Hot Electron Injection Field Effect Transistor (Heifet)

Summary A new method for raising the upper frequency limit of field effect transistors by injection of accelerated electrons has been investigated. While the utilization of the fabricated devices as amplifiers was not possible, 58 GHz- oscillators which need only low supply voltages (around 3 volts) have successfully been developed. Motivation The most important areas where microwaves are applied today are communications engineering and microwave metrology. In order to achieve higher data rates and improved accuracy, in both of these fields a continuous trend towards using higher frequencies can be observed. It is therefore necessary to increase the upper frequency limit of the applied devices, in particular of the transistors used for generating and amplifying microwave signals. Objectives The method usually adopted to accomplish increased speed as described above consists of shrinking the geometrical dimensions of the transistors thereby decreasing the distances to be traversed by the charge carriers in the devices. Another possibility to improve the speed of transistors is to increase the drift velocity of the injected electrons as proposed in the so-called HEIFET (Hot Electron Injection Field Effect Transistor). In conventional field effect transistors an ohmic contact is used from which only slow electrons are injected into the drift zone of the device and acceleration of these electrons to the maximum possible speed (saturation velocity) takes some time. In the proposed HEIFET the ohmic source contact is replaced by a special contact which accelerates the carriers by means of an additional electrode before injecting them into the drift zone of the device. The objective of this project was to verify the concept of fast (`hot`) electron injection experimentally. Technology and Characterization In order to fabricate the HEIFET a considerable improvement of the semiconductor manufacturing technology at the Microelectronics Institute was necessary. In particular it was necessary to produce fine trenches in extended metal layers for realizing the electron injecting contact. As a result of this work trenches with widths down to 0.4 m can now be manufactured at the Microelectronics Institute. The high frequency characterization of our devices was accomplished `on wafer` using a Hewlett-Packard 8510C network analyzer and a Cascade Microtech Summit 9201 wafer prober. Around these instruments a measurement system has been built which allows the semiautomatic characterization of transistors in the frequency range from 45 MHz to 50 GHz. In addition the required software tools for calibrating the acquired data (`deembedding`) have been implemented. Experimental Results It has been shown that fabricating the electron injecting contact is indeed possible. Unfortunately, it has been found that the resulting field configuration inside the device prevents the HEIFET to be used as an amplifier. However, it was possible to build an oscillator by utilizing the Gunn effect in the device which was made of GaAs. By modifying the HEIFET structure in an appropriate way, we have been able to realize oscillators operating at frequencies around 58 GHz at very low supply voltage of only 3 V. A further increase of the operating frequency seems possible which makes the HEIFET a very attractive candidate for applications in the 61 GHz ISM-band and in the vehicular radar band at 77 GHz.