The new two-dimensional electron gas base HBT (2DEG-HBT): two-dimensional numerical simulation - Electron Devices, IEEE Transactions on

The bipolarlFET characteristics of the ZDEG-HBT (TwoDimensional Electron Gas Base p-n-p AIGaAs/GaAs Heterojunction Bipolar Transistor) are analyzed extensively by a two-dimensional numerical simulator based on a drift-diffusion model. For bipolar operations at high collector current densities, it is confirmed that the cutoff frequency f r is determined mainly by the collector transit time of holes and by the charging time of the extrinsic base-collector capacitance CFTT. The charging times of the emitter and base regions, and the base transit time are shown to be negligible. A high cutoff frequency f r (88 GHz) and current gain h,, (760) are obtained for an emitter size of 1 X 10 pm’, an undoped collector thickness of 150 nm, and a collector current density J , of lo5 A/cm2. The FET operation of the same ZDEG-HBT structure shows a threshold voltage Vrh of 0.74 V, the transconductance G:Yx of 80 mS/mm, and maximum cutoff frequencyf? of 15 GHa. Moreover, the buried Si02 structure for reducing the extrinisic basecollector capacitance CF: ’ shows an extremely high cutoff frequency f r of 163 GHz and maximum oscillation frequencyf,,, of 110 GHz for the bipolar mode. The dependence of the device performance on material parameters is analyzed extensively from a device design point of view. Optimization of the epitaxial layer structure enables the design of a higer performance FET (e.g., G,, = 235 mS/mm andf, = 19 GHz for L, = 1 pm) without sacrificing the bipolar function.