MSD2024_25_Aula04_JFET_MESFET PDF

MICROELETRÓNICA: SISTEMAS E DISPOSITIVOS JFET e MESFET  Outline Field effect transistors – Introduction – Types – Comparison with BJTs JFETs – Introduction – Operation principle – Characteristics –...

MICROELETRÓNICA: SISTEMAS E DISPOSITIVOS JFET e MESFET  Outline Field effect transistors – Introduction – Types – Comparison with BJTs JFETs – Introduction – Operation principle – Characteristics – Bias MESFETs – Introduction – Basic structure – Operation Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Field effect transistors Introduction – The invention of the BJT has brought a great twist to the modern era of semiconductor technology. This device, along with its field-effect counterpart, known as the field-effect transistor (FET), has had a huge impact on virtually every area of modern life. – Practical field-effect transistors were first made in the form of JFET in 1953 and MOSFET in 1963. – The field-effect transistor has taken various forms: The junction field-effect transistor (JFET), The metal semiconductor field-effect transistor (MESFET), The metal-insulator-semiconductor field-effect transistor (MISFET), The metal-oxide-semiconductor field-effect transistor (MOSFET). Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Field effect transistors FETs – Field effect transistor is a unipolar-transistor, which acts as a voltage-controlled current device and is a device in which current between two electrodes is controlled by the action of an electric field at another electrode. – Field effect transistor is a device in which the current is controlled and transported by carriers of one polarity only and an electric field near one terminal controls the current between other two. Comparison with BJTs – The conventional bipolar transistor has two types of current carriers of both polarities (majority and minority) and FET has only one type of current carriers, p or n (holes or electrons) – The BJT is current controlled and FET is voltage controlled, with current flowing between two other terminals Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  FET vs BJT BJT FET Charge carriers Minority (in base) Majority (in channel) Flow mechanism Diffusion (in base) Drift (in channel) Barrier control Direct contact made to base Change induced by gate electrode Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Field effect transistors JFET – Junction Field Effect Transistor – FET = Field Effect Transistor – the flow of charge carriers is influenced by electric field – Unipolar device: current is conducted by majority carriers. No minority carrier storage. – High input impedance (109-1012 ), very low power needed for controlling Channel depletion layer JUNCTION FET: depletion layers of pn-junctions close the channel Most important parameter: Vp pinch-off voltage Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor JFETs basic structure – There are two types of JFETs: n- channel and p-channel. The n-channel is more widely used (µ). – There are three terminals: Drain (D) and Source (S) are connected to n- channel, Gate (G) is connected to the p-type material – Metals creating ohmic contacts are deposited at each terminal – The JFET will conduct current equally well in either direction and the source and drain leads are usually interchangeable. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics – The nonconductive depletion region becomes thicker with increased reverse bias at the pn junction. (Note: The two gate regions of each FET are connected to each other.) Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics – Seen now by grounding S and G terminals and setting VD to different values VD> channel height (L>2*2a) one can have 2 independent 1D-problems: Electric field in space charge layer only in y-direction Electric field in channel only in x-direction Gradual-channel approximation (Shockley) Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics – Assuming gate junctions as one-sided step junctions (p++/n), depletion layer width (h) obtained from: 𝑞𝑁𝑑 ℎ2 𝑉𝐶−𝐺 = 2𝜀𝜀0 With built-in potential 𝜓0 and externally applied voltage 𝜓(𝑥) − 𝑉𝐺 : 𝑉𝐶−𝐺 = 𝜓0 + 𝜓 𝑥 − 𝑉𝐺 At the pinchoff point, a=h and 𝑉𝑃 = 𝜓 − 𝑉𝐺 : 1/2 𝑞𝑎2 𝑁𝑑 ℎ(𝑥) 𝜓 𝑥 + 𝜓0 − 𝑉𝐺 𝑉𝑃 + 𝜓0 = = 𝑉𝑃0 = 2𝜀𝜀0 𝑎 𝑉𝑃0 VP – externally applied voltage to reach pinchoff VP0 – internal pinchoff voltage Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics – What about drain current? Only drift component (diffusion neglected as electron distribution in the channel assumed to be uniform before pinchoff): 𝑑𝜓 𝐼𝐷 = −𝑞𝜇𝑛 𝑛𝐴𝐸 = 𝑞𝜇𝑛 𝑁𝑑 2 𝑎 − ℎ 𝑊 𝑑𝑥 𝐿 𝑉𝐷 Integrating: 𝐼𝐷 𝑑𝑥 1 න =න 1− (𝜓 + 𝜓0 − 𝑉𝐺 ) 𝑑𝜓 0 2𝑞𝜇 𝑁 𝑛 𝑑 𝑊𝑎 0 𝑉𝑃0 2 1 3/2 3/2 𝐼𝐷 = 𝐺0 𝑉𝐷 − 𝑉 + 𝜓0 − 𝑉𝐺 − 𝜓0 − 𝑉𝐺 3 𝑉𝑃0 𝐷 2𝑞𝑎𝑊𝜇𝑛 𝑁𝑑 is the channel conductance without depletion layers where 𝐺0 = 𝐿 Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics – Representation of general ID equation before pinchoff (above pinchoff it is assumed for now that drain current is constant) 2 1 3/2 3/2 𝐼𝐷 = 𝐺0 𝑉𝐷 − 𝑉 + 𝜑0 − 𝑉𝐺 − 𝜑0 − 𝑉𝐺 3 𝑉𝑃0 𝐷 2𝑞𝑎𝑊𝜇𝑛 𝑁𝑑 𝐺0 = 𝐿 Let’s now examine in more detail linear and saturation regions Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics in linear region, 𝑉𝐷 ≪ 𝜓0 − 𝑉𝐺 – Binomial series expansion and simplification of general ID equation before pinchoff: 𝜓0 − 𝑉𝐺 𝐼𝐷 = 𝐺0 1 − 𝑉𝐷 𝑉𝑃0 N-JFET characteristics in saturation region, 𝑉𝐷 > 𝑉𝑃 + 𝑉𝐺 Substituting 𝑉𝐷 − 𝑉𝐺 = 𝑉𝑃 into the general ID equation: 2 𝜑0 − 𝑉𝐺 𝐺0 𝑉𝑃0 𝐼𝐷 = 𝐺0 −1 𝜑0 − 𝑉𝐺 + 3 𝑉𝑃0 3 In amplification applications JFET typically operated in saturation region Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics – FET as a Voltage-Controlled Resistor (linear region) The region to the left of the pinch-off point is called the ohmic region (more correctly, only for very low VDS, then curvature begins…). The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (rd) increases. ro rd = 2  VGS   1 -   VP  Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics - saturation – At the pinch-off point: any further increase in VDS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. ID is at saturation or maximum. It is referred to as IDSS. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET characteristics ID VDSmax. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET breakdown – At high levels of VDS the JFET reaches a breakdown situation. ID will increase uncontrollably if VDS > VDSmax. 𝑉𝐵 = 𝑉𝐷0 + 𝑉𝐺 Where VD0 is VB for VG=0 V Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor N-JFET transfer and output characteristics – It is easy to determine the value of ID for a given value of VGS from output curves – It is also possible to determine IDSS and VP by looking at the knee where VGS is 0 Output characteristics Transfer characteristics Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor Small signal parameters Slope / transconductance dI D Out gm = dVGS VDS = const In Output conductance dI D g0 = dVDS VGS = const Voltage gain uout  1  Au = = − g m  Rt   uin  g0  Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Junction Field Effect Transistor Cutoff frequency Maximum operating frequency fco – JFET no longer amplifying the input signal. I.e., current through input capacitance equal to output drain current: 𝐼𝑖 = 2𝜋𝑓𝑐𝑜 𝐶𝑔𝑠 + 𝐶𝑔𝑑 𝑉𝑔 = 2𝜋𝑓𝑐𝑜 𝐶𝐺 𝑉𝑔 𝐼𝑜 = 𝑔𝑚 𝑉𝑔 Remember that: But ideally this 𝑞𝑎2 𝑁𝑑 Equating Ii and Io: = 𝑉𝑃0 term shouldn’t be 2𝜀𝜀0 adjusted for 𝑔𝑚 𝑞𝑎2 𝜇𝑛 𝑁𝑑 maximum fco… 𝑓𝑐𝑜 = ≤ 2𝜋𝐶𝐺 4𝜋𝜀𝜀0 𝐿2 µ and L should be optimized for high-freq application! Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Introduction – MESFET = Metal Semiconductor Field Effect Transistor = Schottky gate FET. – The MESFET consists of a conducting channel positioned between a source and drain contact region. – The carrier flow from source to drain is controlled by a Schottky metal gate. – The control of the channel is obtained by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel and thereby the current. – Remember the differences between Schottky junction vs pn junction? They mostly dictate the difference in performance between JFET and MESFET (lower input capacitance and higher gm for MESFET). MESFET also turns possible the use of higher mobility semiconductors (see next)… Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Introduction – The operation is very similar to that of a JFET. – The p-n junction gate is replaced by a Schottky barrier, and the lower contact and p-n junction are eliminated because the substrate is now a semi-insulating one. By using GaAs instead of Si, a higher electron mobility is available Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Basic structure – GaAs MESFETs are the most commonly used and important active devices in microwave circuits. – In fact, until the late 1980s, almost all microwave integrated circuits used GaAs MESFETs. – Although more complicated devices with better performance for some applications have been introduced, the MESFET is still the dominant active device for power amplifiers and switching circuits in the microwave spectrum. – The base material on which the transistor is fabricated is a GaAs substrate. – A buffer layer is epitaxially grown over the GaAs substrate to isolate defects in the substrate from the transistor. – The channel or the conducting layer is a thin, lightly doped (n) conducting layer of semiconducting material epitaxially grown over the buffer layer. – Since the electron mobility is approximately 20 times greater than the hole mobility for GaAs, the conducting channel is always n-type for microwave transistors. Other common semiconductors in MESFETs: SiC (Silicon Carbide) – wide bandgap, for high-power, high-T applications GaN (Gallium Nitride) – even higher bangap, high-freq communication InP (Indium Phosphide) – higher mobility than GaAs, even higher freq (mmWave, 30-300 GHz) Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Basic operation – By applying a bias to the gate junction, the depletion depth and therefore the resistance of the current flow between the source and drain and the saturation current can be controlled. – If a large enough negative gate bias is applied, the depletion region depth will equal the channel depth, or the channel will be pinched off. – This gate bias is called the pinch-off voltage and is given by qN d 2 Vp = d d – channel depth 2 0 r – For the gate to have effective control of the channel current, the gate length L must be larger than the channel depth, d, or L/d > 1. – This requires a channel depth on the order of 0.05 to 0.3 µm for most GaAs MESFETs. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Basic operation – The most important feature of MESFET is that they may be used to increase the power level of a microwave signal, or that they provide gain. – Because the drain current can be made to vary greatly by introducing small variations in the gate potential, the MESFET can be modeled as a voltage- controlled current source. – The transconductance of the MESFET is defined as Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Basic operation – Using short-channel approximations, it can be shown that the transconductance may be written as – where IS is the maximum current that can flow if the channel was fully undepleted under saturated velocity conditions. – Since IS is proportional to the channel depth, d, and VP is proportional to the square of the channel depth, gm is inversely proportional to the channel depth. – In addition, note that for large IS and gm, the parasitic resistances RS and RD must be minimized. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Basic operation The most commonly used figures of merit for microwave transistors are the gain bandwidth product, the maximum frequency of oscillation, fmax, and the frequency where the unilateral power gain of the device is equal to one, ft. If short gate length approximations are made, ft can be related to the transit time of the electrons through the channel, , by the expression Since vsat is approximately 6 x1010 µm/s for GaAs with doping levels typically used in the channel, the gate length must be less than 1 µm for ft to be greater than 10 GHz. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Basic operation – The parameter fmax may be approximated by – where RG is the gate resistance. – From the above two expressions for ft and fmax, it is apparent that the gate length should be made as small as possible. – Both the limits of fabrication and the need to keep the electric field under the channel less than the critical field strength required for avalanche breakdown set the lower limit on L at approximately 0.1 µm. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Enhancement JFET and MESFET So far, our n-type JFETs and MESFETs required negative VG to deplete channel charge and bring devices to OFF state. But if a lightly doped and narrow conducting channel is used it is possible that the depletion layer established by the built-in potential is wide enough to pinch off the channel without VG. Normally-off or enhancement mode FET MESFET VT needs to be lower than Schottky diode VT, otherwise large vertical current flows (forward bias of junction) Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Enhancement JFET and MESFET So far, our n-type JFETs and MESFETs required negative VG to deplete channel charge and bring devices to OFF state. But if a lightly doped and narrow conducting channel is used it is possible that the depletion layer established by the built-in potential is wide enough to pinch off the channel without VG. Normally-off or enhancement mode FET Advantages/disadvantages of E-FET: In IC – single power supply, lower power consumption, high gain bandwidth product But since it operates in forward-bias region of the Schottky or pn junction, logic swing is small (e.g., large VG results in large vertical Ifwd) – requires very tight process control. It is thus harder to fabricate circuits containing high density of E-MESFETs than E- MOSFETs (next lecture) Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Metal Semiconductor Field Effect Transistor Applications – The high transit frequency of the MESFET makes it particularly of interest for microwave circuits. – While the advantage of the e-MESFET provides a superior microwave amplifier or circuit, the limitation by the diode turn-on is not easily tolerated. – Typically depletion-mode devices are used since they provide a larger current and larger transconductance and the circuits contain only a few transistors, so that threshold control is not a limiting factor. – The buried channel also yields a better noise performance as trapping and release of carriers into and from surface states and defects is eliminated. – The use of GaAs rather than Si MESFETs provides two more significant advantages: room temperature mobility is more than 5 times larger, while the saturation velocity is about twice that in silicon. it is possible to fabricate semi-insulating (SI) GaAs substrates which eliminates the problem of absorbing microwave power in the substrate due to free carrier absorption. if f > 2 GHz: GaAs transistors are usually used. If f < 2 GHz: Si transistors are usually used. Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET  Channel length modulation in JFET and MESFET When VD increases above VP, more free carriers are depleted from the channel. Length of depleted region increases and length of neutral channel decreases.  ( − Vt ) (1 + Vds ) Channel length modulation 2 I ds = V gs 2 Factor in saturation characteristics… Microeletrónica: Sistemas e Dispositivos Transístores: JFET e MESFET