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  • Introduction

GaP for THz

State of the art high-speed IHP SiGe:C HBT BiCMOS technology can be fabricated nowadays up to cut-off frequency (fT)/maximum frequency of oscillation/common-emitter breakdown voltage (BVCEO) values of 300 GHz/500 GHz/1.6 eV [2]. Future developments aim to reach even higher frequency values. Nevertheless, with increasing transistor speed one key problem remains still unsolved: To reach maximum device performance, HBTs run with large current densities in reverse bias at the collector-base (CB) junction. In this way, the CB electric field increases causing a reduction in carrier transit time over the junction. The transit time is determeind by the saturation velocity of the carrier in the material. Unfortunately, this electric field increase increases also the avalanche breakdown due to multiplied impact ionisation in the collector. Here, the avalanche coefficients heavily depend on the band gap value of the material. So, by increasing fT, we observe a drop of BVCEO as described in the material-related Johnson Limit [3]. This trade-off behavior limits the high-speed / high-power performance for classical SiGe:C HBTs.

One possible approach to counteract this effect is to change the Si-based collector material of the HBT device towards new material compounds with suitable saturation velocity / band gap properties to improve fT and BVCEO simultaneously. Within the frame of this project, indium gallium phosphide (InGaP) was identified as potential candidate to replace Si as collector material in future SiGe HBTs. Main physical parameters of the binary materials InP and GaP, important for SiGe HBT speed and power performance increase, are shown in comparison to Si [5] in Table 1:

 

 

Si

InP

GaP

Band gap

[eV]

1.12

1.33

2.26

Breakdown field

[V cm-1]

3×105

5×105

1×106

Electron mobility

[cm² V-1 s-1]

1400

5400

250

Saturation velocity

[cm/s]

1×107

3×107

1.3×107

Lattice constant

[nm]

0.5431

0.5869

0.5451

 

The following main insights can be drawn from these bulk parameters:

 

  • On the one side, InP has a three times higher saturation velocity than Si, offering the potential to increase the HBT speed performance by reducing the CB junction transit time.
  • On the other side, the wide band gap semiconductor GaP with a two times bigger band gap than Si decreases impact ionisation rates in the collector, providing that way a possibility to increase HBT power performance.

 

Together, the ternary In1-xGaxP (x = 0 - 1) material system offers as potential collector material the opportunity to improve speed and power of HBTs in a flexible way as a function of chemical composition x.

 

As starting point for evaluating InGaP as potential SiGe HBT collector, IHP´s heteroepitaxy project focuses on growth and material science characterization studies of GaP/SiGe/Si(001) heterostructures. GaP is chosen as starting material due to its small lattice mismatch with respect to Si (0.36 %) (Tab. 1). As known from GaP on Si growth studies for photonics and microelectronics, heteroepitaxy challenges for achieving high quality GaP/SiGe/Si(001) heterostacks are given by crystallographic and thermal lattice mismatch, polar/non-polar interfaces, 3D versus 2D growth modes, interdiffusion as well as defect formation (stacking faults (SFs), microtwins (MTs), anti-phase domains (APDs) etc.) [4-9].

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