Electroluminescence and gate current components of InAlAs/InGaAs HEFT's

Increasing the I d s mole fraction in InGaAs-base heterostructure FET's W E T ' S) leads to improved device performance due to the superior carrier transport properties of these materials. At the same time, however, the use of narrow band-gap semiconductors results in enhanced impact-ionization, with severe detrimental effects like excessive shot noise in ID and large gate current IG even at regular bias points. Detailed physical understanding of impact-ionization and of the behaviour of the copious amount of holes that are generated in InGaAs channels is crucial to developing guidelines for designing high-performance devices. Gate current measurements 'and electroluminescence spectra have been widely adopted to evaluate hot-electron effects and impact-ionization in GaAs-based MESFET's and HEMT's, but no agreement has been found as of the origin of the different spectral components of the emitted radiation. In any case, no work has been presented, up to now, in InGaAs-based WET'S. In this paper we cam' out a detailed study of gate current and its correlation w i t h the various spectral components of light emitted in InAlAs/InGaAs HFET's at regular bias points. Our work reveals that light emitted in the visible portion of the spectrum is a good signature of impact-ionization in the channel as impact-ionized holes recombine with channel electrons. On the other hand. light emitted in the infrared portion of the spectrum is found to originate in conduction band-to-conduction band transitions of the hot electrons in the channel. These findings establish electroluminescence in the appropriate spectral range as an ideal tool to characterize hot carrier phenomena in InP-based HFET's, and allowed us for the first time to quuntitavely separate the gate current into its electron and hole components. The devices characterized in ths work are n-channel normally-on L = 1 pm InAIAs/lnGaAs HFET's, with an 100 A n+-In0 ;jGaO 47As Si-doped channel (Nsi = 8 x lo1* ~ m-~) , a 300 In0.41Alo. j9As strained insulator'and a '50 A In0 j3Ga0.47As cap layer. When these devices are biased at high Vds (23 V), sigmficant impact-ionizkon takes place in the channel. A detailed study of the gate current reveals that, for negative Vgs, IG is dominated by collection of impact-ionized holes, while for positive Vgs, IG is dominated by electron real-space-transfer at low V h , and by hole collection at high Vds. Light emission both in the infrared and visible region takes place at high Vds. The …