Electrothermal Limitations on the Current Density of High-Frequency Bipolar Transistors

Abstract

Abstract-In this paper, electrothermal consequences of downscaling bipolar transistors, reducing the emitter resistance and implementing substrate modifications are examined by means of electrical measurements, numerical simulations and analytical calculations. A formulation is given for the optimum current density that can be run through the device and still maintain both sufficient transconductance and thermal stability. This expression sets a theoretical limit on the current density and therefore also on the speed of the given technology node. Particularly the lowering of the emitter resistivity is a trade-off between transconductance and thermal stability, and the optimum choice can be estimated from these results along with the maximum emitter area that will allow unconditional thermal stability.

Restabilizing mechanisms after the onset of thermal instability in bipolar transistors

“…B is a fitting parameter needed to account for the different temperature dependence of the diffusion coefficient of holes and electrons [18], and k is the Boltzmann's constant. Moreover, the temperature-dependent gain lowering at high-injection levels is described by (6) through the temperature-dependent parameters I H (T ) and m H (T ). Note that the term R HI is negligible at low and medium currents, whereas it becomes a decisive parameter at high-current levels.…”

“…with R HI and ϕ TOT defined as (6) and (8), respectively. Compared to the single-finger situation, V ∆T now accounts for the mutual thermal coupling R M , while Early effect and external resistors do not play any role due to the symmetry of the system [see Fig.…”

“…The positive current-temperature feedback may not only adversely affect the safe operating area of devices but also threatens to impose a fundamental limit to the speed [6]. Electrothermal effects in bipolar transistors have been the subject of extensive investigations in recent literature.…”

Thermal design of fully-isolated bipolar transistors

“…Particularly, the use of silicon-on-insulator substrates and trench isolation produces clear improvements in terms of reduced parasitics and minimization of crosstalk via the substrate, thus contributing to increased speed of RF devices and circuits. On the other hand, the poor thermal conductivity of most materials used to electrically insulate the devices enhances the thermal issues that could impose a limit on the current density of high-speed transistors in the close future [1].…”

Electrothermal stability of bipolar transistors at medium- and high-current operation regimes

“…The positive current-temperature feedback in bipolar transistors can lead to thermal instability [1], which not only influences the safe operating area of devices, but also threatens to impose a fundamental limit to the speed [2]. It is therefore crucial to determine all the physical effects that are responsible for transition from a thermally stable to thermally unstable regime of operation, and to develop accurate models with which the critical conditions can be predicted.…”