Details of Grant 

EPSRC Reference:	EP/C548825/1
Title:	Feasibility study of zinc-blende AIGaN heterojunction bipolar transistors
Principal Investigator:	Harrison, Dr I
Other Investigators:	Foxon, Professor CT Campion, Dr RP
Researcher Co-Investigators:
Project Partners:
Department:	Sch of Electrical and Electronic Eng
Organisation:	University of Nottingham
Scheme:	Standard Research (Pre-FEC)
Starts:	01 November 2005	Ends:	30 September 2006	Value (£):	62,011
EPSRC Research Topic Classifications:	Electronic Devices & Subsys. Materials Synthesis & Growth
EPSRC Industrial Sector Classifications:	Electronics
Related Grants:
Panel History:
Summary on Grant Application Form
The overall aim of this project is to determine whether high performance heterojunction bipolar transistors (HBTs) can be achieved using zinc-blende AIGaN structures. Over the last 10-years there has been significant interest in high power devices made from wideband gap semiconductors, including GaN and SiC. The large breakdown field of these devices allows them to be operated at high voltages and so, for a given current, the devices are able to generate higher output power. SiC and GaN are the two most promising wideband gap materials for electronic devices. SiC has a higher thermal conductivity, but GaN has a higher saturated velocity and breakdown voltage. Over the last few years there has been significant progress in GaN heterojunction field effect transistors (HFETs). Modem communication systems use complex digital modulation schemes and the signal envelope is not constant. Any non-linearities in the amplifiers will lead to signal distortion that will result in an increase in the bit error rate of the channel as well as adjacent channel interference. In general, HBTs are inherently more linear than HFETs and in recent years GalnP/GaAs HBTs have dominated the mobile phone hand set power amplifier market. If a GaN HBT could be manufactured, it would have superior performance in terms of linearity and have the same power handling capability as GaN HFETs.There have been several attempts to manufacture GaN HBTs, using the common wurzite polytype grown by metal organic vapour phase epitaxy (MOVPE). Wurtzite GaN is normally used in blue lasers and LEDs. The GaN HBTs suffer from two major problems. The mobility of p-type wurzite GaN is very low and the acceptor is very deep. As a result, the HBT base has a high resistivity and consequently the base resistance of the device will be large thus reducing the high frequency performance. The other major problem in wurtzite GaN HBTs is the leakage current across the collector. This is the cause of the anomalously high current leakage, which reduces the performance at high collector voltages. It has been shown that pure screw dislocations cause a high leakage current.We propose to overcome these two obstacles by using the zinc-blende (cubic) GaN polytype, which has high electron and hole mobilities and does not have threading dislocations in the growth direction. At Nottingham, using plasma assisted molecular beam epitaxy (PA-MBE) we developed a unique technique for the reproducible growth of the zinc-blende polytype of GaN, using arsenic as a surfactant. Recently, we developed reliable p-type doping of zinc-blende GaN using Mn as the dopant resulting in hole mobilities of >300cm2s-1 V-1 at a doping level of -2 1018cm-3. We obtained values of the ionisation energy of around 45-60meV. The high mobility and high hole concentration will potentially significantly reduce the base resistance, thus improving fMAX to above 100GHz. Since the depth of the acceptor is shallower in cubic GaN than in wurzite GaN, less dopants are required to achieve the desired hole concentration. This will have an important effect on the transport of the injected electrons through the base and hence the base transit time. The microdefects in wurtzite and zinc-blende GaN are different. Zinc-blende material does not have the vertical dislocations, but does have inclined stacking faults and so may not suffer from the same leakage phenomenon as the wurtzite material.The above suggests that HBTs based on zinc-blende AIGaN grown by PA-MBE may have superior performance, but this needs to be investigated and confirmed experimentally. The overall aim of this project is to determine whether high performance heterojunction bipolar transistors (HBTs) can be achieved using zinc-blende AIGaN structures. This feasibility study attempts to provide an answer to this question. If successful this may open up applications for high frequency GaN based HBTs.
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Organisation Website:	http://www.nottingham.ac.uk