GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

10

HETEROJUNCTION BIPOLAR TRANSISTORS

10.1 INTRODUCTION TO HETEROJUNCTION BIPOLAR TRANSISTORS

Very low logic voltage swings occur when extremely uniform threshold voltage devices experience high nonlinearity. Because of this phenomenon the device structures preferred are those in which the threshold voltage is highly insensitive to normal processing variations [Wel83]. While GaAs MESFET pinch-off voltage is fairly insensitive to horizontal geometry variations (e.g., L_g), V_p is very sensitive to both the thickness (vertical geometry) and doping level in the channel layer. In contrast to the MESFET, an almost ideal device from the standpoint of threshold voltage variations is the bipolar transistor.

Every device has a dimension in the direction of current flow that controls the speed of the device. In MESFETs the current flow is parallel to the surface, and the critical control dimension is established by fine-line lithography. In the bipolar transistor the speed-determining part of the current path is perpendicular to the surface and to the epilayers, and for the first-order, speed is governed by the layer thickness. Since vertical layer thickness can be easily made much smaller than horizontal lithography dimensions, there is, for the horizontal dimensions, an inherently higher-speed potential in bipolar structures than in MESFETs [Kro82]. In this chapter we will discuss the structure of the device and its theoretical performance, different fabrication techniques, the advantages ...