A. Marty

Electrical behavior of an NPN GaAlAs/GaAs heterojunction transistor☆

This paper deals with a theoretical and experimental study of GaAlAs/GaAs heterojunction transistors fabricated by liquid phase epitaxy. Starting from the general transport equations in a structure of variable composition, the expressions for the injection efficiency and for the transfer characteristic (IC, VBE) are first established. Using reasonable simplifying assumptions, allowed by the physical characteristics of the base and emitter regions, the major role played by the zone including the conduction band spike is clearly shown. In particular, it appears that the presence of such a spike entails a substantial decrease of the injection efficiency and also modifies, over a wide bias range, the (IC, VBE) characteristic which then follows a non ideal law. The experimental behaviour of devices fabricated by liquid phase epitaxy are described. The measurements were carried out at temperatures ranging from 77 to 316°K. The results obtained are compared with those predicted by the theoretical model.

Electrical behavior of double heterojunction NpNGaAlAs/GaAs/GaAlAs bipolr transistors

Abstract This paper deals with a theoretical and experimental study of double heterojunction NpN GaAlAs/GaAs/GaAlAs bipolar transistors fabricated by liquid phase epitaxy. Starting from the general transport equations of the macroscopic theory of diffusion, a charge-control-type model is first established. The compact analytical expressions which determine the transfer characteristics (JC′VBE) and intrinsic current gain h FE ∗ are expressed as a function of the technological or geometrical parameters of the structure. A study of the sensitivity to these parameters highlights in particular the influence of the zone including the conduction band spike of the base collector heterojunction. Such an influence may assume the form of a substantial degradation of the devices electrical characteristics (reduction of the base transfer factor, increase in transit time). The experimental behavior observed on characterized devices confirms the theoretical predictions.

Transistor‐based evaluation of conduction‐band offset in GaInP/GaAs heterojunction

The conduction‐band discontinuity for GaInP/GaAs metalorganic chemical vapor deposition (MOCVD) HBTs has been determined from current–voltage characterization. The charge‐control model used permits to compare the respective weights of collector current limitations due to the potential barrier effect and to the carrier diffusion mechanism in the low gap base region. A study of sensitivity to the main sources of error leads to a conduction‐band offset value of ΔEC≊170±16 meV confirming the advantage of the resulting band lineup for the HBT.

Theory and experiment of the temperature dependence of GaAlAs/GaAs HBTs characteristics for power amplifier applications

Abstract Modelling of thermal effects of GaAlAs/GaAs HBTs d.c. and r.f. characteristics is presented. The thermal resistance value is determined experimentally and introduced in a model. Good agreement between measured results and those from modelling is demonstrated. The r.f. performance shows a good stability at high temperature (fmax is reduced by about 12% due to temperature rise). For the high signal mode, the simulation of a power amplifier at 3 GHz is also carried out with and without temperature effects.

Modeling and experimental results for C(V) in an abrupt isotype n Al/sub 0.5/Ga/sub 0.5/As/GaAs heterojunction

The differential capacitance C of an abrupt isotype n Al/sub 0.5/Ga/sub 0.5/As/GaAs heterojunction has been modeled by directly calculating the dependence of the space charge on the voltage V at its terminals. The electron charge distribution was calculated considering the 2-D electron gas by simultaneously solving the Schrodinger and the Poisson equations, DX centers included. Results from this model predict an asymmetric bell-shape dependence of C on V, with a maximum near the contact potential, and are in good agreement with experiment. This further provides experimental evidence of Gamma - Gamma and X-X valley coupling for electrons traveling across the heterojunction. For voltage values not too close to the contact potential, it was possible to find a simple method, based on a total depletion, that gives a good fit to experiment. >

III, V heterojunction bipolar transistors

Abstract After reviewing the properties and the characteristics of GaAlAs/GaAs heterojunctions we describe the forseeable performances of the single and double heterojunction bipolar transistors. A number of examples showing the realization of HF devices or IC logics are described. The results obtained are presented, the prospect for the evolution is outlined and the discussion brings out the fact that the HBT may take over the lead in the race for highest speed.

Fabrication and d.c. characterization of GaAlAs/GaAs double heterojunction bipolar transistors

Abstract The fabrication and d.c. characterization of GaAlAs/GaAs double heterojunction bipolar transistors grown by molecular beam epitaxy with Mg-implanted external base regions are described. These devices exhibit satisfactory forward and reverse operations which define a very low offset voltage, i.e. VCE ⩽ 10 mV. The detailed characterization of these large-size test structures is primarily concerned with investigating the interdependence of the forward and reverse current gains whose maximum values are 290 and 40 respectively.

Evidence of Γ-Γ and X-X coupling using a single abrupt isotype n-GaAs/Al0.5Ga0.5As heterojunction

Several numerical simulation models were developed in order to explain the experimental current-voltage (I–V) characteristics of a heterojunction formed by GaAs and indirect-band AlGaAs at temperatures ranging from 253 to 353 K. Our models take into account the tunneling effect, the 2-D electron gas calculated by solving the Schrodinger and the Poisson equations simultaneously, the DX centers and optionally X-X, Γ-Γ or X-Γ coupling. Abrupt isotoype n-GaAs/Al0.5Ga0.5As heterojunctions with low doping levels, 1022 m−3/5 × 1022 m−3, were fabricated by MBE. Fitting experimental results using our models led to the following main conclusions: X-X and Γ-Γ coupling occurs instead of X-Γ; the band offset ratio for the conduction and valence bands should by 6535 rather than 8515 and the high voltage drop observed for forward bias may be interpreted as a result of the X-X coupling.

Reciprocity in heterojunction bipolar transistors

Abstract By using the principle of current balancing to determine the boundary conditions for minority carriers in the base of heterojunction bipolar transistors, a general equation for the collector current is derived. This equation is then used to examine the degree to which emitter-collector current reciprocity holds for AlGaAs/GaAs single- and double-heterojunction devices. It is shown that reciprocity holds strictly in only a few instances.

Emitter excess resistance in GaAlAs/GaAs heterojunction bipolar transistors

Numerous studies have shown the influence of parasitic elements, particularly emitter access resistance and inductance, on HBTs DC and dynamic performances. Instead the achievement of low specific contact resistances, the apparent emitter resistance presents high values. To explain this discrepancy the longitudinal distribution of the emitter current has been investigated by using a distributed DC-model, taking into account the metal layers resistance of the different contacts. Thus it is shown that only the emitter contact parameters act on the non equipotentiality along the structure.