5.1 Composition of the simulated device geometry

Next: 5.2 Determination of the Transport Models Up: 5 Simulation Previous: 5 Simulation

5.1 Composition of the Simulated Device Geometry

In Figure 5.1 a typical scanning electron microscope (SEM) photograph of a HEMT is shown. The epitaxial layers cap, supply, channel, and buffer are indicated in the picture. The physical interfaces between the epitaxial layers are considered to be abrupt. The ohmic contacts source and drain can be identified by a rough metal/semiconductor interface which is caused by an alloying process. In contrast to source and drain the gate Schottky contact exhibits a smooth interface.

Figure 5.1 SEM photograph of a HEMT. The ohmic source and drain contacts can be identified by the alloy penetrating into the cap layers whereas the Schottky gate contact builds a sharp interface.

Note that at the ohmic contacts no metal penetrating through the cap layer into the supply or even into the channel layer can be observed. It is discussed that some material which behaves like a dopant in the semiconductor might diffuse into the layers or even that metal might reach the channel by spiking. An important issue which is still under discussion is the actual mechanism of current flow from the ohmic contacts into the semiconductor and through the heterojunction layers in HEMTs.

Based on the different assumptions basically three simulation models for the ohmic contacts can be developed and will be investigated in the following:

(A) Ohmic contact only on top of the cap layers.
(B) Ohmic contact on top of the cap layers with high doping between the contacts and the channel.
(C) Ohmic contact directly on the channel.

In Figure 5.2 a schematic cross section of the investigated HEMT_ref is shown. The hatched areas under the ohmic contacts indicate the regions for which the different models are used, i. e., nominal specified MBE-grown layers, nominal layers but with doping and contact metal.

Figure 5.2 Schematic cross section of the simulated HEMT_ref. The region for which different models are investigated are indicated by the hatched areas.

In Figure 5.3 measured transfer characteristics of HEMT_ref are shown (bold line without symbols) along with three different simulations. All three simulations are performed with the nominal layer sequence, HD transport model in the channel and in the supply, and DD model in all other semiconductor layers. One mechanism for electrons to cross the energy barrier between the channel is to heat up and surmount the energy barrier which is referred to as real space transfer (RST). This mechanism is included in all three simulations by applying the HD model in the channel.

Figure 5.3 Measured and simulated transfer characteristics. The simulations are performed with the nominal layer structure and a interface model with and without tunneling.

Using the contact model (A), i. e. ohmic contacts only on top of the cap layers, the simulation can be performed either with a thermionic emission (TE) model, which does not include tunneling, or with a thermionic field emission (TFE) model, i. e. including tunneling as described in Section 4.2.1.

The results using a TE model and contact model (A) (squared symbols in Figure 5.3) reveal very low current. Without tunneling obviously too few electrons are able to surmount the energy barrier between the channel and the supply. Even when a constant doping concentration of N_D = 3.0*10¹⁸cm^-3 is added in the simulation (contact model (B)) the characteristics are not improved significantly (triangle symbols in Figure 5.3). For the simulation applying a TFE interface model without additional doping (circles in Figure 5.3) the simulated and measured data agree very well.

As described before the third possible approach for the ohmic contacts of HEMTs is based on the assumption that metal from the ohmic contacts spikes through the AlGaAs supply layer facilitating an ohmic contact directly on the channel (contact model (C)). In this case most electrons would not have to cross the energy barrier between the channel and the supply but would be conducted directly from source through the channel to the drain. This is a very commonly used assumption for heterostructure devices [52, 53, 54].

Two aspects of this model will be investigated in the following. The first is the consequences of this contact model on the simulated current transport in HEMTs and the resulting characteristics. The second aspect is experimental results to gain more insight into the ohmic contacts of manufactured HEMTs.

In Figure 5.4 simulation results of the device with ohmic contacts directly on the channel (contact model (C)) are compared with simulation results of the device with nominal layer structure (contact model (A)). Both simulations were performed using the TFE interface model. The threshold voltage of the simulation with directly contacted channel was adjusted by assuming a 10 % lower delta doping concentration than for the simulation with contacts only on the cap. The experimental results of HEMT_ref and the two simulated characteristics are very similar near V_T. But for V_GS > 0.0 V the current in case of model (C) is much higher than in case of model (A) as well as the measured values.

Figure 5.4 Measured and simulated transfer characteristics. The simulations are performed with different geometric contact models. Either with source and drain contacts directly on the channel or source and drain only on top of the cap layers.

More insight is gained by Figure 5.5 which shows the two simulated and the measured g_m. The magnitude of the maximum transconductance g_{m max} and the V_GS where it is reached is among the most important DC parameters. Both are overestimated if the channel is contacted directly. Moreover, neither the reduction in g_m for V_GS > 0.0 V nor the second local maximum in g_m, which refers to the parasitic MESFET, is reproduced. Using contact model (A) with realistic parameters for the models the simulated and measured characteristics coincide extremely well. The used model parameters and the way they are obtained will be described in Section 5.3.2.

Figure 5.5 Measured and simulated transconductance. The simulations are performed with different geometric contact models. With source and drain contacts directly on the channel and source and drain only on top of the cap layers.

The simulation results with the contact model (A) and the measurement agree very well. Therefore this contact model seems to be more suitable though this is still not sufficient evidence. It has to be investigated whether the discrepancy of the simulation results with directly contacted channel is a consequence of an unsuitable contact model or just inadequate model parameters.

The different setups for the ohmic contacts lead to significant differences in the simulated carrier transport properties of the HEMTs. In Figure 5.6 and Figure 5.7 the current densities for the two cases are shown both for V_DS =2.0 V and V_GS = 0.5V. This corresponds to a bias point between the two local maxima of the transconductance given in Figure 5.5. In both cases the channel can be identified by the layer with the highest current density.

Figure 5.6 Current density of the HEMT geometry with directly contacted channel at V_DS = 2.0 V and V_GS = 0.5 V. In addition to the current conducted through the cap a large fraction is conducted directly from source through the channel to the drain. The geometry is not in linear scale.

If the ohmic contacts are directly on the channel as shown in Figure 5.6 most of the electrons flow directly from source through the channel to the drain contact. Therefore the current through the channel is still governed almost linearly by the gate and real space transfer is underestimated. If the contact resistance in the simulation would be reduced to zero and the current flow over heterojunctions prevented (no tunneling) this would correspond to the intrinsic transistor.

If the contacts are only on top of the cap layers as shown in Figure 5.7 the electrons have to cross the energy barrier between the channel and the supply beneath the end of the gate. Moreover, for high currents the electrons heat up and start to surmount the barrier already under the gate. Therefore more current is conducted in the barrier layers where their mobility is rather low and thus g_m is reduced. This effect is not only important for modeling the transfer characteristics but also to describe short channel effects as will be discussed in Section 5.3.2.2.

Figure 5.7 Current density of the HEMT geometry with contacts only on top of the cap layer at V_DS = 2.0 V and V_GS = 0.5 V. All electrons from the channel which contribute to I_D have to be partially conducted in AlGaAs layers. The geometry is not in linear scale.

The simulation study shown so far leads to the assumption that the measured device can be modeled very well with contacts only on top of the cap layer and an interface model which includes tunneling. More evidence that this contact model is a very realistic assumption is obtained by aspects obtained from experimental results.

The directly contacted channel model relies on the assumption that contact metal spikes through the cap and the barrier layers along with doping high enough to facilitate an ohmic contact to the channel. This mechanism either depends on the thickness of the cap and the barrier layers through which spiking should occur, or the spiking has to be much deeper than the thickness variations of the layers between ohmic contact and channel.

Figure 5.8 Schematic cross section of a power HEMT with different thickness of undoped supply layer.

To investigate this issue two DH-HEMTs with double recess were fabricated which basically differ only in the thickness d_S of 7 nm and 42 nm indicated in Figure 5.8. In Figure 5.9 the measured transfer characteristics of the two HEMTs are shown. The two devices were produced in different technology runs which resulted in a shift of V_T due to slightly different recess depths. A more positive V_T, obtained for the device with d_S = 42 nm, usually leads to a higher g_{m max} but to a lower I_{D max}. In addition to a more positive V_T this device has a 35 nm larger d_S than the other device, but I_{D max} is significantly larger. If the depth of spiking would be in the order of 120 nm (thickness of cap and supply) one would expect a reduced I_{D max}. Thus, if spiking is an appropriate assumption the depth has to be much deeper than 120 nm. It is believed that the reason for the enhanced I_{D max} is a reduction in crystal damage which usually occurs by etching the double recess in the cap layer as shown in Figure 5.8.

Figure 5.9 Measured transfer characteristics of two devices which differ only in their thickness of undoped AlGaAs supply layer between the ohmic contacts and the channel.

To close the chain of proof against spiking the depth of the ohmic contact metal penetrating into the semiconductor is estimated. Figure 5.10 shows a SEM photograph of a HEMT with alloyed ohmic contacts from the bottom. With a special preparation technique the semiconductor was removed. The Tshaped gate finger can be clearly identified between the ohmic source and drain metals. The light spots on the ohmic contacts show some remaining semiconductor material. The dimensions can be estimated by comparison to the gate length which is about 200 nm. It is evident that the total contact area is much smoother than 200 nm. Therefore it is clear that the HEMT cross section shown on the SEM photograph in Figure 5.1 is not a coincidental but a very typical one.

Figure 5.10 SEM picture of the contact metals of a HEMT from the backside with removed semiconductor. A gate finger as well as alloyed ohmic contacts on both sides are shown. Some remaining GaAs can be observed by the lighter spots on source and drain.

Non alloyed ohmic contacts on top of the cap layer have led to very good HEMT characteristics for InP based HEMTs [55, 56]. This is another argument that contacting the channel is not a prerequisite for excellent performance. In our case, sufficient evidence has be given for the contact model (A). It was further shown that the simulation of the nominal given layer sequence with a thermionic field emission interface model is well suited to describe the DC characteristics of the different HEMTs discussed here. Therefore this setup will be used for all following simulations as well.

Next: 5.2 Determination of the Transport Models Up: 5 Simulation Previous: 5 Simulation

Helmut Brech

1998-03-11