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(Obsolete) UCSD HBT Model: HBT_UCSD



This element is OBSOLETE and is replaced by the UCSD HBT Model (HBT) element with the EXT_FLAG parameter set to "Disabled". The UCSD HBT model, like the Berkeley BSIM3 model, is designed as an "industry standard" SPICE model. Its basic concepts are the same as the Gummel-Poon BJT model; capacitances are a combination of diffusion and depletion components, for example, but the expressions are more complex. This model includes breakdown and self-heating.

Documentation for this model is available in print[1].

Equivalent Circuit


Name Description Unit Type Default
ID Element ID   UH#1
SELFT Flag; 0=no self-heating (T=TOP); 1=self-heating is included   0
BKDN Flag; 0=no breakdown; 1=breakdown is included   0
TBP Base-plate temperature Temperature 26.85 C
TNOM Temperature at which model parameters were extracted Temperature 26.85 C
TOP Best-guess operating temperature Temperature 26.85 C
IS Saturation current Current 10-22
NF Forward ideality factor   1.0
NR Reverse ideality factor   1.0
ISA Collector EB barrier limiting current Current 1013
NA Collector EB barrier ideality factor   2.0
ISB Collector BC barrier limiting current Current 1013
NB Collector BC barrier ideality factor   2.0
VAF Forward early voltage Voltage 1000.0
VAR Reverse early voltage Voltage 1000.0
IK Forward current knee Current 1013
BF Forward ideal current gain   104
BR Reverse ideal current gain   104
ISE Saturation value for nonideal forward base current Current 10-27
NE Ideality factor for nonideal forward base current   2.0
ISEX Saturation current for emitter leakage diode Current 10-27
NEX Ideality factor for emitter leakage diode   2.0
ISC Saturation value for intrinsic BC current Current 10-27
NC Ideality factor for intrinsic BC current   2.0
ISCX Saturation current for extrinsic BC junction current Current 10-27
NCX Ideality factor for extrinsic BC junction current   2.0
FA Factor to specify avalanche voltage   0.9
BVC C-B breakdown voltage (BVcb0); positive Voltage 1000.0
NBC Exponent for BC multiplication factor vs. voltage   8.0
ICS Saturation value for collector-substrate current Current 10-27
NCS Ideality factor for collector-substrate current   2.0
RE Emitter resistance Resistance 0.001
REX Extrinsic emitter leakage diode series resistance Resistance 0.001
RBX Extrinsic base resistance Resistance 0.001
RBI Intrinsic base resistance Resistance 0.001
RCX Extrinsic collector resistance Resistance 0.001
RCI Intrinsic collector resistance Resistance 0.001
CJE BE depletion capacitance at zero voltage Capacitance 2e-8
VJE BE built-in potential Voltage 1.6
MJE BE capacitance exponent (0.1<MJE<0.9)   0.33
CEMIN Min value of intrinsic BC capacitance Capacitance 1e-8
FCE Factor for BE capacitance approximation near Vbi   0.8
CJC BC depletion capacitance at zero voltage Capacitance 2e-8
VJC BC built-in potential Voltage 1.4
MJC BC capacitance exponent (0.1<MJC<0.9)   0.33
CCMIN Min value of intrinsic BC capacitance Capacitance 1e-8
FC Factor for BC capacitance approximation near Vbi   0.8
CJCX Extrinsic BC capacitance at zero voltage Capacitance 2e-8
VJCX Extrinsic BC capacitance Vbi Voltage 1.4
MJCX Extrinsic BC capacitance exponent (0.1<MJCX<0.9)   0.33
CXMIN Minimum extrinsic BC capacitance Capacitance 1e-8
XCJC Factor for partitioning BC capacitance   1.0
CJS Substrate depletion capacitance at zero voltage Capacitance 0.0
VJS Built-in potential for substrate capacitance Voltage 1.4
MJS Substrate capacitance exponent (0.1<MJS<0.9)   0.5
TFB Base transit time Time 0.0
TBEXS Excess BE transit time Time 0.0
TBCXS Excess BC transit time Time 0.0
TFC0 Collector forward transit time Time 0.0
ICRIT0 Critical current for junction capacitance Current 1000000
ITC Characteristic current for TFC Current 0.0
ITC2 Characteristic current for TFC Current 0.0
VTC Characteristic voltage for TFC Current 1000.0
TKRK Forward transit time for Kirk effect Time 0.0
VKRK Characteristic voltage for Kirk effect Voltage 1000.0
IKRK Characteristic current for Kirk effect Current 1000000
TR Reverse storage time for intrinsic BC diode Time 0.0
TRX Reverse storage time for extrinsic BC diode Time 0.0
FEX Excess phase factor   0.0
RTH Thermal resistance Resistance 10-8
CTH Thermal capacitance Capacitance 1e6
KFN BE flicker noise constant   0.0
AFN BE flicker noise exponent for current   1.0
BFN BE flicker noise exponent for frequency   1.0
XTI Exponent for IS temperature dependence   2.0
XTB Exponent for beta temperature dependence   2.0
TNE Coefficient for NE temperature dependence   0.0
TNC Coefficient for NC temperature dependence   0.0
TNEX Coefficient for NEX temperature dependence   0.0
EG Activation energy for IS temperature dependence   1.5
EAA Activation energy for ISA temperature dependence   0.0
EAB Activation energy for ISB temperature dependence   0.0
EAE Activation energy for ISE temperature dependence   0.0
EAC Activation energy for ISC temperature dependence   0.0
EAX Activation energy for ISEX temperature dependence   0.0
XRE Coefficient for RE temperature dependence   0.0
XREX Coefficient for REX temperature dependence   0.0
XRB Coefficient for RB temperature dependence   0.0
XRC Coefficient for RC temperature dependence   0.0
TVJE Coefficient for VJE temperature dependence   0.0
TVJCX Coefficient for VJCX temperature dependence   0.0
TVJC Coefficient for VJC temperature dependence   0.0
TVJS Coefficient for VJS temperature dependence   0.0
XTITC Coefficient for ITC temperature dependence   0.0
XTITC2 Coefficient for ITC2 temperature dependence   0.0
XTTF Coefficient for TF temperature dependence   0.0
XTTKRK Coefficient for TKRK temperature dependence   0.0
XTVKRK Coefficient for VKRK temperature dependence   0.0
XTIKRK Coefficient for IKRK temperature dependence   0.0
XRT Coefficient for RTH temperature dependence   0.0
DTMAX Maximum temperature rise above heatsink in Celsius degrees Temperature 1000.0 C
AFAC Area scale factor   1.0

Parameter Details

DTMAX. A temperature limit used to avert numerical difficulties in the self-heating model. The Cadence® AWR® Microwave Office® implementation also includes a minimum temperature fixed at 100 K (-173 C). You cannot change the minimum temperature.

TOP and TBP. The estimated device temperature is not listed in the tables in [1] because, in SPICE, the global temperature parameter is used for this quantity. In AWR Microwave Office software, however, TOP is included in the model's parameter list, and therefore can be different from the temperature of other parts of the circuit. TOP is your best estimate of device temperature in operation. It is used for parameters that depend on temperature only weakly, or have only a minor effect in the model. Including self-heating for such parameters increases the computational overhead of the model, for very little improvement in accuracy. TOP is essentially the same as the temperature parameter in conventional SPICE models, which employ thermal scaling but not self-heating.

TBP is the baseplate temperature, which you specify. [1] indicates that you should set this value by applying a voltage to a fifth, thermal node in the HBT. AWR Microwave Office software does not use this approach, because treating it as a model parameter is simpler and less error-prone.

CJx and CxMIN. Defaults have been changed to 2·10-20 F for the parameters CJE, CJC, and CJCX; and to 1*10-20 F for the parameters CEMIN, CCMIN, and CXMIN. The default values of zero, listed in [1], cause divide-by-zero errors.

HB_UCSD contains terms of the form (CJx/CxMIN)(1/MJx), where MJx is typically in the range of 0.33 to 0.50. With incorrect values for these parameters, it is easy to "crash" such equations. These errors are very difficult to trap without restricting the use of reasonable parameter values, so you should be aware of this problem and choose parameter values accordingly.

RTH and CTH. HBT_UCSD provides for a nonlinear thermal resistance, RTH, which is a function of temperature. This formulation makes the temperature increase, normally the dependent quantity, a function of itself. This could lead to numerical problems, poor convergence, instability, or other unusual behavior. Although AWR Microwave Office software attempts to make its models conform to the defining documentation (so SPICE and AWR Microwave Office calculations give the same results), in this case, AWR Microwave Office software makes RTH a function of TOP, instead of instantaneous device temperature.

BF and BR. The default values of BF and BR (104) specified in [1] present little problem in most practical cases, but if the base is biased with a current source (as might happen in the initial stages of a design), and BF/BR use the default values, bad convergence and strange results are possible. You should be careful to set these parameters to reasonable values, or make certain that BE leakage currents account for base current correctly.

Parameter Restrictions and Recommendations

If you exceed the following parameter limits, the parameter is modified as necessary to prevent a numerical error. In general, physically reasonable values are always acceptable.

  1. CEMIN > 0.01 CJE.

  2. CCMIN > 0.01 CJC.

  3. 0.1 < MJx < 0.9, where MJx is any of the parameters beginning with MJ.

  4. FC and FCE must be in the range (0.001, 0.999). If either is outside this range, it is set to 0.5.

  5. 0.01 < FA < 0.99.

  6. Device temperature must be greater than 100 K (-173 C).

Implementation Details

Breakdown and Self-heating

HBT_UCSD includes collector-to-base breakdown and self-heating effects. You should exercise caution in making use of these effects. Breakdown is a strong nonlinearity which can cause convergence difficulties in a harmonic-balance simulator. Reference [1] warns that breakdown modeling can cause poor convergence even in time-domain simulators, and [2] gives some of the reasons. Self-heating also can introduce negative-resistance effects that cause difficult conversion in harmonic-balance simulation.

You can easily switch these effects on and off with the BKDN and SELFT flag parameters. If SELFT=0, the device temperature is set to TOP, and unless TOP=TNOM, thermal scaling is still performed.


Scaling is relatively simple:

All charges are scaled in proportion to AFAC;

All resistors are scaled inversely with AFAC;

All currents are scaled in proportion to AFAC.

The model equations are too extensive to include here. See reference [1] for further information.


This element does not have an assigned layout cell. You can assign artwork cells to any element. See “Assigning Artwork Cells to Layout of Schematic Elements” for details.


[1] Rudolph, Matthias. Chapter 6, Section 3 in Introduction to Modeling HBTs, Artech House, 2006.

[2] S. Maas, "Ill Conditioning in Self Heating FET Models," IEEE Microwave and RF Component Letters, March, 2002.

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