Radiation Effects of SiGe and III-V Based Material HBTs Technology: Comparison and trends

C P Jain1, K.B Joshi2

1Department of Electronics, Banasthali University, Banasthali, Rajasthan, India-3040222Department of Physics, M.L.Sukhadia University, Udaipur, India-313001

1cpjain.vlsi@gmail.com

Abstract

Silicon Germanium (SiGe) & III-V based material Hetero-junction Bipolar Transistor (HBT)

technology has been developed as a significant technology for both wired and wireless

application for its excellent performance in analog and RF application and compatibility with

CMOS process.

The study of radiation effect of SiGe & III-V HBT technology is very important to spacecraft

designers to introduce the latest technology into the system. In this paper, the Radiation hardens

characteristics of SiGe HBTs as well as III-V HBTs are measured in rad/s. The characteristics

are compared in with and without radiation for different doses of alpha particle and the

influences on above device are investigated and correlation between base and collector current in

SiGe and III-V HBTs are established to reduce radiation impact.

Keywords— Radiation, SiGe HBT, GaAs HBT, InP HBT, Rad Hard Bipolar devices

1. Introduction

In this paper, we mainly focus on to make device and circuit resistive or harden against

radiation environment and for bipolar process; for years, Si BJT has been widely used all over in

the semiconductor and integrated circuit technologies [1],[2],[3]. A massive research that has

been done into silicon technology over the many years has resulted in incredibly cheap, reliable,

and simple manufacturing and design processes [3]. The mysterious world of the III-V transistor

and other band gap-engineered technologies occupies only a small slice of the overall

semiconductor pie due to the expensive cost and low-yield typical of these advanced platforms

[2]. These technologies are used only for those applications where we require the highest speed,

lowest-noise, and highest gain [2].

2Presently at Department of Physics & Astronomical Science, Central University of Himachal Pradesh, Post Box 21, Dist- Kangra, HP- 176215

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Clearly, a space qualified IC technology must show sufficient radiation tolerance to support high

speed circuit applications as well possess total dose tolerance. SiGe & III-V HBT technology has

generated considerable interest in the space community due to its robustness to total ionizing

dose radiation (TID) without any additional hardening[4]. But, recently it has found that high

speed SiGe HBT digital logic circuits were vulnerable to SEU [5]. Hence it is important to study

the radiation effect on HBTs. To understand these effects in HBTs [6], we use calibrated two

dimensional (2-D) device simulations to assess the characteristics of SiGe HBTs.

2. Simulation Details

Radiation dose is defined as the amount of energy deposited into unit mass of the material of

interest. The units of radiation dose are 1 Gy (Gray) = 1 Joule/Kg = 100 rads. Since this energy

loss is material dependent, the type of the material is appended onto the unit. A dose is always

referenced to a mass of material. The term dose rate is used to indicate the dose per unit time

(e.g. Gy/min or Rad/min) that a device will experience at a position from a radiation source.

In Technology Computer Aided Device (TCAD) tool, Devices simulation is divided into two

different models that are calculated simultaneously at each DC bias point or transient time step.

l. Optical ray trace using real component of refractive index to calculate the optical intensity at

each grid point.

2. Absorption or photo-generation model using the imaginary component of refractive index to

calculate a new carrier concentration at each grid point.

Here we define the photo-generation rate in a C-INTERPRETER function written into a text file

that can be supplied to the program. The file returns a time and position dependent photo-

generation rate to the program. This returned value is multiplied at every node point defined

during mesh initialization.

The X. ORIGIN, Y. ORIGIN and X. END, Y. END parameters set the coordinates of the starting

and the last point of the line segment. The default values correspond to the top left and bottom

left corners of the device considered.

The Standard beam input syntax allows specification of plane waves with Gaussian or flat-top

(top-hat) irradiance profiles.

In this simulation, we have assumed that the radiation on the device has created a uniform Photo-

generation rate of different dose rate vary from 1E10 to 25E30 on devices.

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3. Device Characterization

3.1 Bipolar Technology

In this simulation, the Ic verse Vce characteristic is plotted for different values of base current

of the silicon BJT using Gummel model.

To construct the NPN transistor structure, in the initial stage, we specify the mesh, region and

electrode statement in TCAD ATLUS. In doping state statement, we added analytical doping

profile statement. The structure has a heavy n+ emitter 1.0e18 peak base concentration, a buried

collector layer and heavy p+ extrinsic base contact.

In contact statement, we define the emitter contact as an N-type polysilicon emitter. Surface

recombination is also specified at the polysilicon/silicon interface. Next, the material statement is

used to specify the electron and hole SRH lifetime. The model bipolar statement is used to

specify that the default of bipolar model to be set. The default set of bipolar model includes:

concentration dependent mobility, field dependent mobility, band gap narrowing, concentration

dependent lifetime and auger recombination.

Figure 1 Cross-section of Bipolar Junction Transistor

From figure 1 it is seen that, as we increase the radiation dose, the drain current increases. These

trapped charges produce new interface states at the frontier SiO2-Si interface that also decreases

minority carrier lifetime and increases the junction leakage current. The impact of oxide trapped

charges on bipolar devices is much smaller than for MOSFETs because the oxide is not an active

part and the surface doping is much greater for bipolar transistors than for MOSFETs. Both bulk

and interface defects decrease the overall gain and increases the leakage current. The major

effects of radiation on bipolar transistors are a gain loss and an increase of leakage current. The

degradations are higher for power bipolar transistors, especially at low currents. The damages

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that produced from a high dose rate exposure of a bipolar transistor are smaller to the ones that

can be found with a low dose rate [3].

In this simulation, the syntax for the Ic/Vce simulation is described in considering above NPN

structure. The key point is the use of current boundaries conditions on the base contact as defined

by contact name = base current. This allows constant base current to be forced while Vce is

ramped. The family of Ic/Vce curve can be overlaid in plot.

Figure 2 Overlay results of Id/Vd for dose 7e12 Figure 3 Results of Id/Vd for dose 7e12

Figure 4 Result for dose2E20 Figure 5 Result for dose2.7E25

3.2 The SiGe HBT Technology

The study of radiation effects on SiGe technology can be divided into two main topics. Total

ionizing dose (TID) and single event effects (SEE). TID effects are those caused by long-term

exposure to radiation that slowly ionizes the oxides within a device and usually appears as

increased base leakage current [4].

SiGe HBTs have posed a desirable side benefit of possessing an inherent hardness to ionizing

radiation, and have been shown to be TID tolerant to multi-rad dose levels without any additional

hardening. This unique feature of SiGe HBTs is because of the presence of Ge in the base region

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as the major source of degradation in Si transistors is the ionization damage in the emitter-base

(EB) spacer oxide. The radiation hardness of SiGe HBTs, is a result of their natural structural

properties that is the heavily doped extrinsic base region, very thin EB spacer, and very small

active volume are the main technological features of SiGe HBTs that promisingly help the device

to be became TID tolerant. Yet, minor radiation induced degradation can still be observed in the

performance of SiGe HBTs. In forward-mode operation, ionization damage in SiGe HBTs will

cause a insignificant increase in the base current at low injection that will consequently degrade

the current gain. Although SiGe HBTs are TID tolerant in regard to single-event effects and

these devices lack immunity and require mitigation techniques that can be accomplished at the

circuit level or at the device level or at both levels [5], [6].

Figure 7 to figure 9 shows the Gummel plot characteristics of a SiGe HBT (which has not been

radiation hardened in any way) both before and after exposure.

The majority of this is similar to Silicon bipolar Gummel plot in BJT section, this description

will focus on the specific SiGe syntax.

In region statement, we define the SiGe region in the base of an NPN bipolar transistor. The

composition fraction of the area inside of x.min, x.max, y.min, and y.max is defined by the

x.composition parameter. Outside this box the Ge composition fraction rolls of linearity to zero

over the distance specified by the grad.* parameters.

The material parameters for the SiGe are defined by material material = SiGe… statement. Here

the life time is set differently for the SiGe and Si region. In this model, we specify common

models for Si and SiGe regions, but this is not essential. The material parameter of the model

statement can be used to set separate model for each material.

The electrical part of this file is same as the silicon file above.

Figure 6 Cross section of SiGe HBT Transistor

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Figure 7 Result for dose 1E18 Figure 8 Result for dose 2E20

Figure 9 Result for dose 10E20

3.3 GaAs /AlGaAs HBT Technology

In this simulation we take the simulation of an AlGaAs/GaAs HBT to extract Ic and Ib verse

Vbe and plot the bipolar gain. The majority is similar to the Silicon bipolar Gunmmel plot. Here

we focus on specific AlGa and AlGaAs syntax.

In the region statement we used the graded AlGaAs composition fraction. The parameter x.comp

defines the Al content inside the box defined by x.min, x.max, y.min, y.max statement. Outside

the box the Al content fails linearly to zero in a distance grad.*. Since devices constructed in

ATLAS syntax must be in rectangular shape, a silicon dioxide region is used to fill the space to

the left of the emitter.

The low field motilities for the AlGaAs region is set in the material statement. Keep in mind that

the concentration dependent mobility model ‘conmob’ can be applied to GaAS so two model

statement are used to defined the model used in all region and then just the GaAs region.

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The model statements are used to specify the following set of models: field dependent mobility,

SRH and optical recombination. Optical recombination (also known as band to band

recombination) is important effect in III-V devices. The parameter ‘optr’ sets this model on.

The electrical simulation is same as in silicon BJT.

Figure 10 Cross-section of GaAs HBT Transistor

Figure 11 Characteristic without radiation

From figure 11 to figure 15, it is seen that to illustrate GaAs field effect transistor (FET) does not

have any oxide that can trap charges. Little threshold voltage shift resulting from charge trapping

is experienced. Radiation creates defect centers in the emitter and base materials. These two

effects increase the base current and lead to a small decrease of current gain [7].

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Figure 12 Result for dose 1E18 Figure 13 Result for dose 2E20

Figure 14Rresult for dose 2E15 Figure 15 Result for dose 2E20

3.4 InP/InGaAs HBT technology

An HBT structure based on the InGaAs-InP material created. The structure is then passed to

ATLUS for electrical stimulation. The input file consists of the following two main stages.

The HBT geometry material region, doping profile, and electrodes are defined in the first stage

of the input construct in DEVEDIT. The p-n-n device is based on a lattice matched InGaAs-InP

material system. It consists of highly doped InGaAs cap region followed by another cap region

made of InP. The next region, also made of InP, constitute the actual emitter. As usual in HBTs

the emitter (InP) has wider energy band gap than the base and collector (InGaAs). The base is

followed by the n-subcollector and n+ collector regions. The substrate is made of undoped InP.

In this structure each region was uniformly doped. The Ga composition fraction was also

specified. Finally the mesh was generated automatically by specified basic mesh constraints and

refining it along x- and /or y- direction in the important areas of the device [8].

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Figure 16 Cross section of InP HBT Transistor

The first active statement in the ATLAS portion of the input file is material parameter and

models definition. Here the energy band gap, densities of states, and other fundaments material

parameter for InGaA are defined based on the composition specified. the respective default value

are used for InP. The band alignment is defined using the align parameter on the material

statement. Material and model parameters can be defined in ATLAS on material by material or

region by region basis. The latter possibilities are used here to define carrier lifetime, low field

motilities, and saturated velocities taking into consideration the doping levels in the respective

regions.

The same set of physical models is applied here to all the region/materials. Schockley-Read- Hall

recombination, electric field dependent motilities with GaAs- like velocity-field characteristic,

and Fermi- Dirac statistics. To reflect the different properties of the material with regard to

critical field, ecrtn defines in separate model statement for InP and InGaAs material.

The simulation is first performed to obtain the Gummel plot by biasing simultaneously the base

and collector with respect to the emitter up to 12 V.

Figure 17 Characteristic without radiation

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The simulated ID - VG and IB-VG characteristics of an InP HBT transistor with different levels of

hot carrier damage .As expected the threshold voltage increases as a result of oxide interface

charge. As a result changes in drain current and base current by using different radiation doses

applies.

Figure 18 Result for dose 1E15 Figure 19 Result for dose 10E20

Figure 20 Results of InP HBT Id/Vd for dose 2*10E6 Figure 21 Results of InP HBT Id/Vd for dose 1*1E14

4. Conclusion

In this paper we have given an overview of radiation effects in Silicon-Germanium hetero

junction bipolar transistors (SiGe HBT), BJT and GaAs/AlGaAs HBT & InP/InGaAS HBTs. We

start by reviewing HBTs and study the impact of ionizing radiation with and without radiation.

This effects are pronounced in SiGe HBT circuits ensure adequate tolerance for many orbital

missions. SiGe HBT, GaAs HBT & InP HBT technology thus offers many interesting

possibilities for electronic systems.

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Acknowledgement

The author wishes to acknowledge Prof. A. Shastri for guiding and providing research

facilities at Banasthali University. The author (CPJ) is grateful to Department of Science and

Technology, New Delhi for supported this work through SERC grant.

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