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 E_g. 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 E_C to the material with the lower E_C where they can occupy a lower energy state. This can be a large number of electrons especially if the semiconductor with the high E_C 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 E_C than the channel material.

In  Figure 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 DE_V and a band gap offset in the conduction band DE_C. 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.

In  Figure 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 E_F is determined by DE_C, the doping level N_D, the barrier height of the Schottky contact qF_B, the gate to channel separation d_GC, and the applied voltage on the gate V_GS. To obtain high drain currents I_D and high transconductance g_m it is favorable to maximize qF_B, N_D, DE_C, and to minimize d_GC. If a homogeneously doped upper barrier layer is used qF_B, N_D, and d_GC are directly related to each other. A decrease in d_GC reduces the total doping in the barrier layer which shifts the threshold voltage (V_T) to more positive values and thus reduces I_{D max}. If N_D is increased F_B 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 N_D is not reduced by a reduction of d_GC. 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 .

F_B is about 650 meV and decreases if the separation between the delta doping and the gate gets below 10 nm. The achievable minimum d_GC 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 DE_C as well as the doping concentration in the barrier layers and its distance to the channel. To increase DE_C it is favorable both to reduce E_g of the channel material and to increase E_g of the barrier layers.
 

• 2.1 Limitations of the Channel Material InGaAs
• 2.2 Electron Energy Levels in Strained Quantum Wells
• 2.3 Limitations of the Barrier Material AlGaAs

   
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