Fabrication and Scaling Effects of Very Short Gate-Length GaAs MESFETs

[摘要] A process has been demonstrated for the fabrication of scaled GaAs based metal-semiconductor field effect transistors (MESFETs) suitable for high frequency characterisation with gate-lengths down to 40nm. MESFETs were fabricated with gate-lengths in the range 40 to 300nm on molecular beam epitaxy (MBE) grown layers with GaAs and AlGaAs buffers. The MESFETs were characterised electrically at direct current (DC) and high frequency. The MESFETs have very good DC and high frequency performance, even down to the shortest gate-lengths. The performance is characterised by figures of merit such as transconductance and unity current gain cut-off frequency. MESFETs with a DC extrinsic transconductance of 720mS/mm and unity gain cut-off frequency of 150GHz (for a 40nm gate-length) have been demonstrated. However, the performance of the MESFETs is limited by scaling effects. Some of these effects are of a technological nature such as buffer layer current, a large gate resistance, surface depletion effects, and the high active layer doping. More importantly, the performance of the shortest gate-length MESFETs is restricted by the fundamental short-channel effects of punch-through and hot-electrons. The origin and function of many of these effects are introduced through a review of the operation and modelling of short-channel FETs. The MESFET design included features to minimise the technological scaling effects, i.e. 1) AlGaAs buffer layers to suppress buffer layer current. Experimental comparison was made with the GaAs buffer case. 2) High active layer doping to reduce the effect of surface depletion and maintain the channel aspect ratio, at the cost of deteriorated carrier transport. Devices with three different doping concentrations were compared. 3) A reduced gate-width with shorter gate-length to compensate for the larger series gate resistance. The experimental work described in this thesis investigates the scaling effects by electrical characterisation of the devices at DC and high frequency. The measurements presented and discussed include: 1) DC open channel and subthreshold transfer and output characteristics. 2) DC threshold voltage shift, equivalent to output conductance. 3) High frequency (up to 26.5GHz) S-parameters which yield the short-circuit current gain, maximum frequency of oscillation, and parasitic carrier transit delays through the channel. The design strategy was successful to the extent of overcoming many of the detrimental scaling effects for MESFETs with gate-lengths down to 200nm. The buffer layer current was suppressed by the AlGaAs buffer, and it was shown that interrupted growth of the AlGaAs buffer can surmount the problem of degradation of active layer quality associated with MBE growth of GaAs on AlGaAs. On the negative side, the technological problems of large gate resistance, surface depletion effects and very high active layer doping (which inhibits carrier transport) were found to significantly degrade the high frequency performance of the devices in this work. These are all issues which could be addressed by modifications to the device design. In addition, the parasitic carrier transit delays became more significant in degradation of high frequency performance as the gate-length was reduced. As for the more fundamental effects, the main conclusion of this thesis is that the ultimate scaling limits of hot-electron and punch-through effects govern the behaviour of the MESFETs with very short gate-lengths (in this case, less than 100nm). The most severely affected parameters are output conductance and subthreshold current. For example, the benefits of AlGaAs buffer are greatly reduced for 40nm gate-length MESFETs. Reduction of gate-length can still give an improvement in high frequency performance, but less than predicted by simple scaling of the carrier transit time and the beneficial hot-electron effect of velocity overshoot. No evidence of velocity overshoot effects has been found in these MESFETs.

[发布机构] University:University of Glasgow

[关键词] Electrical engineering, Materials science, Condensed matter physics