microwave solid state power devices yonglai tian

Microwave Solid State Power Devices

Yonglai Tian

Introduction of microwave power devices Performance of Si and GaAs microwave devices Wide bandgap semiconductors for microwave applications Processing of WBG silicon carbide wafers SiC microwave power devices GaN microwave power device

Various types of microwave power devices

Magnetron Traveling wave tube

KlystronGyrotrons

Disadvantages

• Large size• Heavy • Fixed frequency• Complicated power supply (HV)• Poor quality of waveform

spectrum• Slow tuning and coupling

• Cost

Single mode cavity for Microwave sintering of advanced ceramics

Multiple DoD platform will benefit from microwave solid state devices and WBG semiconductors

Electrodeless HID lamps driven by microwaves

1400w magnetron driven HID lamps,

200w aperture HID lamps (7mm) driven by solid state microwave devices

Various types of microwave solid state devices

Bipolar Junction Transistors (BJT)– Si BJT

– HBT (hetero junction bipolar transistor) AlGaAs-GaAs HBT SiGe-Si HBT

Field Effect Transistors– GaAs MESFET (metal-semiconductor field effect transistors)

– HEMT (high electron mobility transistors)

Various types of microwave solid state devices

• Wide Bandgap Transistors– SiC

SIT (static induction transistors) MESFET (metal-semiconductor field effect transistors) HBT (hetero junction bipolar transistor)

– GaN HEMT (high electron mobility transistors)

Performance characterization

• Out put power Pmax

Pmax Vmax x Imax

– Vmax: Voltage breakdown

– Imax: Heat removed, gate width and length

• Power Density PD

PD = Vmax x Current density– Vmax: Voltage breakdown

– Current density: limited by bandgap and thermal conductivity

Performance characterization

• Frequency

f max (Vs/L)

– Vs. saturated carrier velocity

– Gate length

Pma α 1/f2

• Efficiency PAE

Depends on wave shape, impedance, leakage current and power gain

Widely used Si microwave devices

• Si BJT– < 5 GHz– 100-600W at 1 GHz– > 40% Efficiency– Low cost

• Limitation:– Pmax: voltage breakdown and

current (limited by emitter periphery and resistivity of epitaxial layer)

– f : limited by carrier mobility,

capacitance C bc

A typical Si BJT characteristics

– Frequency; 2.7-2.9 GHz

– Output power: 105 W– Pulse width: 50 m– Duty cycle: 10%– Gain: 6.5 db (min)– Efficiency:

40% (min)– Supply voltage: 40V

GaAs MESFET (Metal semiconductor field effect transistors)

• GaAs MSFET– 3-30 GHz– Power density: 0.5-0.8 w/mm– Power level and cost:

Frequency band Power (W) cost ($)

C and S 10 300 20

600 30 900

Ku 10 1000 15 1500

• Limitation: – f and Pmax: gate length, thermal

conductivity

HEMT and HBT

HEMT (High electronic mobility transistor)

• AlGaAs-GaAs heterojunction– 5-100 GHz– High frequency – High Pmax

– High efficiency– Low noise

HBT (heterojunction bipolar junction transistor)

• Similar to BJT, but much higher power and frequency performance

State of the art power output performance

State of the art power density performance

State of the art PAE performance

Evolution of microwave device noise figures

Advantages of wide bandgap semiconductors(SiC, GaN and diamond)

1. Wide bandgap– SiC: 3.2 eV

– GaN: 3.4 eV

– Si : 1.1 eV

– GaAs: 1.4 eV

3 times higher than that of Si and GaAs

– High service temperature of 650 oC due to the high intrinsic temperature

– Low noise

Advantages

2. High breakdown voltage – SiC: 10 times higher than that of

Si and GaAs

– High output power due to high V– High operating frequency– Short-channel MESFETs in SiC

Fmax : 50 GHz

Advantages

3. High thermal conductivity– SiC 4.9 w/(C-cm)


– Si: 1.6 w/(C-cm)

– GaN 0.5 w/(C-cm)

4. High Saturated velocity – SiC 2.2 x107 m/s


– Si and GaAs: 1 x107 m/s

Physical characteristics of Si, GaAs and main wide bandgap semiconductors

WBG semiconductor material challenges

Growth of SiC single crystal

• J. A. Lely , Philips Labs 1955 sublimation process for growing a-SiC crystals

• Davis at North Carolina State University (NCSU), 1987 seeded-growth sublimation process

• Cree Res, started in 1987 by students from the NCSU.

• Cree, 1990, Introduction of 25 mm single crystal wafers of 6H-SiC

1990

Physical vapor transport (PVT) growth of SiC single crystal wafers

• PVT growth process:– Evaporation of SiC charge

materials

– Transport of vapor spices to the growth surface

– Adsorption surface diffusion and incorporation of atoms into crystal.

– Temperature: 2000-2300oC T of 10-30C controlled by

moving RF coil

– Growth rate controlled by T and pressure in reactor

Defects in SiC wafer

• Micropipes– breakdown at low voltage

• Dislocations• Low angle grain boundaries• Stacking faults

GMU WBGS research projects:Ion implantation of SiC wafers

• Ion implantation is the only viable selective area doping techniques for SiC device production

• N and P were implanted in p-type and Al and B were implanted in n-type 6H-SiC using single and multiple ion energy schedules ranged from 50 KeV to 4 MeV

• Second ion mass spectrometry measurements (SIMS) were conducted to obtain the implant depth profiles

• Doping layer theickness.

N-implanted SiC (50 KeV to 4MeV at 700oC)

B-implanted SiC (50 KeV to 4MeV at 700oC)

Multiple energy P-implanted SiC

Rapid annealing of ion implanted SiC

• The crystal lattice is damaged by the penetration of ion energetic ions

• Post annealing is necessary to recover the lattice damage

• Microwave and conventional annealing at 1500C – Microwave: Heating rate; 200oC/min, total time: 20 min.

– Conventional: heating rate: 10oC/min, total time 3 hr.

• Rutherford backscattering (RBS) measurements are conducted before and after ion-implantation to study the recovery of the crystal lattice.

RSB spectra on N-implanted SiC

Sheet resistivity of annealed SiC wafersGMU data

Sheet resistivity of nitrogen-implanted 4H-SiC as a function of time and temperature.

Sheet resistivity of phosphorus -implanted 4H-

SiC as a function of time and temperature.

Best Reported Sheet Resistivity of Ion Implanted SiC

Figure 1. Sheet resistivity of nitrogen implants into 6H silicon carbide at room tmperature

. Figure 2. Sheet resistivity of Al implants into 6H silicon carbide at room tmperature

SiC microwave power devices

High power 4H-SiC static induction transistors (SITs)

– Vertical short channel FET structure

– Current flow vertically by modulating the internal potential of the channel using surrounding gate structure

– Characteristics similar to a vacuum-tube tiode

– 470W (1.36 /mm) at 600 MHz– 38 W (1.2 w/mm) at 3 GHz– PAE ~ 47%

High power 4H-SiC static induction transistors (SITs)

Measured static I-V characteristics of a SIT

Cross section of a SiC SIT

SEM photo of a SIT device. The mesa fingers are 1 µm wide and 100 µm long. The total mesa length is 1 cm (100 fingers).

• The best performance– High output power; 900 W (at 1.3

GHz, drain efficiency = 65%, gain = 11 dB) [Northrop-Grumman/ Cree Inc]

– High frequency performance with a cut-off frequency of 7 GHz [Purdue]

A comparison of SIT with other relevant SiC

microwave devices..

High power SiC MESFET

• Three epitaxial layers– P buffer layer– Channel layer doped

Nd=3x1017cm-3

– Heavily doped n+ cap layer

• Performance– Pmax : 15W

– Frequency: 2.1 Ghz– Power density: 1w/mm– PAE; 54%

a

Cross section of SiC MESFET. The epitaxial layers were grown on a semi-insulating SiC substrate, including p-buffer layer and a n-doped channel layer

GaN Power High electronic mobility transistors

• two dimensional electron gas with a high mobility is formed at the AlGaN-GaN heterojunction interface, the mobility can be in excess of 1000 cm2/Vs

• High frequency 100GHz

• High power density: 10w/mm

• Base station microwave power amplifier

• highly linear mixers

• high power switches

microwave solid state power devices yonglai tian

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