Abstract
Deep Level Transient Spectroscopy (DLTS) has been used to characterise the deep levels in In/sub x/Ga/sub 1-x/P/In/sub 0.20/Ga/sub 0.80/As/GaAs (0.40/spl les/x/spl les/0.48) pseudomorphic high electron mobility transistors (pHEMT) grown by solid source molecular beam epitaxy (SSMBE). Three different pHEMT devices were investigated, with a single InGaP barrier layer, double InGaP barrier layers and strained InGaP barrier layers. Only one electron trap in the InGaP barrier layer was detected in each of the devices. The activation energy of the electron trap is 0.39 eV for the single barrier layer device, 0.40 eV for the double barrier layer device and 0.57 eV for the strained barrier layer device. The trap concentrations are 7.22/spl times/10/sup 18/ cm/sup -3/, 2.38/spl times/10/sup 20/ cm/sup -3/ and 5.02/spl times/10/sup 20/ cm/sup -3/, respectively. The current-voltage (I-V) characteristics and transconductance of the devices were measured at 300 K, 77 K and 30 K. The drain saturation current becomes smaller due to the carriers being captured by the defects, and the transconductance becomes higher due to an increase in carrier mobility in the channel as the temperature was lowered from 300 K to 30 K. No devices showed any collapse in the I-V characteristic or persistent photoconductivity (PPC) at low temperature, suggesting that the trap in the InGaP layer does not have a DX centre-like characteristic. A comparison was made with Al/sub 0.24/Ga/sub 0.76/As/In/sub 0.20/Ga/sub 0.80/As/GaAs pHEMTs. Only one electron trap was detected in the Al/sub 0.24/Ga/sub 0.76/As layers of the latter pHEMTs. The trap concentration is 1.72/spl times/10/sup 20/cm/sup -3/. Drain current collapse at temperature below 77 K at low drain bias and persistent photoconductivity (PPC) effect were evident, indicating the presence of DX centers in the Al/sub 0.24/Ga/sub 0.76/As layers.
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