Low-frequency noises in GaAs MESFET’s currents associated with substrate conductivity and channel-substrate junction

Abstract

Low-frequency noises in GaAs MESFET are usually observed when investigating the drain current and substrate leakage current under sidegate bias conditions. Experimental results show that the magnitude of low-frequency noises is in a direct dependency upon the sidegate bias and the noises in drain current will disappear if sidegate bias increases more negatively beyond a certain voltage. A mechanism associated with the substrate conductivity and the channel-substrate junction modulated by sidegate bias is proposed to explain the fluctuation of low-frequency noises.MESFET, low-frequency noise, substrate conductivity, channel-substrate junction Citation:Ding Y, Yan X L. Low-frequency noises in GaAs MESFET's currents associated with substrate conductivity and channel-substrate junction. Chinese Sci Bull, 2011Bull, , 56: 1267Bull, −1271Bull, , doi: 10.1007 MESFET is considered as one of the important devices for the applications of high power, high frequency and high efficiency microwave electric circuit [1,2]. However, as the demand for higher speeds and increased functionality in circuits, low-frequency noises (LFN) in currents, such as thermal noise, 1/f noise, generation-recombination noise, diffusion noise and shot noise, in various GaAs FET structures have been reported [3][4][5][6][7][8][9][10]. The main characteristics of these noises are: (1) LFN strongly depends upon the substrate materials. (2) The frequency of LFN is in the range of about 10 −2 to 10 2 Hz. (3) LFN is dependent on the fabrication procedure and bias condition. (4) LFN is sensitive to illumination. Recently LFN has received more attention due to its effect on the performance of low-noise microwave integrated circuits (MMICs).Although it is generally accepted that LFN results from the field-enhanced capture of electrons by deep traps, the exact mechanism is not fully understood. Miller and Bujatti attributed LFN directly to the oscillations in leakage currents in the semi-insulating (SI) GaAs substrate [5]. While Wager pointed out that LFN in channel current could not be simply attributed to the oscillations in leakage current of the substrate, but related to the peculiarities of channel-substrate junction [9].In this paper, LFN in MESFET's currents is investigated under sidegating conditions, and the mechanism is identified whereby the current fluctuation is directly related to the sidegate bias. Unlike Birbas's explanation in terms of an unreal negative capacitance of the depletion region in buffer-channel interface [11], we associate LFN with the substrate conductivity and channel-substrate junction modulated by sidegate bias. This study should be of value to elucidate the fundamental nature of noises modulation in GaAs ICs.