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The base of heterobipolar transistor can be doped to much higher levels than can the base of the homojunction transistor. We describe the principle, operation and effusion properties of a home-made boron source for p-type doping of the SiGe base of the heterobipolar transistor. Transistors fabricated from the material grown by molecular beam epitaxy, with a boron-doped base, exhibit good d.c. characteristics. A current gain of β max = 1000 could be obtained with 10 18 cm −3 base doping.

Subthreshold conduction in submicron MODFETs has been measured and can be well described by a model which includes the shifts of both the ideality factor and the threshold voltage with the drain voltage. A method for extracting the model parameters from measured data is described. Experimental results turn out that it is essential to introduce the shift of the ideality factor with the drain voltage to properly describe the subthreshold characteristics observed in submicron MODFETs. The model with all extracted parameters, incorporating gate junction leakage, is in good agreement with the experimental data for 0.25/0.3 μm AlGaAs/GaAs MODFETs.