GaAs-GaAlAs HETEROJUNCTION TRANSISTOR FOR HIGH FREQUENCY OPERATION

A bipolar transistor structure is proposed having application for either high frequency operation or integration with certain types of light emitting devices. The structure involves liquid phase epitaxially grown layers of GaAs for the collector and base regions, and of Ga,_,Al,As for the heterojunction emitter. The high frequency potential of this device results primarily from the high electron mobility in GaAs and the ability to heavily dope the base region with slowly diffusing acceptors. The Ga,_,Al,As emitter region provides a favorable injection efficiency and, because it is etched preferentially relative to GaAs, access to the base layer for making contact. Transistor action with d.c. common emitter current gains of 25 have been thus far observed. Calculations of the high speed capability of this transistor are-presented. NOTATION and Ge bipolar transistors in terms of most of the characteristics that contribute to a high frequency capability. These advantages result partly from the physical and chemical properties of GaAs and Ga,_,Al,As, and partly from the unique approach used in the fabrication of the transistor structure. Some estimates of the highfrequencycharacteristics which might be expected from such a device will also be presented. In addition to its use in high-frequency applications, the GaAs-Ga,_,Al, As structure described here could be integrated with other GaAs based optical devices, for example with GaAs-Ga,_,Al,As light emitting arrays. The ability to integrate transistors and light emitting devices on a common substrate offers several potential advantages including greater simplicity, smaller size, and lower cost. PROPOSED DEVICE STRUCTURE A schematic representation of our proposed device structure is shown in Fig. 1. An n-GaAs collector layer, a p-GaAs base layer, and an n-Ga,_,Al,As emitter layer are grown in sequence on an n+-GaAs substrate [Fig. l(a)]. Using a recently developed liquid phase epitaxial (LPE) growth technique [ 11, the layers are grown from separate melts while maintaining the growing solidliquid interface. The technique allows the controlled growth of uniformly doped layers as thin as 1000 A. The device is fabricated by first depositing onto