Bipolars

All bipolar transistor and Darlington models are based on Spice's modified Gummel-Poon model. A typical model for a single transistor is shown as follows:

*Zetex FMMT493A Spice Model v1.0 Last Revised 30/3/06
*
.MODEL FMMT493A NPN IS =6E-14 NF =0.99 BF =1100 IKF=1.1
+NK=0.7 VAF=270 ISE=0.3E-14 NE =1.26 NR =0.98 BR =70 IKR=0.5
+VAR=27 ISC=1.2e-13 NC =1.2 RB =0.2 RE =0.08 RC =0.08 RCO=8
+GAMMA=5E-9 CJC=15.9E-12 MJC=0.4 VJC=0.51 CJE=108E-12
+MJE=0.35 VJE=0.7 TF =0.8E-9 TR =55e-9 XTB=1.4 QUASIMOD=1
*

In the bipolar model:

• IS and NF control Icbo and the value of Ic at medium bias levels.
• ISE and NE control the fall in hFE that occurs at low Ic.
• BF controls peak forward hFE and XTB controls how it varies with temperature.
• BR controls peak reverse hFE i.e. collector and emitter reversed.
• IKF and NK control the current and the rate at which hFE falls at high collector currents.
• IKR controls where reverse hFE falls at high emitter currents.
• ISC and NC controls the fall of reverse hFE at low currents.
• RC, RB and RE add series resistance to these device terminals.
• VAF controls the variation of collector current with voltage when the transistor is operated in its linear region.
• VAR is the reverse version of VAF.
• CJC, VJC and MJC control Ccb and how it varies with Vcb.
• CJE, VJE and MJE control Cbe Ccb and how it varies with Veb.
• TF controls Ft and switching speeds.
• TR controls switching storage times.
• RCO, GAMMA, QUASIMOD control the quasi-saturation region.

Some standard bipolar transistor Spice models may not include a parameter that allows BF, the hFE parameter, to vary with temperature. If XTB is absent it defaults to zero, e.g. no temperature dependence. If hFE temperature effects are of interest and XTB is not modeled then the following values may be used to provide an estimate or a starting point for further investigation:

It is suggested that the appropriate datasheet hFE profile is examined, and a Spice test circuit created that simulates the device in question and generates a set of hFE curves. Two or three such iterations should normally be sufficient to define a value for XTB in each case.

Please remember that these notes are only a rough guide as to the effect of model parameters. Also, many of the parameters are interdependent so adjusting one parameter can affect many device characteristics.

At Diodes, we have tried to make the models perform as closely to actual samples as possible but some compromises are forced which can result in simulation errors under some circumstances. The main areas of error observed so far have been:

• Spice is often over optimistic in the hFE a transistor will give when operated above its data sheet current ratings. This is particularly true for a high voltage transistor operated at a low collector-emitter voltage and quasi-saturation parameters RCO, GAMMA and QUASIMOD have been introduced to improve the models in this region.
• Spice can be pessimistic when predicting switching storage time when current is extracted from the base of a transistor to speed turn-off.

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