Rd(V)). This feature makes accurate RF CMOS simulation possible.
The following figure shows the schematic of source/drain resistance connection
5. The quantum mechanical charge-layer-thickness model for both IV and CV
As the gate oxide thickness is vigorously scaled down, the finite charge-layer thickness can not be ignored . BSIM4 models this by accepting two of the following three as the model inputs:
the electrical gate oxide thickness TOXE,
the physical gate oxide thickness TOXP,
and their difference DTOX = TOXE - TOXP.
Based on these parameters, the effect of effective gate oxide on IV and CV is modeled.
6. A more accurate mobility model for predictive modeling
A good mobility model is critical to the accuracy of a MOSFET model. Mobility depends on the gate oxide thickness, substrate doping concentration, threshold voltage, gate and substrate voltages. BSIM4 provides three different models of the effective mobility. The mobMod = 0 and 1 models are from BSIM3v3.2.2; the new mobMod = 2, a universal mobility model, is more accurate and suitable for predictive modeling.
7. A gate-induced drain leakage (GIDL) current model, available in BSIM for the first time
The Gate induced drain leakage current and its body bias effect are modeled by
where AGIDL, BGIDL, CGIDL, and EGIDL are model parameters .CGIDL accounts for the body-bias dependence of IGIDL.WeffCJ and Nf are the effective width of the source/drain diffusions and the number of fingers.
8. Different diode IV and CV characteristics for source and drain junctions
Junction Diode IV Model
In BSIM4, there are three junction diode IV models:
1. When the IV model selector dioMod is set to 0 ("resistance-free"), the diode IV is modeled as resistance-free with or without breakdown depending on the parameter values of XJBVS or XJBVD.
2. When dioMod is set to 1 ("breakdown-free"), the diode is modeled exactly the same way as in BSIM3v3.2 with current-limiting feature in the forward-bias region through the limiting current parameters IJTHSFWD or IJTHDFWD; diode breakdown is not modeled for dioMod = 1 and XJBVS, XJBVD, BVS, and BVD parameters all have no effect.
3. When dioMod is set to 2 ("resistance-and-breakdown"), BSIM4 models the diode breakdown with current limiting in both forward and reverse operations.
In general, setting dioMod to 1 produces fast convergence.
Junction Diode CV Model
Source and drain junction capacitances consist of three components: the bottom junction capacitance, sidewall junction capacitance along the isolation edge, and sidewall junction capacitance along the gate edge. An analogous set of equations are used for both sides but each side has a separate set of model parameters
Typical BSIM4 characteristics
IV Characteristics :
a) Ids vs. Vds @ Vgs=1.5V (deep-inversion) Vbs sweep
b) Ids vs. Vds @ Vbs=0.0V; Vgs sweep to test Impact-ionization
CV Characteristics :
Cgd vs. Vgd @ CAPMOD=2 (Vbs=0.0; Vgs=1.0 and 2.0)
So, how many AMS Simulator designers will be taking advantage of these new model capabilities? I'm interested in hearing from you.