The invention relates to a vertical channel field effect transistor, which is also called a permeable base transistor (PBT), as well as to the process for producing this transistor.
A transistor having a structure of this type has already been made. The active part of the grid of such a transistor consists of a comb-shaped portion formed from parallel teeth. A transistor of this type is formed by superimposing four layers, namely a type n.sup.+ gallium arsenide (GaAs) substrate, a type n layer forming the source, a comb-shaped tungsten film forming the grid and a type n layer forming the drain. The regions forming the source and the drain are surrounded by a gallium arsenide layer made insulating by a proton bombardment. The electrons coming from the region forming the source are forced to pass through the grid to reach the drain by the presence of insulating zones. The tungsten layer forming the transistor grid forms a Schottky contact with the gallium arsenide making it possible to control the passage of electrons between source and drain. The concentration of carriers is adjusted in such a way that the width of the transition zone for a zero polarization voltage of the Schottky contact is approximately double the space between the teeth of the comb forming the grid. Metallic coatings provide the ohmic contacts for the different electrodes.
Compared with conventional field effect transistors, the present permeable base transistor offers the following advantages:
a vertical structure permitting a better control of phenomena within the transistor; PA1 the electron flow passing from the source to the drain is controlled by the thickness of the grid, which is smaller than the grid width in planar technology and consequently reduces the electron transit time, PA1 the possibility of epitaxy above the grid.
On the basis of theoretical considerations, the maximum oscillating frequency with devices of this type is approximately 200 GHz. However, such transistors suffer from a number of disadvantages, particularly a high resistance of the grid due to its shape, as well as a considerable risk of the grid teeth breaking, so as to make important parts thereof unusable.