I-V kink in InAlAs/InGaAs MODFETs due to weak impact ionization process in the InGaAs channel

InAlAs-In_0.53Ga_0.47As-InP MODFETs on InP substrates have demonstrated superior high frequency performance in recent years. However, the devices with sub-half-micron gate lengths often exhibit a detrimental “kink effect” in the I-V curves, which is a sudden increase in the saturated drain current with respect to drain bias. The devices also suffer from high output conductance, G ₀, with value as high as 100 mS/mm compared to 30 mS/mm for the GaAs based MODFETs with the same geometry. This excessive G₀ value degrades the voltage gain G_m/G₀, and consequently the power gain and f_max. Another serious problem for the device is the low drain to source breakdown voltage, which imposes a severe limitation on its power applications. Although the anomalous characteristics have been studied extensively, the original cause is still controversial. The speculative theories range from field de-ionization of trapped electrons in the InAlAs buffer layer to buffer layer conduction due to deconfinement of 2DEG from the InGaAs channel. The trap related theory contradicts the fact that when the narrow band gap InGaAs channel layer is replaced by a wider band material such as InGaAsP or InP, while keeping the same InAlAs as the buffer layer, the G ₀ value of the device is much lower and the I-V kink is absent. The buffer conduction theory agrees well to the normally low G ₀ value for the GaAs based MESFETs and MODFETs, but does not explain the excessive G₀ observed on InP based InAlAs/InGaAs MODFETs. Based on the facts that (i) the value of the G₀ is proportional to the electric field in the region near the drain side of the gate edge, and (ii) light emission was observed from the device with high G₀, we believed that the excessive G₀ and I-V kink for this device were related to weak impact ionization process in the InGaAs channel. In order to clearly understand the actual mechanism causing the I-V kink and excessive G₀, we conducted a series of carefully designed experimental and theoretical studies. The influences of InAlAs buffer layer and the InGaAs channel layer on the anomalies were investigated by using several device structures. The experimental results indicated that the weak impact ionization in the narrow band InGaAs channel is responsible for anomalies. The occurrence of impact ionization was confirmed by detection of light emission from the device. We then calculated the impact ionization rate and estimated the accumulated hole concentration in the channel. Based on all the experimental data and calculation results, we constructed the models to explain the anomalies