SiGe HBTs Optimization for Wireless Power Amplifier Applications

Abstract

This paper deals with SiGe HBTs optimization for power amplifier applications dedicated to wireless communications. In this work, we investigate the f_T-BV_CEO tradeoff by various collector optimization schemes such as epilayer thickness and dopant concentration, and SIC and CAP characteristics. Furthermore, a new trapezoidal base Germanium (Ge) profile is proposed. Thanks to this profile, precise control of Ge content at the metallurgical emitter-base junction is obtained. Gain stability is obtained for a wide range of temperatures through tuning the emitter-base junction Ge percent. Finally, a comprehensive investigation of Ge introduction into the collector (backside Ge profile) is conducted in order to improve the f_T values at high injection levels.

1. Introduction

SiGe heterojunction bipolar transistors (HBTs) integrated into BiCMOS technologies have been providing cost-effective solutions to many of the building blocks of RF and microwave transceivers. Power amplifiers have remained an exception to this trend for some time, but SiGe HBTs have emerged as competitive alternatives to III-V devices for RF power applications in wireless handsets and cell phones [1]. State-of-the-art SiGe/Si HBTs dedicated to power amplifier application present a 𝑓_𝑇 of 27 GHz with a BV_CEO of 8.5 V [2].

However, for similar breakdown voltages, the achieved Johnson’s limit using SiGe power HBTs and their power-handling capability per unit emitter area are not comparable with III-V power devices since the minority carrier mobility and the bandgap are lower in Si compared with III-V counterparts [3].

To enhance SiGe HBTs performance in this field while maintaining its very competitive cost, further breakthrough in device design and optimization are mandatory.

In this paper, we focused on three main SiGe HBTs features improvements: device robustness, temperature behavior and HF characteristics.

The paper is organized as follows. First, process flow, RF power device structure and electrical device characteristics are illustrated. Results of static and dynamic performances are presented. Next, different optimization processes are described. These optimizations deal with collector tuning, Ge base profile engineering and Ge introduction into the collector layer. Finally, their impacts on the electrical characteristics are highlighted.

2. Device Structure Design and Fabrication Process

2.1. Process Flow Description

This study has been carried out on the 250 nm SiGe HBT architecture shown in Figure 1, which consists in a double polysilicon quasi-self-aligned structure. Basically, the process starts with a buried subcollector layer and subsequently grown epitaxial collector layer, followed by deep trench isolation and sinker formation. The nonselective SiGe base epitaxy and oxide/poly stack are deposited. Oxide/poly stack is patterned and etched. Next, a polybase/oxide stack is deposited. In this stack, an emitter window is then patterned and etched, stopping on the oxide/poly stack. Selective collector implant is realized and then D-shape inside spacers are fabricated. Afterwards, the pedestal oxide is wet etched and arsenic in-situ doped poly-Si is deposited to form the emitter. After poly-emitter patterning, flash activation anneal, silicidation and contact formation completed the fabrication sequence. Trapezoidal Ge profile use for the SiGe:C base is presented on Figure 2.

Figure 1 

0.25 μm SiGe HBT architecture SEM cross-section.

Figure 2 

Trapezoidal Ge base profile.

The power SiGe HBT cell is presented on Figure 3. It consists in 8 emitter fingers, with drawn dimensions of 1.6∗13.4 μm² for each finger. The values of ballasting resistors are carefully selected to warrant thermally stable device operation at high power levels.

Figure 3 

Power SiGe HBT cell.

2.2. Electrical Characterization of Fabricated Devices

Fabricated devices have ideal static characteristics down to very low V _BE (Figure 4). The current gain is 170. Peak 𝑓_𝑇/𝑓_MAX values are 26/38 GHz (Figure 5). Measured breakdown voltages are 6.45 and 19.25 for BV_CEO and BV_CBO, respectively.

Figure 4 

Static characteristics of power SiGe HBT cell.

Figure 5 

Dynamic characteristics of power SiGe HBT cell.

3. DC/RF Performance Optimizations

3.1. Collector Tuning

In order to achieve high breakdown voltages (BV_CBO and BV_CEO) required for power amplification applications, epitaxial collector layer characteristics (thickness and doping concentration) have been optimized as well as dose and energy of the SIC implant.

The epitaxial collector layer thickness has been varied. Figure 6 shows BV_CBO improvement (from 16 to 23 V) with epitaxial collector layer thickness increase from 0.8 to 1.4 μm. BV_CEO is also improved from 6.5 to 11.2 V. However, due to the transit time rise associated with the increase of epitaxial collector layer thickness, the dynamic device response is reduced. In this case, maximum 𝑓_𝑇 value decreases from 26 to 15 GHz (Figure 6). Regarding BV_CEO optimization, variations on epitaxial collector layer doping concentration between 1⋅10¹⁵ and 1⋅10¹⁶ at/cm³ have been realized. Decreasing epitaxial collector layer doping concentration, BV_CEO has been improved from 6.45 to 7.23 V. This results are obtained at constant 𝑓_𝑇∗BV_CEO product.

Figure 6 

BVCBO and 𝑓𝑇max variations with epitaxial collector thickness.

BV_CEO enhancement also relies on SIC characteristics. Electrical results obtained for different SIC characteristics are presented in Table 1.

Table 1 

BV_CEOand maximum 𝑓_𝑇 value obtained for different SIC characteristics.

Standard implantation consists in phosphorus 2⋅10¹² cm⁻², 400 keV. Variations consist in dose as well as energy implant. Through energy implant increase and dose reduction, higher BV_CEO values are obtained, but always with a constant 𝑓_𝑇∗BV_CEO product. In order to overpass this constant product, another way for optimization has been looked for. CAP thickness has been increased. The CAP thickness layer control the minority charge in the emitter. The latter impacts directly the emitter transit time as well as the Gummel number and so the current gain. Current gain reduction implies a BV_CEO increase. Moreover, the CAP thickness has an impact on the base-emitter space charge region that modifies the pinched base resistance. As a consequence CAP thickness increase reduces the current gain that enhances the breakdown voltage, reduces the emitter transit time that increases the transit frequency 𝑓_𝑇. These results are summarized in Table 2. By that way, the 𝑓_𝑇∗BV_CEO product is improved from 178 to 216 GHz * V.

Table 2 

Principal device parameters for different CAP thicknesses.

3.2. Ge Base Profile Engineering

It has been shown that current gain decrease at elevated temperatures in SiGe:C HBTs has become an issue [4]. Gain characteristic versus temperature obtained for standard germanium base profile device has been plotted in Figure 7.

Figure 7 

Gain characteristics versus temperature for standard Ge base profile.

Strong variation of maximum gain value with temperature is observed. Analyzing the current gain equation given below, we see that the temperature-dependence of 𝛽 can be minimized when the term in the exponential function is close to zero. Δ𝐸_𝐺 represents the bandgap narrowing in the base due to the presence of germanium content and Δ𝐸_𝑔 represents the bandgap narrowing due to high doping effects. Temperature stability can be obtained when Δ𝐸_𝐺≈Δ𝐸_𝑔.𝐼𝛽=^𝐶𝐼_𝐵𝐷=𝛾^𝑛𝐷_𝑝𝑊_𝐸𝑊_𝐵𝑁_𝐷𝐸𝑁_𝐴𝐵expΔ𝐸_𝐺−Δ𝐸_𝑔𝑘𝑇. (1) For this purpose, devices having special germanium profiles in the base with different Ge content at the base-emitter junction have been fabricated (see Figure 8). All along the Ge base profile, constant carbon dose is present. The main advantage of these Ge profiles is a precise control of base-emitter Ge content associated with beneficial Ge grading effect. Base-emitter Ge content has been varied from 3 to 6%. For the 3% Ge profile device, which results in best gain stability, device current gain versus temperature has been plotted in Figure 9. Over a wide range of temperature, (from −25°C to 125°C), the current gain is constant close to 100. Specific efforts have been made to improve the maximum current gain value. Reducing base boron concentration by 15%, gain of 110 with unchanged temperature stability is obtained.

Figure 8 

Special Ge base profile with different base-emitter junction Ge content.

Figure 9 

Current gain versus temperature for 3% Ge profile device.

Input impedance (𝑍_in) of the device for different temperature has been monitored. Connecting a vector network analyzer with couplers at the input of an automated tuner system allows the device under test input impedance to be accurately measured during load and source pull tests. This impedance was characterized for an operating point of 𝑉_CE=3.6 V and 𝐼_𝐶=30 mA. An operation frequency of 900 MHz was used for load-pull comparison. Knowing 𝑍_in temperature-dependence, input and interstage-matching circuits design is facilitated.

The imaginary part of 𝑍_in versus input power as a function of temperature for standard and 3% Ge profile has been plotted on Figures 10 and 11. At −10 dBm input power, input impedance variation with temperature for 3% Ge profile is 12% compared to 18% for standard Ge profile. For low power level, lower temperature-dependence with optimal 3% Ge profile is obtained. This fact is highlighted by Figure 12, which shows the imaginary part variation of the input impedance with temperature for a −10 dBm input power. A temperature-dependence reduction with emitter-base junction Ge percent decrease is clearly observed. Thereby, circuit mismatch linked to input impedance variation with temperature is reduced.

Figure 10 

Imaginary part of 𝑍in versus input power as a function of temperature for standard Ge profile.

Figure 11 

Imaginary part of 𝑍in versus input power as a function of temperature for 3% Ge profile.

Figure 12 

Imaginary part variation of the input impedance with temperature for a −10 dBm input power.

3.3. Investigation of Retrograde Ge Profile in the Collector

Physical simulations for HBTs with three different Ge profiles have been performed. Figure 13 shows the standard Ge profile together with two additional Ge profile shapes that extend deep into the collector (60 and 120 nm retrograde Ge profile) (see [5–7]). The simulated structure is in agreement with the HBT topology presented in Figure 1. Figure 14 represents simulated 𝑓_𝑇 versus 𝐽_𝐶 characteristics. For the device featuring the 120 nm retrograde Ge profile, 𝑓_𝑇 drops only by a value of 8 GHz as 𝐽_𝐶 increases from 0.5 to 1 mA⋅μm⁻² as opposed to a drop of 13 GHz in the case of the standard Ge profile. The high current range showing an improved 𝑓_𝑇 is clearly widened. This studied confirms the work done by Liang et al. [7].

Figure 13 

Simulated Ge profiles.

Figure 14 

Simulated 𝑓𝑇 versus 𝐽𝐶 characteristics.

The three above-described Ge-profile design structures were fabricated in the same wafer lot under identical processing conditions. Figure 15 presents the comparison between the measured 𝑓_𝑇 versus 𝐽_𝐶 characteristics. 𝑓_𝑇 maximum value close to 32 GHz is obtained for 60nm retrograde Ge profile. As the Ge retrograde profile extends wider into the collector, the device shows a higher 𝑓_𝑇 in the high current density range.

Figure 15 

Measured 𝑓𝑇 versus 𝐽𝐶 characteristics.

4. Conclusion

In this work, 𝑓_𝑇-BV_CEO behavior for various collector optimization schemes such as epilayer thickness and dopant concentration, SIC and CAP characteristics has been demonstrated. Furthermore, based on a mature 250 nm technology, an optimized SiGe:C base profile has been fabricated with a special carefully designed Ge profile dedicated to current gain stability for a wide temperature. The associated input impedance temperature-dependence reduction has also been highlighted. That way circuit design is facilitated. Finally, device simulation pointed out the improvement of high injection operation and 𝑓_𝑇 performance through the Ge retrograde profile in the collector, which has been confirmed by measurements. The devices with optimized profile exhibit improved 𝑓_𝑇 maximum value and wider 𝑓_𝑇 operating current density range, suitable for PA applications.