Text 3A

Read text 3A and answer the questions after the text

Memorize the following basic vocabulary and terminology to text 3A

LESSON 3

1. Hall effect sensors – датчики Холла; датчики, работа которых основана на явлении Холла;

2. proximity switches – бесконтактный переключатель;

3. harsh environmental conditions – суровые природные условия;

4. heterojunction – гетеропереход;

5. band gap – запрещенная зона;

6. intrinsic carriers – собственные носители заряда;

7. elevated temperatures – повышенная температура;

8. to sacrifice in mobility – снижение подвижности;

9. breakdown voltage – напряжение пробоя;

10. sheet carrier densities – пленочная плотность заряда;

11. residual charge – остаточный заряд;

12. annealed contacts – отожженные контакты;

13. to dice – нарезать кристаллы из полупроводниковой пластины;

14. biasing current – ток смещения, ток подмагничивания;

15. linear regression fitting technique – метод линейно-регрессионного приближения;

AlGaN/GaN heterojunction for Hall Effect sensors

Hall effect sensors are widely used as proximity switches, position sensors, velocity sensors, and in current sensing applications. Although it is one of the best known solid-state devices, the Hall effect sensor is still attracting considerable attention. One of the areas of interest is to enable Hall effect sensors to function at high temperatures (>250 °C) and other harsh environmental conditions; those operating conditions are of prime importance for a wide range of industrial and military applications. Conventional Hall effect sensors are mainly based on silicon or compound semiconductors, such as InAs, InSb, and GaAs. The advantage of using silicon as the active layer of a Hall effect sensor is its easy integration with signal-conditioning circuits and its low cost. InAs, InSb, GaAs, and related heterojunctions typically have very high electron mobilities. As a result, Hall effect sensors based on these materials can have fairly high magnetic field sensitivity at low temperature up to room temperature. However, due to the narrow band gap of these materials, thermal activation of intrinsic carriers can significantly change their transport properties at higher temperature (>200 °C).

The resulting large temperature cross sensitivity makes these sensors unusable at temperatures above approximately 200 °C. By heavily doping the active layer, the extrinsic state of these semiconductors may be maintained to somewhat elevated temperatures; however, due to a sacrifice in carrier mobility, this approach is quite limited. Currently most Hall effect sensors on the market have a peak specified operation temperature of 200 °C or lower. AlGaN/GaN heterostructures have been a subject of intense investigation recently due to their high potential to be used for high temperature, high power radio frequency (rf) electronics.

The wide band gap of GaN-related materials leads to low intrinsic carrier concentration and high breakdown voltage, which is a requirement for extremely high microwave or millimeter wave power applications. Further contributing to the outstanding performance of AlGaN/GaN-based heterojunction transistors is their ability to form a two-dimensional electron gas (2DEG) with sheet carrier densities of 10¹³ cm⁻² or higher near the interface without intentional doping. This is well in excess of those achievable in most other III-V material systems. It has been demonstrated previously that the spontaneous and piezoelectric polarizations play an important role on the 2DEG formation and confinement at the AlGaN/GaN interface.

The AlGaN/GaN heterojunction structures were grown by metal organic chemical vapor deposition on sapphire substrates. The epitaxial layer consisted of a 2 μm undoped GaN buffer layer and a 25 nm Al_0.3Ga_0.7N barrier layer. A sheet resistance of 250 Ohm/sq was obtained from Leighton measurement at room temperature. A capacitance voltage profile showed a 2DEG at the junction interface and no residual charge was found in the AlGaN barrier or the GaN/sapphire interface.

Square-shaped Hall devices were defined by optical photolithography followed by mesa etching using an inductively coupled plasma reactive ion etching system in a chlorine-based plasma. Ti/Al/Mo/Au Ohmic contacts were deposited by electron beam evaporation and annealed at 800 °C for 1 min. The processed wafer was diced into 3x3 mm² chips. Each chip was mounted on top of a resistive heater element using high temperature epoxy. The combined assembly was mounted onto a chip carrier for testing. The heater element was used to raise and maintain the surface temperature of the Hall sensor and was calibrated using a high-precision infrared camera.

The output Hall voltage of the AlGaN/GaN Hall effect sensor as a function of temperature and magnetic induction curves have good linearity and are very close to one another from room temperature up to 300 °C. With increasing temperature, no degradation of functionality of the sensor is observed. In fact, the Hall voltage is larger at higher temperatures, which is shown by the larger slope at higher temperatures. As the measurements suggest, 300 °C is not a limiting temperature for operation. The magnetic induction and biasing current were fixed at 1.7 kG and 3 mA, respectively. The current-related magnetic sensitivity increases from approximately 54.5 to 56.5 V/(A_*T) between room temperature (RT) and 300 °C. Using a linear regression fitting technique, the temperature coefficient of magnetic sensitivity is calculated to be 103 parts per million (ppm)/°C. This is an excellent value even for conventional Hall effect sensors made for low temperature operation. (3997)