Modulation doped GaAs-AlGaAs quantum well based structures are usually used to achieve very high mobility 2-dimensional electron (or hole) gases. Usually high mobilities ($>10^{7}{rm{cm}^{2}rm{V}^{-1}rm{s}^{-1}}$) are achieved at high densities. A lo
ss of linear gateability is often associated with the highest mobilites, on account of a some residual hopping or parallel conduction in the doped regions. We have developed a method of using fully undoped GaAs-AlGaAs quantum wells, where densities $approx{6times10^{11}rm{cm}^{-2}}$ can be achieved while maintaining fully linear and non-hysteretic gateability. We use these devices to understand the possible mobility limiting mechanisms at very high densities.
We report our work on fabricating lithographically aligned patterned backgates on thin (50-60$mu$m) Roman{roman3}-Roman{roman5} semiconductor samples using {it single sided mask aligners only}. Along with this we also present a way to photograph both
sides of a thin patterned chip using inexpensive infra-red light emitting diodes (LED) and an inexpensive (consumer) digital camera. A robust method of contacting both sides of a sample using an ultrasonic bonder is described. In addition we present a mathematical model to analyse the variation of the electrochemical potential through the doped layers and heterojunctions that are normally present in most GaAs based devices. We utilise the technique and the estimates from our model to fabricate an electron-hole bilayer device in which each layer is separately contacted and has tunable densities. The electron and hole layers are separated by barriers either 25 or 15nm wide. In both cases, the densities can be matched by using appropriate bias voltages.
