# Presentation: 942074

##  GaN-based Micro Pressure and Temperature Sensors for Extreme Planetary Environments 

** ****GaN-based Micro Pressure and Temperature Sensors for Extreme Planetary Environments**** **

**Kyung-ah Son**
**Jet Propulsion Lab**
**June 29**
**IPPW 2006**
**Pasadena, CA**

## Electronic Architecture for CWA specific RF tagging

**Acknowledgement**

-  B. Yang at JPL
-  Y. Liu, P.P. Ruden at University of Minnesota
-  N. Biyikli, X. Ni, J. Xie, H. Morkoç
-   at Virginia Commonwealth University
-  R.E. Young and A. Colaprete at Ames Research Center
-  Financial support from NASA/PIDDP

## Temperature, Pressure, and Radiation in Reference Missions

**Chris Moore**, “Technology Development for Extreme Environments Systems”
- Workshop on Extreme Environments Technologies for Space Explorations, Pasadena, May 14, 2003.

## AlGaN/GaN Hetero Structure-based Micro Sensors 

- Merits:
-  Superior materials properties;
-   Mechanically strong, chemically & thermally inert, radiation hard
-   Minimal in unwanted optical or thermal generation of charge carriers
-     Ideal for operations at extreme environments
-  Small volume, low mass, and low power requirement
-  Novel device concept and simple and reproducible fabrication
-  Monolithic integration of sensors and GaN-based RF transceiver
-   with on-chip amplification; wireless remote sensing (X, Q, and Ka bands)

## AlGaN/GaN Hetero Structure Devices for Pressure Sensing

-  Sheet charge layer (i.e. 2-Dimensional Electron Gas) at the interface of the GaN and AlGaN layers;
-  Sheet charge density ~1x1013 /cm2
-   High electron mobility ~ 1500 cm2/Vs
-  Modulation of the interfacial polarization charge by applied stress /pressure is the basis of pressure sensing.

## AlGaN/GaN based High Pressure Sensors

- Expected operational range:
- 	0-10 kbar with 1 bar accuracy
- 	4 K - 870 K
- Expected volume: ~ 1 cm3
- 	  mass: < 5 g
- 	  power requirement: < 10 mW

## Conduction Band Diagram of GaN/AlxGa1-xN/GaN Device 

**Conduction Band Diagram of ****GaN/Al****x****Ga****1-x****N/GaN Device**** **

**Conduction band diagram of  an n-GaN/Al****0.13****Ga****0.87****N/n-GaN**
- 10 nm thick undoped Al0.13Ga0.87N layer and doping concentration of 2x1017 cm-3 in both n-GaN regions are assumed. The estimated polarization charge formed at the Al0.13Ga0.87N/n-GaN is 7.24 x1012 ecm-2 and a barrier height of 1.33 eV is calculated. A decrease of the barrier height by 12 meV is expected from 1% reduction of polarization charge, which corresponds to 60 % increase in thermionic emission current over the barrier.

## Theoretical Modeling of Pressure Effect onGaN/AlxGa1-xN/GaN (n-I-n) Sensor 

-  Current decreases with
-    increasing pressure.
-  Decrease of the current is more
-   significant with higher x and the
-   thicker AlGaN layer.

- Normalized change of current densities, (J0-J)/J0, calculated for
-  GaN/AlxGa1-xN/GaN heterojunction under 10kbar hydrostatic pressure

## n-GaN/Al0.2Ga0.8N/n-GaN Heterojunction Device

- Optical microscope image of
- n-GaN/Al0.20Ga0.80N/n-GaN heterojunction device.
- Epi layers are grown with MBE.

- top ohmic contact

- 250 m mesa

- bottom contact

- The AlGaN layer is 10 nm thick , and the doping density is 3×1018 cm-3 for the GaN layer above AlGaN layer, and  7×1017 cm-3 for the GaN layer below AlGaN layer.

## I-V Simulations of n-GaN/Al0.2Ga0.8N/n-GaN Sensor

- Calculated band profiles of
- n-GaN/Al0.2Ga0.8N/n-GaN at three applied voltages. The inset shows the effect of 10kbar pressure on band structures.

- Calculated current density at zero pressure, and the change of current density with 10 kbar pressure.  The AlGaN layer is 10 nm thick , and the doping density is 3×1018 cm-3 for the GaN layer above AlGaN layer, and 7×1017 cm-3 for the GaN layer below AlGaN layer.

## Electrical Responses of n-GaN/Al0.2Ga0.8N/n-GaN Sensor to Hydrostatic pressure

- Pressure response measured for the n-GaN/Al0.2Ga0.8N /n-GaN single barrier vertical transport device. Current was measured under hydrostatic pressure at a fixed forward bias (-0.2 V and +0.5 V).

-  Linear decrease of the current with increasing pressure

## High Pressure Test Setup at UMN

**High Pressure Test Setup at UMN**

- Gas pressure cell
- CuBe cell. He gas compressor
- Maximum pressure: 15 kbar

- Liquid pressure cell
- CuBe cell. Hexane/pentane medium
- Piston compressor
- Maximum pressure: 400 bar

## Electrical Responses of n-GaN/Al0.15Ga0.85N/n-GaN Sensor to Hydrostatic pressure

- Pressure response measured for the n-GaN/Al0.15Ga0.85N /n-GaN single barrier vertical transport device. Current was measured under hydrostatic pressure at a fixed forward bias (+0.8 V) while the pressure was increased (solid dots) and decreased (open dots) as well.

- Optical image of
- n-GaN/Al0.15Ga0.85N /n-GaN heterojunction device. Epi layers are grown with MOCVD

-  Linear decrease of the current
-   with increasing pressure
-  Reversible response

## Theoretical Modeling of Temperature Responses of GaN/AlGaN/GaN (n-I-n) Sensor 

- Calculated with
- doping density of 3×1018 cm-3 for top GaN layer and 3×1017 cm-3 for bottom GaN layer.
- 10 nm thick undoped AlGaN layer is assumed.

|  |
| --- |

- Current density vs. voltage plots for GaN/Al0.15Ga0.85N/GaN sensor at zero pressure

- Current decreases strongly
-  with decreasing temperature.

## Theoretical Modeling of Temperature Responses of n-I-n Sensor under High Hydrostatic Pressure

-  Current decreases with the applied
-   pressure.
-  The decrease gets more significant
-   at lower temperatures.

- Normalized change of current density under 10kbar for GaN/Al0.15Ga0.85N/GaN

- Calculated with
- doping density of 3×1018 cm-3 for top GaN layer and 3×1017 cm-3 for bottom GaN layer.
- 10 nm thick undoped AlGaN layer is assumed.

## Test Setup for Measuring Sensor Responses in Wide Temperature Range (JPL)

- Temperature controllable sample stage;
- ~100-670K

## Electronic Architecture for CWA specific RF tagging

**Summary**

-  Investigated n-GaN/AlxGa1-xN/n-GaN (n-I-n) devices for pressure sensors in extreme environments.
-  Theoretical modeling indicates decrease of electrical currents with increasing pressure due to the increase of polarization charge.
-  The modeling predicts more significant decrease of current with higher AlN compositions in the AlxGa1-xN layer and for the thicker AlxGa1-xN layer.
-  The vertical transport current measured with n-GaN/AlxGa1-xN/n-GaN (X=0.15 & 0.2) sensors is consistent with the modeling studies.
-  Linearity and reversibility in pressure response; n-GaN/AlxGa1-xN/n-GaN is promising for high-pressure sensing in extreme environments.

## GaN/AlxGa1-xN/GaN (n-I-n) Sensors Fabricated for Characterization of P & T responses

- Device structure

- Al*x*Ga1-*x*N  thickness: *t*

- Modifying barrier height with AlN mole fraction and thickness

**GaN/Al****x****Ga****1-x****N/GaN (n-I-n) Sensors Fabricated for Characterization of P & T responses**

## AlGaN/GaN HFET for Characterization of P & T responses

**AlGaN/GaN HFET ****for Characterization of P & T responses**

## Pressure Dependence of Al0.3Ga0.7N/GaN HFET Sensor

**Pressure Dependence of Al****0.3****Ga****0.7****N/GaN HFET Sensor**

- Relative change of saturation current with pressure

- Relative change of the drain current under 1.8kbar pressure

## Pressure Dependence of Threshold Voltage & Mobility

**Pressure Dependence of Threshold Voltage & Mobility**

- Threshold voltage versus pressure extracted from -2.5V gate bias data.

- Mobility versus pressure extracted from -2.5V gate bias data.

## Summary of experimental results

| Devices | Gauge factors |
| --- | --- |
| Ga- GaN Schottky diodes on sapphire  | -980 |
| freestanding N- GaN Schottky diodes | -1560 |
| Ga- AlGaN Schottky diodes on sapphire  | -1672 |
| AlGaN/GaN HFETs on sapphire  | +216 |
| GaN/AlGaN/GaN devices on SiC  | -500 |

**Summary of experimental results**

## SOA Pressure Sensors for Planetary Applications

**Vaisala BAROCAP sensor**
-  Proposed for current New Frontiers missions
-  Si-based capacitive pressure sensor
-  operational range: 0.05-1.1 bar
-  accuracy: ± 1.5x 10-4 bar
-  operational temperature range: -40 °C to 60 °C
-  packaged sensor with cooling system: several hundred cm3, ~2Kg
-  power ~2 W

**Kulite/Glenn SiC pressure sensor**
-  SiC diaphragm/resonator based pressure sensor
-  operational range: 1-70 bar
-  operational temperature range: -200 °C to 530 °C
-  Sensitivity at 25 °C: 20-80 V/V/psi
-  Sensitivity at 400 °C -500 °C: 50% of sensitivity at 25 °C