Emitter-Base Separation Techniques for Resonant Tunneling Light Emitting Transistors
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conditions on the growth accuracy of the
7
quantum-well thickness. Two variations
on the layer structure allow us to obtaina high optical peak-to-valley ratio for almost any wavelength. The first layer structure prepares the injected carriers in an additional quantum well. Excellent optical peak-to-valley ratio's have been 3 obtained in triple barrier RTLED's . The second device is the Resonant Tunneling Light Emitting Transistor (RTLET) 4 . This transistor (see figure 1) injects the
Optical output
collector
+
h-
Emitter Figure 1. Principle of operation of the RTLET
431 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
electrons and holes independently in the tunneling quantum well by using a quantum well base layer below the tunneling structure. The charge in the base quantum well is unable to completely screen the emitter-collector voltage due to its quantisation. 4 This makes both the hole current (emitter) and electron current (base) independently controllable. The emitter-collector current can be brought into oscillation, and the optical output of the device will oscillate at the same frequency modulated by the amplitude of the current injected in the base layer. THE CONTACT ON A QUANTUM WELL BASE LAYER Two major technological problems hinder the realization of a good Ohmic contact on a 2-Dimensional Electron Gas (2DEG) acting as the base of a resonant tunneling transistor i.e.: the leakage current to the collector and the emitter-base separation. The first task is to prevent leakage current from the quantum-well base layer to the collector. Conventional contact technologies involving alloying can not be used because their penetration is deeper than the base quantum-well width.5 Non-alloyed6 contacts have proven to deliver good Ohmic contacts on both n- and p-type materials. The consumption of the toý-layer (typical 4 nm) does not disturb the quantisation in the layers directly below . However, also the emitter-base separation is important. An overlap of the base contact layer with the emitter sidewall is not allowed due to the high leakage current to the emitter. An additional Schottky-barrier guard ring on top of the emitter layer (see figure 2.a) could reduce this leakage but its control will never be accurate enough to deplete the emitter layer without having any influence on the base quantum well layer. One also has to take into account that the AlAs barriers are underetched during most of the selective processes.
Guard Ring
Emitter Base
Figure 2.
Base
The contact on a quantum well base layer using a guard ring and an overlap to the emitter (a) and using a free base layer (b). The hatched area is the depletion region.
The other alternative is to leave a free base surface between the base contact and the emitter layer (see figure 2.b). The transistor layer can be designed such that the injected base charge does not recombine under the free base surface 4 . However, it is important to restrict the length of this free base surface to the absolute minimum 8 to maintain a s
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