The present invention pertains to High Electron Mobility Transistors and to Heterojunction Bipolar Transistors.
High Electron Mobility Transistor:
A High Electron Mobility Transistor (HEMT) is a Metal Semiconductor Field Effect Transistor (MESFET) fabricated on a doped aluminum gallium arsenide/undoped gallium arsenide heterostructure. This heterostructure is preferably formed by molecular beam epitaxy.
When, for example, a silicon-doped aluminum gallium arsenide (AlGaAs) layer is grown on top of an undoped gallium arsenide (GaAs) layer, a heterojunction is formed between the two layers. A two-dimensional electron gas is formed on the GaAs side of the heterojunction due to the unique crystal structure of the heterojunction and the greater electron affinity of the GaAs. The AlGaAs layer is fully depleted of mobile charge near the heterojunction and acts like the gate oxide of a metal oxide semiconductor field effect transistor. When a Schottky barrier gate is deposited on the AlGaAs layer, a depletion region is formed beneath the gate. The resulting device functions as a field effect transistor in that the Schottky barrier gate controls the number of electrons in the two-dimensional electron gas formed on the GaAs side of the heterojunction.
Carrier transport in the electron gas is similar to transport in undoped GaAs where the electron mobility is many times greater than doped GaAs because there is little or no impurity scattering. As a result, the electrons travel at twice the saturated velocity of a conventional GaAs MESFET. Thus, ultra high speed digital integrated circuits can be fabricated with HEMT devices. For example, HEMT ring oscillators have been fabricated which exhibit 12 picosecond switching delays at room temperature.
For all their promise, a substantial problem exists in fabricating HEMT devices. The thickness of the doped AlGaAs layer beneath the gate is difficult to control, and yet it is critical to device performance. Specifically, for each variation by 10 angstroms in the thickness of the AlGaAs layer, the saturated source-drain current of the HEMT is changed by 1 milliampere. Present fabrication techniques first form the AlGaAs layer and then alter its thickness during subsequent etching steps. Unfortunately, such techniques cannot produce layers having the uniformity and reproducibility required for large scale production of these devices. Thus, there exists a great need to improve the control over the thickness of the AlGaAs layer in a HEMT.
Heterojunction Bipolar Transistor:
A Heterojunction Bipolar Transistor typically comprises an epitaxial structure of various layers including gallium arsenide and aluminum gallium arsenide. For example, an NPN heterojunction bipolar transistor comprises an n-type GaAs collector layer, a thin p-doped GaAs base layer and an n-type AlGaAs emitter layer.
One problem with bipolar transistors, including GaAs heterojunction bipolar transistors, is that the cut-off frequency and power dissipation are adversely affected by any increase in base resistance. Typically, the extreme thinness of the base layer, which is required for low-power and high-frequency operation, gives rise to current crowding in the base-emitter heterojunction, which increases the base resistance. This problem is worsened during etching to expose the base layer to form the base contact. This is because such etching attacks the base layer and can alter its cross-sectional shape.
Another problem is that, during operation, the majority carrier current from emitter to base is accompanied by a reverse current arising from injection of minority carriers from base to emitter.
Thus, in heterojunction bipolar transistors, there exists a need to form semiconductor layers, such as the base layer, and to expose it without affecting its thickness or shape. Further, there exists the need to prevent minority carrier injection from base to emitter during transistor operation.