ISOTYPE HETEROJUNCTION

An isotype heterojunction is different from an anisotype heterojunction in that the dopants of the two sides are of the same type. It can be an n-n heterojunction or a p-p heterojunction. The first heterojunction was the anisotype, which was suggested by Shockley in 1951, to be incorporated into the emitter-base junction to increase the current gain of a bipolar transistor. This application was analyzed in more detail by Kroemer in 1957. The isotype heterojunction had been studied in different material systems. These include Ge-GaAs by Anderson in 1962, InP-GaAs by Oldham and Milnes in 1963, Ge-GaAsP by Chang in 1965, and GaAs-AlGaAs by Womac and Rediker in 1972, by Chandra and Eastman and Lechner et al. in 1979. Theoretical analysis of the device has been presented by some of these authors, namely Anderson, Chang, and Chandra and Eastman.

An n-n isotype heterojunction uses the GaAs-AlGaAs system as an example. The layers are grown epitaxially. For good-quality heterostructure epitaxy, the lattice constants of the two materials have to be matched within « 5%. The heterointerface must be extremely abrupt to achieve rectification rather than have ohmic characteristics. This transition region has to be less than « 100 A thick.10-12 Also, for best rectification behavior, the doping level in the wide-energy-gap material should be non-degenerate and lighter than that in the narrow-energy-gap counterpart. Isolation between diodes can be achieved by mesa etching down to the substrate layer.

The Fermi level in isolated AlGaAs is higher than that in GaAs. Conceptually, upon contact of these two materials, electrons transfer from AlGaAs to GaAs, causing a depletion layer in AlGaAs and an accumulation layer in GaAs. Such an accumulation layer does not exist in the anisotype heterojunction. In comparison to a standard therm ionic-emission current of a Schottky-barrier diode, a few points are worthy of mentioning. The temperature dependence of the coefficient is now T instead of T2. It causes the forward current to have a more gradual exponential rise with voltage. The reverse current also becomes non-saturating. Another important deviation from a Schottky diode is that the barrier height becomes temperature dependent. This is implied in the derivation of the barrier height. Since the temperature dependence on current is a useful technique to measure parameters for thermionic-emission current, the barrier height can be eliminated. The transition between the two materials at the heterointerface has to be abrupt. This transition region has been shown to decrease the barrier height. A transition region of only * 150 A can reduce the barrier to the extent that rectification vanishes and ohmic behavior results.10-12

The barrier is formed by a thin wide-energy-gap material, sandwiched between two narrow- energy-gap materials. The I-V characteristics show that the current is symmetrical, and, at low temperature, nonlinear. The nonlinearity is due to the decrease of the effective barrier height with bias.