Chapter 1. Background on Microwave Transistors Mar. 7th, 2006

1.1 Introduction Since the invention of the bipolar transistor in 1947, semiconductor electronics has been advancing and evolving at an enormous pace. Microprocessors now contain hundreds of millions of transistors and Gbit DRAMs are commercially available. Si VLSI In past 30 years, the minimum feature size of production stage Si ICs decreased by a factor of about 0.7 and the capacity of DRAMs increased by a factor of 4 every 3 years.  Moore’s law Besides Si VLSI, there are other emerging fields in semiconductor electronics. Microwave electronics with microwave transistors is the most prominent one.

~1980s: military microwave application (performance was most important) 1990s: consumer applications (sufficient performance at lowest cost) The only semiconductor material used in Si VLSI is silicon. Furthermore, only 2 basic types of transistors are widely in the Si VLSI: MOSFETs and BJTs. For microwave electronics, a large variety of different semiconductor materials have been employed: Si, SiGe, GaAs, InP, further III-V compounds, and wide bandgap materials

Microwave transistors MESFETs (Metal-Semiconductor FETs) HEMTs (High Electron Mobility Transistors) MOSFETs BJTs HBTs (Heterojunction Bipolar Transistors) Circuits using microwave transistors: LNA, PA, mixers, frequency converters and multipliers, attenuators, and phase shifters Divided into in principle 2 groups: low-noise transistors and power transistors

1.2 Microwave Transistor Figures of Merit 1.2.1 The concept of 2-Port Networks Y parameters

Input admittance Reverse transfer admittance Forward transfer admittance Output admittance

S parameters are not defined as quotients of currents and voltages but as ratios of the powers of traveling waves. Despite that Y parameters cannot be measured in the microwave range, they are widely used for discussing the properties of microwave transistors. Y parameters are more closely related to device physics and are more interpretable for device engineers than S parameters.

1.2.2 The Problem of Stability A microwave transistor is capable of power amplification or sustained oscillation. Whether the transistor in a circuit will oscillate or not depends on the transistor itself, and on the source and load impedance. The stability behavior of a transistor can be described by the stability factor k: k>1: unconditionally stable k<1: conditionally stable

1.2.3 Power Gain Definitions The power gain is the ratio of the power P2 delivered from the transistor output to the load to the power P1 delivered from the signal source to the transistor input. The matching conditions between the signal source and transistor and between the transistor and load influence the power transfer. If a transistor is to achieve the maximum power gain, then power matching is required. (k>1)  conjugate matching  the maximum available gain

If k <1, auxiliary external admittances have to be connected If k <1, auxiliary external admittances have to be connected. The overall stability factor is

If the input and output of the whole network is conjugately impedance-matched, the maximum stable gain is The unilateral power gain U:

1.2.4 The Characteristic Frequencies fT and fmax The cutoff frequency fT is the frequency at which the magnitude of h21 (short-circuit current gain) = 1. The maximum frequency of oscillation fmax is the frequency at which the U = 1 (still provides a power gain).

1.2.5 Minimum Noise Figure and Associated Gain A figure of merit describing the amount of intrinsic noise produced in microwave transistors The magnitude of NF is dependent on the matching conditions at the input of the transistor, bias condition, and frequency. The power gain obtained from the transistor biased and matched for minimum noise is called associated gain Ga.

1.2.6 Output Power and Power-Added Efficiency Pout [dBm] = 10 log Pout [mW]

1.3 Historical View of Microwave Transistors 1.3.1 The Early Years Ge BJTs developed in 1958-1959 were the first transistors operating above 1 GHz. By 1963, Si BJTs became competitive and in 1970 almost all microwave transistors were Si BJTs.

It became clear in the early 1960s that Si is not the optimal semiconductor for microwave transistors. GaAs (having a 6-fold electron mobility and a higher maximum electron drift velocity compared to Si) is a far better material for high-speed transistors. In 1966 Mead presented the first GaAs MESFET. The first GaAs MESFET with practical microwave performance was reported in 1967 and showed a fmax of 3 GHz. In 1970, the record fmax of GaAs MESFET increased to 30 GHz. Si BJTs and GaAs MESFETs were the 2 only microwave transistor types in use in 1970s and early 1980s.

The critical dimensions to obtain good microwave performance are wB and L, both of which should be as small as possible.

1.3.2 Development of Microwave Transistors with Heterostrutures The use of heterostructures after 1980 offered the opportunity of tremendous progress toward improved high-frequency performance of microwave transistors. A heterostructure is a combination of at least 2 layers of different semiconductors with distinct bandgaps. 2 growth techniques MBE: Molecular beam epitaxy MOCVD: Metal-organic chemical vapor deposition

Mobility of sequences of n-type AlGaAs and undoped GaAs exceeds those of doped bulk GaAs or AlGaAs at room and lower temperatures. Selectively doped heterostructure FET (SDHT), modulation doped FET (MODFET), two-dimensional electron gas FET (TEGFET) The different bandgaps of AlGaAs and GaAs cause a bandgap difference ΔEG resulting in band offsets ΔEC and ΔEV in the conduction and valence bands at the heterointerface. In HEMT, a large ΔEC is desired. The transferred electrons are confined to a region only a few nanometers thick in the GaAs layer near the heterointerface, called the two-dimensional electron gas (2DEG ).

Because the 2DEG electrons are spatially separated from the donors, ionized impurity scattering is suppressed and the electron mobility in the channel is increased.

The second type using heterostructures is the HBT. In the early 1980s, practical HBTs using the AlGaAs/GaAs system could be successfully fabricated. The key part of a HBT is the emitter-base heterojunction with the bandgap of the emitter being larger than that of the base. Hole injection from the base into the emitter is strongly suppressed, and higher current gains compared to those in homojunctions BJTs can be obtained. The emitter injection efficiency (Ge) A large valence band offset ΔEV is desirable for HBT.

Using a high base doping density produces A low base resistance resulting in higher fmax and lower NFmin, A very thin base, leading to a short base transit time and thus a high fT.

When considering the lattice constants of the semiconductors, 3 different heterostructure types can be found: lattice matched, pseudomorphic, metamorphic heterostructures

It is possible to grow good quality heterostructures from materials with different lattice constants, provided the thickness of the grown layer does not exceed a certain critical value tc. If the grown layer is thinner than tc, its crystalline structure accommodates to that of the substrate material.  a lattice deformation in the grown layer (pseudomorphic layer) Since 1986, pseudomorphic AlGaAs/InGaAs/GaAs heterostructures with In contents in the range of 15-25% were successfully grown on GaAs substrates and used in pseudomorphic HEMTs.

Metamorphic type: uses a substrate material (GaAs) and overgrows a graded buffer layer (InAlAs) with a thickness much greater than tc. Because the buffer is extremely thick, dislocations arising at the interface substrate/buffer barely influence the electrical properties of the device layer on top of the buffer. The main advantage is that inexpensive GaAs substrates can be used to obtain high ΔEC values, and thus InP-HEMT-like performance can be attained with GaAs.

2 new research directions in 1990s 1.3.3 Recent Developments 2 new research directions in 1990s The application of the Si MOSFET as a microwave device. CMOS microwave circuits with operating frequencies up to 5 GHz. Si power MOSFET up to 2.5 GHz used in base stations of wireless communication systems. The investigation of wide bandgap semiconductors, such as SiC and III-nitride, for power transistors in the GHz range. Allows operating temperatures far exceeding those for Si and III-V transistors  new application in automobile and aircraft.

1.4 State of the Art of Microwave Transistors in 2001 1.4.1 III-V FETs Both the HEMT and GaAs MESFET are widely used microwave devices due to their simple structure and superior high-frequency performance. When the gate length is reduced down to the deep submicron range, the resistance of the small gate strip becomes large,  a negative influence on the gain and noise behavior at high frequencies.  Mushroom gates are frequently used to achieve a short gate length and a small gate resistance

Since the late 1980s, the focus of HEMT research has been shifted to systems that offer large conduction band offsets, such as pseudomorphic heterostructures on GaAs as well as lattice-matched and pseudomorphic structures on InP, all using InGaAs channel layers.

During the 1990s, m-HEMTs grown on GaAs substrates were investigated.

1.4.2 BJTs and HBTs The first successfully realized HBTs for microwave applications were based on GaAs.

The major disadvantage of InP HBTs in general in the brittle and expensive InP substrate. Also, the technology of InP HBTs is relatively immature compared to that of GaAs HBTs. A main advantage of III-V HBTs compared to III-V FETs is that high fT and fmax can be obtained without the limitation of photolithography.

III-V HBTs have inferior high-frequency noise behavior compared to III-V FETs. The main application of III-V HBTs is in microwave power amplifiers. HBTs offer much higher power densities than III-V FETs, which makes impedance matching easier and leads to smaller chip sizes.

SiGe HBT processes a huge advantage over the III-V HBTs in that SiGe HBT can be fabricated with the existing Si CMOS technology with only a few more steps added. The layer sequence from the bottom: Si substrate, n+-Si subcollector, n-Si collector, p-strained-SiGe base, and n-Si emitter

SiGe HBT – commercial status in the late 1990s. Unlike III-V HBTs, SiGe HBT noise figures are quite low. The main problem of Si and SiGe bipolar power transistors is the relatively low breakdown voltage. From the economical perspective, the main advantage of SiGe HBTs compared to III-V HBTs are (1) large diameter and inexpensive Si substrates, and (2) existing Si technology available for the production of SiGe HBTs.

1.4.3 Wide Bandgap Transistors Since 1990, wide bandgap semiconductors such as SiC and group III nitrides (AlN, GaN and AlGaN) have received increasing attention for high-power microwave applications due to their high breakdown field and their high electron peak and saturation velocities. In the case of SiC, there is more than 100 different polytypes with different crystal structures (4H SiC is the favorite material). The group III nitrides exist in 2 crystal types: wurtzite (hexagonal) and zincblende (cubic) The first commercial SiC MESFET was announced in 1999. Another class of wide bandgap FETs is AlGaN/GaN HEMTs.

1.4.4 Si MOSFETs The advances in CMOS processing, continuous scaling of gate length, progress in SOI (silicon on insulator) technology, and development of Si LDMOSFETs (laterally diffused MOSFET)  suitability of MOSFETs and CMOS for microwave applications The SOI concept seems to be more promising because of the ease of integration with other high-performance microwave components also fabricated on insulators. The noise performance of Si MOSFET will always be inferior to that of advanced III-V FETS because of their poorer electron transport properties.

Another problem of microwave Si MOSFETs is the fact that good fT, fmax, and NFmin can only be obtained with extremely scaled MOSFETS. The breakdown voltages of such MOSFETs may by too low for many practical applications. However, in the mass consumer markets, where cost and the ability for integration are of major concern, microwave Si MOSFETs have clear advantages over other microwave devices.

1.5 Outlook

HW1 Solve and explain the Schrödinger wave equation Explain the energy band, Fermi level and bandgap. Investigate simple crystal structures and explain them.