pHEMTs for Microwave and Millimeter-Wave Applications
Pseudomorphic high electron mobility transistors (pHEMTs) are devices that consist of heterojunctions.
The saturated electron velocity in the semiconductor materials used in pHEMTs is high compared to HEMTs.
pHEMTs are extensively used in monolithic microwave-integrated circuits.
pHEMTs are extensively used in monolithic microwave-integrated circuits
The interest in quantum well structure behavior and developments in crystal growth techniques and modulation doping have led to the creation of pseudomorphic high electron mobility transistors (pHEMTs). Pseudomorphic high electron mobility transistors are devices that consist of heterojunctions, mostly of the semiconductor materials belonging to groups III and IV. The materials are arranged to bring abrupt transitions atomically in composition and doping. The microwave frequency performance of pHEMTs is superior to conventional field effect transistors, and they are often used in military systems.
High Electron Mobility
Electron mobility is one of the crucial aspects of pHEMTs. Fast electron mobility is not practical in conventional technologies such as MESFETs. The room temperature limits electron mobility due to the scattering of ionized impurities and optical phonons. The increase in doping levels is another method employed to improve electron mobility. However, impurity scattering becomes more prominent as the doping level is increased. With these limitations, it is not feasible to increase the electron mobility of MESFETs by increasing the sheet density in the channel.
Pseudomorphic high electron mobility transistors are an exception in semiconductor devices, as they offer high electron mobility.
The Evolution of pHEMTs
The demand for high electron mobility transistors led to the development of a range of transistor technologies such as high electron mobility transistors (HEMTs), modulation doped field effect transistors (MODFETs), selectively doped heterojunction transistors (SDHTs), 2-dimensional electron gas FETs (TEGFETs), and pseudomorphic high electron mobility transistors (pHEMTs).
Among all these technologies, pHEMTs gained popularity due to their high performance in power and low-noise applications. Their performance advantages include:
- Larger band gap discontinuity between the semiconductor materials. For example, AIGaAs and InGaAs. The large bandgap between the materials produces more charge transfer or higher sheet carrier density, thereby increasing the device current. This is why pHEMTs offer high current ratings compared to HEMTs.
- The saturated electron velocity in the semiconductor materials used in pHEMTs is high compared to HEMTs. For example, InGaAs-based pHEMTs exhibit better frequency performance as well as higher gain compared to GaAs-based HEMTs.
- InGaAs-based pHEMTs offer improved carrier confinement in the channel. The larger band gap discontinuity also aids the carrier confinement and produces higher output conductance in pHEMT devices.
- As the pHEMT charge density in the channel is high compared to HEMT, the semiconductor layer (usually AlGaAs) above the channel can be overdoped without introducing excessive parasitic current. The access resistance to the channel is reduced by the higher doping level.
pHEMTs offer high gain, low noise figures, and high power levels with high current compared to HEMTs.
To achieve high electron mobility in a device, quantum well heterostructures are developed in pHEMTs. Previous technologies, such as MESFETs, were limited in their ability to enhance electron mobility, which is not a problem in pHEMT technology due to heterojunctions.
The pHEMT structure is designed so that it decouples the mobile carrier from the scattering mechanism. Decoupling happens through the physical separation of the mobile carriers from the dopant ions. The separation is established by juxtaposing a wide bandgap undoped semiconductor with a narrow bandgap undoped semiconductor.
2-Dimensional Electron Gas (2DEG) in pHEMTs
Wide and narrow bandgap semiconductors differ in their energy levels. The potential energy of the conduction band of narrow-bandgap undoped material is less than wide-bandgap-doped material. The difference in energy levels causes the electrons to move to the lower energy region. However, the electric field between the separated electrons and donor ions opposes the charge transfer. The same electric field changes the band potential and accumulates the carriers in the narrow bandgap material in the immediate vicinity of the wide bandgap material.
A triangular quantum well region confines the carriers in the narrow bandgap material. The thin quantum well region helps in forming the 2-dimensional electron gas (2-DEG). The 2-DEG consists of heterojunctions that significantly reduce scattering and result in high electron mobility in the structure.
pHEMTs are extensively used in monolithic microwave-integrated circuits. They offer good performance at high frequencies and are used in microwave systems, communication systems, and military devices. pHEMT technology showcases high power-added efficiency (PAE) with low noise figures. High PAE and noise indices make their way with satellite communication systems.
The following pHEMT characteristics make them particularly advantageous for millimeter and microwave devices:
- High maximum current for a given gate voltage swing
- Lower minimum current due to sharper pinch-off
- Lower knee voltage
- Higher gain at high power levels
Cadence’s suite of tools can help you design millimeter and microwave circuits using HEMTs and pHEMTs. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.