Ulrich Seiler

High performance 100 nm T-gate strained Si/Si0.6Ge0.4 n-MODFET

Abstract 100 nm T-gate strained Si/Si 0.6 Ge 0.4 n-MODFETs have reached new record cut-off frequency f T of 74 GHz (105 GHz), with maximum oscillation frequency f max of 107 GHz (170 GHz) at temperatures 300 K (50 K). Moreover they show a low noise figure NF min of 0.4 dB and noise resistance R n of 52 Ω at 2.5 GHz and 300 K. The dependence of electric parameters and RF performances of the device on biases and temperature is presented. Experimental results are compared with physical simulations at short gate lengths to analyze carrier transport and further device optimization.

Thermionic diffusion model for abrupt HBTs including self-heating inside the multilayer nonplanar device structure

Abstract A thermo-electrical model which analytically describes the current flow inside abrupt heterojunction bipolar transistors (HBTs) using thermionic diffusion theory was developed. This model calculates numerically the three-dimensional temperature distribution inside the multilayer, nonplanar device structure with the help of a semi-analytical approach. This semi-analytical approach consists of a finite element program computing the heat flow in the mesa structure and an analytical solution of the temperature distribution in the substrate. The influence of the nonplanar and multilayer device structure and the effect of metal airbridges on the HBT performance is determined using this model. The phenomenon of thermal runaway in common emitter configuration is explained.

Current-temperature feedback effects in III-V heterojunction bipolar transistors

The consequence of the reciprocal relation between the temperature and current distributions in heterojunction bipolar transistors (HBTs) has been determined. The dc current voltage (I-V) characteristics, RF small-signal parameters, and temperature distributions of discrete devices with emitter fingers of varying lengths were analyzed empirically and their thermal profiles calculated numerically. The lateral temperature gradient induced in the finger due to power dissipation under normal operating conditions is shown to directly affect the current distribution in the transistor. The negative temperature dependence of the HBT base-emitter junction turn-on voltage results in positive feedback between current and temperature. This current temperature relationship leads to higher localized current densities in the hottest portion of the device, the center of the emitter. The temperature of the hot section rises with increasing power dissipation, continually drawing more current. Ultimately, the current through HBTs is localized to a comparable area at the finger center, independent of the emitter length.

Impact of thermal distribution and emitter length on the performance of microwave heterojunction bipolar transistors

Abstract The relationship between the output characteristics of heterojunction bipolar transistors (HBTs) and emitter finger length is determined on the basis of experimental observations and calculated temperature distributions. The emitter finger geometry is shown to be prone to developing a thermal gradient and ensuing non-uniform current distribution. The degree of thermal degradation is correlated to emitter length. Inherent disadvantages associated with longer emitters are presented.

High performance 100 nm T-gate strained Si/Si/sub 0.6/Ge/sub 0.4/ n-MODFET

100 nm T-gate Si/SiGe n-MODFETs are reported with new record f/sub T/ of 76 GHz (105 GHz), and with f/sub MAX/ of 107 GHz (170 GHz) at temperatures 300 K (50 K), low noise figure NF/sub min/ of 0.4 dB at 2.5 GHz and 300 K. Dependence on biases and temperature of HF performances and main parameters are presented. Experimental results are compared with data from physical simulations at short gate lengths to analyze carrier transport and device optimization.

Self-Aligned SiGe HBT Based on Combined Dry and Wet Etching

In this paper, a self-aligned SiGe/Si HBT was fabricated based on dry & wet etching, in which Si and SiGe materials were etched by KOH (potassium hydroxide) solution and SF6 (sulfur hexafluoride). The measured results are: cutoff frequency fT=103.3GHz, maximum oscillation frequency fmax=124.2GHz. 1. Mechanism of chemical reactions for etch The main chemical composition of etch solution is KOH which reacts with silicon as follows [1][2]: Si +2KOH+H2O=K2SiO3 +H2↑ (1) Germanium is not as active as silicon, and doesn’t react with KOH solution at room temperature. When SiGe is etched in the etching solution, germanium plays an obstructive role in etching, decreasing the etch rate greatly. The hydrolysis in water of H2GeO3 formed by reaction of Ge and etch solution is pasty deposit. The reaction is as follows. Ge + 4OH=GeO2.2H2O (2) Adsorptive power of the substrate is so high that SiGe alloy surface is protected. Therefore, KOH contributes little to etching SiGe alloy. Roughly speaking, KOH doesn’t etch SiGe alloy as an etch endpoint. Silicon was etched by SF6 and oxygen. At RF power, SF6 is dissociated into F groups that have high chemical activity and reacts easily with silicon, germanium, etc. The chemistry is as follows.

Thermally induced current constriction in III-V heterojunction bipolar transistors

The current through HBTs with different emitter lengths is shown to be constricted to practically identical areas as a result of the lateral temperature distribution and negative temperature coefficient of the base-emitter voltage.