Modulation doping is the process of doping the barrier layer in a HEMT device wth donors/n-type dopants to increase 2DEG density by bringing the minimum of conduction band energy below the fermi level. Can a device work without modulation doping? Let us find out using Silvaco TCAD.
Chosen structure
The electrical characteristics of HEMT depend on the entire stack, the simulation of the entire device would require a very high computation time due to heavy calculation of non-critical nodes in the mesh. We shall instaed only simulate the barrier and channel layer since the barrier layer will be doped and the channel layer forms the conduction region.
A channel length of 0.6 microns is utilized. The gate is a schottky contact and the drain and source electrodes are ohmic contacts. The AlGaAs material is light green in colour, that is the barrier layer.
The first simulation will consist of no doping and the second loop will add in a concentration of 4 x 1018 cm-3 n-type dopants. Silicon is an amphoteric dopant of III-V compound semiconductors. Amphoteric dopant means that it can act as an n-type dopant or as a p-type dopant depending on which lattice position it occupies in the structure.
Energy band diagrams and electron concentration
In figure 2, the Schottky contact at gate is reducing electron concentration underneath gate. The undoped barrier layer has a significantly lesser number of electrons near its channel region because of the absence of donor or free electrons in the structure.
Figure 3 shows the poor number of electrons present in the channel layer. A concentration of 4 x 106 electrons is not suitable for a conduction channel for a viable amount of current to flow through.
On comparison with the concentration of electrons in channel region due to modulation doping the difference is clear. Most of the electrons in barrier layer migrate to the upper part of channel layer to form the conduction channel called 2DEG.
The straddling gap heterojunction, also known as the type 1 heterojucntion is formed by AlGaAs and GaAs. The concentration of Al in AlGaAs is 30%. A higher percentage of Al would have increased the difference in energy gaps even more. Notice that the conduction band offset is larger than the valence band offset.
Modulation doping has changed the band structure, donors reduce the magnitude of difference between electron fermi level and the conduction band energy. The correct amount of modulation doping will cause the conduction band energy to decrease beneath the fermi level.
A better understanding of the scale of changes brought by modulation doping is seen in figure 7.
Characteristics of device
Drain current for the undoped HEMT was in the magnitude of 10-17A. It was equivalent to noise in a device, hence the straight line at zero ampere. The gate was unbiased, i.e. no voltage was applied. A considerable magnitude of drain current is flowing in the modulation doped HEMT proving its operation as a depletion mode transistor.
Applying a drain voltage of 0.1V and sweeping the gate voltage from -0.8 to 0.2V gives us a threshold voltage of around -0.57V. In the python code, I specified the point at which drain current reaches 10 microampere as the threshold voltage as extrapolating the slope of the linear region was giving me a very low threshold voltage.
Conclusion
An AlGaAs/GaAs HEMT cannot function without modulation doping. Modulation doping is required to obtain a conduction channel underneath the heterojunction interface. Without modulation doping, the electron density near interface will be very low due to absence of free electrons. As a result, the two-dimensional electron gas (2DEG) necessary for device operation cannot be formed. The channel conductivity decreases significantly, leading to very low drain current and poor transconductance characteristics. Device performance is severely degraded because carrier transport is no longer supported by a high-mobility electron channel. Therefore, modulation doping plays a critical role in supplying electrons to the GaAs channel while maintaining spatial separation between the electrons and ionized donor impurities, thereby enabling the high-speed and high-frequency operation of the HEMT.
Code source: Arjun’s Github
Frequently Asked Questions (FAQ)
Ques: What is Silvaco TCAD?
Silvaco TCAD (Technology Computer-Aided Design) is a software suite used to simulate and analyze semiconductor devices and fabrication processes. It allows engineers and researchers to model device behavior, predict performance, and optimize designs before manufacturing, reducing development time and cost.
Ques: What is Atlas module?
ATLAS is a device simulation module within the Silvaco TCAD environment. It is used to simulate the electrical, optical, and thermal behavior of semiconductor devices such as diodes, MOSFETs, HEMTs, and solar cells. ATLAS solves fundamental semiconductor equations to predict device characteristics under different operating conditions.
Ques: What is mesh?
A mesh is a network of small interconnected elements used to divide a device structure into many discrete regions for numerical simulation. In TCAD, physical equations are solved at the mesh points, allowing accurate calculation of electrical and material properties. A finer mesh generally improves accuracy but increases computation time.
Ques: What is modulation doping?
Modulation doping is a semiconductor doping technique in which dopant atoms are placed in a layer adjacent to, rather than directly within, the active conducting channel. This separation reduces impurity scattering while still supplying free carriers to the channel, resulting in higher electron mobility and improved device performance. The technique is widely used in HEMTs and other high-speed electronic devices.
Ques: What is an amphoteric dopant?
An amphoteric dopant is a dopant that can behave as either a donor or an acceptor depending on the crystal site it occupies within a semiconductor. Its electrical activity is influenced by factors such as growth conditions and material composition. Silicon in gallium arsenide (GaAs) is a common example of an amphoteric dopant because it can act as either n-type or p-type dopant.
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