Current gain collapse in HBTs analysed by transient interferometric mapping method

Abstract

Thermal distribution during a current gain collapse event is investigated in multi-finger InGaP/GaAs HBTs using the transient interferometric mapping method. The onset of the collapse is observed at time of about 1ms in devices with a low emitter ballasting resistance R E , while for HBTs with a high R E , the current is distributed equally over the fingers. 3D thermal simulation supports the results and allows an estimation of temperature at which the collapse occurs. I. INTRODUCTIONThe self-heating effect is recognized as one of the major factor limiting the performance of heterojunction bipolar transistors (HBTs) [1][2][3][4][5][6][7][8][9][10][11][12][13]. In multifinger devices when a critical temperature is reached, the equal current distribution over the fingers can be unstable against a small temperature fluctuation and the current flow over the fingers can become inhomogeneous. The hottest fingers carry the most of the current (current crunching) inducing a positive feedback effect, which may be dangerous. The effect manifests itself as a decrease in the collector current causing a drop in current gain, hence the "gain collapse" effect. Electrical experiments and thermal analysis have been performed under the DC or transient conditions [1][2][3][4]7]. Thermal observations from the device top side under the DC conditions have been performed by infrared (black-body) thermometry [1]. However there is a lack of data of direct observation of the phenomenon under the transient conditions. On the other hand, it has been found that the proper increase in the emitter ballasting resistance [12] and optimization of thermal design [11] can decrease the HBT susceptibility to gain collapse, by keeping acceptable performances.In this paper we use the transient interferometric mapping (TIM) method [14] for the thermal imaging of multifinger InGaP/GaAs HBTs prior and at the onset of the gain collapse. Devices with a high and low R E are investigated.

RF Characterization of GaAs HBT under Load Mismatch with Reverse Wave Injection Technique

“…At the extreme, when the VSWR becomes infinite, this indicates total reflection, where no energy is transmitted at all [16]. The relationship between the VSWR and reflection coefficient can be expressed by (2). As can be seen, the PWR is any possible transformation of the power wave reflection coefficient to a parameter ranging from 1 to infinity.…”

“…This impedance mismatch significantly reduces the device's output power and efficiency and further decreases the performance of communication systems. Furthermore, as in all bipolar devices, HBTs suffer from various feedback phenomena, which may cause instability and device failure in certain operating conditions, such as self-heating [2][3][4].…”