GaN based Heterojunction Bipolar Transistors

University of Notre Dame

GaN based Heterojunction Bipolar Transistors

John Simon

EE 666

April 7, 2005

University of Notre Dame EE666: Advance Solid State Devices

OUTLINE

• Introduction

• Why GaN ?

• First GaN HBT

• Polarization Doping

• Collector up Structure

• Emitter up Structure

• Future Alternatives

• Conclusions


INTRODUCTION

2

2

iE

iB

pE

nB

B

E

n

n

D

D

GN

GN

Improved speeds can also be obtained with graded base technology.

Heterojunctions allow us to dope the base heavily reducing the base resistance and still maintaining a large gain (β).

kT

EE gBgE

e


Why GaN?

Break down Fields ~150kV/cm

Saturation Velocities ~3.5x107cm/sec


HBT Requirements

• High Gain: High Emitter Injection Efficiency (), provided by

Heterojunction(s) High Base Transport Factor (~1), requiring a good quality p-type

base region (in npn structure), high minority lifetime in base, proper base design.

• High Breakdown Voltage: Low doping in collector.

• Good RF Performance: Low base resistance, given by high base conductivity. Good ohmic contacts to base.


First GaN HBT

• First GaN HBT grown by MOCVD at UCSB in 1998.

• Current gain of only 3.• High Acceptor Activation

energies in GaN give poor p-type lager.

• Thick base (200nm) needed for low base resistance.

• Base doping of 4x1019cm-3 resulting in a hole concentration of 1x1018cm-3

McCarthy L S, Kozodoy P, Rodwell M, DenBaars S and Mishra U K 1999 First demonstration of an AlGaN/GaN heterojunction bipolar transistor Proc. Int. Symp. on Compound Semiconductors (Nara, Japan)

Sapphire Substraten+ GaN Subcollector

n- GaN Subcollector

Mg Doped Base

n+ Emitter

AlN Barrier

Regrown Base

Etched Surface


First GaN HBT

• Regrown Base was needed to make ohmic contacts to the base.

• Etch surface was shown to have rectifying effects on contacts.

• Nitrogen vacancies created during RIE have donor like characteristics.

McCarthy L S, Aluminum Gallium Nitride / Gallium Nitride Heterojunction Bipolar Transistors, PhD Dissertation UCSB 2001.


First GaN HBT

• Memory Effect present in all MOCVD grown samples.

• Emitter-Base junction placement is erratic.

• No memory effect in MBE grown samples and no annealing of p-type layer is required.

H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari,R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang,J J Xu, B P Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13 7139 (2001).


Regrown Emitter Structure

• Regrown Emitter structure developed.

• Eliminates memory effects and etch damage of base.

• Base was made thinner (100nm) for improved base transit time.


n- GaN Subcollector

Mg Doped Base

n+ EmitterAlxNy


Regrown Emitter StructureBase Contact I-V

Abrupt Emitter-Base Junction

H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari,R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang, J J Xu, B P Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13 7139 (2001).


RF Performance

• Current gains as large as 10 have achieved with this structure.

• Early voltages as high as 400V are estimated.

• High Emitter-Collector leakage attributed to donor like dislocations in GaN.

• Dislocations are present in both HBT structures.

H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari,R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang,J J Xu, B P Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13 7139 (2001).


LEO HBT

• GaN HBT’s were grown at UCSB via Lateral Epitaxy Overgrowth (LEO)*.

• Devices grown over windows exhibited a much larger leakage current than devices grown on the LEO regions.

• Gain in both devices was comparable.

• Threading Dislocations do not contribute to minority carrier recombination in the base.

* McCarthy L, Smorchkova Y, Fini P, Xing H, Rodwell M, Speck J, DenBaars S and Mishra U 2000 BT on LEOGaN Proc. 58th DRC: Device Research Conf. (Denver, CO, 2000)

H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari, R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang, J J Xu, B P Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13 7139 (2001).


Improved HBTCommon Emitter Operation as high as 330V.

Huili Xing, Prashant M. Chavarkar, Stacia Keller, Steven P. DenBaars and Umesh K. Mishra. IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 3, MARCH 2003.


Polarization in Nitrides

• Polarization fields present in wurtzite structure of nitrides allow for new novel devices.

• Polarization charges are created by differences in Polarization Fields.

Ga

N

P

P In [0001] direction:

σ = n·(P1-P2)



• Two types of Polarization in Nitrides: – Spontaneous Polarization

– Piezoelectric Polarization

• Gives us two degrees of freedom to determine the polarization charge:– Semiconductor Composition

– Layer thickness

Debdeep Jena, Polarization induced electron populations in III-V nitride semiconductors Transport, growth, and device applications. PhD Dissertation UCSB (2003)



• Electrostatic attraction from polarization charges creates regions of mobile charges.

qΦb

ρ

σMET

σPOL

2-DEG x


GaN HEMT

• Polarization doping has been used in High Electron Mobility Transistors (HEMT).

• Polarization doping can increase the effective AlGaN/Gate Barrier.

• No need to introduce dopants.

• Higher gm at higher voltages.

P.M. Asbeck, E.T. Yu, S.S. Lau, W. Sun, X. Dang, C. Shi. Solid-State Electronics 44 (2000) 211±219


Polarization Doping

• By grading the Metal composition we can create 3-D bulk doping.

GaN

AlxGa1-xN

Gra

ded

up

x

Polarization Charges

3-DEG

ρ


Polarization Doping

• Same techniques can be used for p-type doping.• Two configurations of HBT’s result from this:

– Emitter up Configuration– Collector up Configuration

AlxGa1-xN

GaN

Gra

ded

dow

n

xPolarization Charges

ρ

3-DHG


Collector up

• Using the Collector up configuration polarization doping in base is produced.

• Base will produce a dopant free p-type layer improving the base conductivity.

Sapphire Substrate

n+ AlGaN Emitter

AlGaN Graded Base

n+ Subcollector

Graded down

n- Collector


Collector up

• As Collector area scales down so does collector current.

• Extrinsic emitter base current becomes more dominant.

• Minority carriers injected into the base contribute to base current.

• Transistor gain is suppressed.



Collector up



Emitter Up

• Switch crystal orientation.

• N-face GaN gives opposite polarization charge allowing p-type doping of the base.

• Growth issues are present with N-face GaN Sapphire Substrate

n+ GaN Subcollector

n- GaN Subcollector

AlGaN Graded Base

n+ Emitter

Graded up


Alternative InGaN


n- GaN Subcollector

InGaN Graded Base

n+ AlGaN Emitter

Increasing In

Advantages:

• Can keep Emitter up structure and still produce the polarization doped p-type base.

• InGaN smaller band gap, larger band offset.

Disadvantages:

• Spontaneous polarization is almost identical in InN and GaN

•Hard to produce polarization charges.

•Difficult to grow In rich InGaN.

•Higher base transit times.


Conclusions

• GaN HBT’s have tremendous potential for high power applications.

• p-type conductivity is the limiting factor for all GaN base devices today.

• Normally doped GaN HBT’s have been demonstrated, with operational voltages as high as 330V.

• Polarization doping gives a promising solution to the p-type conductivity problem.

• Growth technique as well as device design must be carefully chosen.

Documents

GaN based Heterojunction Bipolar Transistors