2.6.2 InGaN/GaN HBTs

One of the suggested ways to counter the low base conductivity is to employ a lower gap material such as InGaN. InGaN is also more resistant against the damage introduced by the dry etching step used to expose the surface of the base layer [97]. Thus, the potential of an InGaN/GaN HBT was quickly realized, first in theoretical studies [98,79,99] and subsequently in experiments [97]. The latter is the first reported InGaN/GaN (D)HBT. It achieved a $\beta$ of 20 but also a high offset voltage (5 V). Further studies reported breakdown voltages as high as 100 V, mainly due to the low damage of the InGaN base layer [100]. Ongoing improvement of the base ohmic characteristics resulted in a decrease of the offset voltage (from 5 V down to 1 V) in addition to a record $\beta$=2000 [101]. The same group reported a collector current density of 6.7 kA/cm$^2$ corresponding to a power density of 270 kW/cm$^2$ [102].

Several works employed a theoretical approach in exploring and optimizing the device performance. Ensemble Monte Carlo simulations were used to determine the temperature and doping concentration dependence of the low-field mobility [99]. Based on this calculations the temperature dependence of the cut-off frequency was studied. The impact of non-uniform base doping on $\beta$ was studied in [103].

The implementation of a graded emitter layer was experimentally studied by Keogh et al. [104]. A low offset voltage (2-3 V) and $\beta$=27 with breakdown voltage greater than 40 V was reported. The current gain decreased to 10 at 300$^\circ$ C, but no device degradation was observed. The same technique was used by other groups [105,106]. A state-of-the-art device with $\beta$=42, a current density of 5.2 kA/cm$^2$, and a breakdown voltage larger than 75 V was obtained in [106]. A record current gain of 2450 was reported in [107] at room temperature. At 40 K it reaches 5000 due to a reduction of the recombination current in the base layer. By introducing an AlGaN collector Kumakura et al. [108] were able to achieve a breakdown voltage of 190 V albeit the current gain was only 3 due to the low quality AlGaN.