Influence of Emitter Width and Emitter–Base Distance on the Current Gain in 4H-SiC Power BJTs

The influence of the emitter-base geometry on the current gain has been investigated by means of measurements and simulations. Particular attention has been placed on the emitter width and on the distance between the emitter edge and the base contact. When the emitter width is decreased from 40 to 8 m, the current gain is reduced by 20%, whereas when the distance between the base contact and the emitter edge is decreased from 5 to 2 m, the current gain is reduced by 10%. Simulations have been used to investigate the reasons for the current gain reduction. The reduction of the emitter width induces two mechanisms of current gain reduction: earlier forward biasing of the base-collector junction and higher recombination in the emitter region. Both mechanisms result from the higher current density flowing under the emitter region. Placing the base contact very close to the emitter edge increases the base current by increasing the gradient of the electron concentration toward the base contact. The effect of increasing the base doping in the extrinsic region has been simulated, and the results demonstrate that the current gain can be improved if a high doping concentration in the range of 5 × 10¹⁸ cm^-3 is used.