2 The Principles of a HEMT
HEMTs are field effect transistors where the current flow between two ohmic contacts, source and drain, is controlled by a third contact, the gate. Most often the gate is a Schottky contact. In contrast to ion implanted MESFETs HEMTs are based on epitaxially grown layers with different band gaps Eg. A schematic cross section of a HEMT with a Tshaped gate is shown in Figure 2.1.
In the vicinity of a semiconductor heterojunction electrons are transferred from the material with the higher conduction band energy EC to the material with the lower EC where they can occupy a lower energy state. This can be a large number of electrons especially if the semiconductor with the high EC barrier is doped. Near the interface a two dimensional electron gas (2DEG), the channel, is created. This way it is possible to separate the electrons in the channel from their donor atoms which reduces Coulomb scattering and hence increases the mobility of the conducting electrons. If the channel is built only by a single heterojunction the electrons are penetrating into the buffer under the channel very easily where their mobility is usually lower and the control of the gate is poor. To keep the electrons in the channel a second energy barrier below the channel can be introduced by a material with a higher EC than the channel material.
2.2 the band gaps of the most important III-V semiconductors and the
available substrates are shown. AlGaAs/InGaAs on GaAs substrate nowadays
is the most widely used material system and will be investigated in this
work. The principal advantages and disadvantages of HEMTs based on InP
substrates will be addressed in the appropriate sections.
If two semiconductors with different band gap energies are joined together the difference is divided up into a band gap offset in the valence band DEV and a band gap offset in the conduction band DEC. One of the most common made assumptions for the AlGaAs/InGaAs material system is 40 % valence band offset and 60 % conduction band offset. This is only valid for Al contents below about 45 %. For higher Al contents the bandgap of AlGaAs changes from direct to indirect.
2.1 such an AlGaAs/InGaAs HEMT with a delta doped upper barrier layer
is shown. The conduction band energy under the gate along the cutting line
AA' is shown in Figure2.3.
The conduction band of the channel relative to the Fermi level EF
is determined by DEC, the
doping level ND, the barrier height of the Schottky contact
qFB, the gate to channel
separation dGC, and the applied voltage on the gate VGS.
To obtain high drain currents ID and high transconductance
gm it is favorable to maximize qFB,
and to minimize dGC. If a homogeneously doped upper barrier
layer is used qFB,
ND, and dGC are directly related to
each other. A decrease in dGC reduces the total doping
in the barrier layer which shifts the threshold voltage (VT)
to more positive values and thus reduces ID max. If ND
is increased FB of the Schottky
contact is reduced.
This direct trade-off can be overcome if a delta doping is used. A delta doping in an (Al)GaAs layer can be realized by growing pure silicon for a short period of time within the growth of an undoped AlGaAs layer. This way ND is not reduced by a reduction of dGC. The sheet doping concentration can be adjusted by the amount of silicon incorporated in the crystal and the activation of the dopands. The activation depends on various parameters of MBE growth such as substrate temperature. The upper limit of activated sheet doping concentration is in the order of .
FB is about 650 meV and decreases if the separation between the delta doping and the gate gets below 10 nm. The achievable minimum dGC highly depends on the applied technology. Tight process control can yield .
The aim of the channel is to provide a high current density. The electron
concentration is mainly determined by DEC
as well as the doping concentration in the barrier layers and its distance
to the channel. To increase DEC
it is favorable both to reduce Eg of the channel material
and to increase Eg of the barrier layers.
Next: 2.1 Limitations of the Channel Material InGaAs Up: Dissertation Helmut Brech Previous: 1 Introduction
Helmut Brech 1998-03-11