Browse Prior Art Database

High Speed HEMT Memory Device

IP.com Disclosure Number: IPCOM000060524D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Codella, CF: AUTHOR

Abstract

A very fast, high density memory element can be developed by tailoring the doped layer in a high electron mobility transistor (HEMT). The HEMT is a MESFET (metallized semiconductor field-effect transistor) which achieves very high electron mobility in the channel. This is partially due to the physical separation of the conduction electrons from their donor impurities. This effect is achieved by means of a heterojunction comprised of a heavily doped (Nd about 1018Si), wide bandgap semiconductor AlxGal-xAs layer 2 (as seen in Fig. 1) grown epitaxially on top of an undoped, narrow gap semiconductor, GaAs 4. The bandgaps are about 1.7 eV and 1.4 eV, respectively. The conduction electrons 3 are confined to a sheet of charge just below the interface due to this bandgap difference.

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High Speed HEMT Memory Device

A very fast, high density memory element can be developed by tailoring the doped layer in a high electron mobility transistor (HEMT). The HEMT is a MESFET (metallized semiconductor field-effect transistor) which achieves very high electron mobility in the channel. This is partially due to the physical separation of the conduction electrons from their donor impurities. This effect is achieved by means of a heterojunction comprised of a heavily doped (Nd about 1018Si), wide bandgap semiconductor AlxGal-xAs layer 2 (as seen in Fig. 1) grown epitaxially on top of an undoped, narrow gap semiconductor, GaAs 4. The bandgaps are about 1.7 eV and 1.4 eV, respectively. The conduction electrons 3 are confined to a sheet of charge just below the interface due to this bandgap difference. As a result, they experience greatly reduced impurity scattering and higher mobility. The metal gate is indicated by 1. The nature of the doped AlxGal- xAs is the key element in this invention. As the mole fraction of aluminum (x) is changed, there is a marked change in the Si donor level energy as indicated in the graph in Fig. 2. The deeper the donor level, the greater its ionization level and the less likely that it will be ionized. The HEMT device shown in Fig. 3 has been constructed with the AlxGal-xAs layer so tailored in a dual configuration as to have the mole fraction of Al as 0.3 in its upper portion 2A and the mole fraction as 0.2 in the lower portion 2B. With this tailoring, the donors near the heterojunction are shallow and those near the surface are deep. The deep states near the surface can be made to be occupied or unoccupied by an externally applied electric field from the gate electrode. This makes it possible to have the device to be off or on. Source and drain are shown as 12 and 13, respective...