Development of GaN-based Transistors
By Ibrahim Fakih and Stephen Yee, supervised by Prof. Ishiang Shih and Prof. Zetian Mi
McGill University, Department of Electrical, Computer and Software Engineering
Introduction
Simulation and Results
The GaN Transistor
A transistor is a semiconductor device used to switch and amplify electronic signals and
electrical power. It is the most fundamental building block in modern electronics; over the
years, it shrunk in size paving the way to ever smaller yet powerful electronic components
and devices such as today’s smart phones.
The GaN transistor is a very good high electron mobility transistor (Hemt), and is used in high
frequency and voltage applications; to test it, a GaN was fabricated and simulated.
Before fabricating and testing the transistors, the proposed design structure was simulated
and the below results were observed.
Silicon is the most favoured material used in fabricating digital logic transistors. This is
mainly due to its electrical properties (band gap) of switching and low power consumption.
Silicon however does not provide the same properties at high frequencies nor in amplifying areas where compound semiconductors such as GaAs and GaN are more favourable.
The GaAs Transistor
Fig. 8: GaN Electric Density
A standard hetero-structure GaAs transistor was designed and simulated using a new software
(Synopsis Sentaurus). In this transistor, the current flows through the quantum well, InGaAs
channel, and it was formed by using AlGaAs barriers (with spacers).
Fig. 6: GaN Hemt Structure
Fig. 1: GaAs Hemt Structure
The structure was simulated for typical transistor behavior, by plotting drain current versus
both drain and gate voltages. The results were compared with simulations done by another
student, and were found to be very comparable.
The AlGaN and GaN interface is used to form a 2DEG (Two-dimensional electron gas) at the
interface, and allow current to flow through the transistor. Because of their electrical
properties and conduction bands, a two dimensional sheet of electrons are accumulated at
the semiconductor interface.
Fabrication
Fig. 13: Drain Current at High Drain Voltages
Due to technical difficulties, the final stage of fabrication was not completed and the
transistors were not tested. However, the below results were obtained from other
students investigators who fabricated the exact same transistor (1).
By using different masks, the initial GaN wafer was transformed. Source and drain contacts
were created, some of GaN was etched away, and a gate contact was also added.
Fig. 9: Some of the stages during fabrication
Fig. 3: Drain Current v Drain Voltage Saturation
Fig. 12: Drain Current vs Gate Voltage
Fig. 7: GaN Band Structure
Titanium is used for its good property of adhesion between the semiconductor and the metal.
Due to its high conductivity, Gold is used for its property of forming an excellent ohmic
contact to allow for better probing. Finally, nickel is used to form a good Schottky gate
contact.
Fig. 2: Electric Current Density for GaAs Hemt
Fig. 11: Drain Current vs Drain Voltage
Fig. 10: Masks used for Fabrication
Fig. 14: Drain Current at High Drain Voltages
Fig. 15: Drain Current versus Gate Voltages
Future Improvements
Fig. 4: Drain Current v Gate Voltage
Due to technical problems with the diffusion pump, and few missing materials and equipment
at the time, it was not possible to finish fabricating the transistors and test them. Since it was
the first time fabricating, the transistors also became scratched and dirty.
Another simulation was conducted on the GaAs
transistor by varying the cap length and observing
its electrical properties. As expected, the
threshold voltage of the transistor gets smaller as
the cap length is shortened; this is due to the
reduced electric field.
After applying Mask 1
After etching the GaN
After applying Mask 2
With more hands-on experience with the equipment and handling of the samples, future
fabrication efforts should be smoother with more concrete results.
References
Fig. 5: Drain Current for Different Caps
After Applying Mask 3
After Applying Mask 4
(1) Yi Fan Qi, Andy Shih, Jeremy Wong,Wendi Zhou. (2011). Advanced Compound Semiconductors for High speed and High Power
Devices. McGill.
(2) Patel, D. (2011). Nitride semiconductors for efficient transistors and LEDs. McGill.