Influence of Franz Keldysh effect on the modulation depth and distortion characteristics of transistor laser

Numerical analysis of Transistor Laser is carried out to investigate the influence of Franz-Keldysh (F-K) Effect on the non-linear distortion characteristics such as intermodulation (IM). The device considered for the study incorporates an intrinsic GaAs absorption layer in the collector–base junction of a TL emitting around 980 nm. The base current threshold is obtained as 39 mA for the collector to base voltage of 1 V and it is discerned that the threshold current of TL is dependent on electric field in addition to the bias current. The analytical solution of rate equations at steady state is derived and compared with the numerical simulation and the same is plotted for V_CB = 1 V. Moreover, maximum optical light output loss of 12 mW and 1.37 mA additional current is contributed to the collector current for collector base voltage of 3 V, under various base currents due to F-K effect. The modulation bandwidth (f_-3 dB) of TL under voltage modulation is obtained as 9.63 GHz. While, under current modulation, it is found as 8.38 GHz respectively. A maximum modulation depth of 86% is obtained for I_B = 1.06I_th, at 2.4 GHz. The frequency components related to intermodulation product, IMD3 (2.15 GHz) magnitudes are obtained as − 10.26 dBc, − 12.37 dBc, − 13.54 dBc and − 15.92 dBc for V_CB voltage value of 0 V, 1 V, 2 V and 3 V respectively. The intermodulation product, IMD5 (2.05 GHz) magnitudes are obtained as − 18.89 dBc, − 25.76 dBc, − 28.22 dBc and − 36.05 dBc for the collector bias voltage (V_CB) value of 0 V, 1 V, 2 V and 3 V respectively. In both cases, the intermodulation is found to decrease with increase in F-K absorption.