High Electron Mobility Transistors (HEMTs) under optical illumination: A Review Rajesh B. Lohani and Jaya V. Gaitonde Electronics and Telecommunication Department Goa Engineering College Farmagudi-Ponda-Goa-India rblohani@gec.ac.in, jayagaitonde@rediffmail.com Abstract—The use of microwave high electron mobility transistors (HEMTs) as photodetectors and optically controlled circuit elements has attracted interest. This paper reviews the previously published papers of HEMT under optical illumination. There are two effects which are induced in HEMT due to optical illumination-photoconductive and photovoltaic effects. The photoconductive effect is due to transport of photogenerated electrons into the two dimensional electron gas (2DEG). The photovoltaic effect is due to accumulation of photogenerated holes in the buffer layer and/or due to crossing of gate junction by the photogenerated holes. The photovoltaic effect leads to the development of photovoltage at the heterojunction and/or at the gate junction resulting in the shift of the gate bias. Keywords-HEMT, photodetector, 2DEG. I. INTRODUCTION The optical control of high electron mobility transistor (HEMT) has been studied over the years for its potential applications as photodetectors, amplifier gain control, switching, phase shifting, mixing, optically tuning of oscillators and optical injection locking of oscillators. The optical illumination acts as an additional terminal which controls the 2DEG current density. In section II, we will review, in short, the work done by different researchers. In section III, we will describe the HEMT under optical illumination. In section IV, we will review some important results. II. REVIEW In the 1980s, Simons and Bhasin [1] did the anlaysis of optically controlled microwave/millimeter-wave device structures such as GaAs MESFET, InP MESFET, Al0.3Ga0.7As/GaAs high electron mobitity transistor (HEMT), and GaAs permeable base transistor (PBT) in which it was shown that Al0.3Ga0.7As/GaAs HEMT has the highest sensitivity to optical illumination. Simons [2] presented the microwave performance of an optically controlled AlGaAs/GaAs HEMT and GaAs MESFET which included extensive experimental results which showed the light induced voltage, the increase in the drain current, the RF gain, and the change in the microwave scattering parameters of an AlGaAs/GaAs HEMT under optical illumination. The response speed of the HEMT to an intensity modulated optical signal was shown to be of the order of 11 ps. In the 1990s, Salles and Romero [3] considered the photovoltaic and photoconductive effects in the AlGaAs and GaAs layers of the depletion mode Al0.3Ga0.7As/GaAs HEMT and predicted the change in the dc and RF characteristics of the HEMT under illumination. The photoconductive effect was found to increase the drain-to- source current by a factor around 10 percent (at the gate to source voltage, Vgs = 0 V) for the levels of illumination used. Experimental work using the photovoltaic effect was shown in which large control of gain (many decibels) of a 2-6 GHz HEMT amplifier and moderate tuning range (around 12 MHz) of a 2 GHz HEMT oscillator were obtained. Mitra, Singh and Pal [4] presented an analytical modeling for a GaAlAs/GaAs optical HEMT considering the effect of signal modulated optical radiation incident on the transparent and semi- transparent Schottky gate of the device. They showed that the modulating frequency has significant effect on I-V characteristics, offset voltage, sheet concentration and transconductance of the device upto a certain range (~1 MHz) above which the frequency dependent terms become insignificant compared to the dc part. The photovoltaic effect increases the offset voltage and reduces the current and transconductance of the 2DEG compared to the case where only photogeneration is considered. Romero and Herczfeld [5], presented a study of the negative photoresponse of AlGaAs/GaAs HEMT. Extensive experimental characterization of this phenomena regarding its dependence on bias voltages and optical power was performed and a comparison with devices that present positive photoresponse was carried out. The negative photoresponse was explained in terms of the increase of the stored charge in the buffer layer, inducing a reduction in the number of 2-DEG electrons. Yang et al [6], investigated the optical tuning and injection locking characteristics of MMIC oscillators made with InP-based 0.25 μm gate In0.53Ga0.47As/In0.52A10.48As HEMTs. The optical tuning of X- and R-band oscillators were made as a function of drain bias and a maximum tuning range of 8.7 MHz and 11.7 MHz, respectively, were measured. Direct optical subharmonic injection locking of the X-band oscillators were demonstrated with a maximum locking range of 4.8 MHz at 10 GHz and 19 GHz. Romero, Martinez and Herczfeld [7], derived a semi-analytical model for the photodetection mechanisms in HEMTs, including the internal photovoltaic